van der Kooij, J. | Search bioRxiv

bioRxiv2025

Behavioural changes in female northern bats (Eptesicus nilssonii) across gestation and lactation.

Unpredictable environmental conditions affecting prey availability and short summer nights reducing foraging time present nocturnal insectivorous mammals with an energetic dilemma at northerly latitudes during gestation and lactation. This is pronounced for aerial hawking bat species, for which prey activity is highly dependent on ambient temperature. Here, we describe changes in activity of female Eptesicus nilssonii, the northernmost breeding bat species, during gestation and lactation over several years at a roost site in southern Norway (60.1{degrees}N). Roost exits and returns were recorded using an infrared barrier at the entrance, and parturition dates were registered via a camera set inside the roost. Our results indicate significant temporal changes in body mass and behaviour throughout the breeding season. Lactating bats both setting out to forage earlier than during gestation and returning later. This behaviour is dependent on ambient temperature, with bats extending foraging time at elevated temperatures. These behaviours may occur as a response to the increased energetic needs of lactation, and the effect of increased temperature to reflect increased food availability which allows females to avoid increased predation pressure as a consequence of advanced roost exit or delayed return. The mass of females also decreases during lactation, suggesting high energetic demands during this period. Our study reveals insights into underlying mechanisms in high latitude insectivorous bats that can assist in coping with a short season with fluctuating resources.

animal behavior and cognition↗

bioRxiv2024

Individual variation in breeding phenology and postnatal development in northern bats (Eptesicus nilssonii)

Bats inhabiting northern latitudes are faced with short reproductive seasons during which they must produce and rear pups before fattening up in time to survive the winter hibernation. Therefore, the timing of parturition has considerable impacts on future fitness prospects for mother and pup; however, little is known about individual variation in breeding phenology and its consequences for postnatal development within bat populations. Here, we studied the phenology of breeding in Eptesicus nilssonii across seven years using data collected by day-to-day monitoring of a breeding colony in Norway (60.1{degrees}N) for which the identity and age of each mother (N = 8) and pup (N = 28) were known. By applying mixed-effect models, we found that arrival at the colony was largely dependent on late spring temperatures for all females, but that there were strong and consistent individual differences in arrival time across years. Females generally arrived [~]31.6 days ({+/-} 0.8 SE) before giving birth but could delay the timing of parturition by leaving the colony during early gestation if faced with poor weather conditions. However, females arriving late expressed shorter gestations, and pups born later in the season were born smaller but had higher growth rates during the most rapid growth period (<10 days old). The within-individual effects suggest that the higher growth rates could be due to mothers compensating for late parturition rather than by improved food availability. Date of parturition did not influence adult body size in pups. Pups became volant at the earliest only 13.1 days after birth (mean: 15.3 {+/-} 1.6 SD) and approached adult flight patterns during their first flight week. Our unique results suggest that E. nilssonii is highly adapted to a short breeding season and is able to buffer unfavourable weather conditions to avoid slowing pup development, although the mechanistic drivers remain unknown.

ecology↗