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.