Pacas

(Agoutidae)

Class: Mammalia

Order: Rodentia

Suborder: Hystricognathi

Family: Agoutidae

Thumbnail description
Like a giant guinea pig, it is a large square-headed rodent, with small ears, almost no visible tail, and a pattern of white spots and stripes on its flanks

Size
Head and body length, 20–30.5 in (50–77.4 cm); tail, 5–9 in (13–23 cm); weight, 13.2–31 lb (6–14 kg)

Number of genera, species
1 genus; 2 species

Habitat
Forests and semi-open areas

Conservation status
Lower Risk/Near Threatened: 1 species

Distribution
Central and South America

Evolution and systematics

Fossils exist from the Pleistocene of Brazil. The two species are well defined and occur at different altitudes (though they do overlap in altitudinal range and are not always easy to distinguish under field conditions). Part of an ancient rodent family with its origins in South America, pacas are one of the few mammal species that successfully moved north after North and South America became connected by the central American isthmus. Possessing a cone-shaped body, hind limbs longer than the forelimbs, and a diet devoted largely to fruit, pacas show a remarkable convergence with the chevrotain (Hyemoschus aquaticus), a deer-like ungulate of the West African rainforests. Like the paca, the chevrotains also have a pattern of strong horizontal bars and blotches along its flanks. Pacas were formerly grouped with the agoutis in the family Dasyproctidae, under the subfamily Agoutinae, but were eventually given full family status because they differed from agoutis in the number of toes on the feet, the shape of the skull, and the patterning of the fur. The family name Cuniculidae was formerly used instead of Agoutidae.

The name paca comes directly from the Tupi indigenous language, as does the word agouti. The fact that the genus name for this animal is the common English name for another, quite similar-looking animal shows why it is a good idea to have the Linnean naming system as a final arbiter for whichever animal is being talking about.

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.

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.

Distribution

Pacas are found in southern and eastern Mexico to southern Brazil, but not Uruguay. Pacas are not found west of the Andes in Ecuador and Peru.

Habitat

Pacas live in rainforest, cloud forest, and some slightly more open habitats. They prefer to be near water.

Behavior

Pacas are active in the morning and late afternoon in remote areas, but strictly nocturnal in areas of high hunting pressure. They lessen their activity on moonlit nights. Good swimmers, they will try to reach water when threatened, 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.

Feeding ecology and diet

Pacas eat fruits of understory trees and shrubs and fallen fruits of taller trees. They have also been recorded as eating leaves, buds, flowers, and fungi. Studies in Costa Rica identified 33 plant species consumed by pacas. Pacas play a major role in seed predation and in seed dispersal. Home ranges may have an activity core that centers on a cluster of fruiting trees. This center shifts as fruit availability changes. Pacas rarely use their forepaws to manipulate fruits. Accordingly, they are unable to exploit some types of fruits that the smaller agoutis manage to penetrate through the ability to keep gnawing at a chosen spot while dexterously manipulating the fruit in their forepaws. However, pacas do have powerfully muscled jaws and strong teeth; they are even able to break open the species of the very woody hard-shelled fruits of Shelia palms. In addition, pacas browse on seedlings and nibble flowers. The remains of ants and caterpillars have been found in their feces and stomach contents. It is not known if these were ingested intentionally or accidentally.

The loss of pacas in Los Tuxtlas, Mexico is believed to be one of the key reasons for a two-thirds drop in plant diversity in this rainforest remnant. Unlike agoutis, pacas can store fat to see them through seasonal slumps in fruit availability. Consequently, they have less of a need to cache seeds against hard times. Ever alert for predators, pacas like to eat seeds in the darkest possible place (such as a burrow or a hollow log). Pacas disperse seeds by active transportation in their stomachs and then voiding them in their feces, and (more rarely) by forgotten caches and accidental dropping of fruits held in cheek pouches. However, they are less efficient dispersers than agoutis. Competition with agoutis is avoided by different cycles of activity and slightly different food preferences. Pacas supplement their diet with mineral-rich soil from salt licks. Like rabbits, pacas practice caecotrophy, the ingestion of specially produced fecal pellets from which they absorb protein and carbohydrates synthesized by microorganisms living in the caecum, with a second passage through the digestive system. These moist soft pellets are different from the hard dry pellets that are fully excreted, and are a way by which the animal gets an additional bonus of nutrients from their food. Caecotrophy occurs in the burrow when the paca is asleep in its characteristic posture of mouth close to the base of its tail.

Reproductive biology

Litter size is usually one (exceptionally two). Young are precocial, born furred, and with open eyes. They are able to run with the mother from an early age, and they are weaned at around six weeks but may stay with the mother for up to a year afterwards. Gestation lasts between 114–119 days. Inter-birth interval is around 190 days. Females generally breed seasonally, but will bear up to three litters if conditions allow. Sexual maturity occurs at between six and 12 months, whenever the weight of females reaches 14.3 lb (6.5 kg) and males reach 16.5 lb (7.5 kg). Where two litters a year occur, the lactation period of the existing young overlaps the female's second pregnancy. Females in breeding condition will exhibit a "frisky hopping" behavior at the approach of the male. This will become more developed if he sprays her with urine. Copulation frequently takes place in water.

Pacas go to considerable effort to reduce the possibility of predation of their offspring. The young is born away from the burrow, in one of the mother's habitual sleeping spots. The morning after its birth, female leads the 22.9–25 oz (650–710 g) young to various holes in the territory. It will choose one with an entrance hole too small for such predators as coatis (Nasua nasua) and tayras (Eira barbara) to enter. The female will bring it leaves and twigs for its nest. Morning and evening, the mother, who is likewise unable to enter this natal burrow, uses a low rolling vocalization to call the young to suckle. Suckling lasts some 90 days, by which time the young paca weighs about 8.8 lb (4 kg). Before it is allowed to suckle, it must defecate and urinate, stimulated to do this when the female licks it. She ingests the resulting products to reduce the possibility of odors around the natal burrow that might attract predators. Thus, pacas have modified the familiar rodent strategy of high numbers, low individual investment into one very much like deer and most other large mammals, which use a strategy of low fecundity but high survivorship. Apart from human hunters, pacas have a large number of other predators, including jaguar, puma, ocelot, margay, jagarundi, bush dog, boa constrictor, and caiman.

Conservation status

Pacas are being exterminated as they sometimes destroy several different crops such as yams, cassava, sugar cane, and corn. They are also hunted intensively throughout their range for their meat. Their numbers are dwindling over large areas of their distribution due to the hunting and habitat destruction.

Significance to humans

Occasionally, pacas may become pests of agricultural crops. Often a mainstay for rural populations who hunt for meat, it is also a favorite in specialist restaurants. In Amazonas, paca constitutes some 8% of the meat eaten by indigenous people, and is the third most-consumed meat after tapir and peccary.

Some 70% of an adult paca's 22–31 lb (10–14 kg) is usable meat, so it is frequently an important source of meat for rural populations. Because of the fat stored in the flesh, the meat has a very high caloric value (1,620 calories per 2.2 lb [1 kg]). It often sells for a higher price than beef. Habitat fragmentation makes things worse, concentrating populations and increasing ease of access for hunters. This is true even in supposedly protected areas. Sometimes an agricultural pest, pacas eat maize, manioc, and on plantations, cocoa pods. In Costa Rica, its flesh is served on special occasions such as baptisms and weddings. Populations are locally threatened by over hunting. Pacas are important dispersers of many tree species used by humans, including Virola surinamensis, an important commercial timber tree. Attempts have been made to domesticate the paca for use in rural populations, as a means of increasing rural health, conserving wild paca populations, and protecting the ecosystem services they provide.

Species accounts

Resources

Eisenberg, J. F., and K. H. Redford. Mammals of the Neotropics. Vol. 3, The Central Tropics: Ecuador, Peru, Bolivia, Brazil. Chicago: University of Chicago Press, 1999.

Janzen, D. H. Costa Rican Natural History. Chicago: University of Chicago Press, 1983.

Reid, F. A. A Fieldguide to the Mammals of Central America and Southeast Mexico. Oxford: Oxford University Press, 1997.

Beck-King, H., O. von Helversen, and R. Beck-King. "Home Range, Population Density, and Food Resources of Agouti paca (Rodentiia: Agoutidae) in Costa Rica: A Study Using Alternative Methods." Biotropica 31: (1999): 675–685.

Estrada, A., R. Coates-Estrada, and D. Merrit Jr. "Non-flying Mammals and Landscape Changes in the Tropical Rain Forest Region of Los Tuxtlas, Mexico." Ecography 17: (1994): 229–241.

Peres, E. M. "Agouti paca." Mammaian Species 404: (1992): 1–7.

Sabatini, V., and M. J. R. Paranhos de Costa. "Caecotrophy in Pacas (Agouti paca Linnaeus, 1766)." Mammalian Biology 66: (2001): 305–307.

Terbough, J., and J. S. Wright. "Effects of Mammalian Herbivores on Plant Recruitment in Two Neotropical Forests." Ecology, 75 (1994): 1829–1833.

[Article by: Adrian A. Barnett, PhD]