Sexual dimorphism in steppe tortoises (Testudo horsfieladii): influence ofthe environment and sexual selection on body shape and mobility

Selective forces shape sexes differently, with male body proportions facing strong selection to enhance mate searching and male-to-male combat traits, and female fitness being influenced by the ability to assimilate large amo unts of nutrients necessary for vitellogenesis (and/or gestation), and thei r ability to carry the eggs or embryos. We evaluated the sexual dimorphism of body proportion of more than 800 wild steppe tortoises (Testudo horsfiel dii) in Uzbekistan. The thick, well-developed shell offers protection from predators but pronounced digging habits probably also constrain body shape (e.g. a shell that is dorso-ventrally flattened, although round from a dors al view helps to penetrate into, and move within the soil). Thus, in this s pecies, natural selection might favour a heavy and flat shell that is 'clos ed' with small openings for appendages. In males, these environmental influ ences appear to be countered by sexual selection. Compared to females, they weigh less (absolutely and relative to shell dimensions), have longer legs , have shell structure allowing wider movements for their legs, and they wa lk faster. Males were also able to right themselves more quickly than femal es did in experimental tests. This quick righting ability is critical becau se intra-sexual combats frequently result in males being flipped onto their backs and becoming prone to hyperthermia or predation. Females are heavily built, with wide shells (relative to male shells), which may provide space for carrying eggs. From our results, a number of simple hypotheses can be tested on a wide range of chelonian species. (C) 2001 The Linnean Society o f London.