Delphinidaedolphins, killer whales, pilot whales, and relatives

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With 32 species placed in 17 genera, this is by far the largest family of cetaceans. Delphinids are small to medium-sized cetaceans, ranging from about 1.5 m in length and 50 kg weight to almost 10 m in length and 7000 kg. Males are usually larger than females. The shape of the head of many delphinids is distinctive; the forehead appears to bulge over the beak-like rostrum due to the presence of a lens-shaped fatty deposit called a "melon." This structure may help focus the sound emitted by these animals in echolocation and feeding. Other delphinids possess a melon, but their rostrum is short and the bulging forehead merely gives the head a squared-off appearance. The bodies of most species are sleek and streamlined. Most have dorsal fins, which are usually curved (falcate), but much variation exists. Some species have striking color patterns over their bodies; others are more or less uniform. The group includes bottlenose dolphins, killer whales, pilot whales, Pacific striped dolphins, and many more.

In general, the common name "porpoise" is given to the related phocoenids, which lack a well-formed beak and have a squarish head and relatively chunky body. Most "dolphins" or delphinids generally have a distinct beak and a relatively slender, streamlined body.

In the delphinid skull, the facial depression is broadly expanded. The posterior end of the maxilla rises up above the rostrum. The zygomatic process of the squamosal is small and hidden from dorsal view by the expanded maxillae and frontals. The rostrum varies from short and broad to long and narrow. The lower jaws are fused for less than 40% of the length of their rami. The upper toothrows diverge posteriorly. The teeth are peg-like, circular in cross section, and sometimes numerous; the dental formula varies from 0/2 to 65/58.

Delphinds are found in all oceans and seas and in some river systems. They generally live in shallow water or at least stay near the surface, not making the deep and prolonged dives that characterize some other groups of cetaceans. They are fast and acrobatic swimmers, feeding on fishes and squids, which they pursue actively. Killer whales also prey on mammals such as other cetaceans and pinnipeds, as well as on birds and large fish. Like other odontocetes, delphinids echolocate and may even use high-intensity sound to stun their prey.

At least some members of this family are highly social, living in large groups (sometimes over 100,000 individuals!) and exhibiting a number of fascinating behaviors related to group living. Individuals appear to cooperate in a number of ways. One example is that these schools sometimes attack sharks, killing them by ramming them. Dolphins may work cooperatively to assist an individual that is ill or injured. Groups of dolphins often follow ships, riding the bow wave and sometimes making spectacular, acrobatic leaps. Several individuals in a school may leap in concert.

Delphinids appear to be highly intelligent, adapting quickly and flexibly to novel situations. Most species, unfortunately, are poorly studied.

References and literature cited:

Nowak, R.M. and J.L. Paradiso. 1983. Walker's Mammals of the World, 4th edition . John Hopkins University Press, Baltimore, MD.

Savage, R. J. G. and M. R. Long. 1986. Mammal Evolution: An Illustrated Guide. Facts on File Publications, UK. 251 pp.

Rice, D. W. 1984. Cetaceans. Pp. 447-490 in Anderson, S. and J. K. Jones, Jr. (eds). Orders and Families of Recent Mammals of the World. John Wiley and Sons, N.Y. xii+686 pp.

Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, N.Y. vii+576 pp.

Wilson, D. E., and D. M. Reeder. 1993. Mammal Species of the World, A Taxonomic and Geographic Reference. 2nd edition. Smithsonian Institution Press, Washington. xviii+1206 pp.


Species included in database:

Glossary

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

motile

having the capacity to move from one place to another.

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate