Melon-headed whale

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. 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). 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. ==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. Melon-headed whales grow up to in length, and weigh up to , adult males being slightly larger than females. Length at birth is approximately . Melon-headed whales are physically mature at 13–15 years and live up to 45 years. 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. 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. While both species have white around the mouth, in adult pygmy killer whales this can extend onto the face. 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. From a distance melon-headed whales could also be confused with false killer whales (Pseudorca crassidens), but the much larger size– 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. ==Geographic range and distribution==

between Balicasag Island and Alona Beach Melon-headed whales occur in deep tropical/subtropical oceanic waters, between 40°N and 35°S. Sightings or strandings at the extremes of their range are likely associated with extensions of warm currents. Melon-headed whales are not known to be migratory. ==Behavior==

Foraging Melon-headed whales feed primarily on pelagic and mesopelagic squid and small fish. Crustaceans (shrimp) have also been reported from stomach contents. 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. 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. 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). 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. The melon-headed whale calf was observed suckling from the bottlenose dolphin female, and was repeatedly sighted with its adoptive/foster mother until 2018. 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. 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. Movement 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. Melon-headed whales can be wary of boats, but in some regions will approach boats and bow-ride. ==Population status==

The world population is unknown, but abundance estimates for large regions are approximately 45,000 in the eastern tropical Pacific Ocean, 2,235 in northern Gulf of Mexico and in the Philippines 920 in the eastern Sulu Sea and 1,380 in Tañon Strait between Cebu and Negros Islands. 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, particularly in light of U.S. Navy activities in the region, given the potential link between sonar and mass stranding events. ==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, and in driftnet fisheries in the Philippines, Sri Lanka, Ghana and India. In the eastern tropical Pacific purse-seine tuna fisheries, melon-headed whales have been rarely taken as bycatch. 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. Hunting Individuals are taken for bait or human consumption in small cetacean subsistence and harpoon fisheries in several regions, including Sri Lanka, the Philippines and Indonesia. 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. 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. In high concentrations these pollutants can interfere with overall health, hormone levels and affect both the immune and reproductive systems. Females with high contaminant levels can pass contaminant loads across the placenta or via lactation from mother to calf, leading to calf mortality. Blubber samples from melon-headed whales stranded in Japan and Hawaiʻi were found to have PCB concentrations above thresholds considered toxic. Off Japan the levels of PBDE and chlordane related compounds (CHL) in blubber increased during 1980–2000. 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. Based on previous stranding events linking mass strandings with sonar, ==Conservation status==

The melon-headed whale is listed as Least Concern on the IUCN Red List. 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). == See also ==