Variation in Relative Clutch Mass in Snakes among and within Species

We examined the relationship between relative clutch mass (RCM) and female body size (SVL) both within and among different species of snakes. In 10 of 12 within-species comparisons, RCM varied independently of SVL; only viviparous elapids showed a significant regression between these two variables. We suggest that the independence of body size and RCM is explainable on ecological grounds alone. RCM decreased with increasing body size in both viviparous colubrids (n = 16 species) and viviparous viperids (n = 7 species), but only the colubrid sample was statistically significant. In 20 egg-laying colubrids, RCM increased slightly (but not significantly) with increasing female SVL. We suggest that the differences in the SVL/RCM relationship between reproductive modes is related to the increased costs of viviparity resulting from long gestation times. In addition, small viviparous species may be able to bear a greater RCM because of differences in either longevity or habitat utilization.