Melon-headed whale

The melon-headed whale (Peponocephala electra), also known less commonly as the electra dolphin, little killer whale, or many-toothed blackfish, is a toothed whale of the oceanic dolphin family (Delphinidae). The common name is derived from the head shape. Melon-headed whales are widely distributed throughout deep tropical and subtropical waters worldwide, but they are rarely encountered at sea. They are found near shore mostly around oceanic islands, such as Hawaii, French Polynesia, and the Philippines.

Taxonomy

The melon-headed whale is the only member of the genus Peponocephala. First recorded from a specimen collected in Hawaiʻi in 1841, the species was originally described as a member of the dolphin family and named Lagenorhynchus electra by John Edward Gray in 1846. The melon-headed whale was later determined to be sufficiently distinct from other Lagenorhynchus species to be accorded its own genus.<ref>Template:Cite journal</ref> A member of the subfamily Globicephalinae, melon-headed whales are closely related to long-finned and short-finned pilot whales (Globicephala melas and G. macrorhynchus, respectively) and the pygmy killer whale (Feresa attenuata).<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Collectively, these species (including killer whales Orcinus orca, and false killer whales Pseudorca crassidens) are known by the common name 'blackfish'. Melon-headed whales are one of the smallest species of cetacean (after pygmy killer whales) to have the word 'whale' in their common name.Template:Citation needed

Description

Melon-headed whales have a robust, dolphin-like body, a tapering, conical head (head shape triangular when viewed from above) with no discernible beak and a relatively tall, falcate (sickle-shaped) dorsal fin located near the middle of the back. Body coloration is charcoal-gray to dark-gray body. A dark face 'mask' extends from around the eye to the front of the melon and larger animals have whitish lips. Melon-headed whales have a dark colored dorsal cape that starts narrowly at the front of the head and dips down at a steep angle below the dorsal fin. The boundary between the darker cape and coloration on the flanks is often faint or diffuse. Both the mask and dorsal cape are often only visible in good lighting conditions. Compared to females, adult males have more rounded heads, longer flippers, taller dorsal fins, broader tail flukes and some have a pronounced ventral keel posterior to the anus.<ref name="Best">Template:Cite journal</ref><ref name="Miyazaki"/><ref name="Encyc MM">Template:Cite encyclopedia</ref>

Melon-headed whales grow up to Template:Cvt in length, and weigh up to Template:Cvt, adult males being slightly larger than females.<ref name="Miyazaki"/><ref name="Handbook">Template:Cite book</ref> Length at birth is approximately Template:Cvt.<ref name="Bryden">Template:Cite journal</ref><ref name="Amano">Template:Cite journal</ref> Melon-headed whales are physically mature at 13–15 years and live up to 45 years.<ref name="Miyazaki">Template:Cite journal</ref>

Similar species

At-sea, melon-headed whales can be confused with pygmy killer whales, which are very similar in appearance and share almost identical habitat and range.<ref name="MC">Template:Cite book</ref> The shape of the head, flippers and dorsal cape can be useful diagnostic features. Melon-headed whales have flippers with sharply pointed tips whereas pygmy killer whales have rounded flipper tips, and viewed from above, the head shape is more triangular than the rounded head of the pygmy killer whale.<ref name="Encyc MM"/> The dorsal cape of melon-headed whales is rounded and dips much lower below the dorsal fin than that of pygmy killer whales, which dips at a relatively shallow angle and is more sharply demarcated in color between the dark cape and lighter flanks.<ref name="Jefferson">Template:Cite book</ref> While both species have white around the mouth, in adult pygmy killer whales this can extend onto the face.<ref>Template:Cite journal</ref>

Many of these traits are difficult to distinguish in challenging sea and/or lighting conditions, and behavior is often a useful aid to identification. Pygmy killer whales are slower moving (although melon-headed whales often log motionless at the surface), and are usually found in much smaller groups than melon-headed whales. Large groups (more than 100) are more likely to be melon-headed whales.<ref name="MC"/><ref name="Handbook"/> From a distance melon-headed whales could also be confused with false killer whales (Pseudorca crassidens), but the much larger size–Template:Cvt adult length–long slender body shape and relatively smaller dorsal fin of false killer whales should distinguish them from melon-headed whales.

Stranded/post-mortem individuals can be easily identified by tooth number: melon-headed whales have 20 – 25 pairs of slender teeth (more similar to the teeth of smaller dolphins than other blackfish) in both the upper and lower jaws compared to 8–13 pairs of robust teeth in both the upper and lower jaws for pygmy killer whales.<ref name="Jefferson"/>

Geographic range and distribution

A pod in Bohol Sea between Balicasag Island and Alona Beach

Melon-headed whales occur in deep tropical/subtropical oceanic waters, between 40°N and 35°S.<ref name="Handbook"/> Although considered an offshore pelagic species, in some regions there are island-associated populations (e.g., Hawaiʻi) and they can be found close to shore associated with oceanic islands and archipelagoes, such as Palmyra and the Philippines.<ref name="Brownell">Template:Cite journal</ref> Sightings or strandings at the extremes of their range are likely associated with extensions of warm currents.<ref name="Handbook"/> Melon-headed whales are not known to be migratory.Template:Citation needed

Behavior

Foraging

Melon-headed whales feed primarily on pelagic and mesopelagic squid and small fish.<ref name="West">Template:Cite journal</ref><ref name="Jefferson&Barros">Template:Cite journal</ref> Crustaceans (shrimp) have also been reported from stomach contents.<ref name="Encyc MM" /><ref name="West"/> Mesopelagic squid and fish species exhibit diel vertical migration behavior, inhabiting greater depths during the day and moving hundreds of meters to shallower depths after dusk to feed on plankton. Melon-headed whales feed at night, when their prey is within the upper Template:Cvt of the water column.<ref name="West"/> In the Hawaiian Islands, individuals fitted with depth-transmitting satellite tags made night-time foraging dives that had an average range of Template:Cvt, with a maximum dive depth of Template:Cvt recorded.<ref name="West"/>

Social

Melon-headed whales are a highly social species and usually travel in large groups of 100 – 500 individuals, with occasional sightings of herds as large as 1000–2000.<ref name="Jefferson"/> Large herds appear to consist of smaller subgroups that aggregate into larger groups.<ref>Template:Cite journal</ref> Data from mass strandings in Japan suggest melon-headed whales may have a matrilineal social structure (i.e., related through female kin/groups organized around an older female and their relatives); the biased sex ratio (higher number of females) of the stranding groups suggesting mature males may move between groups.<ref name="Amano"/> While melon-headed whales associate in large groups (a common trait amongst the oceanic dolphins, in contrast to the smaller group sizes of other blackfish species) their social structure may be more stable and intermediate between the larger blackfish (pilot whales, killer whales and false killer whales) and smaller oceanic dolphins.<ref name="Amano"/> However, genetic studies of melon-headed whales across the Pacific, Indian and Atlantic Ocean basins suggest that there is relatively high level of connectivity (inter-breeding) between populations.<ref name="Martien">Template:Cite journal</ref> This indicates that melon-headed whales may not show strong fidelity to their natal group (the group into which the individual was born) and that there are higher rates of movement of individuals between populations than in other blackfish species.<ref name="Martien"/> Larger group sizes may increase competition for prey resources, requiring large home ranges and broad-scale foraging movements.<ref name="Martien"/>

Observations of daily activity patterns of melon-headed whales near oceanic islands suggest they spend the mornings resting or logging in near-surface waters after foraging at night.<ref name="Brownell"/> Surface activity (such as tail slapping and spyhopping) and vocalizations associated with socializing (communication whistles, rather than echolocation clicks used for foraging) increase during the afternoons.<ref name="Brownell"/><ref>Template:Cite journal</ref> The daily pattern of behavior observed in island-associated populations, combined with the larger group sizes of melon-headed whales (compared to that typically seen in other blackfish species) is more similar to the fission-fusion social structure of spinner dolphins (Stenella longirostris).<ref name="Brownell"/><ref name="Martien"/> These behavioral traits may relate to predation avoidance (bigger groups offer some protection from large oceanic sharks) and foraging habits (both species are nocturnal predators that prey on predictable, relatively abundant mesopelagic squid and fish that make diel vertical migrations from the deep-sea to the surface).

Melon-headed whales frequently associate with Fraser's dolphins (Lagenodelphis hosei), and are also sighted, although less commonly, in mixed herds with other dolphin species such as spinner dolphins, common bottlenose dolphins (Tursiops truncatus), rough-toothed dolphins (Steno bredanensis), short-finned pilot whales and pantropical spotted dolphins (Stenella attenuata).<ref name="Aschettino">Template:Cite journal</ref><ref name="Dolar">Template:Cite journal</ref>

A unique case of inter-species adoption between (presumably) an orphaned melon-headed whale calf and a common bottlenose dolphin mother was recorded in French Polynesia. The calf was first observed in 2014 at less than one month of age, swimming with the bottlenose dolphin female and her own biological offspring.<ref name="Carzon">Template:Cite journal</ref> The melon-headed whale calf was observed suckling from the bottlenose dolphin female, and was repeatedly sighted with its adoptive/foster mother until 2018.<ref name="Carzon"/>

In August 2017 off the island of Kauaʻi, Hawaiʻi, a hybrid between a melon-headed whale and rough-toothed dolphin was observed travelling with a melon-headed whale amongst a group of rough-toothed dolphins.<ref name="Baird HI">Template:Cite journal</ref> The hybrid superficially resembled a melon-headed whale, but closer observation revealed it had features of both species and some features intermediate between the two species, particularly in head shape. Genetic testing of a skin biopsy sample confirmed that the individual was a hybrid between a female melon-headed whale and a rough-toothed dolphin male.<ref name="Baird HI"/>

Predators

Melon-headed whales may be preyed upon by large sharks and killer whales.<ref name="Baird 2016">Template:Cite book</ref><ref name="Aschettino Msc"/> Scars and wounds from non-lethal bites of cookie cutter sharks (Isitius brasiliensis) have been observed on free-ranging and stranded animals.<ref name="Best"/><ref name="Baird 2016"/>

Breeding

Little is known about the reproductive behavior of melon-headed whales. The most information comes from analyses of large stranding groups in Japanese waters, where sexual maturity for females is reached at 7 years of age.<ref name="Best"/> Females give birth to a single calf every 3–4 years after a gestation of approximately 12 months.<ref name="Bryden"/><ref name="Amano"/> Off Japan, the calving season appears to be long (from April to October) without an obvious peak.<ref name="Amano"/> In Hawaiian waters newborn melon-headed whales have been observed in all months except December, suggesting births occur year-round, but sightings of newborns peak between March and June.<ref name="Baird 2016"/> Newborn melon-headed whales have been observed in April and June in the Philippines.<ref name="Handbook"/> In the Southern Hemisphere calving also appears to occur over an extended period, from August to December.<ref name="Bryden"/>

Strandings

Melon-headed whales are known to mass strand, often in groups numbering in the hundreds, indicative of the strong social bonds within herds of this species.<ref name="Jefferson&Barros"/> Mass strandings of melon-headed whales have been reported in Hawaiʻi, eastern Japan, the Philippines, northern Australia, Madagascar, Brazil and the Cape Verde Islands. Two of these mass stranding events have been linked to anthropogenic sonar, associated with naval activities in Hawaiʻi and high frequency multi-beam sonar used for oil and gas exploration in Madagascar.<ref name="Southall06">Template:Cite journal</ref><ref name="Southall13">Template:Cite journal</ref> The mass stranding at Hanalei Bay, Kauaʻi, Hawaiʻi is more precisely described as a 'near' mass stranding event, as the group of >150 melon-headed whales was prevented from stranding by human intervention.<ref name="Southall06"/> The animals occupied the shallow waters of a confined bay for over 28 hours before being herded back into deeper waters by stranding response staff and volunteers, community members, state and federal authorities.<ref name="Baird 2016"/><ref name="Southall06"/> Only a single calf is known to have died on this occasion. The frequency of mass strandings of melon-headed whales appears to have increased over the past 30+ years.<ref>Template:Cite journal</ref>

Movement

Template:See also Melon-headed whales are fast swimmers; they travel in large, tightly packed groups and can create a lot of spray when surfacing, often porpoising (repeatedly leaping clear of the water surface at a shallow angle) when travelling at speed, and are known to spyhop and also may jump clear out of the water.<ref name="MC"/> Melon-headed whales can be wary of boats, but in some regions will approach boats and bow-ride.<ref name="Handbook"/><ref name="Brownell"/>

Population status

The world population is unknown, but abundance estimates for large regions are approximately 45,000 in the eastern tropical Pacific Ocean,<ref>Template:Cite journal</ref> 2,235 in northern Gulf of Mexico<ref>Template:Cite journal</ref> and in the Philippines 920 in the eastern Sulu Sea and 1,380 in Tañon Strait between Cebu and Negros Islands.<ref name="Dolar"/> There are two known populations in Hawaiʻi: a population of approximately 450 individuals resident to shallower waters of the northwest side of Hawaiʻi Island (the 'Kohola resident population') and a much larger population of approximately 8,000 individuals that moves among the main Hawaiian Islands in deeper waters.<ref name="Aschettino Msc">Template:Cite journal</ref>

Because the Hawaiʻi Island resident population has a restricted range (sightings have only been recorded off the northwest side of Hawaiʻi Island), and at times most of, or the entire resident population can be together in a single group, there is some concern that this population may be at risk from fisheries interactions, and exposure to anthropogenic noise,<ref name="Aschettino"/><ref name="Baird 2016"/> particularly in light of U.S. Navy activities in the region, given the potential link between sonar and mass stranding events.<ref name="Southall06"/>

Interactions with humans

Fisheries bycatch

Small cetaceans such as melon-headed whales are vulnerable to fisheries bycatch and may be injured or killed through interactions with fisheries or entanglement in lost or discarded netting. Small numbers of melon-headed whales have been caught incidentally in the longline fishery targeting tuna and swordfish off Mayotte,<ref name="Kiszka">Template:Cite journal</ref> and in driftnet fisheries in the Philippines,<ref>Template:Cite journal</ref> Sri Lanka,<ref name="Ilangakoon">Template:Cite journal</ref> Ghana<ref name="Van Waerebeek">Template:Cite journal</ref> and India.<ref>Template:Cite journal</ref> In the eastern tropical Pacific purse-seine tuna fisheries, melon-headed whales have been rarely taken as bycatch.<ref name="Encyc MM" /> and have not been recorded in the southwest Indian Ocean purse-seine fishery <ref name="Kiszka"/> Individual melon-headed whales exhibit injuries such as body scars and dorsal fin disfigurements likely due to interactions with fisheries in Hawaiʻi <ref name="Aschettino Msc"/> and near Mayotte in the Mozambique Channel Islands.<ref name="Kiszka b">Template:Cite journal</ref> The small numbers of injured individuals observed near Mayotte suggests that either interactions with the pelagic longline fishery in this region are rare for this species, or that individuals are more often killed rather than injured.<ref name="Kiszka b"/> Mortalities are also likely in other countries where gill- or driftnet fisheries occur, however data on bycatch in many regions are sparse.<ref name="Kiszka"/> Animals caught as bycatch are sometimes used as bait in fisheries.<ref>Template:Cite journal</ref>

Hunting

Individuals are taken for bait or human consumption in small cetacean subsistence and harpoon fisheries in several regions, including Sri Lanka,<ref name="Ilangakoon"/> the Caribbean,<ref>Template:Cite journal</ref> the Philippines <ref>Template:Cite journal</ref> and Indonesia.<ref>Template:Cite journal</ref> At Dixcove port in Ghana, melon-headed whales are the third highest cetacean species caught for 'marine bushmeat' by artisanal fishermen, through both bycatch from drift gillnets and occasional directed catch.<ref name="Van Waerebeek"/> The Japanese drive fishery has taken herds of melon-headed whales occasionally in the past.<ref name="Miyazaki"/> In the early 2020s, between 67 and 230 melon-headed whales were hunted annually in the Taiji dolphin drive hunt.<ref name="fa180125">Template:Cite web</ref><ref name="fa24">Template:Cite web</ref>

Pollution

Environmental contaminants stemming from plastic debris, oil spills and dumping of industrial wastes at-sea, in addition to agricultural run-off from terrestrial sources, can lead to bioaccumulation in marine ecosystems and pose a threat to melon-headed whales (as with all marine mammals and long-lived, high trophic level consumers). Persistent organic pollutants (POPs)–include environmental contaminants such as polychlorinated biphenyls (PCBs), organochlorine pesticides e.g. dichlorodiphenyltrichloroethanes (DDTs) and hexachlorocyclohexanes (HCHs) and organobromine compounds such as polybrominated diphenyl ethers (PBDEs)–are lipophilic (fat-soluble) and can accumulate in the blubber of marine mammals.<ref name="Bossart">Template:Cite journal</ref> In high concentrations these pollutants can interfere with overall health, hormone levels and affect both the immune and reproductive systems.<ref name="Bossart"/><ref>Template:Cite journal</ref> Females with high contaminant levels can pass contaminant loads across the placenta or via lactation from mother to calf, leading to calf mortality.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Blubber samples from melon-headed whales stranded in Japan and Hawaiʻi were found to have PCB concentrations above thresholds considered toxic.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Off Japan the levels of PBDE and chlordane related compounds (CHL) in blubber increased during 1980–2000.<ref>Template:Cite journal</ref>

Noise

Melon-headed whales may be vulnerable to impacts from anthropogenic (human generated) noise, such as those associated with military sonar activities, seismic surveys and high power multi-beam echosounder operations.<ref name="Brownell"/><ref name="Forney">Template:Cite journal</ref> Based on previous stranding events linking mass strandings with sonar,<ref name="Southall06"/><ref name="Southall13"/> melon-headed whales appear to be one of the more sensitive species to mid-frequency active sonar (1 to 10 kHz) used in military operations and other types of sonar.<ref name="Forney"/> For island-associated populations, such as those in the Hawaiian archipelago,<ref name="Aschettino"/> Palmyra Atoll and the Marquesas Islands,<ref name="Brownell"/> exposure to anthropogenic noise could result in displacement from important habitat.<ref name="Forney"/>

Whale watching

Regions in which melon-headed whales can be reliably sighted are few, however Hawaiʻi, the Maldives, the Philippines, and in the eastern Caribbean, especially around Dominica, are the best places to see them.<ref name ="MC"/> The International Whaling Commission (IWC) has guidelines for whale watching to ensure minimum disturbance to wildlife, but not every operator adheres to them.<ref>Template:Cite web</ref>

Conservation status

The melon-headed whale is listed as Least Concern on the IUCN Red List.<ref name="iucn status 19 November 2021" /> There is little information available on current levels of bycatch and commercial hunting, therefore the potential effects on melon-headed whale populations are undetermined.<ref name="Encyc MM" /> The current population trend is unknown.<ref name="iucn status 19 November 2021" />

The species is listed on Appendix II <ref name="UNEP">Template:Cite web</ref> of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). The melon-headed whale is included in the Convention on the Conservation of Migratory Species of Wild Animals (CMS) Memorandum of Understanding Concerning the Conservation of the Manatee and Small Cetaceans of Western Africa and Macaronesia (Western African Aquatic Mammals MoU) and the Memorandum of Understanding for the Conservation of Cetaceans and Their Habitats in the Pacific Islands Region (Pacific Cetaceans MoU).<ref name="UNEP"/> As with all other marine mammal species, the melon-headed whale is protected in United States waters under the Marine Mammal Protection Act (MMPA).<ref>Template:Cite web</ref>

See also

• List of cetaceans
• Marine biology

Further reading

• Carwardine, M., 2019. The Handbook of Whales, Dolphins and Porpoises. Bloomsbury, London. 528pp.
• Jefferson, T.A., M.A. Webber, and R.L. Pitman, 2015. Marine mammals of the world: a comprehensive guide to their identification. 2nd ed, London: Academic Press. 616pp.

External links

• Dolphin mom adopts whale calf—a first
• Rare dolphin-whale hybrid spotted near Hawaii
• Melon-headed whale sounds
• Society for Marine Mammalogy
• Cascadia Research
• NOAA Fisheries
• IWC Responsible Whale Watching/handbook
• Memorandum of Understanding Concerning the Conservation of the Manatee and Small Cetaceans of Western Africa and Macaronesia.
• Official webpage of the Memorandum of Understanding for the Conservation of Cetaceans and Their Habitats in the Pacific Islands Region