Change in body size is one of the universal responses to global warming, with most species becoming smaller. While small size in most species corresponds to low individual fitness, small species typically show high population growth rates in cross-species comparisons. It is unclear, therefore, how climate-induced changes in body size ultimately affect population persistence. Unravelling the relationship between body size, ambient temperature and individual survival is especially important for the conservation of endangered long-lived mammals such as bats. Using an individual-based 24-year dataset from four free-ranging Bechstein's bat colonies ( Myotis bechsteinii ), we show for the first time a link between warmer summer temperatures, larger body sizes and increased mortality risk. Our data reveal a crucial time window in June–July, when juveniles grow to larger body sizes in warmer conditions. Body size is also affected by colony size, with larger colonies raising larger offspring. At the same time, larger bats have higher mortality risks throughout their lives. Our results highlight the importance of understanding the link between warmer weather and body size as a fitness-relevant trait for predicting species-specific extinction risks as consequences of global warming.

Whether species can cope with environmental change depends considerably on their life history. Bats have long lifespans and low reproductive rates which make them vulnerable to environmental changes. Global warming causes Bechstein’s bats (Myotis bechsteinii) to produce larger females that face a higher mortality risk. Here, we test whether these larger females are able to offset their elevated mortality risk by adopting a faster life history. We analysed an individual-based 25-year dataset from 331 RFID-tagged wild bats and combine genetic pedigrees with data on survival, reproduction and body size. We find that size-dependent fecundity and age at first reproduction drive the observed increase in mortality. Because larger females have an earlier onset of reproduction and shorter generation times, lifetime reproductive success remains remarkably stable across individuals with different body sizes. Our study demonstrates a rapid shift to a faster pace of life in a mammal with a slow life history.

Many animal populations around the globe struggle with the magnitude and speed of current climate change (Radchuk et al., 2019). The capacity of populations to cope with changing environments is determined by their ability to adapt through evolutionary change, or via phenotypic plasticity (Williams et al., 2008). While plasticity may enable local populations to overcome short-term environmental changes, it is genetic adaptation that enables populations to survive over longer periods in a changing world (Hoffmann & Sgró, 2011). Rapid genetic change, however, is limited in species with slow life histories, potentially increasing extinction risk (Chevin et al., 2010;Hoffmann & Sgró, 2011).Heritability is a common measure of a trait's potential for evolutionary change (de Villemereuil et al., 2018;Wilson & Poissant, 2016).Measuring and interpreting heritability, however, is notoriously impeded by the fact that it changes over space and time, depending on variation in genetic and environmental factors (Visscher et al., 2008;. To predict evolutionary change in populations, it is therefore crucial to investigate how variable environmental conditions affect heritability (Charmantier & Garant, 2005). To aid in the comparison of heritability estimates across studies, an additional standardized parameter has been suggested: 'evolvability', which is defined as the additive genetic variance of a trait, standardized by the trait value (Houle, 1992).In many animal species, body size is tightly linked to reproductive success and survival, typically with higher fitness in larger individuals (Green & Rothstein, 1991;Kruuk et al., 1999). In contrast, in the long-lived Bechstein's bat (Myotis bechsteinii), larger females exhibit

Climate change may force organisms to adapt genetically or plastically to new environmental conditions. Invasive species show remarkable potential for rapid adaptation. The ovoviviparous New Zealand mud snail (NZMS), Potamopyrgus antipodarum , has successfully established across Europe with two clonally reproducing mitochondrial lineages since its arrival in the first half of the 19th century. Its remarkable variation in shell morphology was shown to be fitness relevant. We investigated the effects of temperature on shell morphology across 11 populations from Germany and the Iberian Peninsula in a common garden across three temperatures. We analyzed size and shape using geometric morphometrics. For both, we compared reaction norms and estimated heritabilities. For size, the interaction of temperature and haplotype explained about 50% of the total variance. We also observed more genotype by environment interactions indicating a higher degree of population differentiation than in shape. Across the three temperatures, size followed the expectations of the temperature‐size rule, with individuals growing larger in cold environments. Changes in shape may have compensated for changes in size affecting space for brooding embryos. Heritability estimates were relatively high. As indicated by the very low coefficients of variation for clonal repeatability ( CV A ), they can probably not be compared in absolute terms. However, they showed some sensitivity to temperature, in haplotype t more so than in z, which was only found in Portugal. The low CV A values indicate that genetic variation among European populations is still restricted with a low potential to react to selection. A considerable fraction of the genetic variation was due to differences between the clonal lineages. The NZMS has apparently not been long enough in Europe to accumulate significant genetic variation relevant for morphological adaptation. As temperature is obviously not the sole factor influencing shell morphology, their interaction will probably not be a factor limiting population persistence under a warming climate in Europe.