Artificially raised roost temperatures lead to larger body sizes in wild bats

Climate Change-Driven Heatwaves Pose Lethal Risks to Newborn Forest Bats

Climate change poses a significant threat to biodiversity, with extreme weather events such as heatwaves exacerbating the risks to animal populations. Temperature extremes can cause high physiological stress in animals, particularly in species or life stages with limited thermoregulatory abilities. While available evidence pertains to flying foxes and bats using bat boxes or dwelling in urban environments, heatwave-induced mortality in forest-dwelling species in temperate forests has not been reported. We present the first evidence of heatwave-related mortality in temperate forest bats, specifically in common noctules Nyctalus noctula, observed in northeast Italy during the summers of 2023 and 2024. Our fieldwork, conducted in a forest fragment in the Friuli-Venezia Giulia Region (Northeastern Italy), identified 17 dead juvenile bats found at the base of roost trees during periods of extreme heat (Tmax ≥ 30°C). Laboratory necropsies revealed that the cause of death was consistent with heat-related stress, as no viral infections were detected, and recent feeding evidence was present in a few individuals. Dead bats are difficult to find in forests, especially when mortality occurs in unsurveyed areas, scavengers remove carcasses, or deaths go unnoticed within roost cavities. Consequently, our observations likely represent only a limited fraction of actual mortality. The phenomenon may be quantitatively significant and widespread. The findings highlight the vulnerability of bat populations to heatwaves, particularly in fragmented forest habitats where roosting opportunities are limited. Our results allow us to hypothesise that forest fragmentation increases exposure to heat stress, particularly along forest edges. In the context of climate change, roosts deemed suitable may act as ecological traps, making this a hypothesis worth testing.

Field experiment reveals that female Bechstein's bats (Myotis bechsteinii) select bat boxes based on the space available for roosting

Roosts are a crucial resource for bats, which choose them based on many factors, including the surrounding habitat, microclimate, and space available for communal roosting. The latter is important because many bat species benefit from social thermoregulation by forming colonies in their roosts. However, it remains unclear whether bats can base their roost choice directly on the space available for roosting when other characteristics, such as roost microclimate do not vary. We present results from a field experiment in which RFID-tagged Bechstein's bats (Myotis bechsteinii) in a maternity colony were given paired bat boxes with identical external dimensions but differing internal roosting space. This allowed us to control for other factors that might influence roost choice and to record the bats' nightly visits to the boxes prior to their occupation as day roosts. To assess whether roost temperature influenced roost choice in our setup, we measured the internal temperature of the boxes. Female Bechstein's bats showed a clear preference for boxes with more roosting space, as evidenced by a higher frequency of nightly visits and more frequent use as day roosts. As there was no significant difference in temperature between boxes with different internal volumes, roost temperature cannot explain the bats' preference for spacious roosts in our experiment. Our data provide evidence that bats can directly assess the volume of their roosts. Our results are consistent with the argument that small volume roosts may limit roosting group size potentially reducing social thermoregulation benefits.

In-situ responses of temperate-zone bats to climate change

There is growing evidence that human-induced climate change poses a major threat to bats. As climate change progresses, we can only hope to mitigate its negative effects on bat populations by gaining a more comprehensive understanding of the complex interactions of all the factors involved. Drawing on recent evidence, largely from long-term field studies of individually marked bats, we discuss the multiple impacts-positive and negative-of climate change on temperate heterothermic bats and their responses to climate change in situ. For example, there is increasing evidence that warmer summers and milder winters are leading to changes in the seasonal phenology of bats, which in turn may lead to species-specific changes in demography, morphology, physiology, food availability, and roost use. We also highlight open research questions on the responses of bats to climate change. This includes better data on population trends and the underlying direct and indirect climate-related causes for changes in mortality and reproductive success. In order to assess the long-term impacts of climate change on bats, more information is needed about the relative importance of phenotypic plasticity and evolutionary adaptation in the responses of bats to climate change.

Animal research revisited - the case of behavioural studies

Animal research is a matter of intense public debate, with some people supporting and some opposing it. Drawing from examples of behavioural biology, we argue that such an 'all-or-nothing' debate falls short. We highlight the potential of better science communication and tailored ethics assessments to foster a more nuanced view.

Field respirometry in a wild maternity colony of Bechstein's bats (Myotis bechsteinii) indicates high metabolic costs above but not below the thermoneutral zone

In a warming world, it is crucial to understand how rising temperature affects the physiology of organisms. To investigate the effect of a warming environment on the metabolism of heterothermic bats during the costly lactation period, we characterised metabolic rates in relation to roost temperature, the bats' thermoregulatory state (normothermia or torpor), time of day and age of juveniles. In a field experiment, we heated the communal roosts of a wild colony of Bechstein's bats (Myotis bechsteinii) every other day while measuring metabolic rates using flow-through respirometry. As expected, metabolic rates were lowest when the bats were in torpor. However, when bats were normothermic, colder temperatures had little effect on metabolic rates, which we attribute to the thermoregulatory benefits of digestion-induced thermogenesis and social thermoregulation. In contrast, metabolic rates increased significantly at temperatures above the thermoneutral zone. Contrary to our expectations, metabolic rates were not lower in heated roosts, where temperatures remained close to the bats' thermoneutral zone, than in unheated roosts, where temperatures were more variable. Our results show that torpor and digestion-induced thermogenesis are effective mechanisms that allow bats to energetically buffer cold conditions. The finding that metabolic rates increased significantly at temperatures above the thermoneutral zone suggests that the physiological and behavioural abilities of Bechstein's bats to keep energy costs low at high temperatures are limited. Our study highlights that temperate-zone bats are well adapted to tolerate cold temperatures, but may lack protective mechanisms against heat, which could be a threat in times of global warming.

Optimally warm roost temperatures during lactation do not improve body condition in a long-lived bat

Lactation is the most energetically demanding time in the life of female mammals. To maximize lifetime reproductive success, females of long-lived species, such as bats, face a trade-off between investing in current and future reproduction. However, it is unclear whether global warming could influence this trade-off through shifts in the energy budget: warmer temperatures may reduce thermoregulatory costs, leaving mothers with more energy available for maternal care or for improving their own body condition (BC), which may increase survival and ensure future reproduction. Here, we investigated whether lactating Bechstein's bats (Myotis bechsteinii) allocate the energy saved in optimally warm roosts into their own BC. We analysed a 14-year dataset on the individual BC of 237 females marked with radio-frequency identification tags from four wild maternity colonies. In two of the colonies, the temperature in the roosts, in which the females raised their offspring, was artificially kept in the bats' thermoneutral zone to reduce their thermoregulation costs. We found that BC shortly after the lactation period did not differ between mothers from heated and non-heated colonies. Our results suggest that mothers do not invest the energy saved in warmer roosts in their own BC, consistent with an increased investment in maternal care.

Higher and bigger: How riparian bats react to climate change

The altitudinal distribution of animals and changes in their body size are effective indicators of climate change. Bats are sensitive to climate change due to their dependence on temperature during critical life stages. However, long-term studies documenting responses over extended periods are rare. We present a 24-year investigation of Myotis daubentonii, a riparian bat known for altitudinal sexual segregation, along a river course in Central Italy. While males occupy the entire river course, females are confined to downstream warmer areas supporting successful reproduction due to improved foraging site productivity. In 2000, females were absent above 900 m a.s.l in our study area. We hypothesise that a) this altitude threshold is now higher, due to thermal gradient changes along the river course; and b) thermoregulatory costs for reproductive females have declined, leading to increased energy investment in offspring and subsequent generational growth in bat body size. Confirming our hypotheses, females exhibited a 175-m upward shift in altitude limit. Furthermore, we found a concurrent increase in body size (but not condition). Temperatures increased in the 24 years, likely allowing females to extend their range to higher elevations and favouring an increase in newborn body mass. Riparian vegetation remained unchanged, excluding habitat quality changes as the cause for the observed responses. The rapid female elevation rise might imply future disruption of established social structures, altering intra- and intersexual competition for roosts and food. Given the global decline in insect populations, larger bats might face future difficulties in finding food to sustain their body size, increasing mortality. However, the full impact of such changes on bat fitness remains unexplored and warrants further investigation, including other bat populations. This knowledge is crucial for informing conservation in the face of ongoing climate change and preserving the ecosystem services bats deliver in riparian ecosystems.

Climate is changing, are European bats too? A multispecies analysis of trends in body size

Animal size, a trait sensitive to spatial and temporal variables, is a key element in ecological and evolutionary dynamics. In the context of climate change, there is evidence that some bat species are increasing their body size via phenotypic responses to higher temperatures at maternity roosts. To test the generality of this response, we conducted a >20-year study examining body size changes in 15 bat species in Italy, analysing data from 4393 individual bats captured since 1995. In addition to examining the temporal effect, we considered the potential influence of sexual dimorphism and, where relevant, included latitude and altitude as potential drivers of body size change. Contrary to initial predictions of a widespread increase in size, our findings challenge this assumption, revealing a nuanced interplay of factors contributing to the complexity of bat body size dynamics. Specifically, only three species (Myotis daubentonii, Nyctalus leisleri, and Pipistrellus pygmaeus) out of the 15 exhibited a discernible increase in body size over the studied period, prompting a reassessment of bats as reliable indicators of climate change based on alterations in body size. Our investigation into influencing factors highlighted the significance of temperature-related variables, with latitude and altitude emerging as crucial drivers. In some cases, this mirrored patterns consistent with Bergmann's rule, revealing larger bats recorded at progressively higher latitudes (Plecotus auritus, Myotis mystacinus, and Miniopterus schreibersii) or altitudes (Pipistrellus kuhlii). We also observed a clear sexual dimorphism effect in most species, with females consistently larger than males. The observed increase in size over time in three species suggests the occurrence of phenotypic plasticity, raising questions about potential long-term selective pressures on larger individuals. The unresolved question of whether temperature-related changes in body size reflect microevolutionary processes or phenotypic plastic responses adds further complexity to our understanding of body size patterns in bats over time and space.