Space use by Río Negro tuco-tucos (Ctenomys rionegrensis): Excursions and spatial overlap

Abstract

Patterns of spatial overlap can provide important insights into a species’ social organization. Subterranean rodents are notable for their lack of intermediate social systems in which individuals overlap spatially with one another but occupy distinct burrows. Previous studies of Río Negro tuco-tucos (Ctenomys rionegrensis) have revealed that while multiple adults can be captured at the same burrow entrance, these animals do not display other traits characteristic of social tuco-tucos, leading us to suspect that C. rionegrensis may exhibit an intermediate form of spatial and social structure. To test this hypothesis, we used radiotelemetry to monitor spatial relationships among adult C. rionegrensis during May, September, and November of 2005. Comparisons of minimum convex polygons for these animals revealed spatial overlap between 3 pairs of neighboring adult females. Most instances of spatial overlap occurred during November, which was the sampling period characterized by the largest number of radio fixes per animal and, accordingly, the largest individual home ranges. The observed examples of overlap appeared to reflect occasional excursions by one individual into the burrow system of another adult. These findings are consistent with an intermediate pattern of space use, suggesting that C. rionegrensis exhibits a spatial structure not previously documented among ctenomyids or other subterranean rodents.

Introduction

The social systems of subterranean rodents are typically divided into two categories based upon patterns of space use by adults. The majority of subterranean species are believed to be solitary (Nevo, 1979), meaning that each animal occupies its own burrow system, with effectively no spatial overlap among adult conspecifics (Lacey, 2000). In contrast, in social species, multiple animals share the same burrow system, as evidenced by extensive spatial overlap among adults and use of a single, shared nest site (Lacey, 2000). While many surface-dwelling and fossorial mammals exhibit a variety of intermediate patterns of space use (Michener, 1983, Lidicker and Patton, 1987), truly subterranean rodents are notable for the absence of examples in which adults exhibit moderate spatial overlap but retain areas of exclusive space use (Lacey, 2000). While the apparent lack of such intermediate systems may reflect conditions associated with life in underground burrows (e.g., movement is constrained to open tunnels), it may also reflect the limited data on space use available for most subterranean species.

Studies of tuco-tucos (Rodentia: Ctenomyidae) provide an important opportunity to explore comparative patterns of spatial organization in subterranean rodents. The genus Ctenomys, which contains >50 species, is widely distributed in South America, ranging from southern Peru and Bolivia to Tierra del Fuego and from the Andes mountains to southwestern Brazil. As this distribution suggests, the animals occur in a variety of habitats, including high elevation deserts, lower elevation grasslands, and coastal sand dunes. Although detailed field research on these animals is limited, the few studies that have employed radiotelemetry to examine spatial organization have revealed that the genus contains both strictly solitary (C. haigi: Lacey et al., 1998; C. talarum: Cutrera et al., 2006) and highly social species (C. sociabilis: Lacey et al., 1997, Lacey and Wieczorek, 2004). To date, however, more intermediate forms of spatial organization have not been documented using telemetry or other non-invasive methods for monitoring individuals.

Several aspects of the biology of the Río Negro tuco-tucos (Ctenomys rionegrensis) suggest that this species may display a more intermediate form of spatial organization than previously studied ctenomyids. First, in the relict sand dunes in western Uruguay where this species occurs, population densities can be high (>40 adults/ha; Tassino, 2006), with adjacent burrow systems tightly packed together. Second, previous field studies have revealed that multiple animals, including multiple adults, are sometimes captured at the same burrow entrance (Lessa et al., 2005). Third, laboratory observations of social interactions have revealed that males of this species show markedly reduced levels of aggression toward each other compared to other captive ctenomyids (B. Tassino, unpubl. data). Unlike the group-living C. sociabilis, however, Río Negro tuco-tucos do not display obvious signs of sociality such as multiple adults foraging together or alarm calling to one another in response to predators (Pearson and Christie, 1985, Lacey and Ebensperger, 2007).

Collectively, these observations led us to predict that C. rionegrensis may be characterized by an intermediate form of social structure in which adults resident in adjacent burrow systems engage in some degree of spatial overlap but do not share burrows or nest sites as do members of social species. Here, we test this prediction using radiotelemetry data obtained from a population of C. rionegrensis from El Rincon, Uruguay. In addition to providing the first quantitative description of the spatial organization of this species, our analyses reveal the first evidence that adult tuco-tucos may engage in spatial relationships that are intermediate to being either strictly solitary or truly social.