Physical characteristics

The third and fourth largest living rodents, both paca species have a heavy and robust appearance. The flanks are characteristically patterned with four to seven horizontal lines of pale blotches and stripes. The ears are small and set high on the square-looking head. The legs are long and, though sturdy, appear almost too dainty for such a chunky animal. There are four toes on the forefoot and five on the hind (two of which are short and rarely touch the ground). Both are adapted to swift movement with toes that are broad, down-pointing, and powerful, and stout nails that resemble small hooves. In body form, the paca closely resembles a small forest deer. The streamlined cone-shaped body form allows swift passage through dense vegetation. The skin of young pacas is covered with horny scales about 0.08 in (2 mm) in diameter. These disappear in adults. They are thought to have a protective function against some of the smaller paca predators.

where they may remain submerged for up to 15 minutes. Other escape mechanisms include the ability to leap up to 3.28 ft (1 m) into the air; they can also rush away from the source of disturbance and then freeze for as long as 45 minutes. Normal travel is by a series of established paths. A new route will be made if a path shows any sign of disturbance. Pacas sleep in burrows up to 9.8 ft (3 m) deep. Burrows are either a hole in the soil (often adapted from a hole originally dug by an armadillo), in hollow logs, or among rocks. They are often near water, but above the seasonal flood line. There are generally at least two entrances, which will be plugged by the paca with wads of leaves (to hinder the dispersion of their body smell and so avoid alerting their many predators, and to provide an early warning system if a predator should try to enter while the paca is asleep). Pacas make a low growling noise that is surprisingly loud for the size of the animal. The call is amplified by bony expansions on the side of the skull and by the fleshy sacs contained within them. Modified in duration and pulse pattern, this is used to indicate territorial ownership, aggression, threat, or, by the female, to call to her young. At around 1 kHz, this call is audible to humans. When threatened, pacas may grind their teeth, a noise that is also amplified by the cheek-side resonating chambers. A pair of pacas marks their mutual territory with urine. Paca densities may reach 70 adults per 0.4 mi2 (per 0.2 km2). Because of this high density, pacas often constitute nearly 20% of the biomass of terrestrial mammals in a forest. The number of pacas in an area is often determined by the availability of territories, the size of which is ultimately determined by the productivity of the region's soil. If they escape their numerous predators, pacas can live up to 13 years in the wild.

Seeing pacas in the field is difficult; there is little size difference between adults, no color differences, and all genitalia is hidden in an anal pouch.

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