Locomotor speeds of gravid lizards: placing 'costs of reproduction' within an ecological context

1. Mathematical models suggest that 'costs of reproduction' (decrements in an organism's probable future reproductive output due to investing in current reproduction) are major determinants of life-history evolution. 

2. Pregnancy decreases locomotor performance in many taxa, and could render reproducing females more vulnerable to predators. To evaluate the importance of this 'cost of reproduction', however, we need to know the magnitude of performance decrement induced by reproduction relative to that induced by other factors in the animal's biology. 

3. Studies on adult female skinks (Lampropholis guichenoti) confirm that pregnancy significantly impairs maternal locomotion, but also show that factors such as a moderate decrease in body temperature, a large meal or loss of the tail reduced locomotor speeds even more than did pregnancy. Thus, reproductive state probably causes only a minor proportion of the total temporal variation in a female skink's locomotor ability. 

4. In such a system, even a large effect of pregnancy on running speeds may not impose a significant selective pressure on reproductive investment. 

5. Climate and foraging modes may affect the degree to which locomotor speeds are influenced by pregnancy vs. other factors, offering a potential explanation for the lower overall reproductive investment per clutch in tropical vs. temperate-zone reptiles, and in lizards vs. snakes.