Chinchilla rats
(Abrocomidae)
Class: Mammalia
Order: Rodentia
Suborder: Hystricognathi
Family: Abrocomidae
Thumbnail description
Similar in appearance to members of the chinchilla family, but for a more elongated head and ears, which presents a more rat-like appearance
Size
6–10 in (15–25 cm) from the tip of the nose to the rump; tail 2.5–6 in (6.0–14.5 cm); 7–11 oz (200–300 g)
Number of genera, species
2 genera; 6 species (1 species presumed extinct)
Habitat
From sea level to elevations up to 16,000 ft (5,000 m); rock crevices and underground tunnels; small shrubs and trees
Conservation status
Vulnerable: 1 species
Distribution
Coastal regions and Andes mountains of South America, encompassing southern Peru, northern Chile, northwestern Argentina, and central Bolivia
Evolution and systematics
The chinchilla rats, or chinchillones (genus Abrocoma), belong to the Abrocomidae family, in the order Rodentia. Within the genus Abrocoma, there are three species. The two best-studied are the ashy chinchilla rat (Abrocoma cinerea) and Bennett's chinchilla rat (Abrocoma bennettii). In 1990, a third species was designated, namely the Bolivian chinchilla rat (Abrocoma boliviensis). Another species, Abrocoma vaccarum, has been the subject of only a few academic reports and is considered a systematic development, rather than a true species.
In 1999, a new genus, Cuscomys, was described in the Peruvian Andes. As of 2003, two species are included in the genus. Of these, one (Cuscomys oblativa) is presumed to be extinct, and is represented only by remains discovered in burial sites in Peru. The other species is the Ashaninka rat, Cuscomys ashaninka. To date, only the recently-killed corpse of a single member of the species has been discovered. Until further specimens are recovered, information concerning Cuscomys is pending, and this species designation is not widely accepted.
The evolutionary history of the family Abrocomidae dates back to the Early Miocene Epoch (from 23.8 million to 16.4 million years ago), in what is present-day Argentina. Evolutionary diversification of the octodont rodents, which led to the development of the six families, including Abrocomidae, is thought to have had its genesis after the separation of the South American continent from other continental masses.
Physical characteristics
Abrocoma have large eyes and ears on an elongated head. The ears are round rather than pointed. However, the nose assumes the typical long and pointed rat-like countenance. The cylindrical tail is covered by short and fine hairs, unlike the bushier and squirrel-like tail of true chinchillas. Another distinguishing feature is the double pair of nipples present on the upper belly of females.
Members of the genus Abrocoma have four digits on the forelimbs and five digits on the hindlimbs. Soles are uncovered, while the tops of the feet are fur-covered. Small claws are present on the fore and hind feet. The claws are not exceptionally powerful, owing to their hollow structure. However, some stiffness is provided in the middle three toes of the hind feet by protruding upper hairs that act to form a grooming or utility comb.
Chinchilla rats are covered in fur that is gray or gray-brown in color. The belly is yellowish white or brown. The silky, thick fur resembles that of the highly prized fur of the family Chinchillidae, but is not as woolly in texture. Indeed, chinchilla rats are a source of false fur sold to unsuspecting buyers by unscrupulous merchants.
For its size, the members of Abrocoma possess a long intestinal tract. The combined small and large intestines coil for 8.2 ft (2.5 m), and the caecum extends for a further 7.9 in (20 cm). The intestines and other internal organs are well protected by the 17 pairs of ribs. No other rodent has as many ribs.
Distribution
Chinchilla rats are distributed throughout the Andes from southern Peru to west central Argentina, northern Chile, and into Bolivia. Depending on the species, they range from the foothills of the west coast of Peru and Chile, to the 12,000–15,000 ft (3,660–4,570 m) elevations of the Altiplano of western Bolivia and southeastern Peru.
Abrocoma cinerea dwells exclusively in the Altiplano, and is found from southeastern Peru to the north of Chile and Argentina. In contrast, A. bennettii prefers the coastal foothills of Chile. As implied by its name, A. boliviensis is found in Bolivia, specifically at higher elevations, up to 10,000 ft (3,050m) in the country's central region.
Habitat
All three species of Abrocoma are agile climbers of rocks, shrubs, and even small trees. They can make their dens in crevices in the rock- or cliff-face. Sometimes these crevices are supplemented by the construction of sidewalls that can reach up to 10 ft (3 m) high. The walls are built of feces that are bonded together using urine and other fluids. When dry, the walls become almost as hard as the surrounding rock. Underground tunnels can also provide a home. The tunnel entrances are typically concealed under a rock or at the base of a bush.
Behavior
Chinchilla rats are thought to be colonial. A field study observed a half dozen A. cinerea burrowing within 59 ft (18m) of one another, and small colonies can even occupy a single burrow. Further attesting to their colonial proclivity have been observations of co-dwelling degu, another rodent that has similar size and behavior.
Both males and females are nocturnal, and keep to their burrows or crevices by day.
Communication includes the issuing of a grunt prior to or following an attack, a higher-pitched squeak when under pursuit, and a gurgling sound when being groomed.
Feeding ecology and diet
Diet for these herbivores consists of plant material such as buds, leaves, nuts, seeds, stems, and fruit. The search for food can even lead them to climb bushes and low-lying trees. Being nocturnal, the search for food occurs at night.
Reproductive biology
Little is known about reproduction in Abrocoma. Gestation times have been documented for A. cinerea, ranging from 115 to 118 days, with litters of one to two young. Litters of up to six have been described, and indeed may be more the norm, as shown by field work that collected pregnant females in Chile with embryo counts of four, five, and six.
Pregnant A. cinerea females, or females with newborns, have been collected during the months of April, the end of July and August, and at the beginning of June. Mating would have occurred in the early months of the year. Details of mating are unknown.
Conservation status
The Bolivian chinchilla rat (Abrocoma boliviensis) is Vulnerable; the other species are not listed by the IUCN. Hunting and destruction of their habitats, in order to secure the fur, is responsible for declining populations. The fur, which resembles the prized fur of the true chinchilla, is sold to the unsuspecting buyer.
Significance to humans
Other than as a supply of fur for unscrupulous marketers, the species of chinchilla rat have no outstanding significance to humans. The fur itself has no marked economic value.
Species accounts
Ashy chinchilla ratBennett's chinchilla rat
Bolivian chinchilla rat
Resources
Books:Nowak, R. M. "Rodentia: Ambrocomidae Chinchilla Rats or Chinchillones." In Walker's Mammals of the World. Vol. 2. Baltimore: Johns Hopkins University Press, 1991.
Nowak, R. M., and J. L. Paradiso. Walker's Mammals of the World. Baltimore: Johns Hopkins University Press, 1983.
Vaughn, T. A., J. M. Ryan, and N. J. Czaplewski. Mammalogy. 4th ed. Philadelphia: Saunders College Publishing, 2000.
Periodicals:Eisenberg, J. F. "The Function and Motivational Basis of Hystricomorph Vocalizations." Symposium of the Zoological Society of London 34 (1974): 211–247.
Fulk, G. W. "Notes on the Activity, Repoduction, and Social Behavior of Octodon degus." Journal of Mammalogy 57 (1976): 495–505.
Kuch, M., N. Rohland, J. L. Betanourt, C. Latorre, S. Steppens, and H. N. Poinar. "Molecular analysis of a 11700-year-old Rodent Midden from the Atacama Desert, Chile." Molecular Ecology 11: 913–924.
Miller, S. D., J. Rottmann, K. J. Raedeke, and R. D. Taber. "Endangered Mammals of Chile: Status and Conservation." Biological Conservation 25: 335–352.
Pine, R. H., S. D Millar, and M. L. Schamberger "Contributions to the Mammalogy of Chile." Mammalia 43: 339–376.
Weir, B. J. "Reproductive Characteristics of Hystricomorph Rodents." Journal of Mammalogy 57: 495–505.
[Article by: Brian Douglas Hoyle, PhD]
