(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
Red viscacha ratRock rat
Coruro
Chozchoz
Viscacha rat
Degu
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.
[Article by: Rodney L. Honeycutt, PhD]
