Higher and bigger: How riparian bats react to climate change

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.

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.

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.

Learning to Hunt on the Go: Dietary Changes During Development of Rhinolophid Bats

Mammals may experience physical changes from birth, and their diet varies at different stages of life. This study investigates the impact of development on the diet composition of three horseshoe bats: Rhinolophus euryale, R. hipposideros, and R. ferrumequinum in the Basque Country, north of the Iberian Peninsula. The diets of juvenile and adult individuals of each species were obtained by analysing their droppings using metabarcoding and then compared at (1) the taxonomic and (2) prey trait levels (size, flying speed, hardness). The diets of juvenile and adult individuals of R. euryale and R. hipposideros showed significant differences at the taxonomic level and regarding prey traits. In contrast, in the case of R. ferrumequinum, we could only observe discernible diet patterns through the trait analysis. Additionally, we discovered a shared pattern: younger individuals tend to feed on easier-to-hunt and/or handle smaller and smoother prey. The varying degrees of dissimilarity between juvenile and adult diets observed in this study suggest that the relative importance of psychomotor development, foraging strategies, prey discrimination, and/or spatial learning may differ among species. These findings contribute to conservation efforts, especially by recognising the dietary needs of juveniles for their survival and successful development.

Re-establishing historic ecosystem links through targeted species reintroduction: Beaver-mediated wetlands support increased bat activity

Despite the global significance of wetlands, conservation strategies often fall short in preserving these ecosystems due to failures in incorporating processes that sustain the ecosystem functioning, hydrological dynamics, ecological processes, and biodiversity of wetlands. Nature-based solutions, such as the reintroduction of beavers, have emerged as effective tools for promoting wetland restoration. Whilst the impact of beavers on wetland restoration is well known, their broader influence on ecosystem health, particularly in modifying habitats for other species, remains inadequately understood. Here we assess the impact that habitat modification through the reintroduction of beavers has on bat populations. There were significantly greater activity levels within beaver-modified wetland habitats for multiple bat species, including higher activity levels of 393 % for Barbastella barbastellus and 313 % for Plecotus spp.. Additionally, we observed positive effects on bat populations in the woodland habitat surrounding beaver-modified wetland for certain taxa. In the face of escalating challenges posed by climate change and habitat loss, addressing biodiversity loss necessitates a shift toward ecosystem-centric mitigation measures. Our study demonstrates that the reintroduction of keystone species like beavers can re-establish historical facilitative links between aquatic and terrestrial food webs, highlighting the importance of such interventions in fostering the resilience and sustainability of entire 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.