Relative reproductive effort drives metabolic changes and maternal emaciation during pregnancy in a viviparous snake

Abstract

AbstractMost viviparous squamates are lecithotrophic, and maternal effort during pregnancy mainly involves behavioural and thermoregulatory shifts to optimize developmental conditions. Still, pregnancy also imposes specific metabolic demands on the female, known as the metabolic cost of pregnancy (MCP). Contrary to the thermoregulatory shift, these energy constraints should be directly fecundity dependent and their evaluation is important to assess the ‘costs’ of viviparity. We measured the metabolic rate of aspic vipers Vipera aspis at three stages (early pregnancy, late pregnancy and post parturition), and we examined the determinants of maternal metabolic changes over time. We found a 55% metabolic increase over the course of pregnancy that was better explained by maternal relative reproductive effort (relative litter mass) when compared with absolute estimates (litter mass, litter size). After parturition, female metabolism dropped below values recorded at early pregnancy and this decrease was closely related to maternal relative reproductive effort. Our estimates for MCP ranged from 13.9 to 14.7% of maternal metabolic rate, suggesting that specific energetic demands of pregnancy are significant. It appears crucial to consider both direct (MCP) and indirect (thermoregulatory shift) components to evaluate overall maternal metabolic demand during pregnancy. Because females are already emaciated at the onset of pregnancy, these combined constraints are likely costly by inducing structural protein mobilization and altered performances after parturition.