View source for Zoology

{{Short description|Scientific study of animals}}
{{Other uses}}
{{redirect-multi|2|Animal biology|Zoologist|the journals|Animal Biology{{!}}''Animal Biology''|and|The Zoologist{{!}}''The Zoologist''}}
{{use dmy dates|cs1-dates=ly|date=May 2023}}
{{TopicTOC-Biology|Branches}}

'''Zoology''' ({{IPAc-en|z|oʊ|ˈ|ɒ|l|ə|dʒ|i|}} {{respell|zoh|OL|ə|jee}}, {{IPAc-en|UKalso|z|u|ˈ|-}} {{respell|zoo|-}})<ref name="Cambridge">{{cite dictionary|title=zoology|url=https://dictionary.cambridge.org/dictionary/english/zoology|dictionary=[[Cambridge Advanced Learner's Dictionary|Cambridge Dictionary]]}}</ref> is the scientific study of [[animal]]s. Its studies include the [[anatomy|structure]], [[embryology]], [[Biological classification|classification]], [[Ethology|habits]], and distribution of all animals, both living and [[extinction|extinct]], and how they interact with their [[ecosystem]]s. Zoology is one of the primary branches of [[biology]]. The term is derived from [[Ancient Greek]] {{lang|grc|ζῷον}}, {{Transliteration|grc|zōion}} ('animal'), and {{lang|grc|λόγος}}, {{lang|grc|logos}} ('knowledge', 'study').<ref name=OnlineEtDict>{{cite dictionary|title=zoology|url=http://www.etymonline.com/index.php?term=zoology&allowed_in_frame=0|dictionary=[[Online Etymology Dictionary]]|access-date=2013-05-24 |archive-date=2013-03-08 |archive-url=https://web.archive.org/web/20130308153604/http://etymonline.com/index.php?term=zoology&allowed_in_frame=0|url-status=live}}</ref>

Although humans have always been interested in the natural history of the animals they saw around them, and used this knowledge to domesticate certain species, the formal study of zoology can be said to have originated with [[Aristotle]]. He viewed animals as living organisms, studied their structure and development, and considered their adaptations to their surroundings and the function of their parts. Modern zoology has its origins during the [[Renaissance]] and early modern period, with [[Carl Linnaeus]], [[Antonie van Leeuwenhoek]], [[Robert Hooke]], [[Charles Darwin]], [[Gregor Mendel]] and many others.

The study of animals has largely moved on to deal with form and function, adaptations, relationships between groups, behaviour and ecology. Zoology has increasingly been subdivided into disciplines such as [[Taxonomy|classification]], [[physiology]], [[biochemistry]] and [[evolution]]. With the discovery of the structure of [[DNA]] by [[Francis Crick]], [[James Watson]] and [[Rosalind Franklin]] in 1953, the realm of [[molecular biology]] opened up, leading to advances in [[cell biology]], [[developmental biology]] and [[molecular genetics]].

==History==
{{Main|History of zoology through 1859|History of zoology (1859–present)}}
{{For timeline}}
[[File:Gessner Conrad 1516-1565.jpg|thumb|left|[[Conrad Gessner]] (1516–1565). His {{Lang|la|[[Historia animalium (Gessner book)|Historiae animalium]]}} is considered the beginning of modern zoology.]]

The history of zoology traces the study of the [[animal|animal kingdom]] from ancient to modern times. Prehistoric people needed to study the animals and plants in their environment to exploit them and survive. [[Cave painting|Cave paintings]], engravings and sculptures in France dating back 15,000 years show bison, horses, and deer in carefully rendered detail. Similar images from other parts of the world illustrated mostly the animals hunted for food and the savage animals.<ref name="Fellowes2020">{{cite book|author=Mark Fellowes|title=30-Second Zoology: The 50 most fundamental categories and concepts from the study of animal life|url=https://books.google.com/books?id=gkzODwAAQBAJ|year=2020|publisher=Ivy Press|isbn=978-0-7112-5465-7|access-date=2021-06-04 |archive-date=2024-06-12 |archive-url=https://web.archive.org/web/20240612073743/https://books.google.com/books?id=gkzODwAAQBAJ|url-status=live}}</ref>

The [[Neolithic Revolution]], which is characterized by the [[domestication of animals]], continued throughout Antiquity. Ancient knowledge of wildlife is illustrated by the realistic depictions of wild and domestic animals in the Near East, [[Mesopotamia]], and Egypt, including husbandry practices and techniques, hunting and fishing. The invention of writing is reflected in zoology by the presence of animals in Egyptian hieroglyphics.<ref>{{cite web |url=https://www.um.es/cepoat/egipcio/wp-content/uploads/egyptianhierogly.pdf |title=Egyptian Hieroglyphic Dictionary: Introduction |author=[[E. A. Wallis Budge]] |year=1920 |publisher=John Murray |access-date=10 June 2021 |archive-date=2021-07-21  |archive-url=https://web.archive.org/web/20210721132433/https://www.um.es/cepoat/egipcio/wp-content/uploads/egyptianhierogly.pdf |url-status=live }}</ref>

Although the concept of ''zoology'' as a single coherent field arose much later, the zoological sciences emerged from [[natural history]] reaching back to the [[Aristotle's biology|biological works of Aristotle]] and [[Galen]] in the ancient [[Greco-Roman world]]. In the fourth century BC, [[Aristotle]] looked at animals as living organisms, studying their structure, development and vital phenomena. He divided them into two groups: animals with blood, equivalent to our concept of [[vertebrate]]s, and animals without blood, [[invertebrate]]s. He spent two years on [[Lesbos]], observing and describing the animals and plants, considering the adaptations of different organisms and the function of their parts.<ref>{{cite book |last=Leroi |first=Armand Marie |title=The Lagoon: How Aristotle Invented Science |date=2015 |author-link=Armand Marie Leroi |title-link=Aristotle's Lagoon |publisher=Bloomsbury |isbn=978-1-4088-3622-4 |pages=135–136}}</ref> Four hundred years later, Roman physician [[Galen]] dissected animals to study their anatomy and the function of the different parts, because the dissection of human cadavers was prohibited at the time.<ref>{{cite book|author=Claudii Galeni Pergameni|title="That the best physician is also a philosopher" with a Modern Greek Translation
|editor=Odysseas Hatzopoulos|publisher=Odysseas Hatzopoulos & Company: Kaktos Editions|location=[[Athens]], [[Greece]]|year=1992}}</ref> This resulted in some of his conclusions being false, but for many centuries it was considered [[Heresy|heretical]] to challenge any of his views, so the study of anatomy stultified.<ref>{{cite book |title=Medecine's 10 Greatest Discoveries |last1=Friedman |first1=Meyer |last2=Friedland |first2=Gerald W. |year=1998 |publisher=Yale University Press |isbn=0-300-07598-7 |page=2 |url= }}</ref>
 
During the [[Post-classical history|post-classical era]], [[Medicine in the medieval Islamic world|Middle Eastern science and medicine]] was the most advanced in the world, integrating concepts from Ancient Greece, Rome, Mesopotamia and Persia as well as the ancient Indian tradition of [[Ayurveda]], while making numerous advances and innovations.<ref>{{cite journal |first=Mehmet |last=Bayrakdar |year=1986 |title=Al-Jahiz and the rise of biological evolution |journal=Ankara Üniversitesi İlahiyat Fakültesi Dergisi |publisher=[[Ankara University]] |volume=27 |issue=1 |pages=307–315 |doi=10.1501/Ilhfak_0000000674 |doi-broken-date=12 July 2025 |doi-access=free }}</ref> In the 13th century, [[Albertus Magnus]] produced commentaries and paraphrases of all Aristotle's works; his books on topics like [[botany]], zoology, and minerals included information from ancient sources, but also the results of his own investigations. His general approach was surprisingly modern, and he wrote, "For it is [the task] of natural science not simply to accept what we are told but to inquire into the causes of natural things."<ref>{{Cite book|url=https://archive.org/details/308059821ALBERTUSMAGNUSTheBookOfMinerals|title=Book of Minerals|last=Wyckoff|first=Dorothy|publisher=Clarendon Press|year=1967|location=Oxford|pages=Preface}}</ref> An early pioneer was [[Conrad Gessner]], whose monumental 4,500-page encyclopedia of animals, {{Lang|la|[[Historia animalium (Gessner book)|Historia animalium]]}}, was published in four volumes between 1551 and 1558.<ref>{{cite web|last1=Scott|first1=Michon|title=Conrad Gesner|url=https://www.strangescience.net/gesner.htm|website=Strange Science: The rocky road to modern paleontology and biology|access-date=27 September 2017|date=26 March 2017|archive-date=2021-06-16 |archive-url=https://web.archive.org/web/20210616080201/https://www.strangescience.net/gesner.htm|url-status=live}}</ref>

In Europe, Galen's work on anatomy remained largely unsurpassed and unchallenged up until the 16th century.<ref>{{Cite book|title=Thinking about Life: The History and Philosophy of Biology and Other Sciences|url=https://archive.org/details/thinkingaboutlif00agut_532 |author1=Agutter, Paul S. |author2=Wheatley, Denys N.  |publisher=Springer |year=2008 |isbn=978-1-4020-8865-0 |page=[https://archive.org/details/thinkingaboutlif00agut_532/page/n56 43]}}</ref><ref>{{cite book |author=Saint Albertus Magnus |title=On Animals: A Medieval Summa Zoologica |year=1999 |publisher=Johns Hopkins University Press |isbn=0-8018-4823-7 }}</ref> During the [[Renaissance]] and early modern period, zoological thought was revolutionized in [[Europe]] by a renewed interest in [[empiricism]] and the discovery of many novel organisms. Prominent in this movement were [[Andreas Vesalius]] and [[William Harvey]], who used experimentation and careful observation in [[physiology]], and naturalists such as [[Carl Linnaeus]], [[Jean-Baptiste Lamarck]], and [[Georges-Louis Leclerc, Comte de Buffon|Buffon]] who began to [[Taxonomy (biology)|classify the diversity of life]] and the [[Fossil#Dating|fossil record]], as well as studying the development and behavior of organisms. [[Antonie van Leeuwenhoek]] did pioneering work in [[microscopy]] and revealed the previously unknown world of [[microorganism]]s, laying the groundwork for [[cell theory]].<ref>{{cite book |author=Magner, Lois N. |title=A History of the Life Sciences, Revised and Expanded |year=2002 |publisher=CRC Press |pages=133–144 |isbn=0-8247-0824-5 }}</ref> [[Antonie van Leeuwenhoek|van Leeuwenhoek]]'s observations were endorsed by [[Robert Hooke]]; all living organisms were composed of one or more cells and could not generate spontaneously. Cell theory provided a new perspective on the fundamental basis of life.<ref>{{cite book |author=Jan Sapp |title=Genesis: The Evolution of Biology |year=2003 |publisher=Oxford University Press |chapter=Chapter 7 |isbn=0-19-515619-6 |chapter-url=https://archive.org/details/genesisevolution00sapp }}</ref>

Having previously been the realm of gentlemen naturalists, over the 18th, 19th and 20th centuries, zoology became an increasingly professional [[Branches of science|scientific discipline]]. Explorer-naturalists such as [[Alexander von Humboldt]] investigated the interaction between organisms and their environment, and the ways this relationship depends on geography, laying the foundations for [[biogeography]], [[ecology]] and [[ethology]]. Naturalists began to reject [[essentialism]] and consider the importance of [[extinction]] and the [[history of evolutionary thought|mutability of species]].<ref>{{cite book |author=William Coleman |title=Biology in the Nineteenth Century |year=1978 |publisher=Cambridge University Press |chapter=Chapter 2 |isbn=0-521-29293-X }}</ref>

These developments, as well as the results from [[embryology]] and [[paleontology]], were synthesized in the 1859 publication of [[Charles Darwin]]'s theory of [[evolution]] by [[natural selection]]; in this Darwin placed the theory of organic evolution on a new footing, by explaining the processes by which it can occur, and providing observational evidence that it had done so.<ref>{{cite book |title=Why Evolution is True |author=Coyne, Jerry A. |year=2009 |publisher=Oxford University Press |location=Oxford |isbn=978-0-19-923084-6 |page=[https://archive.org/details/isbn_9780199230846/page/17 17] |url=https://archive.org/details/isbn_9780199230846/page/17 }}</ref> [[Darwinism|Darwin's theory]] was rapidly accepted by the scientific community and soon became a central axiom of the rapidly developing science of biology. The basis for modern genetics began with the work of [[Gregor Mendel]] on peas in 1865, although the significance of his work was not realized at the time.<ref>{{cite book|last=Henig|first=Robin Marantz|title=The Monk in the Garden: The Lost and Found Genius of Gregor Mendel, the Father of Modern Genetics |publisher=Houghton Mifflin |year=2009 |isbn=978-0-395-97765-1 |url=https://archive.org/details/monkingardenlost00heni}}</ref>

Darwin gave a new direction to [[Morphology (biology)|morphology]] and [[physiology]], by uniting them in a common biological theory: the theory of organic evolution. The result was a reconstruction of the classification of animals upon a [[Genealogy|genealogical]] basis, fresh investigation of the development of animals, and early attempts to determine their genetic relationships. The end of the 19th century saw the fall of [[spontaneous generation]] and the rise of the [[germ theory of disease]], though the mechanism of [[Heredity|inheritance]] remained a mystery. In the early 20th century, the rediscovery of [[Gregor Mendel|Mendel's]] work led to the rapid development of [[genetics]], and by the 1930s the combination of [[population genetics]] and natural selection in the [[Modern synthesis (20th century)|modern synthesis]] created [[evolutionary biology]].<ref>{{cite book |title=Science and Creationism: a view from the National Academy of Sciences |url=https://archive.org/details/sciencecreationi0000unse/page/28 |format=php |access-date=September 24, 2009 |edition=Second |year=1999 |publisher=The National Academy of Sciences |location=Washington, DC |isbn=-0-309-06406-6 |page=[https://archive.org/details/sciencecreationi0000unse/page/28 28] |chapter=Appendix: Frequently Asked Questions |chapter-url=http://www.nap.edu/openbook.php?record_id=6024&page=27#p200064869970027001 |ref=NAS }}</ref>

Research in cell biology is interconnected to other fields such as genetics, [[biochemistry]], [[medical microbiology]], [[immunology]], and [[cytochemistry]]. With the determination of the double helical structure of the [[DNA]] molecule by [[Francis Crick]] and [[James Watson]] in 1953,<ref>{{cite journal |vauthors=WATSON JD, CRICK FH |title=Molecular structure of nucleic acids; a structure for deoxyribose nucleic acid |journal=Nature |volume=171 |issue=4356 |pages=737–8 |date=April 1953 |pmid=13054692 |doi=10.1038/171737a0 |bibcode=1953Natur.171..737W |url=}}</ref> the realm of [[molecular biology]] opened up, leading to advances in [[cell biology]], [[developmental biology]] and [[molecular genetics]]. The study of [[systematics]] was transformed as [[DNA sequencing]] elucidated the degrees of affinity between different organisms.<ref>{{Cite news|url=http://www.biologydiscussion.com/animals-2/systematics-meaning-branches-and-its-application/32374|title=Systematics: Meaning, Branches and Its Application|date=27 May 2016|work=Biology Discussion|access-date=10 June 2021|archive-date=2017-04-13 |archive-url=https://web.archive.org/web/20170413073150/http://www.biologydiscussion.com/animals-2/systematics-meaning-branches-and-its-application/32374|url-status=live}}</ref>

Zoology plays a vital role in addressing [[Contemporary history|contemporary]] environmental challenges, including climate change, [[habitat destruction]], and species extinction. By studying animal diversity and ecosystem interactions, zoologists inform conservation strategies and ecosystem restoration efforts.<ref>{{Cite web |last=Pathak |first=Neha Adikane, Dr Charudatta |date=2024-03-27 |title=Zoology: Exploring the Wonders of Animals |url=https://entechonline.com/zoology-exploring-the-wonders-of-animals/ |access-date=2025-07-22 |website=Science and Technology Magazine to Explore Your Passion |language=en-US}}</ref> For example, research into species behavior, genetics, and habitat requirements has directly contributed to the protection of [[Endangered species|endangered]] populations and the design of wildlife corridors.

==Scope==
Zoology is the branch of science dealing with [[animal]]s. A [[species]] can be defined as the largest group of organisms in which any two individuals of the appropriate sex can produce fertile offspring; about 1.5 million species of animal have been described and it has been estimated that as many as 8 million animal species may exist.<ref>{{Cite journal |last1=Mora |first1=Camilo |last2=Tittensor |first2=Derek P. |last3=Adl |first3=Sina |last4=Simpson |first4=Alastair G. B. |last5=Worm |first5=Boris |date=2011-08-23 |title=How Many Species Are There on Earth and in the Ocean? |journal=PLOS Biology |volume=9 |issue=8 |article-number=e1001127 |doi=10.1371/journal.pbio.1001127 |issn=1545-7885 |pmc=3160336 |pmid=21886479 |doi-access=free }}</ref>
An early necessity was to identify the organisms and group them according to their characteristics, differences and relationships, and this is the field of the [[taxonomy|taxonomist]]. Originally it was thought that species were immutable, but with the arrival of Darwin's theory of evolution, the field of [[cladistics]] came into being, studying the relationships between the different groups or [[clade]]s. [[Systematics]] is the study of the diversification of living forms, the evolutionary history of a group is known as its [[Phylogenetic tree|phylogeny]], and the relationship between the clades can be shown diagrammatically in a [[cladogram]].<ref name="Ruppert">{{cite book |title=Invertebrate Zoology, 7th edition |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard S. |last3=Barnes |first3=Robert D. |year=2004 |publisher=Cengage Learning |isbn=978-81-315-0104-7 |page=2 }}</ref>

Although someone who made a scientific study of animals would historically have described themselves as a zoologist, the term has come to refer to those who deal with individual animals, with others describing themselves more specifically as physiologists, ethologists, evolutionary biologists, ecologists, pharmacologists, endocrinologists or parasitologists.<ref name=Campbell>{{cite book |author=Campbell, P.N. |title=Biology in Profile: A Guide to the Many Branches of Biology |url=https://books.google.com/books?id=tQTLBAAAQBAJ |year=2013 |publisher=Elsevier |isbn=978-1-4831-3797-1 |pages=3–5 |access-date=2021-06-18  |archive-date=2024-06-12  |archive-url=https://web.archive.org/web/20240612073719/https://books.google.com/books?id=tQTLBAAAQBAJ |url-status=live }}</ref>

==Branches of zoology<!-- Link here found at: Animal#top -->==

Although the study of animal life is ancient, its scientific incarnation is relatively modern. This mirrors the transition from [[natural history]] to [[biology]] at the start of the 19th century. Since [[John Hunter (surgeon)|Hunter]] and [[Georges Cuvier|Cuvier]], comparative [[Anatomy|anatomical]] study has been associated with [[Morphogram|morphography]], shaping the modern areas of zoological investigation: [[anatomy]], [[physiology]], [[histology]], [[embryology]], [[teratology]] and [[ethology]].<ref>{{Cite encyclopedia|url=https://www.britannica.com/science/zoology#toc48756|title=zoology|encyclopedia=Encyclopedia Britannica|access-date=2017-09-13|language=en|archive-date=2017-09-13 |archive-url=https://web.archive.org/web/20170913185844/https://www.britannica.com/science/zoology#toc48756|url-status=live}}</ref> Modern zoology first arose in German and British universities. In Britain, [[Thomas Henry Huxley]] was a prominent figure. His ideas were centered on the [[morphology (biology)|morphology]] of animals. Many consider him the greatest comparative anatomist of the latter half of the 19th century. Similar to [[John Hunter (surgeon)|Hunter]], his courses were composed of lectures and laboratory practical classes in contrast to the previous format of lectures only.

===Classification===
[[Taxonomy (biology)|Scientific classification in zoology]], is a method by which zoologists group and categorize [[organism]]s by [[Type (biology)|biological type]], such as [[genus]] or [[species]]. Biological classification is a form of [[Taxonomy (biology)|scientific taxonomy]]. Modern biological classification has its root in the work of [[Carl Linnaeus]], who grouped species according to shared physical characteristics. These groupings have since been revised to improve consistency with the [[Charles Darwin|Darwinian]] principle of [[common descent]]. [[Molecular phylogenetics]], which uses [[nucleic acid sequence]] as data, has driven many recent revisions and is likely to continue to do so. Biological classification belongs to the science of [[systematics|zoological systematics]].<ref>{{Cite web |url=http://www.biologydiscussion.com/animals-2/systematics-meaning-branches-and-its-application/32374 |title=Systematics: Meaning, Branches and Its Application |date=27 May 2016 |work=Biology Discussion |access-date=12 April 2017 |language=en-US |archive-date=2017-04-13  |archive-url=https://web.archive.org/web/20170413073150/http://www.biologydiscussion.com/animals-2/systematics-meaning-branches-and-its-application/32374 |url-status=live }}</ref>

[[File:Linnaeus - Regnum Animale (1735).png|thumb|Linnaeus's table of the animal kingdom from the first edition of ''[[Systema Naturae]]'' (1735)]]

Many scientists now consider the [[Kingdom (biology)#Five kingdoms|five-kingdom system]] outdated. Modern alternative classification systems generally start with the [[three-domain system]]: [[Archaea]] (originally Archaebacteria); [[Bacteria]] (originally Eubacteria); [[Eukaryote|Eukaryota]] (including [[protist]]s, [[Fungus|fungi]], [[plant]]s, and [[animal]]s)<ref name="domain">{{cite journal |vauthors=Woese C, Kandler O, Wheelis M | title = Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya | journal = Proc Natl Acad Sci USA | volume = 87 | issue = 12 | pages = 4576–4579 | year = 1990 | pmid = 2112744 | doi = 10.1073/pnas.87.12.4576 | pmc = 54159 | bibcode=1990PNAS...87.4576W| doi-access = free }}</ref> These domains reflect whether the cells have nuclei or not, as well as differences in the chemical composition of the cell exteriors.<ref name="domain"/>

Further, each kingdom is broken down recursively until each species is separately classified. The order is:
[[Domain (biology)|Domain]]; [[Kingdom (biology)|kingdom]]; [[phylum]]; [[Class (biology)|class]]; [[Order (biology)|order]]; [[Family (biology)|family]]; [[genus]]; [[species]]. The scientific name of an organism is generated from its genus and species. For example, humans are listed as ''[[Homo sapiens]]''. ''Homo'' is the genus, and ''sapiens'' the specific epithet, both of them combined make up the species name. When writing the scientific name of an organism, it is proper to capitalize the first letter in the genus and put all of the specific epithet in lowercase. Additionally, the entire term may be italicized or underlined.<ref>{{cite book | url = https://books.google.com/books?id=hVUU7Gq8QskC&q=species+epithet+capitalize&pg=PA198 | page = 198 | title = Writing for Science and Engineering: Papers, Presentation | author = Heather Silyn-Roberts | year = 2000 | isbn = 0-7506-4636-5 | publisher = Butterworth-Heinemann | location = Oxford | access-date = 2020-11-07  | archive-date = 2024-06-12  | archive-url = https://web.archive.org/web/20240612073723/https://books.google.com/books?id=hVUU7Gq8QskC&q=species+epithet+capitalize&pg=PA198#v=snippet&q=species%20epithet%20capitalize&f=false | url-status = live }}</ref>

The dominant classification system is called the [[Linnaean taxonomy]]. It includes ranks and [[binomial nomenclature]]. The classification, [[Taxonomy (biology)|taxonomy]], and nomenclature of zoological organisms is administered by the [[International Code of Zoological Nomenclature]]. A merging draft, BioCode, was published in 1997 in an attempt to standardize nomenclature, but has yet to be formally adopted.<ref>{{cite conference | title=The BioCode: Integrated biological nomenclature for the 21st century? | book-title=Proceedings of a Mini-Symposium on Biological Nomenclature in the 21st Century | author=John McNeill | date=4 November 1996}}</ref>

===Vertebrate and invertebrate zoology===
[[Vertebrate zoology]] is the [[biology|biological]] [[discipline]] that consists of the study of [[vertebrate]] animals, that is animals with a [[vertebral column|backbone]], such as [[fish]], [[amphibian]]s, [[reptile]]s, [[bird]]s and [[mammal]]s. The various taxonomically oriented disciplines i.e.
[[mammalogy]], [[biological anthropology]], [[herpetology]], [[ornithology]], and [[ichthyology]] seek to identify and classify [[species]] and study the structures and mechanisms specific to those groups. The rest of the animal kingdom is dealt with by [[invertebrate zoology]], a vast and very diverse group of animals that includes [[sponge]]s, [[echinoderm]]s, [[tunicate]]s, [[worm]]s, [[mollusc]]s, [[arthropod]]s and many other [[Phylum|phyla]], but [[Unicellular organism|single-celled organisms]] or [[protist]]s are not usually included.<ref name="Ruppert"/>

===Structural zoology===
[[Cell biology]] studies the structural and [[physiology|physiological]] properties of [[cell (biology)|cells]], including their [[behavior]], interactions, and [[natural environment|environment]]. This is done on both the [[microscope|microscopic]] and [[molecule|molecular]] levels for single-celled organisms such as [[bacteria]] as well as the specialized cells in [[multicellular organism]]s such as [[human]]s. Understanding the structure and function of cells is fundamental to all of the biological sciences. The similarities and differences between cell types are particularly relevant to molecular biology.

[[Anatomy]] considers the forms of macroscopic structures such as [[organ (biology)|organs]] and organ systems.<ref>{{cite book |author=Henry Gray |title=Anatomy of the Human Body |year=1918 |publisher=Lea & Febiger |url=http://www.bartleby.com/107/1.html |access-date=2011-01-01  |archive-date=2007-03-16  |archive-url=https://web.archive.org/web/20070316005206/http://www.bartleby.com/107/1.html |url-status=live }}</ref> It focuses on how organs and organ systems work together in the bodies of humans and other animals, in addition to how they work independently. Anatomy and cell biology are two studies that are closely related, and can be categorized under "structural" studies. [[Comparative anatomy]] is the study of similarities and differences in the [[anatomy]] of different groups. It is closely related to [[evolutionary biology]] and [[Phylogenetic tree|phylogeny]] (the [[evolution]] of species).<ref>{{cite journal |author1=Gaucher, E.A. |author2=Kratzer, J.T. |author3=Randall, R.N. | title = Deep phylogeny—how a tree can help characterize early life on Earth | journal = Cold Spring Harbor Perspectives in Biology | volume = 2 | issue = 1 | article-number = a002238 | date = January 2010 | pmid = 20182607 | pmc = 2827910 | doi = 10.1101/cshperspect.a002238 }}</ref>

===Physiology===
[[File:Dog anatomy anterior view.jpg|thumb|Animal anatomical engraving from ''Handbuch der Anatomie der Tiere für Künstler''.]]

Physiology studies the mechanical, physical, and biochemical processes of living organisms by attempting to understand how all of the structures function as a whole. The theme of "structure to function" is central to biology. Physiological studies have traditionally been divided into [[plant physiology]] and [[animal physiology]], but some principles of physiology are universal, no matter what particular [[organism]] is being studied. For example, what is learned about the physiology of [[yeast]] cells can also apply to human cells. The field of animal physiology extends the tools and methods of [[Human body#Physiology|human physiology]] to non-human species. Physiology studies how, for example, the [[nervous system|nervous]], [[immune system|immune]], [[endocrine system|endocrine]], [[respiratory system|respiratory]], and [[circulatory system|circulatory]] systems function and interact.<ref>{{Cite web |url=https://www.biology.cam.ac.uk/undergrads/nst/courses/physiology-of-organisms/what-is-physiology |title=What is physiology? — Faculty of Biology |website=biology.cam.ac.uk |date=16 February 2016 |language=en |access-date=19 June 2021 |archive-date=2018-07-07  |archive-url=https://web.archive.org/web/20180707231044/https://www.biology.cam.ac.uk/undergrads/nst/courses/physiology-of-organisms/what-is-physiology |url-status=live }}</ref>

===Developmental biology===
[[Developmental biology]] is the study of the processes by which animals and plants reproduce and grow. The discipline includes the study of [[embryonic development]], [[cellular differentiation]], [[Regeneration (biology)|regeneration]], [[asexual reproduction|asexual]] and [[sexual reproduction|sexual]] [[reproduction]], [[metamorphosis]], and the growth and differentiation of [[stem cell]]s in the adult organism.<ref>{{cite web |url=https://plato.stanford.edu/entries/biology-developmental/ |title=Developmental biology |date=14 February 2020 |publisher=Stanford Encyclopedia of Philosophy |access-date=20 June 2021 |archive-date=2021-04-30  |archive-url=https://web.archive.org/web/20210430230644/https://plato.stanford.edu/entries/biology-developmental/ |url-status=live }}</ref> Development of both animals and plants is further considered in the articles on [[evolution]], [[population genetics]], [[heredity]], [[genetic variability]], [[Mendelian inheritance]], and [[reproduction]].

===Evolutionary biology===
Evolutionary biology is the subfield of biology that studies the evolutionary processes (natural selection, common descent, speciation) that produced the diversity of life on Earth. Evolutionary research is concerned with the origin and descent of [[species]], as well as their change over time, and includes scientists from many [[Taxonomy (biology)|taxonomically]] oriented disciplines. For example, it generally involves scientists who have special training in particular [[organism]]s such as [[mammalogy]], [[ornithology]], [[herpetology]], or [[entomology]], but use those organisms as systems to answer general questions about evolution.<ref>Gilbert, Scott F.; Barresi, Michael J.F. (2016) "Developmental Biology" Sinauer Associates, inc.(11th ed.) pp. 785–810. {{ISBN|9781605354705}}</ref>

Evolutionary biology is partly based on [[paleontology]], which uses the [[fossil]] record to answer questions about the mode and tempo of evolution,<ref name=Jablonski>{{cite journal |author=Jablonski D |title=The future of the fossil record |journal=Science |volume=284 |issue=5423 |pages=2114–2116 |year=1999 |doi=10.1126/science.284.5423.2114 |pmid=10381868 |s2cid=43388925}}</ref> and partly on the developments in areas such as [[population genetics]]<ref>{{cite book |author=John H. Gillespie |title=Population Genetics: A Concise Guide |year=1998 |publisher=Johns Hopkins Press |isbn=978-0-8018-8008-7 }}</ref> and evolutionary theory. Following the development of [[DNA profiling|DNA fingerprinting]] techniques in the late 20th century, the application of these techniques in zoology has increased the understanding of animal populations.<ref>{{Cite journal |last1=Chambers |first1=Geoffrey K. |last2=Curtis |first2=Caitlin |last3=Millar |first3=Craig D. |last4=Huynen |first4=Leon |last5=Lambert |first5=David M. |date=2014-02-03 |title=DNA fingerprinting in zoology: past, present, future |journal=Investigative Genetics |volume=5 |issue=1 |at=3 |doi=10.1186/2041-2223-5-3 |doi-access=free |issn=2041-2223 |pmc=3909909 |pmid=24490906}}</ref> In the 1980s, [[developmental biology]] re-entered evolutionary biology from its initial exclusion from the [[Modern synthesis (20th century)|modern synthesis]] through the study of [[evolutionary developmental biology]]. Related fields often considered part of evolutionary biology are [[phylogenetics]], [[systematics]], and [[Taxonomy (biology)#Alpha and beta taxonomy|taxonomy]].<ref>{{cite book |author=Vassiliki Betty Smocovitis |title=Unifying Biology: The Evolutionary Synthesis and Evolutionary Biology |year=1996 |isbn=978-0-691-03343-3 |publisher=Princeton University Press}}</ref>

===Ethology===
[[File:Larus Dominicanus with young.jpg|right|thumb|[[Kelp gull]] chicks peck at red spot on mother's beak to stimulate the regurgitating reflex.]]

[[Ethology]] is the [[scientific method|scientific]] and objective study of animal behavior under natural conditions,<ref>{{cite dictionary |title=Definition of Ethology |url=http://www.merriam-webster.com/dictionary/ethology |dictionary=Merriam-Webster |access-date=30 October 2012 |quote=2 : the scientific and objective study of animal behaviour especially under natural conditions |archive-date=2009-04-25  |archive-url=https://web.archive.org/web/20090425151624/http://www.merriam-webster.com/dictionary/ethology |url-status=live }}</ref> as opposed to [[behaviorism]], which focuses on behavioral response studies in a laboratory setting. Ethologists have been particularly concerned with the [[evolution]] of behavior and the understanding of behavior in terms of the theory of [[natural selection]]. In one sense, the first modern ethologist was [[Charles Darwin]], whose book, ''[[The Expression of the Emotions in Man and Animals]],'' influenced many future ethologists.<ref name="black">{{Cite journal |pmc=1279921 |pmid=12042386 |date=Jun 2002 |author=Black, J. |title=Darwin in the world of emotions |volume=95 |issue=6 |pages=311–313 |issn=0141-0768 |journal=Journal of the Royal Society of Medicine |url=http://www.jrsm.org/cgi/pmidlookup?view=long&pmid=12042386 |format=Free full text |doi=10.1177/014107680209500617 |access-date=2011-01-01  |archive-date=2016-08-10  |archive-url=https://web.archive.org/web/20160810043441/http://jrsm.rsmjournals.com/cgi/pmidlookup?view=long&pmid=12042386 |url-status=live }}</ref>

A subfield of ethology is [[behavioral ecology]] which attempts to answer [[Nikolaas Tinbergen]]'s [[Tinbergen's four questions|four questions]] with regard to animal behavior: what are the [[proximate cause]]s of the behavior, the [[ontogeny|developmental history]] of the organism, the [[Adaptation|survival value]] and [[phylogeny]] of the behavior?<ref>{{Cite journal |last=MacDougall-Shackleton |first=Scott A. |date=2011-07-27 |title=The levels of analysis revisited |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=366 |issue=1574 |pages=2076–2085 |doi=10.1098/rstb.2010.0363 |pmc=3130367 |pmid=21690126}}</ref> Another area of study is [[animal cognition]], which uses laboratory experiments and carefully controlled field studies to investigate an animal's intelligence and learning.<ref>{{cite book| author=Shettleworth, S.J. |year=2010 |title=Cognition, Evolution and Behavior |edition=2nd |publisher=Oxford Press |location=New York |citeseerx=10.1.1.843.596}}</ref>

===Biogeography===

[[Biogeography]] studies the spatial distribution of organisms on the [[Earth]],<ref>{{cite book |author-last=Wiley | author-first=R. Haven | editor1= P.P.G. Bateson | editor2= Peter H. Klopfer |title=Perspectives in Ethology |chapter=Social Structure and Individual Ontogenies: Problems of Description, Mechanism, and Evolution |year=1981 |volume=4 |pages=105–133 |chapter-url=http://www.unc.edu/home/rhwiley/pdfs/IndividualOntogenies.pdf |access-date=21 December 2012 |doi=10.1007/978-1-4615-7575-7_5 |isbn=978-1-4615-7577-1 |archive-date=2013-06-08  |archive-url=https://web.archive.org/web/20130608072253/http://www.unc.edu/home/rhwiley/pdfs/IndividualOntogenies.pdf |url-status=deviated | publisher= Plenum }}</ref> focusing on topics like [[Biological dispersal|dispersal]] and [[Animal migration|migration]], [[plate tectonics]], [[climate change]], and [[cladistics]]. It is an integrative field of study, uniting concepts and information from [[evolutionary biology]], [[taxonomy (biology)|taxonomy]], [[ecology]], [[physical geography]], [[geology]], [[paleontology]] and [[climatology]].<ref name="Cox_et_al_2016">{{cite book |title=Biogeography:An Ecological and Evolutionary Approach |last1=Cox |first1=C. Barry |last2=Moore |first2=Peter D. |last3=Ladle |first3=Richard J. |year=2016 |publisher=Wiley |location=Chichester, UK |isbn=978-1-118-96858-1 |page=xi |url=https://books.google.com/books?id=GP5HeCwkV2IC |access-date=22 May 2020 |archive-date=2024-06-12  |archive-url=https://web.archive.org/web/20240612073719/https://books.google.com/books?id=GP5HeCwkV2IC |url-status=live }}</ref> The origin of this field of study is widely accredited to [[Alfred Russel Wallace]], a British biologist who had some of his work jointly published with [[Charles Darwin]].<ref name=Browne>{{cite book |last=Browne |first=Janet |year=1983 |title=The secular ark: studies in the history of biogeography |publisher=Yale University Press |location=New Haven |isbn=978-0-300-02460-9 }}</ref>

===Molecular biology===
[[File:Phylogenetic tree of dogs.png|thumb|upright=1.35|A clade representation of seven dog breeds in relation to wolves.]]

[[Molecular biology]] studies the common [[genetics|genetic]] and developmental mechanisms of animals and plants, attempting to answer the questions regarding the mechanisms of [[Heredity|genetic inheritance]] and the structure of the [[gene]]. In 1953, [[James Watson]] and [[Francis Crick]] described the structure of DNA and the interactions within the molecule, and this publication jump-started research into molecular biology and increased interest in the subject.<ref>{{Cite encyclopedia |last1=Tabery |first1=James |last2=Piotrowska |first2=Monika |last3=Darden |first3=Lindley |title=Molecular Biology (Fall 2019 Edition) |url=https://plato.stanford.edu/archives/fall2019/entries/molecular-biology/ |encyclopedia=The Stanford Encyclopedia of Philosophy |date=19 February 2005 |editor-last=Zalta |editor-first=Edward N. |publisher=Metaphysics Research Lab, Stanford University |access-date=2020-04-19 |archive-date=2024-06-12  |archive-url=https://web.archive.org/web/20240612073728/https://plato.stanford.edu/archives/fall2019/entries/molecular-biology/ |url-status=live }}</ref> While researchers practice techniques specific to molecular biology, it is common to combine these with methods from [[genetics]] and [[biochemistry]]. Much of molecular biology is quantitative, and recently a significant amount of work has been done using [[computer science]] techniques such as [[bioinformatics]] and [[computational biology]].

[[Molecular genetics]], the study of gene structure and function, has been among the most prominent sub-fields of molecular biology since the early 2000s. Other branches of biology are informed by molecular biology, by either directly studying the interactions of molecules in their own right such as in [[cell biology]] and [[developmental biology]], or indirectly, where molecular techniques are used to infer historical attributes of [[population]]s or [[species]], as in fields in [[evolutionary biology]] such as [[population genetics]] and [[phylogenetics]]. There is also a long tradition of studying [[biomolecule]]s "from the ground up", or molecularly, in [[biophysics]].<ref name="Tian_2013">{{cite book |editor=Tian, J. |date=2013 |title=Molecular Imaging: Fundamentals and Applications |publisher=Springer-Verlag Berlin & Heidelberg GmbH & Co. |isbn=978-3-642-34303-2 |url=https://books.google.com/books?id=cBXIBAAAQBAJ&pg=PA542 |page=542 |access-date=2019-07-08 |archive-date=2024-06-12  |archive-url=https://web.archive.org/web/20240612073745/https://books.google.com/books?id=cBXIBAAAQBAJ&pg=PA542#v=onepage&q&f=false |url-status=live }}</ref>

===Reproduction===

Animals generally reproduce by [[sexual reproduction]], a process involving the union of a male and female [[ploidy|haploid]] [[gamete]], each gamete formed by [[meiosis]]. Ordinarily, gametes produced by separate individuals unite by a process of fertilization to form a diploid [[zygote]] that can then develop into a genetically unique individual progeny. However, some animals are also capable, as an alternative reproductive process, to reproduce parthenogenetically. Parthenogenesis has been described in snakes and lizards (see Wikipedia [[Parthenogenesis in squamates]]), in amphibians (see Wikipedia [[Parthenogenesis in amphibians]]) and in numerous other species (see Wikipedia [[Parthenogenesis]]). Generally, meiosis in parthenogenetically reproducing animals occurs by a similar process to that in sexually reproducing animals, but the diploid zygote nucleus is generated by the union of two haploid genomes from the same individual rather than from different individuals.

=== Animal cognition ===
Animal cognition encompasses the mental processes and abilities of non-human animals, including perception, learning, memory, decision-making, and problem-solving. Research in this field has revealed remarkable cognitive capabilities across diverse species, from the tool use and [[Self-awareness|self-recognition]] observed in great apes and elephants<ref>{{Cite journal |last1=Plotnik |first1=Joshua M. |last2=de Waal |first2=Frans B. M. |last3=Reiss |first3=Diana |date=2006-11-07 |title=Self-recognition in an Asian elephant |journal=Proceedings of the National Academy of Sciences |volume=103 |issue=45 |pages=17053–17057 |doi=10.1073/pnas.0608062103 |pmid=17075063 |doi-access=free |pmc=1636577}}</ref> to the complex communication systems of dolphins <ref>{{Cite web |title=What Laboratory Research has Told Us about Dolphin Cognition |url=https://www.researchgate.net/publication/228644936 |archive-url=https://web.archive.org/web/20220508222305/https://www.researchgate.net/publication/228644936_What_Laboratory_Research_has_Told_Us_about_Dolphin_Cognition |archive-date=2022-05-08  |access-date=2025-07-22 |website=ResearchGate |language=en |url-status=live }}</ref>and the [[Spatial memory|spatial]] memory feats of food-caching birds like [[Clark's nutcracker]]. Studies have demonstrated that many animals possess forms of consciousness, can understand cause-and-effect relationships, exhibit cultural transmission of behaviors, and show evidence of [[metacognition]]—the ability to think about thinking. While debates continue regarding the extent and nature of animal consciousness compared to human cognition, mounting evidence suggests that cognitive abilities exist on a [[Continuum mechanics|continuum]] rather than representing a sharp divide between humans and other species. Modern research methods, including [[neuroimaging]] and carefully controlled behavioral experiments, continue to expand our understanding of how different species process information and navigate their environments, challenging traditional [[Anthropocentrism|anthropocentric]] views of intelligence and consciousness.

==See also==
* [[Animal science]], the biology of domesticated animals
* [[Astrobiology]]
* [[Animal cognition|Cognitive zoology]]
* [[Evolutionary biology]]
* [[List of zoologists]]
* [[Outline of zoology]]
* [[Palaeontology]]
* [[Timeline of zoology]]
* [[Zoopharmacognosy]]

==References==
{{reflist|30em}}

==External links==
{{Sister project links}}
* [http://www.gutenberg.org/browse/loccs/ql Books on Zoology] at [[Project Gutenberg]]
* ''[http://digitalcommons.unl.edu/onlinedictinvertzoology/ Online Dictionary of Invertebrate Zoology]''

{{Zoology}}
{{Animalia}}
{{Branches of biology}}
{{Biology nav}}
{{Zoos}}
{{Portal bar|Animals|Biology|Palaeontology}}
{{Authority control}}

[[Category:Zoology| ]]
[[Category:Branches of biology]]