Reproductive success is the ultimate measure of individual quality; however, it is difficult to determine in free-living animals. therefore, indirect measures that are related to reproduction are generally employed. in snakes, males typically possess longer tails than females and this sexual size dimorphism in tail length (tL) has generally been attributed to the importance of the tail in mating and reproduction. Thus, intra-sexual differences in tail length, specifically within males, were hypothesized to reflect individual quality. We used a body condition index (BCI) as a measure of quality in snakes and predicted that tail length would be correlated with BCI in males. We tested our prediction by determining BCI in the free-ranging adult male and female crowned leafnose snake (Lytorhynchus diadema), a colubrid species that inhabits mainly desert sand dunes. the relative tL was correlated positively and significantly to BCI in males (F 1,131 = 11.05; r 2 adj = 0.07; P < 0.01) but not in females, thus supporting our prediction. This is the first time that the relationship between TL and body condition was tested in a free-ranging species. In addition, sexual size dimorphism of TL increased intra-specifically with body size, which was also found in interspecific analyses following Rensch's rule. Sexual size dimorphism (SSD) occurs in many species of invertebrate and vertebrate taxa 1,2. In general, males are larger than females in bird and mammal species 3,4 , whereas females are larger than males in terrestrial vertebrate ectotherms 5,6. According to Rensch's rule, when males are the larger sex, SSD increases with size; 1,4 however, when females are the larger sex, SSD decreases with size 1,7. Although these trends seem contradictory, they both reflect greater variance in males than in females 8. A number of hypotheses have been proposed to explain these evolutionary interspecific trends including sexual selection and mating success, fecundity selection favoring large females, and natural selection for resource partitioning 8-11. Sexual size dimorphism has also been observed in lengths of appendages such as in tails. In most snake species, males have relatively longer tails than females. The reason for this difference between sexes was initially explained structurally, as males possess a hemipenis in an elongated pocket at the base of the tail 12,13. Several hypotheses for the evolution of sexual differences in tail length (TL) have been suggested. Interspecific analyses of tail and body lengths, as well as clutch mass in colubrid snakes, supported both the morphological constraint hypothesis and the female reproductive hypothesis. Consequently, these analyses indicated a male-biased TL dimorphism in taxa having relatively short tails 14. In addition, the TL was found to be associated with gravitational habitat categories, where terrestrial snake species have shorter tails than arboreal species. This hypothesis was supported in females using 226 snake species in 139 genera and 15 families 15. Tails of snakes are...
