Endocrinology reveals seasonal reproductive dynamics in three Malagasy fruit bats

Abstract

Bats (Chiroptera) are a highly diverse mammalian order with over 1,500 species, roughly 200 of which are Old World Fruit Bats (family: Pteropodidae). Many pteropodid bats, especially those residing in seasonal environments, experience seasonal reproductive cycles, including discrete periods of gestation and lactation, which underpin cyclic energetic costs. These discrete seasonal life history events have consequences for conservation management (e.g., population viability) among wild fruit bat populations; however, little is known about the seasonality of reproduction in many pteropodid bat species. Here, we aimed to elucidate the reproductive biology of three species of pteropodid bats endemic to the island of Madagascar: Pteropus rufus, Eidolon dupreanum, and Rousettus madagascarensis. We leveraged plasma samples collected in part with a longitudinal field study, from 2019 and 2020. We adapted three standard reproductive assays previously validated in humans to quantify seasonal changes in reproductive hormones for bats and applied a mixture model approach to determine hormone cutoffs indicative of pregnancy in females and mating readiness in males. As predicted, we found that pregnant females showed the highest levels of estradiol and progesterone and adult males the highest levels of testosterone. Additionally, female P. rufus and R. madagascariensis showed clear seasonality in reproduction with peaks in estradiol and progesterone in August and October, respectively. Seasonality was less clearly discernible in female E. dupreanum and in male testosterone data. Our results demonstrate that hormone-based reproductive classification is a powerful tool for identifying reproductive status in free-ranging bats. These physiological data refine previously reported reproductive calendars critical for demographic modeling and conservation management of threatened Pteropodidae bats.