By Phil Myers
There are 2 species of pacas, placed in a single genus. They can be found in the tropical forests of Central and South America, from sea level up to about 3000m elevation.
Pacas are very large, ranging up to around 12kg in males (females are somewhat smaller). They have chunky bodies, short and stout legs (the hind limbs are longer than the forelimbs), and a large, blunt head. Eyes are large and external ears are small. Pacas are nearly tailless. The forefeet have 4 functional toes, and the hindfeet 3 (digits 1 and 5 are present but reduced). Claws are thick and hoof-like.
The pelage of pacas is coarse and without underfur. Dorsally, pacas are brown, with whitish spots arranged in longitudinal lines. Their bellies are whitish.
The 
skulls of pacas are unmistakeable. They are broad and massively constructed, with short nasals and long, broad frontals. A sagittal crest may sometimes be found over the posterior part of the braincase. A postorbital process is present, but it seems to be located unusually far back over the orbit. The zygomatic arches are enormously inflated, with jugal and maxillary forming a large, rough-surfaced plate on the outside and a smooth, hollow chamber on the inside. While large, the jugal does not contact the lacrimal. The infraorbital foramen is reduced in size, although these animals are hystricomorphous. A separate canal conducts nerves from the orbit to the rostrum. Pacas have small auditory bullae and long and heavily-built paroccipital processes. Lower jaws are hystricognathous, but the angular process is offset less than in many other hystricognaths.
The cheekteeth of pacas are hypsodont. They are flatcrowned and have a complex pattern of re-entrant folds. The first two molars have 1 labial and 3 lingual folds, while the third molar has 3 labial and 1 lingual. These folds become isolated to form enamel islands as the teeth wear. The dental formula is 1/1, 0/0, 1/1, 3/3 = 20.
Pacas are strictly terrestrial, preferring the wooded banks of streams and ponds. They are good diggers, constructing burrows in banks, slopes, or among tree roots or rocks. These have several entrances and are often plugged with leaves. Pacas forage at night, feeding on foliage, roots, nuts, seeds, and fruits. They are excellent swimmers, using water as a means of escaping predators, but generally they do not dive or forage in the water. While generally solitary, pacas do communicate by means of vocalizations, foot thumping, and tooth grinding. It has been suggested that the hollow chambers formed by the expanded zygomatic arches are resonating chambers to amplify sounds. They also support cheek pouches, which may be used in food storage, but this is not certain.
Throughout their range, pacas are much sought for their flesh, which is tender and mild-flavored.
References and literature 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.
Macdonald, David. 1984. The encyclopedia of mammals. Facts on File Publications, New York.
Nowak, R. M. and J. L. Paradiso. 1983. Walker's mammals of the world. The Johns Hopkins University Press, Baltimore and London, pp 803-810.
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.
Wilson, D. E. and D. M. Reeder (eds.). 1993. Mammal species of the world: A taxonomic and geographic reference, 2nd ed.. Smithsonian Institution Press, Washington and London.
Woods, C. A. 1984. Hystricognath rodents. Pp. 389-446 in Anderson, S. and J. K. Jones, Jr. (eds.). Orders and familes of mammals of the world. John Wiley and Sons, New York.
Contributors
Phil Myers (author), Museum of Zoology, University of Michigan.
