Trade-Off Between Growth and Energy Storage in Male Vipera berus (L.) Under Different Prey Densities

Abstract

There are constraints on the rate of energy intake of foraging animals, and allocation of energy, for example, to growth therefore must be at the expense of allocation to other purposes, such as energy storage. Moreover, the relative amount of energy allocated to different kinds of activities is likely to depend upon food availability. We have compared the patterns of energy allocation between adders, Vipera berus (L.), in populations inhabiting two different island groups in the Baltic Sea. These adders primarily feed on field voles, Microtus agrestis (L.), which undergo great population density fluctuations. In order to survive periods of low food supply adders allocate some energy to storage, and they can be expected to increase the proportion of energy devoted to storage when food levels and rate of energy intake decrease. Individuals allocating much energy to storage have less energy available for growth, reproduction and maintenance, and one can expect a trade-off between growth and energy storage, especially during resource shortage. Since the size of energy reserves influences fasting endurance, while body size influences future reproductive success, the pattern of energy allo- cation between growth and storage might have important fitness consequences for these animals. We compared the size of energy reserves (body mass in relation to body length) and the growth rates of individual male adders from two localities with different densities of field voles. Relative body mass was similar to both adder populations, but individuals grew faster where vole density was higher. This suggests that the pattern of energy allocation changed with resource availability. We also investigated the relationships between rela- tive growth rate and relative mass within the two adder populations. We found a significant nega- tive relationship where prey density was low, and a negative but non-significant relationship where prey density was high, indicating a trade-off between growth and energy storage in male adders that changes with food availability.