CONSTRAINTS ON REPRODUCTIVE INVESTMENT: A COMPARISON BETWEEN AQUATIC AND TERRESTRIAL SNAKES

Life-history theory predicts that "costs" of reproduction may be important evolutionary determinants of reproductive investment; previous studies on reptiles indicate that decrements to maternal mobility may be among the most important components of such costs. Biomechanical models suggest that reproductive investment in aquatic snakes may be constrained by the important locomotory role of the posterior part of the body during swimming: carrying eggs or offspring in this region would more seriously impair locomotory efficiency in swimming than in terrestrial lateral undulation. If this constraint is important, aquatic snakes would be expected to have lower clutch masses relative to body mass than terrestrial species and to carry the clutch in a more anterior position (commencing at the same proportion of maternal body length anteriorly, but not extending as far posteriorly). Comparisons between aquatic and terrestrial snakes of several families confirm these predictions. Phylogenetic analysis suggests that this pattern of reduced reproductive investment has evolved independently in each of the four ophidian lineages that contain marine species (acrochordids, homalopsine colubrids, laticaudid sea snakes, and hydrophiid sea snakes). Although it thus seems likely that these patterns represent adaptations to aquatic versus terrestrial life, the nature of the selective forces involved remains speculative. The hypothesis based on locomotory impairment of gravid females has better empirical support than any alternative hypothesis, as it successfully predicts modifications in the position of the clutch within the female's body, as well as overall reduced reproductive investment.