FUNCTIONAL-MORPHOLOGY, COMPARATIVE BEHAVIOR, AND ADAPTATION IN 2 SYMPATRIC SUBTERRANEAN RODENTS GENUS CTENOMYS (CAVIOMORPHA, OCTODONTIDAE)

Two species of tuco-tucos, Ctenomys talarum and C. australis, overlap geographically in south-eastern Buenos Aires Province, Argentina. C. t alarum (100-180 g) occupies habitats with clayey and harder soils, whi le the larger C. australis (300-600 g) occupies sandy and quite friabl e soils. This study compares the digging apparatus and digging perform ance of both species, and also provides additional ecological data to describe the adaptations of both species. When confronted with sandy, friable soils, both tuco-tucos exclusively use their forelimbs ('scrat ch digging' mode) to break the soil. When confronted with harder and c layey soils, C. australis used its forelimbs and, less frequently, its incisors, but no appreciable effectiveness was observed in breaking t he soil. On the other hand, C. talarum behaved as a 'scratch' and 'chi sel-tooth digger' species, using both forelimbs and incisors to dig tu nnels in extremely hard soils. C. talarum showed relatively larger for elimb extensors than C. australis. Moreover, I hypothesize that the st ructure of the ulna in C. talarum may increase the effective forces of the digital and carpal flexors, thus accounting for the enhanced digg ing ability in this species. Outward forces exerted by the distal elem ents (claws-digits-carpals) when forelimbs impact the soil appear to b e a critical factor in loosening soil fragments. In addition, the incr eased upper incisor procumbency of C. talarum correlated with an effec tive chisel-tooth digging behaviour. Finally, I briefly examine phylog enetic and biogeographical factors that may account for some C. austra lis morphological and physiological traits. It is suggested that the d igging ability of C. australis could be a by-product of its larger siz e, a likely adaptation to evade predation.