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Pygmy killer whale

Opening Summary
The pygmy killer whale (Feresa attenuata) is a poorly known and rarely seen oceanic dolphin. It derives its common name from sharing some physical characteristics with the killer whale. It is the smallest species that has "whale" in its common name. Although the species has been known to be extremely aggressive in captivity, this aggressive behavior has not been observed in the wild.

The species had been described by John Gray in 1874, based on two skulls identified in 1827 and 1874. The next recorded sighting was in 1952 which lead to its formal naming by Japanese cetologist Munesato Yamada in 1954.



Description
The pygmy killer whale is dark gray to black on the cape and has a sharp change to lighter gray on the sides. The flesh around their lips and on the end of their snout is white while pinkish white skin surrounds the genitals. The average length is just over two meters (6.5 ft.). Upon reaching 2 meters in length, males are considered sexually mature. They have around 48 teeth, with 22 teeth on the top jaw and 26 on the lower jaw.

Satellite tagging of two pygmy whales showed a travelling speed around 3 km/h. Both satellite tagging and boat observations have indicated that they are predominately found in deeper waters ranging from 500 m to 2000 m (1600-6500 ft.) in depth.

What little is known about pygmy killer whale feeding indicates a diet of cephalopods and fish. Blood analysis has shown they have a higher mercury:selenium ratio of 1.6:1 than the typical 1:1 ratio thought common in other odontocetes.

Early Records of Pygmy Killer Whales
Prior to the 1950s, the only record of pygmy killer whales was from two skulls identified in 1827 and 1874. In 1952, a specimen was caught and killed in Taiji, Japan which is know for its annual dolphin hunts. Six years later, in 1958, an individual was killed off the coast of Senegal. In 1963, there were two recorded events involving pygmy killer whales. The first was in Japan, where 14 individuals were caught and brought into captivity; all 14 animals were dead within 22 days. The second was off the coast of Hawaii where an individual animal was caught and successfully brought into captivity. In 1967, a single pygmy killer whale off of Costa Rica was entangled purse seine net and died. Finally, in 1969 a pygmy killer whale was killed of of St. Vincent and a group of individuals was recorded in the Indian Ocean.

Differentiating Pygmy Killer Whales from Other Dolphin Species
Pygmy killer whales are easily confused with other dolphin species, most commonly the melon-headed whale and the false killer whale. For instance, a published paper describing an encounter with a school of pygmy killer whales was later determined to be either a mixture of pygmy and false killer whales or solely false killer whales.

The species can be differentiated by physical differences between them. One defining difference between pygmy killer whales and melon-headed whales is that although both species have white around the mouth, on pygmy killer whales the white extends back onto the face. Pygmy killer whales also have rounded tips as opposed to pointed tips on their dorsal fins. When compared to false killer whales, pygmy killer whales have a larger dorsal fin. Also pygmy killer whales have a clearly defined line where the dark dorsal color changes to the lighter lateral color than either of the other two species.

Behavioral differences can also be used to differentiate pygmy killer whales from false killer whales. Pygmy killer whales usually move slowly when at the surface whereas false killer whales are often highly energetic. Pygmy killer whales are rarely seen to bow ride but is common in boat encounters with false killer whales.

Echolocation and Hearing
Like other oceanic dolphins, pygmy killer whales make use of echolocation. The centroid frequencies of the clicks used for echolocation are between 70-85 kHz and can range from 32 to 100 kHz. This is similar to the range of other odontocetes such as the bottlenose dolphin but is slightly higher than false killer whales. While echolocating, they produce 8-20 clicks per second with a 197-223 decibel sound level at the production source. The directionality of pygmy killer whales is better than in porpoises but lower than is found in bottlenose dolphins. Based on similarities to the acoustic parameters of other odontocetes, it is presumed that they use a similar mechanism for producing echolocation clicks.

The anatomy for auditory reception is similar to other odontocetes with a hollow mandible and a mandibular fat body composed of a low density outer layer and a denser inner core. The inner core comes into direct contact with the tympanoperiotic complex. Hearing tests performed on two live individuals brought in for rehabilitation exhibited frequency response range and temporal resolution similar to that found in other echolocating dolphins. During those tests, one individual exhibited low frequency hearing loss that might have been related to treatment with the antibiotic amikacin although the researchers believed the more likely cause was slight differences in testing setup.

Population and Distribution
Pygmy killer whales have been observed in groups ranging from 4 to 30 or more individual animals. Due to the infrequent sightings of this species, population estimates have been calculated. The only population estimate is of 38,900 individuals in the eastern tropical Pacific Ocean. However, this estimate had a large coefficient of variation meaning the true population size could be much lower or much higher.

The species has a wide distribution. They have been observed along the coast of South America and as far north as the Gulf of Mexico where they have been known to breed during the spring season. They have also been spotted around the Hawaiian islands and in Sri Lanka coastal waters.

There is a resident population of pygmy killer whales in the waters around Hawaii. Most sightings have been around the main island, however there have been occasional sightings around several of the other islands. This population has a tightly connected social structure with affiliations between individuals that can last up to 15 years. Despite the existence of this resident population, sightings of pygmy killer whales around Hawaii are still quite rare; in a study lasting from 1985 to 2007, sightings of pygmy killer whales accounted for less than 1.5% of all sightings. The Hawaiian population has been observed associated with false killer whales, short-finned pilot whales, and bottlenose dolphins.

Conservation
Pygmy killer whales have been known to be incidental bycatch in fishing operations. A 1984 study of commercial fisheries indicated that pygmy killer whales might represent as much as 4% of the cetacean bycatch from drift gill nets.

Like other cetaceans, they are hosts to parasitic worms such as cestodes and nematodes. The cestode species, Trigonocotyle sexitesticulae was first discovered in the corpse of a pygmy killer whale. An individual found stranded on the coast of New Caledonia was diagnosed as from parasitic encephalitis caused by nematodes. They are also opportunistic victims of cookie cutter sharks.

Pygmy killer whales are occasionally involved in mass strandings. Often, as seen in other cetaceans, these strandings involve a sick or injured individual where the other stranded animals refuse to leave until the death of the individual in declining health. In such cases, they will often strand again if rescuers push them out to sea.

The pygmy killer whale is covered by the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS) and the Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and Contiguous Atlantic Area (ACCOBAMS). The species is further included in the 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).