Variations in gestation length in a colony of pipistrelle bats (Pipistrellus pipistrellus) from year to year.

Gestation length in a colony of pipistrelle bats varied by 10 days in two consecutive years when the weather conditions during early gestation differed appreciably. Plasma progesterone concentrations and corpus luteum volume increased during pregnancy to reach a maximum about 6 days before the first births. Pipistrelles maintained high body temperatures during pregnancy if the food supply was adequate. The rate of fetal development therefore appears to be determined by maternal body temperature, which becomes reduced by shortage of food in cold weather.