Social networks of threatened Chaco tortoises ( Chelonoidis chilensis ) in the wild

Abstract

We studied encounters between Chaco tortoises (Chelonoidis chilensis) that are in a threatened state of conservation in Patagonia, Argentina, which constitutes the southern limit of their geographical distribution. We monitored six individuals during different behavioural periods of the year (mating, nesting, and brumation), recording 24 344 encounters after tagging the animals with GPS tags. Constructing bipartite networks of burrows and tortoises, in addition to their projections, we discovered a spatial clustering of the burrows. Moreover, our research revealed that as tortoises approach the brumation period they use fewer unique burrows, engage in less burrow sharing, and increasingly favour spending time in their preferred burrow. During the mating and nesting periods some burrows are shared, whereas this was not the case during the brumation period. We studied the daily interactions through proximity-based networks, accounting for the daytime encounters during the different annual behavioural periods. We found the highest network density during the mating period, decreasing to zero, as expected, for the brumation period, when tortoises remain in their burrows during both night and day. Male–male interactions were more probable during the post-brumation and mating periods. Moreover, the number of daily encounters after sharing a burrow for a given night was found to be significant, which could be an indication of social behaviour and memory. Performing numerical simulations, we found that the proximity-based network densities were much higher than expected by chance, which could be attributable to individual selection of preferred sites or could be indicative of social behaviour among these threatened reptiles.