View source for Axolotl

{{Short description|Species of salamander}}
{{Other uses}}
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{{Speciesbox
| name = Axolotl
| image = Axolotl ganz.jpg
| image_caption = The [[wild type]] form
| status = CR
| status_system = IUCN3.1
| status_ref =<ref name="iucn status 12 November 2021">{{cite iucn |author=IUCN SSC Amphibian Specialist Group |date=2020 |title=''Ambystoma mexicanum'' |volume=2020 |article-number=e.T1095A53947343 |doi=10.2305/IUCN.UK.2020-3.RLTS.T1095A53947343.en |access-date=12 November 2021}}</ref>
| status2 = CITES_A2
| status2_system = CITES
| status2_ref =<ref name = "CITES">{{Cite web|title=Appendices {{!}} CITES|url=https://cites.org/eng/app/appendices.php|access-date=2022-01-14|website=cites.org}}</ref>
| genus = Ambystoma
| species = mexicanum
| authority = ([[George Shaw (biologist)|Shaw]] and [[Frederick Polydore Nodder|Nodder]], 1798)
| synonyms = {{species list
 | Gyrinus mexicanus | Shaw and Nodder, 1798
 | Siren pisciformis | Shaw, 1802
 | Siredon axolotl | Wagler, 1830
 | Axolotes guttata | Owen, 1844
 | Siredon Humboldtii | Duméril, Bibron, and Duméril, 1854
 | Amblystoma weismanni | Wiedersheim, 1879
 | Siredon edule | Dugès, 1888 }}
| synonyms_ref =<ref name=Frost>{{cite web |url=http://research.amnh.org/vz/herpetology/amphibia/Amphibia/Caudata/Ambystomatidae/Ambystoma/Ambystoma-mexicanum |title=''Ambystoma mexicanum'' (Shaw and Nodder, 1798) |author=Frost, Darrel R. |year=2018 |website=Amphibian Species of the World: an Online Reference. Version 6.0 |publisher=American Museum of Natural History |access-date=10 August 2018}}</ref>
| range_map = {{maplink|frame=yes|plain=yes|from=Ambystoma mexicanum range.map}}
| range_map_caption = IUCN range of the axolotl
{{leftlegend|#FF0000|Axolotl (''Ambystoma mexicanum'')}}
}}

The '''axolotl''' ({{IPAc-en|ˈ|æ|k|s|ə|l|ɒ|t|əl|audio=en-us-axolotl.oga}}; from {{langx|nci|āxōlōtl}} {{IPA|nah|aːˈʃoːloːtɬ||Axolotl.ogg}}) ('''''Ambystoma mexicanum''''') is a [[neoteny|paedomorphic]] [[salamander]], meaning they [[Sexual maturity|mature]] without undergoing [[metamorphosis]] into the terrestrial adult form; adults remain [[Aquatic animal|fully aquatic]] with obvious [[external gills]]. This trait is somewhat unusual among [[amphibian]]s, though this trait is not unique to axolotls, and this is apparent as they may be confused with the [[Tadpole|larval stage]] or [[Ambystoma#Species|other neotenic adult mole salamander]]s (''Ambystoma'' spp.), such as the occasionally paedomorphic tiger salamander (''[[Ambystoma tigrinum|A. tigrinum]]'') widespread in North America; or with mudpuppies (''[[Necturus]]'' spp.), which bear a superficial resemblance but are from a different [[Family (biology)|family]] of salamanders.<ref name="axolotl bio">{{Cite journal| last= Malacinski| first= George M. |journal=American Zoologist |title=The Mexican Axolotl, ''Ambystoma mexicanum'': Its Biology and Developmental Genetics, and Its Autonomous Cell-Lethal Genes |date= Spring 1978 |doi=10.1093/icb/18.2.195 |volume=18 |issue=2 |pages=195–206|doi-access=free }}</ref>

Axolotls originally inhabited a system of interconnected [[wetland]]s and lakes in the Mexican [[highland]]s; they were known to inhabit the smaller lakes of [[Lake Xochimilco|Xochimilco]] and [[Lake Chalco|Chalco]], and are also presumed to have inhabited the larger lakes of [[Lake Texcoco|Texcoco]] and [[Lake Zumpango|Zumpango]]. These waterways were mostly [[Hydrogeology|drained]] by [[Spanish colonization of the Americas|Spanish settlers]] after the [[Spanish conquest of the Aztec Empire|conquest of the Aztec Empire]], leading to the [[Habitat destruction|destruction]] of much of the axolotl's natural habitat, which is now largely occupied by [[Mexico City]]. Despite this, they remained abundant enough to form part of the [[Staple food|staple]] in the diet of native [[Mexica]] during the [[Spanish colonization of the Americas#Mexico|colonial era]].<ref name="NYT">{{cite news|title=Mythic Salamander Faces Crucial Test: Survival in the Wild|url=https://www.nytimes.com/2012/10/31/world/americas/struggle-of-axolotls-mexicos-mythical-salamander.html?pagewanted=all&_r=1|access-date=30 July 2015|newspaper=The New York Times| date=30 October 2012 | last1=Tickell | first1=Sofia Castello Y. }}</ref> Due to continued urbanization in Mexico City, which causes [[water pollution]] in the remaining waterways, as well as the introduction of invasive species such as [[tilapia]] and [[carp]], the axolotl is near [[extinction]], the species being listed as [[critically endangered]] in the wild, with a decreasing population of around 50 to 1,000 adult individuals, by the [[International Union for Conservation of Nature]] (IUCN) and is listed under [[Appendix II]] of the Convention on International Trade in Endangered Species (CITES).<ref name="CITES"/>

A large captive population of axolotls currently exist, with the specimens being [[Animal testing|used extensively in scientific research]] for their remarkable ability to [[Regeneration (biology)|regenerate]] parts of their body, including limbs, gills and parts of their eyes and brains. In general, they are [[model organism]]s that are also used in other research matters, and as [[aquarium]] technology developed, they have become a common exhibit in [[zoo]]s and [[Public aquarium|aquariums]], and as an occasional pet in home aquaria. Axolotls are also a popular subject in contemporary culture, inspiring a number of [[Creative work|works]] and characters in media.

==Nomenclature==
{{Expand section|taxonomic history (look at those synonyms)|date=June 2025}}
The term "axolotl" is a [[Nahuatl]] word which has been translated variably; it may be interpreted as "water slave", "water servant", "water sprite", "water player", "water monstrosity", "water twin", or "water dog".<ref name="ADW-UM"/><ref>{{cite web |title=Meet the Peter Pan of salamanders, the axolotl |url=https://www.worldwildlife.org/magazine/issues/summer-2021/articles/meet-the-peter-pan-of-salamanders-the-axolotl |website=worldwildlife.org |publisher=World Wildlife Fund |access-date=4 June 2025}}</ref><ref name="Handbook"/> The word refers to the [[Aztec mythology|Aztec God]], [[Xolotl]], who holds dominion over multiple aspects such as [[List of fire deities|fire]], [[List of thunder deities|lightning]], [[List of death deities|the dead]] and those [[Resurrection|resurrected]], dogs, games, grotesque or ugly beings, and twins, as he is the twin of [[Quetzalcōātl]].<ref>{{cite web |title=The axolotl in pre-Hispanic mythology |url=https://museodelaxolote.org.mx/en/the-axolotl-in-pre-hispanic-mythology/ |website=museodelaxolote.org.mx |publisher=Axolotitlán |access-date=4 June 2025}}</ref><ref name="ADW-UM">{{cite web |last1=Majchrzak |first1=Amy |title=Ambystoma mexicanum Salamandra ajolote |url=https://animaldiversity.org/accounts/Ambystoma_mexicanum/ |website=animaldiversity.org |publisher=University of Michigan |access-date=4 June 2025}}</ref>

Some sources prefer the term "'''Mexican axolotl'''" to refer to this species unambiguously, as "axolotl" may be used for unmetamorphosed individuals of other ''Ambystoma'' species,<ref name="Handbook">{{cite book |last1=Humphrey |first1=Rufus R. |editor=Robert C. King |title=Handbook of Genetics Volume 4: Vertebrates of Genetic Interest |date=1975 |publisher=Springer |location=New York |isbn=978-1-4613-4470-4 |doi=10.1007/978-1-4613-4470-4 |pages=3–17 |edition=4 |url=https://link.springer.com/chapter/10.1007/978-1-4613-4470-4_1 |access-date=4 June 2025}}</ref><ref>{{Cite Merriam-Webster|Axolotl|access-date=2025-06-04}}</ref> though the word is most commonly used to refer to wild ''A. mexicanum'' and captive individuals.

==Description==
{{Multiple image
| image1            = Axolot's head (Ambystoma mexicanum).jpg
| direction         = vertical
| image2            = Axolot's_gills_(Ambystoma_mexicanum).jpg
| footer            = Head of a dark-colored, perhaps wild type, axolotl
}}
A sexually mature adult axolotl, at age 18–27 months, ranges in length from {{convert|15| to |45|cm|0|abbr=on}}, although a size close to {{convert|23|cm|0|abbr=on}} is most common and greater than {{convert|30|cm|abbr=on}} is rare. Axolotls possess features typical of salamander larvae, including [[external gills]] and a caudal fin extending from behind the head to the vent.<ref>San Francisco Examiner (San Francisco, California) 7 August 1887, page 9, authored by [[Yda Addis]]</ref><ref>{{Citation|last1=McIndoe|first1=Rosemary|title=Functional morphology of gills in larval amphibians|date=1984|work=Respiration and metabolism of embryonic vertebrates: Satellite Symposium of the 29th International Congress of Physiological Sciences, Sydney, Australia, 1983|pages=55–69|editor-last=Seymour|editor-first=Roger S.|series=Perspectives in vertebrate science|place=Dordrecht|publisher=Springer Netherlands|language=en|doi=10.1007/978-94-009-6536-2_4|isbn=978-94-009-6536-2|last2=Smith|first2=D. G.}}</ref> External gills are usually lost when salamander species mature into adulthood, although the axolotl maintains this feature.<ref name=":1">{{Cite book|last=Kardong|first=Kenneth V|title=Vertebrates: comparative anatomy, function, evolution|date=2019|publisher=McGraw-Hill Education |isbn=978-1-259-70091-0|language=English|oclc=1053847969}}</ref> This is due to their neoteny, where axolotls are much more aquatic than other salamander species.<ref name=":5"/>
Their heads are wide, and their eyes are [[Eyelid|lidless]]. Their limbs are underdeveloped and possess long, thin digits. Three pairs of [[External gills|external gill stalks (rami)]] originate behind their heads and are used to move oxygenated water. The external gill rami are lined with filaments (fimbriae) to increase surface area for gas exchange.<ref name=":1" /> Four-gill slits lined with [[gill rakers]] are hidden underneath the external gills, which prevent food from entering and allow particles to filter through. Males can be identified by their swollen [[cloaca]]e lined with papillae, while females have noticeably wider bodies when [[Gravidity and parity|gravid]] and full of eggs. 
[[File:Buccal pumping.jpg|thumb|upright=1.3|Buccal pumping]]
Axolotls have barely visible [[Vestigiality|vestigial]] teeth, which develop during metamorphosis. The primary method of feeding is by [[suction feeding|suction]], during which their rakers interlock to close the gill slits. External gills are used for respiration, although [[buccal pumping]] (gulping air from the surface) may also be used to provide oxygen to their lungs.<ref name=":1" /> Buccal pumping can occur in a two-stroke manner that pumps air from the mouth to the lungs, and with four-stroke that reverses this pathway with compression forces.
{{Multiple image
| image1            = AxolotlBE.jpg
| direction         = vertical
| image2            = Three Colors of Axolotl.jpg
| footer            = Captive axolotl [[color morph]]s
}}
The [[wild type]] animal (the "natural" form) is brown or tan with gold speckles and an olive undertone, and possess an ability to subtly alter their color by changing the relative size and thickness of their [[melanophore]]s, presumably for [[camouflage]].<ref>{{Cite journal |last1=Pietsch |first1=Paul |last2=Schneider |first2=Carl W. |date=1985 |title=Vision and the skin camouflage reactions of ''Ambystoma'' larvae: the effects of eye transplants and brain lesions |journal=Brain Research |volume=340 |issue=1 |pages=37–60 |doi=10.1016/0006-8993(85)90772-3|pmid=4027646 |s2cid=22723238 }}</ref> 
Axolotls have four pigmentation genes; when mutated, they create different color variants.{{Citation needed|date=May 2025}} The five most common mutant colors are listed below:{{Clarify|reason=There are four color morphs listed|date=May 2025}}

# [[Leucistic]]: pale pink with black eyes.
# [[Xanthochromism|Xanthic]]: grey, with black eyes.
# [[Albinism]]: pale pink or white, with red eyes.
# [[Melanism]]: all black or dark blue with no gold speckling or olive tone.

In addition, there is wide individual variability in the size, frequency, and intensity of the gold speckling, and at least one variant develops a black and white [[piebald]] appearance upon reaching maturity.<ref>{{Cite web|url=https://exopetguides.com/axolotl/axolotl-colors/|title=18 Types of Axolotl Colors You Can Own (Axolotl Color Guide)|date=August 14, 2019}}</ref> Because [[Breeder|pet breeders]] frequently [[Crossbreed|cross]] the variant colors, double [[homozygous]] mutants are common in the [[pet trade]], especially white/pink animals with pink eyes that are double homozygous mutants for both the albino and leucistic genes.<ref name="Color Atlas of Pigment Genes">{{Cite journal |last1=Frost |first1=Sally K. |last2=Briggs |first2=Fran |last3=Malacinski |first3=George M. |date=1984 |title=A color atlas of pigment genes in the Mexican axolotl (''Ambystoma mexicanum'') |journal=Differentiation |volume=26 |issue=1–3 |pages=182–188 |doi=10.1111/j.1432-0436.1984.tb01393.x}}</ref> 
[[File:Cromatóforos de larva de axolote pardo (Ambystoma mexicanum).jpg|thumb|Melanophores of a larva axolotl]]
The 32&nbsp;billion [[base pair]] long sequence of the axolotl's [[genome]] was published in 2018 and was the largest animal genome completed at the time. It revealed species-specific [[genetic pathway]]s that may be responsible for limb regeneration.<ref name=":0">{{Cite journal |last1=Nowoshilow |first1=Sergej |last2=Schloissnig |first2=Siegfried |last3=Fei |first3=Ji-Feng |last4=Dahl |first4=Andreas |last5=Pang |first5=Andy W. C. |last6=Pippel |first6=Martin |last7=Winkler |first7=Sylke |last8=Hastie |first8=Alex R. |last9=Young |first9=George |date=2018-01-24 |title=The axolotl genome and the evolution of key tissue formation regulators |journal=[[Nature (journal)|Nature]] |volume=554 |issue=7690 |pages=50–55 |doi=10.1038/nature25458 |pmid=29364872 |issn=1476-4687 |bibcode=2018Natur.554...50N |doi-access=free |hdl=21.11116/0000-0003-F659-4 |hdl-access=free }}</ref> Although the axolotl genome is about 10 times as large as the [[human genome]], it encodes a similar number of proteins, namely 23,251<ref name=":0" /> (the human genome encodes about 20,000 proteins). The size difference is mostly explained by a large fraction of [[repeated sequence (DNA)|repetitive sequences]], but such repeated elements also contribute to increased median [[intron]] sizes (22,759&nbsp;bp) which are 13, 16 and 25 times that observed in human (1,750&nbsp;bp), mouse (1,469&nbsp;bp) and [[Nanorana parkeri|Tibetan frog]] (906&nbsp;bp), respectively.<ref name=":0" />

===Physiology===
==== Regeneration ====
The feature of the axolotl that attracts most attention is its healing ability: the axolotl does not heal by [[scar]]ring, but is capable of [[regeneration (biology)|tissue regeneration]]; entire lost appendages such as limbs and the tail can regrow over a period of months, and, in certain cases, more vital structures, such as the tissues of the eye and [[heart]] can be regrown.<ref name="nickbaker">{{cite video |date= 2009-11-11 |title= Weird Creatures with Nick Baker |medium= Television series |publisher= [[The Science Channel]] |location= Dartmoor, England, UK }}</ref><ref>{{Cite journal|last1=Caballero-Pérez|first1=Juan|last2=Espinal-Centeno| first2= Annie|last3=Falcon|first3=Francisco|last4=García-Ortega|first4=Luis F.|last5=Curiel-Quesada|first5=Everardo|last6=Cruz-Hernández| first6= Andrés| last7=Bako|first7=Laszlo|last8=Chen|first8=Xuemei|last9=Martínez|first9=Octavio|last10=Alberto Arteaga-Vázquez| first10= Mario| last11= Herrera-Estrella|first11=Luis|date=January 2018|title=Transcriptional landscapes of Axolotl (Ambystoma mexicanum)|journal= [[Developmental Biology]]| language= en| volume=433|issue=2|pages=227–239|doi=10.1016/j.ydbio.2017.08.022|pmid=29291975|doi-access=}}</ref> They can restore parts of their [[central nervous system]], such as less vital parts of their brains. They can also readily accept [[Organ transplantation|transplants]] from other individuals, including eyes and parts of the brain—restoring these alien organs to full functionality. In some cases, axolotls have been known to repair a damaged limb, as well as regenerating an additional one, ending up with an extra appendage that makes them attractive to pet owners as a [[novelty]]. Their ability to regenerate declines with age but does not disappear, though in metamorphosed individuals, the ability to regenerate is greatly diminished. Axolotls experience [[indeterminate growth]], their bodies continuing to grow throughout their life, and some consider this trait to be a direct contributor to their regenerative abilities.<ref>{{Cite journal |last=Sandoval-Guzmán |first=Tatiana |date=August 2023 |title=The axolotl |url=https://www.nature.com/articles/s41592-023-01961-5 |journal=Nature Methods |language=en |volume=20 |issue=8 |pages=1117–1119 |doi=10.1038/s41592-023-01961-5 |pmid=37553398 |s2cid=260699417 |issn=1548-7091|url-access=subscription }}</ref> The axolotl is therefore used as a model for the development of limbs in vertebrates.<ref name=PMID18814845>{{cite journal |last1=Roy |first1=S |last2=Gatien |first2=S |title=Regeneration in axolotls: a model to aim for! |journal= [[Experimental Gerontology]] |date=November 2008 |volume=43 |issue=11 |pages=968–73 |pmid=18814845 |doi=10.1016/j.exger.2008.09.003 |s2cid=31199048 }}</ref> There are three basic requirements for regeneration of the limb: the wound [[epithelium]], nerve signaling, and the presence of cells from the different limb axes.<ref>{{cite journal |last1=Vieira |first1=Warren A. |last2=Wells |first2=Kaylee M. |last3=McCusker |first3=Catherine D. |title=Advancements to the Axolotl Model for Regeneration and Aging |journal=Gerontology |date=2020 |volume=66 |issue=3 |pages=212–222 |doi=10.1159/000504294 |pmid=31779024|pmc=7214127 |doi-access=free}}</ref> A wound epidermis is quickly formed by the cells to cover up the site of the wound. In the following days, the cells of the wound epidermis divide and grow, quickly forming a [[blastema]], which means the wound is ready to heal and undergo patterning to form the new limb.

It is believed that during limb generation, axolotls have a different system to regulate their internal [[macrophage]] level and suppress [[inflammation]], as scarring prevents proper healing and regeneration.<ref>{{cite journal |last1=Goodwin |first1=James W. |last2=Pinto |first2= Alexander R. |last3=Rosenthal |first3=Nadia A. |editor-first= Eric N.| editor-last= Olson |title=Macrophages are required for adult salamander limb regeneration |journal= [[Proceedings of the National Academy of Sciences of the United States of America]]|date=June 4, 2013 |volume=110 |issue=23 |pages=9415–9420 |doi=10.1073/pnas.1300290110 |pmid=23690624 |pmc=3677454 |bibcode=2013PNAS..110.9415G |doi-access=free }}</ref> However, this belief has been questioned by other studies.<ref>{{cite journal |last1=Pedersen |first1=Katherine |last2=Rasmussen |first2=Rikke Kongsgaard |last3=Dittrich |first3=Anita |last4=Pedersen |first4= Michael |last5=Lauridsen |first5=Henrik |title=Modulating the immune response and the pericardial environment with LPS or prednisolone in the axolotl does not change the regenerative capacity of cryoinjured hearts |journal=[[The FASEB Journal]] |date=April 17, 2020 |volume=34 |issue= S1 |page=1 |doi=10.1096/fasebj.2020.34.s1.04015 |s2cid=218792957 |doi-access=free }}</ref> The axolotl's regenerative properties leave the species as the perfect model to study the process of [[stem cell]]s and its own neoteny feature. Current research can record specific examples of these regenerative properties through tracking cell fates and behaviors, lineage tracing skin [[triploid]] cell [[Graft (surgery)|grafts]], pigmentation imaging, [[electroporation]], tissue clearing and lineage tracing from dye labeling. The newer technologies of [[germline modification]] and [[transgenesis]] are better suited for live imaging the regenerative processes that occur for axolotls.<ref>Masselink, Wouter, and Elly M. Tanaka. "Toward Whole Tissue Imaging of Axolotl Regeneration." Developmental Dynamics, vol. 250, no. 6, 2020, pp. 800–806., https://doi.org/10.1002/dvdy.282.</ref> In a 2025 study, scientist found a new way to insert and activate the genes inside the axolotl’s brain and nervous system using special, harmless viruses called [https://pmc.ncbi.nlm.nih.gov/articles/PMC11912378/ Adeno-Associated Viruses (AAVs)]. Before this, it was hard for researchers to make specific genes work inside the axolotl, but this discovery allows them to explore how the axolotl’s nervous system helps it regrow body parts like its brain and spinal cord. Additionally, they found that the axolotl’s nervous system has a unique two-way communication between the brain and eye.<ref>{{Cite journal |last=Lust |first=Katharina |last2=Tanaka |first2=Elly M. |date=2025-03-11 |title=Adeno-associated viruses for efficient gene expression in the axolotl nervous system |url=https://pnas.org/doi/10.1073/pnas.2421373122 |journal=Proceedings of the National Academy of Sciences |language=en |volume=122 |issue=10 |doi=10.1073/pnas.2421373122 |issn=0027-8424 |pmc=11912378 |pmid=40042904}}</ref>

==== Neoteny ====
{{Main|Neoteny}}

<!-- This is the sort of stuff that gets put in a box in fancy journal articles. -->
{{Side box|metadata=no||text='''Role of iodine'''
In animals with functioning thyroid glands, iodine in the form of iodide is selectively gathered into the colloid of the thyroid. Inside the colloid, iodide is reduced to elemental iodine (I<sub>2</sub>), which reacts with the [[tyrosyl]] residues of [[thyroglobulin]]. Two iodinated tyrosyl residues are conjugated together. When they are cleaved from the thyroglobulin chain, thyroid hormone is obtained.<ref name=boron2012>Chapter 49, "Synthesis of Thyroid Hormones" in: {{cite book |author1=Walter F. Boron |author2=Emile L. Boulpaep |title=Medical Physiology |edition= 2nd|publisher=Elsevier/Saunders |year=2012 |isbn=978-1-4377-1753-2}}{{page needed|date=April 2016}}</ref>

[[Diiodotyrosine]], an analogue of the iodinated [[thyroglobulin]] precursor in thyroxine biosynthesis, causes metamorphosis in axolotls that have their thyroids removed.<ref>{{cite journal |last1=Swingle |first1=W. W. |title=Iodine and Amphibian Metamorphosis |url=https://archive.org/details/sim_biological-bulletin_1923-11_45_5/page/n3 |journal=The Biological Bulletin |date=November 1923 |volume=45 |issue=5 |pages=229–253 |doi=10.2307/1536749|jstor=1536749 }}</ref> [[Lugol's solution]], which contains both iodide and I<sub>2</sub>, triggers metamorphosis when injected.<ref>{{cite journal |last1=Ingram |first1=W. R. |title=Metamorphosis of the Colorado Axolotl by Injection of Inorganic Iodine. |journal=Experimental Biology and Medicine |date=1 December 1928 |volume=26 |issue=3 |page=191 |doi=10.3181/00379727-26-4212}}</ref> This is because diiodotyrosine and thyroxine is produced when I<sub>2</sub> reacts with proteins other than thyroglobulin. If given in a bath instead of injected, I<sub>2</sub> has no effect on axolotls.<ref>{{cite journal |last1=Dvoskin |first1=Samuel |title=The Thyroxine-Like Action of Elemental Iodine in the Rat and Chick1 |url=https://archive.org/details/sim_endocrinology_1947-05_40_5/page/334 |journal=Endocrinology |date=May 1947 |volume=40 |issue=5 |pages=334–352 |doi=10.1210/endo-40-5-334|pmid=20245954 }}</ref> [[Iodide]], which does not react with proteins, does not trigger metamorphosis. It does speed up the rate of metamorphosis, once it has been triggered by thyroid hormone extract.<ref>{{cite journal |last1=Krylov |first1=O. A. |title=The role of haloids (bromine and iodine) in the metamorphosis of amphibia |journal=Bulletin of Experimental Biology and Medicine |date=January 1961 |volume=50 |issue=1 |pages=724–727 |doi=10.1007/BF00796048}}</ref>
}}
Most amphibians begin their lives as [[aquatic animal]]s which are unable to live on dry land, often being dubbed as [[tadpole]]s. To reach [[adult]]hood, they go through a process called [[metamorphosis (biology)|metamorphosis]], in which they lose their gills and start living on land. The axolotl is unusual in that it has a lack of [[thyroid-stimulating hormone]], which is needed for the [[thyroid]] to produce [[thyroxine]] for the axolotl to go through metamorphosis; it keeps its gills and lives in water all its life, even after it becomes an adult and is [[sexual maturity|able to reproduce]]. Neoteny is the term for reaching sexual maturity without undergoing metamorphosis.<ref name="ley196802">{{Cite magazine |last=Ley
 |first=Willy |date=February 1968 |title=Epitaph for a Lonely Olm |department=For Your Information|url=https://archive.org/stream/Galaxy_v26n03_1968-02_modified#page/n37/mode/2up |magazine=Galaxy Science Fiction |pages=95–104}}</ref>

The genes responsible for neoteny in laboratory axolotls may have been identified; they are not linked to the genes of wild populations, suggesting [[artificial selection]] is the cause of complete neoteny in laboratory and pet axolotls.<ref name=":3">{{Cite journal|last=Malacinski |first=George M.|date=1978-05-01|title=The Mexican Axolotl, ''Ambystoma mexicanum'': Its Biology and Developmental Genetics, and Its Autonomous Cell-lethal Genes|journal=American Zoologist|volume=18|issue=2|pages=195–206|doi=10.1093/icb/18.2.195 |doi-access=free}}</ref> The genes responsible have been narrowed down to a small chromosomal region called ''met1'', which contains several candidate genes.<ref name=Crowner>{{cite journal |last1=Crowner |first1=Anne |last2=Khatri |first2=Shivam |last3=Blichmann |first3=Dana |last4=Voss |first4=S. Randal |title=Rediscovering the Axolotl as a Model for Thyroid Hormone Dependent Development |journal=Frontiers in Endocrinology |date=12 April 2019 |volume=10 |page=237 |doi=10.3389/fendo.2019.00237 |doi-access=free|pmid=31031711 |pmc=6473073 }}</ref>

Many other species within the axolotl's genus are also either entirely neotenic or have neotenic populations. [[Siren (amphibian)|Siren]]s, ''[[Necturus]]'' mudpuppies, and the troglobitic [[olm]] are other examples of neotenic salamanders, although unlike axolotls, they cannot be induced to metamorphose by an injection of iodine or thyroxine hormone.

Neoteny has been observed in all [[salamander]] families in which it seems to be a survival mechanism, in aquatic environments only of mountain and hill, with little food and, in particular, with little iodine. In this way, salamanders can reproduce and survive in the form of a smaller larval stage, which is aquatic and requires a lower quality and quantity of food compared to the big adult, which is terrestrial. If the salamander larvae ingest a sufficient amount of iodine, directly or indirectly through [[cannibalism]], they quickly begin metamorphosis and transform into bigger terrestrial adults, with higher dietary requirements, but an ability to disperse across dry land.<ref>{{cite web|last=Venturi|first= S.|year=2004 |url= https://sites.google.com/site/iodinestudies/morosini |archive-date=4 March 2017 |archive-url=https://web.archive.org/web/20170304010444/https://sites.google.com/site/iodinestudies/morosini|access-date = 25 September 2020|title= Iodine and Evolution. DIMI-Marche}}</ref> In fact, in some high mountain lakes there live dwarf forms of [[salmonids]] that are caused by deficiencies in food and, in particular, iodine, which causes [[cretinism]] and [[dwarfism]] due to [[hypothyroidism]], as it does in humans.

==== Metamorphosis ====
The axolotl's body has the capacity to go through metamorphosis if given the necessary hormone, but axolotls do not produce it, and must obtain it from an external source, after which an axolotl undergoes an induced metamorphosis and begins living on land.<ref>{{Cite journal |last1=Demircan |first1=Turan |last2=Ovezmyradov |first2=Guvanch |last3=Yıldırım |first3=Berna |last4=Keskin |first4=İlknur |last5=İlhan |first5=Ayşe Elif |last6=Fesçioğlu |first6=Ece Cana |last7=Öztürk |first7=Gürkan |last8=Yıldırım |first8=Süleyman |date=2018-07-20 |title=Experimentally induced metamorphosis in highly regenerative axolotl (''Ambystoma mexicanum'') under constant diet restructures microbiota |journal=Scientific Reports |language=en |volume=8 |issue=1 |page=10974 |doi=10.1038/s41598-018-29373-y |pmid=30030457 |pmc=6054665 |bibcode=2018NatSR...810974D }}</ref> Research on this phenomenon has been performed for over a century; in modern laboratory conditions, metamorphosis is reliably induced by administering either the thyroid hormone [[thyroxine]] or a [[thyroid-stimulating hormone]]. The former is more commonly used.<ref name=Crowner/>
[[File:Ambystomas.jpg|thumb|Metamorphosed axolotls]]
In the absence of induced metamorphosis, larval axolotls start absorbing iodide into their thyroid glands at 30 days post-fertilization. Larval axolotls do produce thyroid hormone from iodide, but the amount appears highly variable. Adult axolotls do not produce thyroid hormone unless metamorphism is triggered.<ref name="pmid9371791">{{cite journal |last1=Brown |first1=Donald D. |title=The role of thyroid hormone in zebrafish and axolotl development |journal=Proceedings of the National Academy of Sciences |date=25 November 1997 |volume=94 |issue=24 |pages=13011–13016 |doi=10.1073/pnas.94.24.13011 |doi-access=free |pmid=9371791 |pmc=24254|bibcode=1997PNAS...9413011B }}</ref>

An axolotl undergoing metamorphosis experiences a number of physiological changes that help them adapt to life on land. These include increased muscle tone in limbs, the absorption of gills and fins into the body, the development of eyelids, and a reduction in the skin's permeability to water, allowing the axolotl to stay more easily hydrated when on land. The lungs of an axolotl, though present alongside gills after reaching non-metamorphosed adulthood, develop further during metamorphosis.<ref name=":6">{{Cite web|title=Axolotls – Metamorphosed & Tiger Salamanders|url=https://www.axolotl.org/tiger_salamander.htm|access-date=2022-01-25|website=www.axolotl.org}}</ref> Axolotl that complete their metamorphosis resembles an adult [[plateau tiger salamander]], though the axolotl differs in its longer toes.{{Citation needed|date=February 2011}}

==Wild population==
{{Multiple image
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| image2            = Lake Xochimilco in Southern Mexico.jpg
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| footer            = Lake Xochimilco, one of the last [[Refugium (population biology)|refuges]] of the wild axolotl
}}

Axolotls are within the same genus as the tiger salamander (''[[Ambystoma tigrinum]]''), being part of its [[species complex]] along with all other Mexican species of ''Ambystoma''.<ref>{{Cite journal |last1=Woodcock |first1=M. Ryan |last2=Vaughn-Wolfe |first2=Jennifer |last3=Elias |first3=Alexandra |last4=Kump |first4=D. Kevin |last5=Kendall |first5=Katharina Denise |last6=Timoshevskaya |first6=Nataliya |last7=Timoshevskiy |first7=Vladimir |last8=Perry |first8=Dustin W. |last9=Smith |first9=Jeramiah J. |last10=Spiewak |first10=Jessica E. |last11=Parichy |first11=David M. |last12=Voss |first12=S. Randal |date=2017-01-31 |title=Identification of Mutant Genes and Introgressed Tiger Salamander DNA in the Laboratory Axolotl, Ambystoma mexicanum |journal=Scientific Reports |volume=7 |issue=1 |page=5 |doi=10.1038/s41598-017-00059-1 |issn=2045-2322 |pmc=5428337 |pmid=28127056|bibcode=2017NatSR...7....6W }}</ref><ref name=aquariumindustries>{{cite web |url= http://www.aquariumindustries.com.au/wp-content/uploads/2012/07/Mexican-Walking-Fish.pdf |archive-url= https://web.archive.org/web/20180315144303/https://www.aquariumindustries.com.au/wp-content/uploads/2017/07/Mexican-Walking-Fish.pdf |archive-date= 15 March 2018 |title=Mexican Walking Fish, Axolotls ''Ambystoma mexicanum''}}</ref><ref name="aquariumonline">{{cite web |url= http://www.aquariumonline.com.au/topics/creatures/mexican.htm |archive-date=10 April 2013 |archive-url= https://web.archive.org/web/20130410234310/http://www.aquariumonline.com.au/topics/creatures/mexican.htm |title=Axolotols (Walking Fish) |publisher= Aquarium Online |access-date=2013-09-12}}</ref> Their habitat is like that of most{{Verify quote|date=June 2025}} neotenic ''Ambystoma'' species; a high-altitude body of water surrounded by a risky terrestrial environment, with these conditions thought to favor the development of [[neoteny]]. However, a population of terrestrial [[Mexican tiger salamander]]s occupies and breeds in the axolotl's habitat (being [[Sympatry|sympatric]]).{{Citation needed|date=January 2017}} The axolotl is native to the freshwater [[Lake Xochimilco|Lakes Xochimilco]] and [[Lake Chalco|Chalco]] in the [[Valley of Mexico]] (though the species may have also inhabited the larger Lakes of [[Lake Texcoco|Texcoco]] and [[Lake Zumpango|Zumpango]]),<ref name="iucn status 12 November 2021"/> and is currently native only to the former two; Lake Chalco is an unstable ecosystem, often being drained as a [[flood control]] measure, and Lake Xochimilco remains a remnant of its former self, existing mainly as [[canal]]s. The water temperature in Xochimilco rarely rises above {{convert|20|C|F}}, although it may fall to {{convert|6|-|7|C|F}} in the winter, and perhaps lower.<ref name=":2">{{Cite web|title=Lake Xochimilco, Borough of Xochimilco in southern México City, 162 L • Biotope Aquarium|url=https://biotope-aquarium.info/aquariums/lake-xochimilco-borough-of-xochimilco-in-southern-mexico-city-162-l/|access-date=2021-04-30|website=Biotope Aquarium|language=en-GB}}</ref> An additional population of ''Ambystoma'' inhabiting the [[artificial lake]] at [[Chapultepec]] was confirmed to contain axolotls; thus the extent of occurrence as of 23 October 2019 was {{Convert|467|km2|abbr=out}}.<ref name="iucn status 12 November 2021"/> Overall, the wild axolotl prefers a system of [[Channel (geography)|water channel]]s and deep-water lakes with abundant [[aquatic vegetation]].<ref name="iucn status 12 November 2021"/>

=== Biology ===
[[File:Axolotl Wild Type.jpg|thumb|upright=1.3|Wild form]]
The axolotl is carnivorous, consuming small prey such as [[Mollusca|mollusks]],<ref name="animaldiversity.org">{{Cite web|url=https://animaldiversity.org/accounts/Ambystoma_mexicanum/|title=Ambystoma mexicanum (Salamandra ajolote)|website=[[Animal Diversity Web]] }}</ref> [[worm]]s, [[Aquatic insect|insect]]s, other arthropods,<ref name="animaldiversity.org"/> and small fish in the wild. Axolotls locate food by smell, and will "snap" at any potential meal, [[Suction feeder|sucking]] the food into their stomachs with vacuum force.<ref>{{Cite journal |last1=Wainwright |first1=P. C. |last2=Sanford |first2=C. P. |last3=Reilly |first3=S. M. |last4=Lauder |first4=G. V. |date=1989 |title=Evolution of motor patterns: aquatic feeding in salamanders and ray-finned fishes |journal=Brain, Behavior and Evolution |volume=34 |issue=6 |pages=329–341 |doi=10.1159/000116519 |pmid=2611639}}</ref> The wild axolotl is thought to reach [[sexual maturity]] at 1.5 years of age, with a [[Biological life cycle|generation length]] of around 5.5 years.<ref name="iucn status 12 November 2021"/>

=== Threats ===
Axolotls are only native to the Mexican Central Valley. Although the population once extended through most lakes and wetlands in this region, its habitat is now limited to Lake Xochimilco as a result of the expansion of Mexico City, and is under pressure by the [[Urban sprawl|city's growth]]. Lake Xochimilco is not a large body of water, but a small series of artificial [[Channel (geography)|channel]]s, small lakes, and temporary [[wetland]]s. The axolotl is on the International Union for Conservation of Nature's Red List of threatened species.

Surveys in 1998, 2003, and 2008 found 6,000, 1,000, and 100 axolotls respectively, per square kilometer in Lake Xochimilco.<ref name = "Stevenson2014">{{cite web | last = Stevenson | first = M.
 | title = Mexico's 'water monster' may have disappeared | agency= Associated Press| website= SFGate.com
 | date = 2014-01-28 | url = http://www.sfgate.com/news/science/article/Mexico-s-water-monster-may-have-disappeared-5182823.php | access-date = 2014-01-29}}</ref> A four-month-long search in 2013 found no surviving individuals in the wild, but just a month later, two were spotted in a network of canals leading from Xochimilco.<ref>{{Cite news|url=https://www.independent.co.uk/environment/axolotl-found-in-mexico-city-lake-after-scientists-feared-it-only-survived-in-captivity-9148775.html|title=Endangered 'water monster' Axolotl found in Mexico City lake|date=2014-02-24|work=The Independent|access-date=2017-06-02|language=en-GB}}</ref>

Lake Xochimilco has poor [[water quality]]; tests reveal a low nitrogen-phosphorus ratio and a high concentration of [[chlorophyll a]], which are indicative of an oxygen-poor environment not well-suited to axolotls. This is caused by the demands of industries such as [[aquaculture]] and agriculture in the region, which maintain the water levels of the Lake through inputs of partially [[treated wastewater]].<ref>{{Cite journal |last1=Nandini |first1=Sarma |last2=García |first2=Pedro Ramirez |last3=Sarma |first3=S. S. S. |date=2016 |title=Water quality in Lake Xochimilco, Mexico: zooplankton indicators and Vibrio cholerae |url=https://www.jlimnol.it/index.php/jlimnol/article/view/jlimnol.2015.1213 |journal=Journal of Limnology |language=en |volume=75 |issue=1 |doi=10.4081/jlimnol.2015.1213 |issn=1723-8633|doi-access=free }}</ref> Agricultural pesticides from its intensive use eventually enters the lake through [[Runoff (hydrology)|runoff]]; these pesticides contain chemical compounds that sharply increase mortality in axolotl embryos and larvae. Of the surviving embryo and larvae, there is also an increase of morphological, behavior, and activity abnormalities.<ref>{{Cite journal |last1=Robles-Mendoza |first1=C. |last2=García-Basilio |first2=C. |last3=Cram-Heydrich |first3=S. |last4=Hernández-Quiroz |first4=M. |last5=Vanegas-Pérez |first5=C. |date=2009-02-01 |title=Organophosphorus pesticides effect on early stages of the axolotl Ambystoma mexicanum (Amphibia: Caudata) |url=https://www.sciencedirect.com/science/article/pii/S004565350801223X |journal=Chemosphere |language=en |volume=74 |issue=5 |pages=703–710 |doi=10.1016/j.chemosphere.2008.09.087 |pmid=19012946 |bibcode=2009Chmsp..74..703R |issn=0045-6535|url-access=subscription }}</ref>

With such a small native or wild population, there is a large loss of [[genetic diversity]]. This can be dangerous for the remaining population, causing an increase in [[inbreeding]] and decrease in [[Fitness (biology)|fitness]] and [[adaptive potential]]. Studies found indicators of a low interpopulation [[gene flow]] and higher rates of [[genetic drift]]. These are likely the result of multiple "[[Population bottleneck|bottleneck]]" incidents wherein the number of individuals in the population and its genetic diversity are sharply reduced. The offspring produced after bottleneck events have a greater risk of showing decreased fitness and are often less capable of adaptation, and multiple bottleneck events can have disastrous effects for the population, potentially leading to extinction. Studies have found high rates of relatedness that are indicative of inbreeding, which can be especially harmful as it can cause an increase in the presence of [[Deleterious mutation|deleterious]], or harmful, genes.<ref>{{Cite journal |last1=Parra-Plea |first1=G |last2=Zamudio |first2=K.R. |last3=Recuero |first3=E. |last4=Aguilar-=Miguel |first4=X. |last5=Huaxuz |first5=D. |last6=Zambrano |first6=L. |date=2011 |title=Conservation genetics of threatened Mexican axolotls (Ambystoma) |journal= Animal Conservation|volume=15 |issue=1 |pages=61–72|doi=10.1111/j.1469-1795.2011.00488.x |s2cid=46992721 }}</ref> The detection of [[introgressed]] tiger salamander (''A. tigrinum'') DNA in the laboratory axolotl population raises concerns about the suitability of the captive population as an "[[wikt:ark#English|ark]]" for potential reintroduction purposes.<ref>{{Cite journal |last1=Woodcock |first1=M. Ryan |last2=Vaughn-Wolfe |first2=Jennifer |last3=Elias |first3=Alexandra |last4=Kump |first4=D. Kevin |last5=Kendall |first5=Katharina Denise |last6=Timoshevskaya |first6=Nataliya |last7=Timoshevskiy |first7=Vladimir |last8=Perry |first8=Dustin W. |last9=Smith |first9=Jeramiah J. |last10=Spiewak |first10=Jessica E. |last11=Parichy |first11=David M. |last12=Voss |first12=S. Randal |date=2017-01-31 |title=Identification of Mutant Genes and Introgressed Tiger Salamander DNA in the Laboratory Axolotl, Ambystoma mexicanum |journal=Scientific Reports |language=en |volume=7 |issue=1 |page=6 |doi=10.1038/s41598-017-00059-1 |pmid=28127056 |issn=2045-2322|pmc=5428337 |bibcode=2017NatSR...7....6W }}</ref>

Another factor that threatens the population is the [[Introduced species|introduction]] of [[Invasive species|invasive fish species]] such as [[Tilapia as exotic species|Nile tilapia]] and [[common carp]]. These eat the axolotls' young, as well as compete for their primary source of food.<ref>{{cite news|url=http://www.uswaternews.com/archives/arcglobal/8mexicity11.html |title=Mexico City's 'water monster' nears extinction |date=November 2008 |access-date=2010-06-28 |archive-url=https://web.archive.org/web/20110723194731/http://www.uswaternews.com/archives/arcglobal/8mexicity11.html |archive-date=2011-07-23 }}</ref> The presence of these species has changed the behavior of axolotls, causing them to be less active to avoid predation. This reduction in activity greatly impacts the axolotl's foraging and mating opportunities.<ref>{{Cite journal |last1=Alcaraz |first1=Guillermina |last2=López-Portela |first2=Xarini |last3=Robles-Mendoza |first3=Cecilia |date=2015-07-01 |title=Response of a native endangered axolotl, Ambystoma mexicanum (Amphibia), to exotic fish predator |journal=Hydrobiologia |language=en |volume=753 |issue=1 |pages=73–80 |doi=10.1007/s10750-015-2194-4 |bibcode=2015HyBio.753...73A |s2cid=254550469 |issn=1573-5117}}</ref>

The fungus ''[[Batrachochytrium dendrobatidis]]'' has been detected in axolotls; ''B. dendrobatidis'' is a fungus that causes [[chytridiomycosis]] in amphibians, and is a major concern for [[amphibian conservation]] worldwide. However, the axolotl displays [[Disease resistance|resistance]] to both ''B. dendrobatidis'' and ''[[Batrachochytrium salamandrivorans|B. salamandrivorans]]'', so chytridiomycosis is thought to not be a threat to this species.<ref name="iucn status 12 November 2021"/>

===Conservation efforts===
There has been little improvement in the conditions of the population of native axolotls over the years.<ref>{{cite news|url= https://news.bbc.co.uk/earth/hi/earth_news/newsid_8220000/8220636.stm |publisher= BBC |title= Axolotl verges on wild extinction |author= Matt Walker |date=2009-08-26 |access-date=2010-06-28}}</ref><ref>{{Cite web|last=PetAquariums.com|title=Are Axolotls Endangered? You Need To Be Careful…|url=https://www.petaquariums.com/2020/10/21/are-axolotls-endangered-you-need-to-be-careful/|access-date=2021-06-26|website=PetAquariums.com|date=22 April 2020 |language=en-US}}</ref> Many scientists are focusing their conservation efforts on translocation of captive-bred individuals into new habitats or [[reintroduction]] into Lake Xochimilco. The ''Laboratorio de Restauracion Ecologica'' ({{Translation|Laboratory of Ecological Restoration}}), of the [[National Autonomous University of Mexico]], has built up a population of 100 captive-bred individuals as of 2021. These axolotls are mostly used for research, but there are plans for a semi-artificial wetland inside the university, to establish a viable population of axolotls within it. Studies have shown that captive-bred axolotls that are raised in a semi-natural environment can catch prey, survive in the wild, and have moderate success in escaping predators. These captive-bred individuals can be introduced into unpolluted bodies of water or back into Lake Xochimilco itself.<ref>{{Cite journal |last1=Ramos |first1=A.G. |last2=Mena-Gonzalez |first2=H. |last3=Zambrano |first3=L |date=2021 |title=The potential of temporary shelters to increase survival of the endangered Mexican axolotl |journal=Aquatic Conservation: Marine and Freshwater Ecosystems |volume=31 |issue=6 |pages=1535–1542|doi=10.1002/aqc.3520 |bibcode=2021ACMFE..31.1535R |s2cid=235587173 }}</ref><ref>{{Cite news |last=Paúl |first=María Luisa |date=2023-12-01 |title=Mexico wants you to adopt an axolotl, the amphibian that never grows up |language=en-US |newspaper=Washington Post |url=https://www.washingtonpost.com/nation/2023/12/01/axolotl-mexican-salamander-adoption-campaign/ |access-date=2023-12-01 |issn=0190-8286}}</ref>

A 2025 study confirmed the viability of releasing captive-bred axolotls into the wild, with recaptured animals putting on weight compared to their release weight, though this practice risks the loss of the axolotls through predation, as several released axolotls were preyed upon by [[great egret]]s.<ref>{{cite journal |author1=Ramos |first=Alejandra G. |author2=Horacio Mena |author3=David Schneider |author4=Luis Zambrano |date=30 April 2025 |title=Movement ecology of captive-bred axolotls in restored and artificial wetlands: Conservation insights for amphibian reintroductions and translocations |journal=PLOS ONE |volume=20 |issue=4 |article-number=e0314257 |doi=10.1371/journal.pone.0314257 |doi-access=free |pmid=40305450 |pmc=12043180 |bibcode=2025PLoSO..2014257R }}</ref><ref>{{cite web |title=Good news for the adorable axolotl — ones born in captivity could survive in the wild |url=https://www.npr.org/2025/05/06/nx-s1-5383896/good-news-for-the-adorable-axolotl-ones-born-in-captivity-could-survive-in-the-wild |website=npr.org |date=6 May 2025 |publisher=NPR |access-date=12 May 2025 |last1=Fink |first1=Kathryn }}</ref><ref>{{cite web |last1=Gill |first1=Victoria |title=Endangered axolotl release raises hopes for rare amphibian |url=https://www.bbc.com/news/articles/cm2xr2jzelyo |website=bbc.com |date=30 April 2025 |publisher=British Broadcasting Corporation |access-date=12 May 2025}}</ref>

==Relation to humans==
=== Research history ===
[[Alexander von Humboldt]] noted that the Mexicans, having been vanquished by the [[Spanish Empire]], lived "in great want, compelled to feed on roots of [[aquatic plant]]s, [[Entomophagy in humans|insects]] and a problematical reptile called axolotl."<ref name="NYT"/>

Six adult axolotls (including a leucistic specimen) were shipped from [[Mexico City]] to the ''[[Jardin des Plantes]]'' in Paris in 1863. Unaware of their neoteny, [[Auguste Duméril]] was surprised when, instead of the axolotl, he found in the vivarium a new species, similar to the salamander.{{verify source|reason=French language verification needed|date=November 2021}} This discovery was the starting point of research about neoteny. It is not certain that ''[[Ambystoma velasci]]'' specimens were not included in the original shipment.{{Citation needed|date=October 2021}} Vilem Laufberger in Prague used thyroid hormone injections to induce an axolotl to grow into a terrestrial adult salamander. The experiment was repeated by Englishman [[Julian Huxley]], who was unaware the experiment had already been done, using ground thyroids.<ref name=":4">{{Cite journal|last1=Reiß|first1=Christian|last2=Olsson|first2=Lennart|last3=Hoßfeld|first3=Uwe|date=2015|title=The history of the oldest self-sustaining laboratory animal: 150 years of axolotl research|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/jez.b.22617|journal=Journal of Experimental Zoology Part B: Molecular and Developmental Evolution|language=en|volume=324|issue=5|pages=393–404|doi=10.1002/jez.b.22617|pmid=25920413|bibcode=2015JEZB..324..393R |issn=1552-5015|url-access=subscription}}</ref> Since then, experiments have been done often with injections of iodine or various thyroid hormones used to induce metamorphosis.<ref name=":5">{{Cite journal|last1=Safi|first1=Rachid|last2=Bertrand|first2=Stéphanie|last3=Marchand|first3=Oriane|last4=Duffraisse|first4=Marilyne|last5=de Luze|first5=Amaury|last6=Vanacker|first6=Jean-Marc|last7=Maraninchi|first7=Marie|last8=Margotat|first8=Alain|last9=Demeneix|first9=Barbara|last10=Laudet|first10=Vincent|date=2004-02-01|title=The Axolotl (''Ambystoma mexicanum''), a Neotenic Amphibian, Expresses Functional Thyroid Hormone Receptors|journal=Endocrinology|volume=145|issue=2|pages=760–772|doi=10.1210/en.2003-0913|pmid=14576183 |doi-access=free}}</ref>

===Use as a model organism===
[[File:Desarrollo de huevo de axolote Ambystoma Mexicanum.jpg|thumb|Stages of development]]
Today, the axolotl is still used in research as a [[model organism]], and large numbers are bred in captivity. They are especially easy to breed compared to other salamanders in their family, which are rarely captive-bred due to the [[husbandry]] demands of terrestrial life. One attractive feature for research is the large and easily manipulated [[embryo]], which allows viewing of the [[Prenatal development|full development]] of a vertebrate. Axolotls are used in [[heart defect]] studies due to the presence of a mutant gene that causes heart failure in embryos. Since the embryos survive almost to hatching with no heart function, the defect is very observable. Further research has been conducted to examine their heart as a model of a single human [[Ventricle (heart)|ventricle]] and excessive trabeculation.<ref>{{Cite journal |last1=Meyer |first1=Sophie |last2=Lauridsen |first2=Henrik |last3=Pedersen |first3=Kathrine |last4=Andersson |first4=Sofie Amalie |last5=van Ooij |first5=Pim |last6=Willems |first6=Tineke |last7=Berger |first7=Rolf M. F. |last8=Ebels |first8=Tjark |last9=Jensen |first9=Bjarke |date=2022-11-28 |title=Opportunities and short-comings of the axolotl salamander heart as a model system of human single ventricle and excessive trabeculation |journal=Scientific Reports |language=en |volume=12 |issue=1 |page=20491 |doi=10.1038/s41598-022-24442-9 |issn=2045-2322 |pmc=9705478 |pmid=36443330|bibcode=2022NatSR..1220491M }}</ref> The axolotl is also considered an ideal animal model for the study of [[neural tube]] closure due to the similarities between human and axolotl [[neural plate]] and tube formation; the axolotl's neural tube, unlike a [[frog]]'s, is not hidden under a layer of superficial [[epithelium]].<ref>{{Cite journal |last=Gordon |first=R. |date=1985 |title=A review of the theories of vertebrate neurulation and their relationship to the mechanics of neural tube birth defects |url= https://journals.biologists.com/dev/content/89/Supplement/229 |journal= [[Journal of Embryology and Experimental Morphology]] |volume=89 |issue=Supplement |pages= 229–255 |pmid=3913733}}</ref> There are also mutations affecting other organ systems some of which are not well characterized and others that are.<ref>{{Cite journal |last=Armstrong |first=John B. |date=1985 |title= The axolotl mutants |journal= [[Developmental Genetics]] |volume=6 |issue=1 |pages=1–25 |doi=10.1002/dvg.1020060102}}</ref> The genetics of the color variants of the axolotl have also been widely studied.<ref name= "Color Atlas of Pigment Genes"/>

===Captive care===
{{Missing information|section|discussion on captive breeding and reproduction in general|date=September 2025}}
{{See also|Herpetoculture}}
[[File: Axolotl with Grit XRay.png|thumb|upright=0.6|This animal was X-rayed several times as part of a research project over a period of two years. It was a normal healthy adult (26.3&nbsp;cm; 159.5&nbsp;gm) at the beginning of the project and lived several more years after the project ended.<ref name=kulbisky>{{Cite journal |last1=Kulbisky |first1=Gordon P |last2=Rickey |first2=Daniel W |last3=Reed |first3=Martin H |last4=Björklund |first4=Natalie |last5=Gordon |first5=Richard |date=1999 |title=The axolotl as an animal model for the comparison of 3-D ultrasound with plain film radiography |journal=Ultrasound in Medicine and Biology |volume=25 |issue=6 |pages=969–975 |doi=10.1016/s0301-5629(99)00040-x|pmid=10461726 }}</ref>]]

The axolotl is a popular [[exotic pet]] like its relative, the tiger salamander (''Ambystoma tigrinum''). As for all [[poikilotherm]]ic organisms, lower temperatures result in slower metabolism and a very unhealthily reduced appetite. Temperatures at approximately {{convert|16|C|F}} to {{convert|18|C|F}} are suggested for captive axolotls to ensure sufficient food intake; stress resulting from more than a day's exposure to lower temperatures may quickly lead to disease and death, and temperatures higher than {{convert|24|C|F}} may lead to metabolic rate increase, also causing stress and eventually death.<ref>{{Cite web|url=https://www.axolotl.org/requirements.htm|title=Axolotls – Requirements & Water Conditions in Captivity|website=axolotl.org|access-date=2016-03-14}}</ref><ref>{{Cite web|url=http://www.caudata.org/cc/species/Ambystoma/A_mexicanum.shtml|title=Caudata Culture Species Entry – ''Ambystoma mexicanum'' – Axolotl|website=www.caudata.org|access-date=2016-03-14|archive-date=2016-03-15|archive-url=https://web.archive.org/web/20160315031956/http://www.caudata.org/cc/species/Ambystoma/A_mexicanum.shtml}}</ref> [[Chlorine]], commonly added to [[tapwater]], is harmful to axolotls. A single axolotl typically requires a {{convert|150|L|gal|adj=on|abbr=off}} tank. Axolotls spend the majority of the time at the bottom of the tank.<ref>{{Cite web |url=https://www.tfhmagazine.com/articles/freshwater/axolotls-keeping-a-water-monster-full |last=Wiegert |first=Joshua |title=Axolotls: Keeping a Water Monster}}</ref>

In captivity, axolotls eat a variety of readily available foods, including trout and salmon pellets, frozen or live [[Chironomidae|bloodworm]]s, [[earthworm]]s, and [[waxworm]]s. Axolotls can also eat [[feeder fish]], but care should be taken as fish may contain parasites.<ref>{{Cite journal | doi=10.1577/03632415.2011.10389070|title = The Aquarium Trade as an Invasion Pathway in the Pacific Northwest| journal=Fisheries| volume=36| issue=2| pages=74–85|year = 2011|last1 = Strecker|first1 = Angela L.| last2=Campbell| first2=Philip M.| last3=Olden| first3=Julian D.| bibcode=2011Fish...36...74S |url = https://pdxscholar.library.pdx.edu/esm_fac/40}}</ref>

[[Substrate (aquarium)|Substrate]]s are another important consideration for captive axolotls, as axolotls (like other amphibians and reptiles) tend to ingest bedding material together with food<ref name=Pough /> and are commonly prone to gastrointestinal obstruction and foreign body ingestion.<ref>{{cite journal|title=Amphibian Emergency Medicine|journal=Veterinary Clinics of North America: Exotic Animal Practice|volume=10|issue=2|pages=587–620|doi=10.1016/j.cvex.2007.02.004|pmid=17577564|year=2007|last1=Clayton|first1=Leigh Ann|last2=Gore|first2=Stacey R.}}</ref> Some common substrates used for animal enclosures can be harmful for amphibians and reptiles. Gravel (common in aquarium use) should not be used, and is recommended that any sand consists of smooth particles with a grain size of under 1mm.<ref name=Pough>{{cite web|last=Pough|first=F. H.|url=https://netvet.wustl.edu/species/reptiles/pough.txt|title=Recommendations for the Care of Amphibians and Reptiles in Academic Institutions|publisher=National Academy Press|location=Washington, D.C.|year=1992}}</ref> One guide to axolotl care for laboratories notes that bowel obstructions are a common cause of death, and recommends that no items with a diameter below 3&nbsp;cm (or approximately the size of the animal's head) should be available to the animal.<ref name=Gresens>{{cite journal|last1=Gresens|first1=Jill|title=An Introduction to the Mexican Axolotl (''Ambystoma mexicanum'')|journal=Lab Animal|volume=33|issue=9|year=2004|pages=41–47|doi=10.1038/laban1004-41|pmid=15457201|s2cid=33299160}}</ref>

There is some evidence that axolotls might seek out appropriately-sized gravel for use as [[gastroliths]]<ref>Wings, O [http://www.dinosaurhunter.org/files/app-2007-wings-gastrolith_function_classification.pdf A review of gastrolith function with implications for fossil vertebrates and a revised classification] {{Webarchive|url=https://web.archive.org/web/20160304062304/http://www.dinosaurhunter.org/files/app-2007-wings-gastrolith_function_classification.pdf |date=2016-03-04 }} Acta Palaeontologica Polonica 52 (1): 1–16</ref> based on experiments conducted at the University of Manitoba axolotl colony.<ref>Gordon, N, [https://embryogenesisexplained.org/2015/09/10/gastroliths-how-i-learned-to-stop-worrying-and-love-gravel/ Gastroliths – How I Learned to Stop Worrying and Love Gravel.] {{Webarchive|url=https://web.archive.org/web/20200922142827/https://embryogenesisexplained.org/2015/09/10/gastroliths-how-i-learned-to-stop-worrying-and-love-gravel/ |date=2020-09-22 }}</ref><ref>Björklund, N.K. (1993). Small is beautiful: economical axolotl colony maintenance with natural spawnings as if axolotls mattered. In:  Handbook on Practical Methods. Ed.: G.M. Malacinski & S.T. Duhon. Bloomington, Department of Biology, Indiana University: 38–47.</ref> As there is no conclusive evidence pointing to gastrolith use, gravel should be avoided due to the high risk of [[Impaction (animals)|impaction]].<ref>{{Cite journal|last=Loh|first=Richmond|date=2015-05-15|title=Common Disease Conditions in Axolotls|website=Vin.com |url=https://www.vin.com/apputil/content/defaultadv1.aspx?pId=14365&catId=73681&id=7259254|url-status=live|access-date=2022-01-21|archive-url=https://web.archive.org/web/20200804110231/https://www.vin.com/apputil/content/defaultadv1.aspx?pId=14365&catId=73681&id=7259254 |archive-date=2020-08-04 }}</ref>

Salts, such as [[Holtfreter's solution]], are often added to the water to prevent infection.<ref>{{cite web|last=Clare|first= John P. |url=https://www.axolotl.org/health.htm |title=Health and Diseases|website=axolotl.org}}</ref> Among hobbyists, the process of artificially inducing metamorphosis can often result in death during or even following a successful attempt, and so casual hobbyists are generally discouraged from attempting to induce metamorphosis in pet axolotls.<ref name=":6" /> Morphed pet axolotls should be given solid footholds in their enclosure to satisfy their need for land. They should not be given live animals as food.<ref>{{cite web |title=Transition & Feeding |url=https://www.morphedaxolotls.com/transition-and-feeding |website=Morphed Axolotls |language=en}}</ref>

<gallery widths=200 heights=160 mode="packed">
File: Ambystoma mexicanum at Vancouver Aquarium.jpg|These axolotls at [[Vancouver Aquarium]] are [[leucistic]], with less pigmentation than normal.
File:Axolotl in a Pet store in Melbourne.jpg|Axolotl in a pet store in Melbourne, Australia
File:Axolotls in Kew Gardens.jpg|Axolotls in a pond with ''[[Pistia]]'', [[Kew Gardens]]
</gallery>

=== Cultural significance ===
[[File:Edoardo Aldama.jpg|thumb|As [[graffiti]], Mexico City]]
The species is named after the [[Aztec deity]] [[Xolotl]], the god of fire and lightning, who transformed himself into an axolotl to avoid being sacrificed by fellow gods. They continue to play an outsized cultural role in Mexico.<ref name="Reuters2018">{{Cite news |last=Garcia |first=David Alire |date=2018-11-20 |title=Mexico's axolotl, a cartoon hero and genetic marvel, fights for survival |url=https://www.reuters.com/article/us-mexico-axolotl-idUSKCN1NP0F6 |access-date=2022-08-16 |work=Reuters |language=en}}</ref> Axólotl also means water monster in the [[Nahuatl]] language.

[[Julio Cortázar]] wrote a book titled "''Axolotl''".{{Clarify|reason=What is the book about? There is currently no wikipage about it|date=June 2025}}<ref>{{cite web |title=Axolotl: The Real Julio Cortazar |url=https://hum11c.omeka.fas.harvard.edu/exhibits/show/readings/axolotl--the-real-julio-cortaz |website=hum11c.omeka.fas.harvard.edu |publisher=Harvard University |access-date=4 June 2025}}</ref><ref>{{cite web |title=Axolotl, Xolotl, and Religion |url=https://hum11c.omeka.fas.harvard.edu/exhibits/show/readings/axolotl--xolotl--and-religion |website=hum11c.omeka.fas.harvard.edu |publisher=Harvard University |access-date=4 June 2025}}</ref>

They appear in the works of Mexican muralist [[Diego Rivera]]. In 2021, Mexico released a new design for its 50-[[Mexican peso|peso]] banknote featuring an axolotl along with maize and [[chinampa]]s on its back.<ref>{{Cite web |date=2020-02-21 |title=Mexican axolotl will be the new image of the 50 peso bill |url=https://www.theyucatantimes.com/2020/02/mexican-axolotl-will-be-the-new-image-of-the-50-peso-bill/ |access-date=2025-04-24 |website=The Yucatan Times |archive-url=https://web.archive.org/web/20250210125136/https://www.theyucatantimes.com/2020/02/mexican-axolotl-will-be-the-new-image-of-the-50-peso-bill/ |archive-date=10 February 2025 |language=en-US |last1=Times |first1=Yucatan }}</ref><ref>{{Cite web |title=Billete de 50 pesos de la familia G |url=https://www.banxico.org.mx/billetes-y-monedas/billete-50-pesos-familia-g.html |access-date=2023-02-20 |website=www.banxico.org.mx |language=es}}</ref> It was recognized as "Bank Note of the Year" by the [[International Bank Note Society]].<ref>{{Cite web |title=Banknote of 2021 Nominations |url=https://www.theibns.org/joomla/index.php?option=com_content&view=article&id=809:banknote-of-2021-nominations&catid=39:banknote-of-2021&Itemid=51 |access-date=2023-02-20 |website=www.theibns.org}}</ref> [[HD 224693]], a [[star]] in the [[Celestial equator|equatorial]] [[constellation]] of [[Cetus (constellation)|Cetus]], was named Axólotl in 2019.<ref>{{Cite web |title=Approved names |url=https://www.nameexoworlds.iau.org/final-results |access-date=2020-01-02 |website=www.nameexoworlds.iau.org |language=en}}</ref><ref>{{Cite web |date=December 17, 2019 |title=100 000s of People from 112 Countries Select Names for Exoplanet Systems In Celebration of IAU's 100th Anniversary |url=https://www.iau.org/news/pressreleases/detail/iau1912/ |access-date=2020-01-02 |website=[[International Astronomical Union]]}}</ref>
[[File:Minecraft Axolotl.jpg|thumb|Axolotl as seen in [[Minecraft]]]]
In the 21st century, axolotls became renowned as a [[cultural icon]], with the species' [[wikt:likeness|likeness]] appearing in or inspiring various aspects of contemporary media, such as television shows, movies, or video games. The [[List of Pokémon|Pokémon]] [[Mudkip]] and its evolutions, added in ''[[Pokémon Ruby and Sapphire]]'' (2002), take some visual inspiration from axolotls.<ref name="Reuters2018" /> Additionally, the Pokémon [[Wooper]], added in ''[[Pokémon Gold, Silver, and Crystal|Pokémon Gold, Silver and Crystal]]'' (1999), is directly based on an axolotl.<ref name="Reuters2018" />{{Additional citation needed|date=May 2023}} The dragon Toothless in the ''[[How to Train Your Dragon]]'' movies was modeled after axolotls as well.<ref name="Reuters2018" /> Following [[Mojang Studios]]' trend of adding endangered species to the game to raise awareness, axolotls were added to the video game ''[[Minecraft]]'' in 2020 (depicted as [[troglofauna]] in-game),<ref>{{Cite web |last=Minecraft |date=October 3, 2020 |title="Minecraft Live: Caves & Cliffs – First Look" |url=https://www.youtube.com/watch?v=DBvZ2Iqmm3M&t=2486s |website=YouTube |quote="And then we also found out that axolotls are endangered in the real world, and we think it's good to add endangered animals to Minecraft to create awareness about that." – Agnes Larsson}}</ref> and were included in its spin-offs ''[[Minecraft Dungeons|Minecraft: Dungeons]]'' and ''[[Lego Minecraft]]''.<ref>{{Cite web |title=The Guardian Battle 21180 |url=https://www.lego.com/en-us/product/the-guardian-battle-21180 |access-date=2023-02-20 |website=www.lego.com |language=en}}</ref><ref>{{Cite web |title=The Axolotl House 21247 |url=https://www.lego.com/en-us/product/the-axolotl-house-21247 |access-date=2024-12-27 |website=www.lego.com |language=en}}</ref>

==See also==
* {{annotated link|Amphibious fish}}
* {{annotated link|Barred tiger salamander}}
* {{annotated link|Eurycea neotenes}}
* {{annotated link|Handfish}}
* {{annotated link|Lake Patzcuaro salamander}}
* {{annotated link|Regenerative medicine}}
* {{annotated link|Texas blind salamander}}

==References==
{{Reflist|30em}}

==External links==
{{Commons category|Ambystoma mexicanum}}
* [https://web.archive.org/web/20160805192629/http://www.fishtankforum.co.uk/forum/viewtopic.php?f=190&t=5263 Follow the Eggs, Hatchlings and Juveniles]
* [https://web.archive.org/web/20160310105933/http://fishtankforum.co.uk/forum/viewtopic.php?f=190&t=6455 Mating Dance and Laying Eggs]
* [https://web.archive.org/web/20160307055355/http://fishtankforum.co.uk/forum/viewtopic.php?f=190&t=6509 Follow the Eggs and Hatchlings (2nd Batch)]
* [https://web.archive.org/web/20140517152123/http://www.biologydir.com/indiana-university-axolotl-colony-info-11384.html Indiana U Axolotl Colony]
* [https://ambystoma.uky.edu/genetic-stock-center/ University of KY Axolotl Colony]
* [https://america.aljazeera.com/articles/2015/2/15/mystical-amphibian-venerated-by-aztecs-nears-extinction.html Mystical amphibian venerated by Aztecs nears extinction]
* [https://www.vox.com/22877353/axolotl-salamander-pet-extinction-mexico The animal that's everywhere and nowhere]
*{{cite web |last1=Sánchez |first1=Aminetth |title=Scientists and farmers restore Aztec-era floating farms that house axolotls |url=https://news.mongabay.com/2024/05/scientists-and-farmers-restore-aztec-era-floating-farms-that-house-axolotls/ |website=news.mongabay.com |date=31 May 2024 |publisher=Conservation News |access-date=4 June 2025}}
{{Taxonbar|from=Q22718}}
{{Authority control}}

[[Category:Amphibians described in the 18th century]]
[[Category:Animal models]]
[[Category:Animal testing on amphibians]]
[[Category:Animals bred for albinism on a large scale]]
[[Category:Critically endangered biota of Mexico]]
[[Category:Critically endangered fauna of North America]]
[[Category:Endemic amphibians of Mexico]]
[[Category:Mole salamanders]]
[[Category:Population genetics]]
[[Category:Regenerative biomedicine]]
[[Category:Species that are or were threatened by pollution]]
[[Category:Species that are or were threatened by invasive species]]
[[Category:Taxa named by Frederick Polydore Nodder]]
[[Category:Taxa named by George Shaw]]
[[Category:Valley of Mexico]]