Life history and habitat do not mediate temporal changes in body size due to climate warming in rodents

Morphological variations of auditory bullae in otomyine rodents (Rodentia: Otomyini) in southern African biomes

Mammalian middle ear cavities differ from those of other taxa as they comprise three ossicles and in rodents, can be encapsulated by an auditory bulla. In small mammals, the middle ear cavity (bulla) was found to be enlarged in the desert-dwelling species; however, differences in bullar size could have been due to ancestry. In this study, we sampled seven species from three genera (Myotomys, Otomys, and Parotomys) of the African murid tribe Otomyini (laminated-toothed rats), and compared the bullar volumes and shapes between the otomyine species and within the species Myotomys unisulcatus. Photographs of museum skull specimens were taken from ventral and lateral views, and the volumes of the bullae were estimated digitally from the photographs. No sexual dimorphism in bullar volumes was found in any of the species. Corrected bullar volumes were significantly different between species and larger bullae were seen in individuals inhabiting regions with lower annual rainfall. Bullar shape (estimated using geometric morphometrics) was significantly different between the genera and the species. Parotomys have tympanic meatuses that face more anteriorly compared to both, Otomys and Myotomys. When comparing bullae within M. unisulcatus, those inhabiting regions with lower annual rainfall had significantly larger bullar volumes, but no significant difference was found in bullar shape between the regions. This study shows that otomyine rodents in more xeric habitats have different auditory structures to those inhabiting wetter habitats.

Counter-gradient variation and the expensive tissue hypothesis explain parallel brain size reductions at high elevation in cricetid and murid rodents

To better understand functional morphological adaptations to high elevation (> 3000 m above sea level) life in both North American and African mountain-associated rodents, we used microCT scanning to acquire 3D images and a 3D morphometric approach to calculate endocranial volumes and skull lengths. This was done on 113 crania of low-elevation and high-elevation populations in species of North American cricetid mice (two Peromyscus species, n = 53), and African murid rodents of two tribes, Otomyini (five species, n = 49) and Praomyini (four species, n = 11). We tested two distinct hypotheses for how endocranial volume might vary in high-elevation populations: the expensive tissue hypothesis, which predicts that brain and endocranial volumes will be reduced to lessen the costs of growing and maintaining a large brain; and the brain-swelling hypothesis, which predicts that endocranial volumes will be increased either as a direct phenotypic effect or as an adaptation to accommodate brain swelling and thus minimize pathological symptoms of altitude sickness. After correcting for general allometric variation in cranial size, we found that in both North American Peromyscus mice and African laminate-toothed (Otomys) rats, highland rodents had smaller endocranial volumes than lower-elevation rodents, consistent with the expensive tissue hypothesis. In the former group, Peromyscus mice, crania were obtained not just from wild-caught mice from high and low elevations but also from those bred in common-garden laboratory conditions from parents caught from either high or low elevations. Our results in these mice showed that brain size responses to elevation might have a strong genetic basis, which counters an opposite but weaker environmental effect on brain volume. These results potentially suggest that selection may act to reduce brain volume across small mammals at high elevations but further experiments are needed to assess the generality of this conclusion and the nature of underlying mechanisms.

Long-term field study reveals that warmer summers lead to larger and longer-lived females only in northern populations of Natterer's bats

Animals often respond to climate change with changes in morphology, e.g., shrinking body size with increasing temperatures, as expected by Bergmann's rule. Because small body size can have fitness costs for individuals, this trend could threaten populations. Recent studies, however, show that morphological responses to climate change and the resulting fitness consequences cannot be generalized even among related species. In this long-term study, we investigate the interaction between ambient temperature, body size and survival probability in a large number of individually marked wild adult female Natterer's bats (Myotis nattereri). We compare populations from two geographical regions in Germany with a different climate. In a sliding window analysis, we found larger body sizes in adult females that were raised in warmer summers only in the northern population, but not in the southern population that experienced an overall warmer climate. With a capture-mark-recapture approach, we showed that larger individuals had higher survival rates, demonstrating that weather conditions in early life could have long-lasting fitness effects. The different responses in body size to warmer temperatures in the two regions highlight that fitness-relevant morphological responses to climate change have to be viewed on a regional scale and may affect local populations differently.

Fifty years of data show the effects of climate on overall skull size and the extent of seasonal reversible skull size changes (Dehnel's phenomenon) in the common shrew

Global climate change affects many aspects of biology and has been shown to cause body size changes in animals. However, suitable datasets allowing the analysis of long‐term relationships between body size, climate, and its effects are rare. The size of the skull is often used as a proxy for overall body size. Skull size does not change much in fully grown vertebrates; however, some high‐metabolic small mammals shrink in winter and regrow in spring, including their skull and brain. This is thought to be a winter adaptation, as a smaller brain size reduces energy requirements. Climate could thus affect not only the overall size but also the pattern of the size change, that is, Dehnel's phenomenon, in these animals. We assessed the impact of the changes in climate on the overall skull size and the different stages of Dehnel's phenomenon in skulls of the common shrew, Sorex araneus, collected over 50 years in the Białowieża Forest, E Poland. Overall skull size decreased, along with increasing temperatures and decreasing soil moisture, which affected the availability of the shrews' main food source, earthworms. The skulls of males were larger than those of females, but the degree of the decrease in size did not differ between sexes. The magnitude of Dehnel's phenomenon increased over time, indicating an increasing selection pressure on animals in winter. Overall, climate clearly affected the common shrew's overall size as well as its seasonal size changes. With the current acceleration in climate change, the effects on the populations of this cold‐adapted species may be quite severe in a large part of its distribution range.

Mammalian body size is determined by interactions between climate, urbanization, and ecological traits

Anthropogenically-driven climate warming is a hypothesized driver of animal body size reductions. Less understood are effects of other human-caused disturbances on body size, such as urbanization. We compiled 140,499 body size records of over 100 North American mammals to test how climate and human population density, a proxy for urbanization, and their interactions with species traits, impact body size. We tested three hypotheses of body size variation across urbanization gradients: urban heat island effects, habitat fragmentation, and resource availability. Our results demonstrate that both urbanization and temperature influence mammalian body size variation, most often leading to larger individuals, thus supporting the resource availability hypothesis. In addition, life history and other ecological factors play a critical role in mediating the effects of climate and urbanization on body size. Larger mammals and species that utilize thermal buffering are more sensitive to warmer temperatures, while flexibility in activity time appears to be advantageous in urbanized areas. This work highlights the value of using digitized, natural history data to track how human disturbance drives morphological variation.

Anthropogenically-driven climate change is responsible for body size decreases in mammals. Using an important dataset of historically-collected data and data from continental-scale survey efforts from the National Ecological Observatory Network, Hantak et al. show that urbanization plays an important role in mediating this dynamic.