Ctenomys sociabilis
social tuco-tuco

Lanin and Nahuel Huapi National Parks, located in the Limay Valley, at the western part of the Rio Negro and Neuquen provinces, Argentina and the surrounding area (Pearson and Christie, 1985). Elevation range is ca. 800 - 1600 meters (E. A. Lacey, 1999, personal communication).

The general distribution of C. sociabilis is in the interface between the Andes and Patagonian Steppe in very dry grassland. Within the 1500 sq. kilometer range there is a very strong rainfall gradiant from east to west. C. sociabilis inhabits wet patches in the steppe where seep-water collects underground to form wet patches of relaitvely lush vegetation. In Argentina these patches are call "mallines". The entire range is patchy in terms of hospitable habitat which is associated with increased moisture (E. A. Lacey, 1999, personal communication). The "mallines" have a characteristic suit of associated plants composed principally of scattered growths of thatching grass (/Festuca pallescens/) and rushes (/Juncus balticus/) that form nets of rhizomes. The soil is black but extremely light and easy to work with (Pearson and Christie, 1985).

A large-bodied tuco-tuco with bright ochraceous-orange spots of fur on the sides of the nose, a conspicuous black and white moustache and with exceptionally narrow, flat ear pinnae. The face is long (the upper "diastema" is more than 12 mm long), the upper incisor arch is large (more than 18 mm), and the incisors are very wide (more than 5.3 mm across at the tip). The upper molar toothrow is relatively long (more than 9.4 mm). The claws are long, relatively thin, and barely curved. The sperm have a post-"acrosomico" process as in C. maulinus. Mean and range of measurements for six adults are: head and body, 192 (168-247) mm; tail, 68 (67-80) mm; hind feet including claws, 34 (32-36) mm; weight, 182 (180-234) g; length of skull base, 40.1 (40.0-46.8) mm; width of brain case, 16.7 (16.4-17.5) mm; width of zygomatic arch, 27.5 (27.3-30.3) mm; diameter of the upper incisor arch, 19.4 (19.7-24.5) mm; length of upper diastema, 12.6 (12.8-16.1) mm; width across the tips of both upper incisors, 5.78 (5.54-7.34) mm; length of the upper molar toothrow, 9.8 (9.6-10.8) mm; length of the ear pinnae, 14.6 (14.0-15.8) mm; and width of ear pinnae, 5.9 (5.8-6.1) mm. The back is ohcraceous-tawny, mixed with black hairs, especially on the front of the head. Ventrally it is ochraceous-tawny without the black mix. One very distinctive identifying feature is a spot of ochraceous-orange fur on both sides of the nose. All the specimens have a conspicuous moustache of short black hairs on the upper lip with stiff, longer white hairs underneath. As seen from the front, the orange spot on the nose and the contrasting black and white moustache form a characteristic design (Pearson and Christie, 1985).

C. sociabilis is a seasonal breeder. Sexual maturity is reached at about nine months of age. Breeding takes place during the austral winter months of July and August. Since this time of year is before an increase in ambiant temperature and before any vegetation flush, it is likely that reproductive physiology and behavior are triggered by photoperiod cues. Females give birth once a year during the austral spring/summer following a gestation period of 90 to 100 days. Litter size ranges from 2 to 6 young. The young are born very precocial. In captivity they have been observed to be fully furred, have their eyes open, and to move about the tunnel system within the first 24 hours. Female reproductive output in the wild appears to be limited to approximately 2 litters, as females in their 4th year, after two reprductive seasons, seem to be spent, their fur dull and unhealthy looking. Males in the wild often are also limited to two breeding seasons probably because they do not survive three dispersal episodes and are likely victims of predation (E. A. Lacey, 1999, personal communication).

Relative to other Ctenomys species, C. sociabilis displays high levels of above-ground activity. It is a strictly diurnal animal and has large eyes. Many individuals have been seen at the surface at one time and have been observed to give alarm calls above ground to other individuals. C. sociabilis lives in colonies which are distinct spatial and social units. These colonies arise from female natal philopatry, with males that are resident in the colonies for one breeding season only. Between seasons the males disperse to neighboring colonies that are anywhere between 5 to 300 meters apart (E. A. Lacey, 1999, personal communication). While other tuco-tucos may be social, C. sociabilis is the only one for which quantitative data are available. In a 3-year study on the social behavior (Lacey et al., 1997), two or more adults were captured in nearly 50% of the colonies surveyed with their activity confined to only one colony. Within a colony, the areas used by different individuals overlapped extensively. Furthermore, multiple adults shared a single nest site. Based on animals captured, 46.1% of colonies surveyed contained two or more adults. However, based on direct and indirect visual observations, it was possible that as much as 69.2% of colonies surveyed may have contained multiple adults. Additional data suggested that extensive overlap existed for burrow-system use and that a single nest is used by all resident adults. Although it may be due to data collecting bias, it is interesting to note that single-female colonies were prevalent and that males were often absent from the colonies sampled (Lacey et al., 1997, see fig. 1).

Ctenomys sociabilis is strictly herbivorous, eating a variety of grasses and sedges. The grass genus Poa comprises the primary component of its diet. C. sociabilis forages just outside the burrow opening, emerging to about its waist. It crops vegetation in a circle around the entrance. Small piles of grass are found inside the burrow suggesting that C. sociabilis hoards the vegetation in its burrow (E. A. Lacey, 1999, personal communication).

The range of C. sociabilis is used by humans as grazing land for cattle and sheep. Often burrowing animals have a negative impact on grazing animals due to the holes of their burrows which may cause injury to either cattle or horses. However, the presence of the tuco-tucos does not seem to have a direct negative impact on locals. Nontheless, the current Argentian conservation status of C. sociabilis as a species of special concern might lead to regulation of human acitvites such as sheep running (E. A. Lacey, 1999, personal communication).

The IUCN listing is lower risk/near threatened. Local status in Argentina is as a "species of special concern." This is likely due to the narrow endemism of the species (E. A. Lacey, 1999, personal communication).