Octodonts

(Octodontidae)

Class Mammalia

Order Rodentia

Suborder Hystricognathi

Family Octodontidae

Thumbnail description
Smallest of hystricognath rodents; classic figure-eight pattern on occlusal surface of molar teeth; diverse ecologically and morphologically; specializations for burrowing in many forms

Size
Head and body length 5.0–8.7 in (125–221 mm); tail 1.5–7 in (40–180 mm); 2.8–10.6 oz (80–300 g)

Number of genera, species
8 genera; 12 species

Habitat
Scrubland, savanna, arid, semiarid, montane, and coastal areas

Conservation status
Vulnerable: 2 species

Distribution
Southwest Peru, Chile, Argentina, and southwest Bolivia

Evolution and systematics

Members of the family Octodontidae are considered to represent some of the most morphologically primitive species of the suborder Hystricognathi. Depending upon the assignment of fossils to the family, the fossil record for the family extends from either the early Miocene or early Oligocene to recent. All recent systematic treatments place the families Octodontidae, Capromyidae, Ctenomyidae, Echimyidae, Myocastoridae, and Abrocomidae in the monophyletic (sharing a common ancestry) superfamily Octodontoidea. A molecular phylogenetic study in 2003 by Honeycutt and others suggested that the families Octodontidae and Ctenomyidae, commonly known as tuco-tucos, share a common ancestry, followed by an association with a group containing the families Myocastoridae, Echimyidae, and Capromyidae.

Speciation within the family was influenced by environmental changes occurring in the Pliocene and Pleistocene. Within the family, species of octodontids are chromosomally diverse ranging in diploid chromosome number from a low of 38 to a high of 102. The red viscacha rat, Tympanoctomys barrerae, has the highest diploid number, and in 1999, Gallardo and others provided genetic evidence for this species being a tetraploid with a genome size twice that seen in related species. Honeycutt and others provided evidence for relationships among genera within the family. Two major groups were observed, one representing the desert specialists from Argentina, Octomys and Tympanoctomys, and the other containing Octodontomys and a group represented by Octodon, Aconaemys, and Spalacopus, three genera restricted to Chile. The latter two genera contain species adapted for semi-fossorial and fossorial lifestyles, and according to the molecular data, they are each other's closest relatives. Although in 1987 Contreras and others questioned the species-level distinction between O. lunatus and O. bridgesi, estimates of genetic and phylogenetic divergence between these forms support these taxa as being distinct species. In 1994, Hutterer named a new species, Octodon pacificus, restricted to Mocha Island off the coast of Chile. Evidence from standard karyotypes and morphological comparisons by Gallardo and Mondaca in 2001 and the molecular phylogenetic study of Honeycutt and others support species-level status for the two currently recognized subspecies of Aconaemys, A. fuscus fuscus, and A. fuscus porteri. Mares and others in 2000 described two new genera and species, Pipanacoctomys aureus and Salinoctomys loschalchalerosorum, both of which are desert specialists occurring in perisaline

shrublands in Argentina. These two new species are most closely related to Tympanoctomys barrerae.

Physical characteristics

A figure-eight cusp pattern on the cheek teeth is the major diagnostic characteristic for the family. Members of the family are rat-like in appearance with stocky bodies, short legs, relatively large heads, and rounded ears. Relative to other families of hystricognath rodents, octodontids are small, not exceeding a head and body length of 7.7 in (195 mm) and a weight of 10.6 oz (300 g). The fur is dense and silky, and the dorsal pelage varies in color from pale gray to black. The ventral pelage is lighter, appearing almost white in some species. The coruro, Spalacopus cyanus, has the darkest pelage with many individuals being entirely black, and species living in arid regions tend to have paler-colored pelage. Species show little digit reduction, and have clawed digits for digging as well as long bristles between the digits. With the exception of coruros and rock rats of the genus Aconaemys, most species have relatively long tails that are distinctly tufted.

Distribution

Members of this family occur primarily in the southern cone of South America and, according to Contreras and others in 1987, most species occur between 16°S and 41°S latitude. Redford and Eisenberg in 1992 described the family's distribution as encompassing primarily areas in Bolivia, Chile, and Argentina, and eight of the 12 currently recognized species have ranges that are restricted almost exclusively to the western side of the Andes in Chile. Several species occur in Andean zones at higher elevations ranging 9,840–16,400 ft (3,000–5,000 m), whereas others maintain distributions from near sea level in coastal areas up to approximately 3,940 ft (1,200 m).

Habitat

Members of the family are diverse in terms of ecology and morphology. All species construct burrow systems under either shrubs or rocks, and coruros and rock rats are the most modified for a subterranean lifestyle. Degus of the genus Octodon are terrestrial, but are capable of climbing low-lying trees and shrubs. Most species prefer scrub habitat occurring in coastal, montane, and desert regions. This habitat varies in elevation from sea level to Andean regions ranging in elevation 9,840–16,400 ft (3,000–5,000 m). Most areas where species occur have shrub and grasses that are patchily distributed and intermixed with some rocks. The genera Tympanoctomys, Salinoctomys, and Pipanacoctomys are restricted to arid regions in Argentina characterized by desert plants with high salt content. Some species of rock rats occur in Nothofagus and Araucaria forests.

Behavior

Degus are diurnal, whereas most species appear to be nocturnal. Studies of activity patterns of coruros appear contradictory. Reig's field observations in 1970 indicated that coruros were active during the day, whereas Begall and others in 2002 observed nocturnal activity patterns in captive coruros. Several species are social, forming colonies occupying a communal burrow system. According to Fulk in 1976, colonies of degus usually consist of two to four adult males and females that share a common burrow system. Individuals within a colony defend territory and display a wide array of social activities related to social grooming, play, and courtship. Coruros also form colonies that occupy a mutual burrow system. Begall and others in 1999 observed one colony containing 26 individuals, whereas Torres-Mura and Contreras in 1998 indicated that the typical colony consisted of either one or more pairs of adults and their young. The colonial species display a diverse repertoire of vocalizations associated with alarm calls and other social interactions. Although little is known about them, rock rats appear to be colonial, while the remaining species are solitary. Both the degu and the chozchoz, representing the genus Octodontomys, take dust baths.

Feeding ecology and diet

All species are herbivorous, and with the exception of cururos, most species forage aboveground. Species living in less arid environments feed on a variety of plant materials, including grasses, leaves, bark, fruits, and forbs. Highly fossorial

forms like cururos feed on underground portions of plants, and desert-adapted species like the red viscacha rat are adapted for feeding on the stems and leaves of plants rich in salt. In 1997, Mares and others described adaptations that allow the red viscacha rat to feed on halophytic plants. This species has bundles of hairs on each side of the mouth that function as an extra pair of incisors for the removal of salt from leaves. Other species feed on fruits of cacti, seeds, and other plant materials.

Reproductive biology

For their body size, octodontids have a long gestation period ranging from 77–105 days. Young are precocial and weaned at an early age. The average litter size ranges between two and five offspring, and the females of many species are capable of postpartum estrus. The viscacha rat of the genus Octomys produces multiple litters per year, whereas coruros, red viscacha rats, and degus appear to be more seasonal in their breeding patterns. The mating system for octodontids are not known.

Conservation status

Most species of octodontids are locally common and not listed as species of concern by either IUCN or CITES. Two species, Octodon pacificus and Tympanoctomys barrerae, are listed as Vulnerable by IUCN, primarily because they display restricted distributions and habitat specializations that make them susceptible to extinction in the near future. In the case of O. pacificus, populations are confined to forest habitats, and recent agricultural activities associated with clear-cutting for cultivation are contributing to loss of habitat. Given the restricted distributions of Salinoctomys and Pipanacoctomys, these new species probably should be considered potential candidates for listing.

Significance to humans

Both degus and coruros are considered potential agricultural pests. According to Torres-Mura and Contreras, coruros can damage tuberous plants like potatoes, and in their 1975 review, Woods and Boraker suggested that degus cause damage to a variety of cash crops, including orchards, grains, and vineyards.

Species accounts

List of Species

Red viscacha rat

Tympanoctomys barrerae

taxonomy

Tympanoctomys barrerae (Lawrence, 1941), La Paz, Mendoza Province, Argentina.

other common names

English: Plains viscacha rat.

physical characteristics

Head and body length averages 5 in (128 mm); tail 5.7 in (145 mm); weight 5 oz (143 g). Enlarged tympanic bullae and head. Dorsal pelage pale, white hind feet, and tail darker and possessing longer hairs near the tip.

distribution

Restricted to arid area the Mendoza Province in Argentina.

habitat

Occupies arid regions characterized by shrubs and halophytic (salt-rich) vegetation. Plants usually patchily distributed.

behavior

Shows physiological and morphological adaptations to arid climates. Constructs complex burrow system and caches food. Nocturnal and solitary.

feeding ecology and diet

Herbivore that specializes on diets consisting of leaves and stems from plants with high salt content, specifically Heterostachys ritteriana. Uses lower incisors and bristle brush located on each side of the mouth to remove excess salt prior to ingesting plant material.

reproductive biology

Precocial young that eat solid food within days of being born.

conservation status

Listed as Vulnerable by IUCN as a result of restricted range and specialized habitat requirements.

significance to humans

None known.

Rock rat

Aconaemys fuscus

taxonomy

Aconaemys fuscus (Waterhouse, 1842), Valle de las Cuevas, Mendoza Province, Argentina.

other common names

None known.

physical characteristics

Head and body length 5.3–7.4 in (135–187 mm); tail 2.2–2.7 in (55–70 mm); males 4.2 oz (120 g), females 4.7–5.3 oz (135–152g). Dark brown dorsally, with bi-colored tail and stout body. Fewer specializations for fossoriality. Hind feet with fringe of hairs.

distribution

Occurs in Andean regions of Chile and Argentina between 33° and 41°S latitude.

habitat

Montane scrub habitat as well as forests consisting of Nothofagus and Araucaria.

behavior

Nocturnal and colonial. Semi-fossorial producing runways.

feeding ecology and diet

Herbivorous feeding on roots and other vegetation. Caches food.

reproductive biology

Nothing is known.

conservation status

Abundant where it occurs, not threatened.

significance to humans

None known.

Coruro

Spalacopus cyanus

taxonomy

Spalacopus cyanus (Molina, 1782), Valparaiso Province, Chile.

other common names

None known.

physical characteristics

Head and body length 4.5–6.5 in (115–165 mm); tail averages 1.9 in (49 mm); 1.7–5.3 oz (50–150 g). Body stocky with short tail, and brown to black pelage. Specialized for digging with large incisors, claws on digits, large front feet, and small ears.

distribution

Occurs west of the Andes in central Chile between approximately 27°S and 36°S latitude.

habitat

Occupies complex, communal burrow system. Fossorial, and prefers more temperate coastal and montane areas with shrub cover that does not exceed 60%.

behavior

Fossorial-excavating burrow systems with complex arrangement of tunnels and multiple openings that are not plugged. Forms colonies containing up to 26 individual adults and young. Highly vocal, with individuals producing a series of trills and other calls associated with socializing and warning. In field, observed to be diurnal, whereas lab colonies display nocturnal activities.

feeding ecology and diet

Herbivorous, foraging below on tubers and stems. Especially fond of the lily, Leucoryne ixiodes. Caches food in burrow system.

reproductive biology

Reproduces twice per year with average litter size of two. Gestation is 77 days.

conservation status

Common locally. Subspecies S. c. maulinus considered Endangered by Chilean Red List.

significance to humans

Considered an agricultural pest in some areas.

Chozchoz

Octodontomys gliroides

taxonomy

Octodontomys gliroides (Gervais and d'Orbigny, 1844), La Paz, La Paz Department, Bolivia.

other common names

Spanish: Rata cola pincel.

physical characteristics

Head and body length 6.3–7.1 in (161–182 mm); tail 5.1–7 oz (130–180 mm); 3.5–7 oz (100–200 g). Pelage is thick with grayish dorsal color and white venter. Tail bicolored and tufted. Ears relatively large and auditory bullae enlarged.

distribution

Higher elevations in north Chile, southwest Bolivia, and northwest Argentina.

habitat

Occupies burrows in xeric scrub areas comprised of various cacti, bushes, and rocks.

behavior

Nocturnal species. Can be solitary or live in small groups. Vocalizes and bathes in dust.

feeding ecology and diet

Forages aboveground on fruits of cacti and acacia pods as well as other plants.

reproductive biology

Gestation approximately 101 days and litter size ranges from one to three. Seasonal breeder producing two litters per year.

conservation status

Considered common, not threatened.

significance to humans

None known.

Viscacha rat

Octomys mimax

taxonomy

Octomys mimax Thomas, 1920, La Puntilla, Catamarca Province, Argentina.

other common names

Spanish: Rata cola peluda.

physical characteristics

Head and body length 5.5–6.7 in (140–170 mm); tail 5.9–7 in (151–180 mm); 4.2–5 oz (121–144 g). Dorsal pelage pale, under parts white, and a tufted tail that is partially bi-colored. Large auditory bullae.

distribution

Argentina in parts of the Monte desert in Catamarca and several provinces including San Juan, La Rioja, and Mendoza.

habitat

Higher altitudes along the slopes of the Andes in desert scrub habitat. Ecologically converges on the North American wood rat genus Neotoma.

behavior

Burrows under rocks and is nocturnal. Appears to be solitary.

feeding ecology and diet

Herbivorous feeding on a variety of plant materials, including seeds and cacti.

reproductive biology

Produce several litters per year.

conservation status

Not threatened.

significance to humans

None known.

Degu

Octodon degus

taxonomy

Octodon degus (Molina, 1782), Santiago, Santiago Province, Chile.

other common names

Spanish: Chozchoris, rata de las cercas, bori.

physical characteristics

Head and body length 9.8–12.2 in (250–310 mm); tail 2.9–5.1 in (75–130 mm); 6.0–10.5 oz (170–300 g). Compact body and large head. Dorsal color brown and ventral side light yellow, with black tufted tail. Digits with claws, with little digit reduction. Hind feet with bristles protruding beyond claws.

distribution

Western slope of Andes in Chile between Vallenar and Curico.

habitat

Usually restricted to elevations at 3,937 ft (1,200 m) and below in shrub and savanna habitats ranging from subtropical to semiarid. Burrows placed at the base of bushes and rocks and prefers microhabitats that are thermally less stressful.

behavior

Diurnal species active in mornings and afternoons. Constructs multi-chambered and colonial burrow systems housing several adults and young. Highly social with allogrooming among males and females, group huddling, and complex vocalizations consisting of warning alarms and other calls.

feeding ecology and diet

Herbivorous diet consisting of forbs, grasses, leaves, and bark of shrubs. Animals forage aboveground and cache food within the burrow system. Individuals capable of climbing to forage.

reproductive biology

Generally breeds twice per year, but demonstrates variation relative to the number and timing of litters. Sexual maturity of females reached in six months, gestation is 90 days, and average litter size is five. Young are precocial, and females experience postpartum estrus.

conservation status

Common throughout its range in Chile; not threatened.

significance to humans

Potential animal model for studying human diseases such as cataracts and diabetes mellitus. Known to harbor some diseases threatening to humans. Agricultural pest in some areas.

Resources

Books

Kleiman, D. G. "Patterns of Behaviour in Hystricomorph Rodents." In The Biology of Hystricomorph Rodents, edited byI. W. Rowlands and Barbara J. Weir. London: Academic Press, 1974.

Nowak, R. M. Walker's Mammals of the World. Vol. 2. Baltimore: Johns Hopkins University Press, 1991.

Redford, K. H., and J. F. Eisenberg. Mammals of the Neotropics: The Southern Cone. Vol. 2. Chicago: Chicago Press, 1992.

Wilson, D. E., and D. M. Reeder. Mammal Species of The World. Washington, DC: Smithsonian Institution Press, 1993.

Periodicals

Begall, S., H. Burda, and M. H. Gallardo. "Reproduction, Postnatal Development, and Growth of Social Coruros, Spalacopus cyanus (Rodentia: Octodontidae), from Chile." Journal of Mammalogy 80 (1999): 210–217.

Begall, S., S. Daan, H. Burda, and G. J. F. Overkamp. "Activity Patterns in a Subterranean Social Rodent, Spalacopus cyanus (Octodontidae)." Journal of Mammalogy 83 (2002): 153–158.

Begall, S., and M. H. Gallardo. "Spalacopus cyanus (Rodentia: Octodontidae): An Extremist in Tunnel Constructing and Food Storing Among Subterranean Mammals." Journal Zoology London 251 (2000): 53–60.

Contreras, L. C., J. C. Torres-Mura, and J. L. Yanez. "Biogeography of Octodontid Rodents: An Eco-Evolutionary Hypothesis." Fieldiana: Zoology 39 (1987): 401–411.

Diaz, G. B., R. A. Ojeda, M. H. Gallardo, and S. M. Giannoni. "Tyampanoctomys barrerae." Mammalian Species 646 (2000): 1–4.

Ebensperger, L. A., and F. Bozinovic. "Communal Burrowing in the Hystricognath Rodent, Octodon degus: A Benefit of Sociality?" Behavioral Ecology and Sociobiology 47 (2000): 365–369.

Fulk, G. W. "Notes on the Activity, Reproduction, and Social Behavior of Octodon degus." Journal of Mammalogy 57 (1976): 495–505.

Gallardo, M. H., J. W. Bickham, R. L. Honeycutt, R. A. Ojeda, and N. Kohler. "Discovery of Tetraploidy in a Mammal." Nature 401 (1999): 341.

Honeycutt, R. L., D. L. Rowe, and M. H. Gallardo. "Molecular Systematics of the South American Caviomorph Rodents: Relationships Among Species and Genera of the Family Octodontidae." Molecular Phylogenetics and Evolution 26 (2003): 476–489.

Hutterer, R. "Island Rodents: A New Species of Octodon from Isla Mocha, Chile (Mammalia: Octodontidae)." Zeitschrift fuer Saeugetierkunde 59 (1994): 27–41.

Lagos, V. O., F. Bozinovic, and L. C. Contreras. "Microhabitat Use by a Small Diurnal Rodent (Octodon degus) in a Semiarid Environment: Thermoregulatory Constraints or Predation Risk?" Journal of Mammalogy 76 (1995): 900–905.

Mares, M. A., J. K. Braun, R. M. Barquez, and M. M. Diaz. "Two New Genera and Species of Halophytic Desert Mammals from Isolated Salt Flats in Argentina." Occasional Papers, Museum of Texas Tech University 203 (2000): 1–27.

Mares, M. A., R. A. Ojeda, C. E. Borghi, S. M. Giannoni, G. B. Diaz, and J. K. Braun. "How Desert Rodents Overcome Halophytic Plant Defenses." BioScience 47 (1997): 699–704.

Reig, O. A. "Ecological Notes on the Fossorial Octodont Rodent Spalacopus cyanus (Molina)." Journal of Mammalogy 51 (1970): 592–601.

Torres-Mura, J. C., and L. C. Contreras. "Spalacopus cyanus." Mammalian Species 594 (1998): 1–5.

Torres-Mura, J. C., M. L. Lemus, and L. C. Contreras. "Herbivorous Specialization of the South American Desert Rodent Tympanoctomys barrerae." Journal of Mammalogy 70 (1989): 646–648.

Woods, C. A., and D. K. Boraker. "Octodon degus." MammalianSpecies 67 (1975): 1–5.

Rodney L. Honeycutt, PhD