Biology subjects

Fjelldal, M. A.

Publications and source records attributed to Fjelldal, M. A..

5 recordsLinked to original sources

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

Supersize me: torpor assists pre-hibernation fattening in a boreal bat

Hibernators face an energetic dilemma in the autumn at northern latitudes; while temperatures and food availability decrease, hibernating species need to build fat deposits to survive the winter. During this critical fattening phase, insectivorous boreal bats use torpor to build and conserve their reserves. However, we still know little about temporal variability in torpor use employed by bats in autumn and how decreasing temperatures and food availability in combination with increasing individual body mass impact this. Here we present two general hypotheses for explaining temporal torpor patterns observed in a boreal bat (Eptesicus nilssonii), in which torpor use I) facilitates rapid mass gain or II) conserves stored body mass. Although temporally separated in our dataset, temperature, insect abundance and body mass throughout autumn in the study system indicate that E. nilssonii reaches a majority of its overwintering mass before the onset of increasing daily and nightly torpor use. In combination with generally low food availability by this point in time, these observations suggest that torpor expression might be intended to conserve gained reserves. Our study is intended as a first proof-of-concept for disentangling temporal drivers of torpor in bats during the pre-hibernation fattening phase.

ecology↗

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↗

bioRxiv2024

Thermoregulation and Diurnal Roost Selection of Boreal Bats During Pre-Hibernation Period

Living in a seasonal environment poses challenges for small mammals, such as bats, reliant on insects as their primary food source. Bats may adeptly navigate these energetic challenges by reducing their metabolism and body temperature, entering a state of torpor. Particularly during the winter, bats remain torpid for extended periods, but are dependent on sufficient energy reserves to survive until spring. With the onset of autumn and declining temperature, bats face the challenge of building their fat deposits during a time of decreasing food availability. Bats may therefore transition to cooler roosts to initiate torpor, thereby reducing energy expenditure. However, little is still known about torpor use or roost selection by bats in autumn. This study explores the factors influencing roost selection and torpor use and -duration in two bat species during this critical transition period between the breeding and overwintering season. We show that date in autumn is a stronger driver of torpor use than prevailing ambient temperature, and that bats employ specific strategies in which they first increase daytime torpor use before also increasing the use of night-time torpor during the pre- hibernation fattening period, most likely to facilitate rapid fat accumulation. Notably, bats commenced night-time torpor use after spending entire days in torpor. These findings underscore the dynamic nature of torpor and the energy-saving strategies employed during the crucial pre- hibernation period, marking the transition from summer to winter.

ecology↗

bioRxiv2022

The small-bat-in-summer paradigm: energetics and adaptive behavioural routines of bats investigated through a stochastic dynamic model

Strong seasonality at high latitudes represents a major challenge for many endotherms as they must balance survival and reproduction in an environment that varies widely in food availability and temperature. Being heterotherms, bats spend long cold winters in hibernation, avoiding the challenges faced by many animals. To avoid energetic mismatches caused by limited foraging time and stochastic weather conditions, bats can also employ this energy-saving state of torpor during summer to save accumulated energy reserves. However, at high latitudes small-bats-in-summer face a particular challenge: as nocturnal foragers they rely on the darkness of the night to avoid predators and/or interspecific competition, but for many the summer involves short nights of mostly twilight, and even a lack of true night at the northernmost distributions of some bat species. To investigate optimal individual behaviour across diurnal cycles, we constructed a stochastic dynamic model of bats living at high latitudes. Using a detailed parameterized model framework with values that are representative for our study system, we show that individual energetic reserves are a strong driver of day-time use of torpor and night-time foraging behaviour alike, with these linked effects being both temperature and photoperiod dependent. We further used the model framework to predict survival probabilities at five locations across a latitudinal gradient (60.1{degrees}N to 70.9{degrees}N), finding that photoperiod is the main limiting factor to bat species distributions. To verify the accuracy of our model results, we compared predictions for optimal decisions with our own empirical data collected on northern bats (Eptesicus nilssonii) from two latitudes in Norway. The similarities between our predictions and observations provide strong confirmation that this model framework incorporates the most important drivers of diurnal decision-making in bat physiology and behaviour. Our model findings regarding state-dependent decisions in bats should therefore contribute to the understanding of how bats cope with the summer challenges at high latitudes.

ecology↗