Influence of sex and reproductive condition on terrestrial and aquatic locomotor performance in the semi‐aquatic snake Seminatrix pygaea

Abstract

1 Most life-history models assume a trade-off between reproductive investment and parental survival. Several studies have documented reproductive costs in terms of reduced locomotor performance in terrestrial habitats. However, few studies have determined the reproductive costs of pregnancy in aquatic environments, or compared pregnancy-induced locomotor costs among habitats. This knowledge gap is important because many organisms rely on multiple habitat types during pregnancy. 2 We compared sexual differences in maximum locomotor velocity and the relative impacts of pregnancy on locomotor performance in aquatic and terrestrial environments for a semi-aquatic snake (Seminatrix pygaea). In addition, because most life-history models predict a direct trade-off between reproductive investment and reproductive costs, we quantified the relationship between reproductive investment and postpartum increase in velocity for both habitat types. 3 Both males and non-pregnant females always swam faster than they crawled, but males were significantly faster for their size than were non-pregnant females. These results mirror sexual differences known to exist in other snakes, but differ in that the degree of sexual divergence in velocity does not vary with habitat for S. pygaea. 4 Pregnancy significantly reduced both crawling and swimming velocity. Moreover, pregnancy impaired crawling velocity significantly more than swimming velocity. The mean (± 1 SE) percentage increases in crawling and swimming velocity after parturition were 72·8 ± 21·6 and 59·4 ± 12·8%, respectively. There was a direct trade-off between reproductive investment and aquatic locomotor impairment: snakes that invested more in offspring experienced larger decreases in swimming velocity. However, evidence for such a trade-off in the terrestrial habitat was weaker. 5 Our results demonstrate that the cost of reproduction for semi-aquatic organisms may differ between aquatic and terrestrial habitats in complex ways. Swimming may be more effective than crawling for escaping predators during pregnancy, because swimming results in faster velocities and is less impaired by pregnancy. However, the assumption of a direct trade-off between reproductive investment and locomotor impairment may be stronger for swimming performance compared with crawling performance.