Capital-breeding and reproductive effort in a variable environment: a longitudinal study of a viviparous snake

1. The ways that fluctuations in prey abundance and weather conditions can affect reproductive output in a 'capital breeding' ectotherm, the aspic viper ( Vipera aspis) were examined. 2. Our longitudinal study confirms that female aspic vipers adjust reproductive investment by integrating allocations of energy from stores ('capital') and facultative feeding ('income'). Thus, long-term energy storage enabled females to reproduce successfully even in years when prey were scarce. 3. Not surprisingly, temporal changes in body reserves of female vipers preparing for reproduction depended upon current feeding rates. However, the mean environmental temperature during the active season also affected mass gain. 4. Allometric patterns suggest that reproductive output was limited by energy availability in 8 out of the 9 years of our study. In the other year, high prey availability in the preceding season meant that reproductive output was maximized within the constraints set by maternal body size (and thus, abdominal volume). 5. High summer temperatures increased basking opportunities of gravid vipers and thus accelerated gestation. However, maternal metabolic costs also increased in such situations, resulting in low postpartum body condition.