Savanna-dwelling social grazers averaging a hefty 50kg (110lb), capybaras are unusual animals. They are found only in South America, where they live in groups near water. Members of the suborder known as caviomorphs, which also includes cavies and chinchillas, they are in fact the largest of all the rodents.
The first European naturalists to visit South America called capybaras “water pigs” or “Orinoco hogs,” and the first of those names has carried over into their present scientific designation as hydrochoerids. Yet in truth, they are neither pigs nor totally aquatic; their nearest relatives are actually the cavies. The other hydrochoerids, now all extinct, were larger than present-day capybaras; the biggest were twice as long and probably weighed eight times as much, making them heavier than the largest North American grizzly bear.
Capybaras are ponderous, barrel-shaped animals. They have no tail, and their front legs are shorter than their back legs. Their slightly webbed toes, four on the front feet and three on the back, make them very strong swimmers, able to stay under water for up to 5 minutes. Their skin is extremely tough and covered by long, sparse, bristlelike hairs. The nostrils, eyes, and ears are situated near the top of the large, blunt head, and hence protrude out of the water when the animal swims. Two pairs of large, typically rodent incisors enable capybaras to eat very short grasses, which they grind up with their molar teeth. There are four molars on each side of each lower jaw. The fourth molar is characteristic of the subfamily in being as long as the other three.
Two kinds of scent glands are present in the capybara. One gland, highly developed in males but almost nonexistent in females, is located on top of the snout and is known as the morrillo (literally, “hillock” in Spanish). This is a dark, oval-shaped, naked protrusion that secretes a copious, white, sticky fluid. Both sexes also produce odors from two glandular pockets located on either side of the anus. Male anal glands are filled with easily detachable hairs abundantly coated with layers of hard, crystalline calcium salts. Female anal pockets also have hairs, but theirs are not detachable and are coated in a greasy secretion rather than with crystalline layers. The proportions of each chemical present in the secretions of individual capybaras are different, providing a potential for individual recognition via personal “olfactory fingerprints.” The snout scent gland also plays a role in signaling dominance status, while the anal gland appears to be important in group membership recognition and perhaps in territoriality.
Capybaras have several distinct vocalizations. Infants and the young constantly emit a guttural purr, probably to maintain contact with their mothers or other group members. This sound is also made by losers in altercations, perhaps to appease their adversary. Another vocalization, the alarm bark, is given when a predator is detected. This coughing sound is often repeated several times, and the reaction of nearby animals may be to stand alert or to rush into the water.
Capybaras are exclusively herbivorous, feeding mainly on grasses that grow in or near water. They are very efficient grazers, and can crop the short, dry grasses left at the end of the tropical dry season. Because a large proportion of the grasses they eat consists of cellulose, which is indigestible by any mammal's digestive enzymes, capybaras possess a huge fermentation chamber called the cecum, equivalent to the tiny human appendix. However, since the cecum is located between the small and large intestine, the animal cannot absorb the products of the fermentation carried out by microbial symbionts. To solve this problem, capybaras resort to coprophagy – reingestion of feces – to take advantage of the work of their symbionts. Thus, for a few hours every morning during their resting period, capybaras recycle what they ate the previous evening and night. Usually they spend the morning resting, then bathe during the hot midday hours; in the late afternoon and early evening they graze. At night they alternate rest periods with feeding bouts. They never sleep for long, instead dozing in short bouts throughout the day.
Capybaras live in groups of 10–30 animals, apparently depending on the habitat: greener and more homogeneous pastures promote larger groups. Pairs are rarely seen, but a proportion of adult males are solitary or loosely associated to one or more groups. In the dry season groups coalesce around the dwindling pools, forming temporary aggregations of 100 or more animals. When the wet season returns, these large aggregations split up into the original groups that formed them. Thus, capybara social units may last three years, and probably more.
Groups of capybaras are closed social units, in which little variation in core membership is observed. A typical group is composed of a dominant male (often distinguishable by his large morrillo), one or more females, several infants and young, and one or more subordinate males. Among the males there is a dominance hierarchy, maintained by aggressive interactions that usually take the form of simple chases. Dominant males repeatedly shepherd their subordinates to the periphery of the group, but fights are rarely seen. Females are much more tolerant of each other, although the precise details of their social relationships, hierarchical or otherwise, are unknown. Territories are defended by all adult members of the group against conspecific intruders. Any animal of either sex may chase an interloper away, irrespective of its sex, as long as the chaser is within its own territory.
Capybaras are found in a wide variety of habitats, ranging from open grasslands to tropical rain forest. Groups may occupy an area varying in size from 2–200ha (4–494 acres), with 10–20ha (24.7–49.4 acres) being most common. Each home range is used mainly, but not exclusively, by one group and can therefore be considered a territory. Territories are defended against conspecific intruders by all adult members of the group. Particularly in the dry season, but at other times as well, two or more groups may be seen grazing side by side. In some areas density may reach two individuals per hectare, but lower densities of fewer than one per hectare are more frequent.
Capybaras reach sexual maturity at 18 months. In Venezuela and Colombia they appear to breed year round, with a marked peak at the beginning of the wet season in May. In Brazil, in more temperate areas, they probably breed just once a year. When a female becomes sexually receptive, a male will start a pursuit that may last for an hour or more. The female will walk in and out of the water, repeatedly pausing while the male follows close behind. Mating takes place in the water; the female stops, and the male clambers on her back, sometimes pushing her underwater with his weight. As is usual in rodents, copulation lasts only a few seconds, but each sexual pursuit typically involves several mountings.
150 days later, up to seven babies are born; four is the average litter size. To give birth the female leaves her group and walks to nearby cover. Her young are born a few hours later, and are precocial, able to eat grass within their first week. A few hours after the birth the mother rejoins her group, the young following as soon as they become mobile, which should occur when the babies are still very young. Females seem to share the burden of nursing by allowing infants other than their own to suckle. The young in a group spend most of their time within a tight-knit crèche, moving between nursing females. When active, they constantly emit a churring purr.
Capybara infants tire quickly, and are therefore vulnerable to predators. They have most to fear from vultures and feral or semiferal dogs, which prey on them. Caymans and foxes may also take young capybaras. Jaguar and smaller cats were certainly important predators in the past, though today they themselves are nearly extinct in most of Venezuela and Colombia and are therefore less of a threat to capybara populations. In some areas of Brazil, however, jaguars seize capybaras in substantial numbers.
When a predator approaches a group the first animal to detect it will emit an alarm bark. The normal reaction of other group members is to stand alert, but if the danger is very close, or the caller keeps barking, they will all rush into the water, where they form a close aggregation with young in the center and adults facing outward.
Capybara populations have dropped so substantially in Colombia that, from 1980 onward, the government prohibited capybara hunting. In Venezuela they have been killed since colonial times in areas that are devoted to cattle ranching. In 1953 hunting became subject to legal regulation and controlled, but to little effect until 1968 when, after a 5-year moratorium, a management plan was devised, based on a study of the species' biology and ecology. Since then, 20–30 percent of the annually censused population in licensed ranches with populations of over 400 animals have been harvested every year. This has apparently resulted in local stabilization of capybara populations. Capybaras are now listed as conservation dependent by the IUCN, in recognition of the fact that control on hunting and harvesting must remain if population levels are to be maintained.
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