Proboscideaelephants

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Elephants are the survivors of a radiation of giant herbivores that once were diverse and widely distributed, including as many as seven families and, through the Tertiary, many dozens of species. They are classified as "subungulates" and believed to be related to hyraxes and sirenians. Throughout their history, proboscideans have shown a tendency to increase in size. The earliest known members of the order were moderately large, probably weighing around 120 kg; modern species are very large (an adult male African elephant can weigh more than 6000 kg); and the largest elephant, an extinct species called Mammuthus trogontherii, was huge, weighing around 9000 kg, 1/3 again as much as African elephants. The earliest proboscideans lacked any trunk or tusks. These structures appeared in several different forms, with trunks of varying length and massiveness (inferred from the morphology of the skull; trunks are soft tissue that is not fossilized) and tusks in a variety of shapes, sometimes including lower as well as upper teeth.

Modern elephants have a long, muscular trunk that functions almost as a fifth limb, and males have a pair of huge tusks derived from upper incisors. Their cheek teeth are uniquely adapted to their highly abrasive diet. As was the case in their ancestors, modern elephants have 6 cheek teeth. The eruption pattern in modern elephants, however, is much modified. The first three cheek teeth are small and relatively simple. The fourth erupts and begins to function during the animal's fourth or fifth year. It moves forward gradually in the jaw, and is replaced after eight years by the fifth tooth. The sixth and last molar erupts when the elephant is about 25 years old. Only one molar is fully functional at a time. Each successive tooth is larger and more complex. The fourth through sixth molars are massive and include numerous transverse lophs or ridges, giving the tooth a "washboard" appearance. Elephants chew with a fore and aft motion of the jaw, grinding the food across the lophs.

The skull of modern elephants is short and high. The lamdoidal crest across the back of the skull is especially well developed; to it attach the muscles that bear the weight of the massive head, tusks, and trunk. The bone over the top of the head contains numerous air cells.

The skeleton of elephants is adapted to bear massive weight, and as well, to provide attachments for the muscles required by an animal that may feed by pushing over and uprooting entire trees. The limb bones are stout; neither ulna nor tibia is reduced in size, and the toes are splayed and supported at the heel by a pad of dense connective tissue. The orientation of the pelvis is also modified.

Elephants require huge amounts of food, over 400 kg per individual per day. They are capable of pushing down large trees in order to get at their foliage and bark. Foraging herds can be very destructive of cropland or forest.

Elephants live long lives (60-70 years). They have complex social system that involves groups of related invidividuals led by mature females. They have an excellent sense of smell but relatively poor sight and hearing. They communicate by means of a variety of sounds, some produced vocally and others by tapping or stamping on the ground. Elephants (especially Asian elephants) are economically extremely important in some parts of the world as beasts of burden.

There is a single family of elephants (Elephantidae) with two living species (but mammoths, which coexisted with and were hunted by early humans, belonged to this family). African elephants are found in Africa south of the Sahara; Asian elephants occur in India, Nepal, and Southeast Asia.

Literature and references cited

Feldhamer, G. A., L. C. Drickamer, S. H. Vessey, and J. F. Merritt. 1999. Mammalogy. Adaptation, Diversity, and Ecology. WCB McGraw-Hill, Boston. xii+563pp.

Jones, C. 1984. Tubulidentates, proboscideans, and hyracoideans. Pp. 523-535 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.

Savage, R. J. G. and M. R. Long. 1986. Mammal Evolution, an Illustrated Guide. Facts of File Publications, New York. 259 pp.

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

Vaughan, T. A., J. M. Ryan, N. J. Czaplewski. 2000. Mammalogy. Fourth Edition. Saunders College Publishing, Philadelphia. vii+565pp.

Contributors

Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor.

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