Does energy or calcium availability constrain reproduction by bats?

Bats are unusual mammals in being small but having long lives and small litters (typically only one or two young). I hypothesize that litter size is constrained by the need to raise young to near adult size before they can be independent. Our studies, and those of others, on a variety of species of bats indicate that juveniles typically start to fly at over 70% of adult mass and over 95% of adult skeletal size. This constraint appears to be associated with flight in vertebrates, since young birds also do not fly until fully grown. This means that each young is very costly and restricts the number that can be raised. Although energetic demands may be the proximate constraint, I argue that calcium is more important. For bats, calcium demand on reproductive females is high and calcium availability in most diets (insects, fruit, pollen) is low. Birds can at least partially overcome this by supplementing their diet with calcium-rich inanimate objects that are unavailable to bats because of their inability to fora ge on the ground and detect such items. This may help to explain why the reproductive output of birds exceeds that of bats. If the hypothesis is correct, bat foraging strategies may be based on the calcium content of prey in addition to-energy content, and female and male foraging strategies may be based on different currencies. Vertebrate-pollinated and seed-dispersed plants may attract bats by offering high calcium rewards. In addition, however, it would mean that flight could only have evolved in bats in association with long lifespans, thereby constraining the possible life histories available to these mammals.