Constraints on Reproductive Investment: A Comparison Between Aquatic and Terrestrial Snakes

Abstract

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.

Pregnancy is a drag: hydrodynamics, kinematics and performance in pre- and post-parturition bottlenose dolphins (Tursiops truncatus)

“…Diminished locomotor performance can in turn increase the susceptibility of pregnant females to predation. Ultimately, the results of this study support the notion that reproduction is a costly endeavour that may increase energetic expenditure, increase risk of predation and decrease longevity (Shine, 1988).…”

Reproduction of the water snake Helicops infrataeniatus (Colubridae) in southern Brazil

“…However, little is known about the reproduction of viviparous colubrids, probably because viviparity is uncommon within this group. Shine (1988a) has emphasized the lack of basic information regarding the reproduction of aquatic viviparous snakes and suggested the Neotropical genus Helicops as a good study model.…”

Swimming and pregnancy in Tiger snakes, Notechis scutatus

“…Reproductive investment in aquatic snakes may be constrained by the important locomotory role of the posterior part of the body during swimming, and to a larger degree than during crawling on land (Jayne, 1985;Gans, 1986;Shine, 1988). Indeed, clutches of aquatic snakes are located more anterior in the body, and overall reproductive investment is reduced compared to terrestrial species (Shine, 1988).…”

“…Reproductive investment in aquatic snakes may be constrained by the important locomotory role of the posterior part of the body during swimming, and to a larger degree than during crawling on land (Jayne, 1985;Gans, 1986;Shine, 1988). Indeed, clutches of aquatic snakes are located more anterior in the body, and overall reproductive investment is reduced compared to terrestrial species (Shine, 1988). If such modifications are genuinely related to a reduction of the "costs" associated with pregnancy in swimming snakes, then it may be possible to record an impairment in swimming performances in pregnant snakes compared to non pregnant individuals, either in terms of stamina or swimming speed.…”