The contribution of genetic and environmental effects to Bergmann’s rule and Allen’s rule in house mice

Distinguishing between genetic, environmental, and genotype-by-environment effects is central to understanding geographic variation in phenotypic clines. Two of the best-documented phenotypic clines are Bergmann’s rule and Allen’s rule, which describe larger body sizes and shortened extremities in colder climates, respectively. Although numerous studies have found inter- and intraspecific evidence for both ecogeographic patterns, we still have little understanding about whether these patterns are driven by genetics, environment, or both. Here, we measured the genetic and environmental contributions to Bergmann’s rule and Allen’s rule across introduced populations of house mice (Mus musculus domesticus) in the Americas. First, we documented clines for body mass, tail length, and ear length in natural populations, and found that these conform to both Bergmann’s rule and Allen’s rule. We then raised descendants of wild-caught mice in the lab and showed that these differences persisted in a common environment, indicating that they have a genetic basis. Finally, using a full-sib design, we reared mice under warm and cold conditions. We found very little plasticity associated with body size, suggesting that Bergmann’s rule has been shaped by strong directional selection in house mice. However, extremities showed considerable plasticity, as both tails and ears grew shorter in cold environments. These results indicate that adaptive phenotypic plasticity as well as genetic changes underlie major patterns of clinal variation in house mice and likely facilitated their rapid expansion into new environments across the Americas.

Revisiting classic ecogeographical rules, using a widely distributed mouse species (Apodemus draco)

Ecogeographical rules predict an association between specific adaptive morphological/physiological traits and latitude, elevation or cooler climates. Such ecogeographical effects are often expressed most clearly in widely distributed species due to continuous selective adaptation occurring over their geographic range. Based on 40 population sampling sites of 116 adult individuals (female, ; male, ) across an elevational range of 191–2573 m, we tested whether morphological traits accorded with predictions of Bergmann’s rule, Allen’s rule and Hesse’s rule for the South China field mouse (Apodemus draco). The effects of elevation on body size, appendage length and heart size were tested by fitting Linear Mixed-Effects Models. None conformed to Bergmann’s, Allen’s or Hesse’s rule. Clines in body size opposed Bergmann’s rule, and foot and snout length ratios opposed Allen’s rule. We conclude that South China field mice, a widely distributed species, exhibit an acute thermoregulation mechanism in which in colder conditions body sizes decrease – as opposed to altering heart sizes or surface area to volume ratios – requiring less energy to regulate body temperatures. Also, there was a stronger selective pressure to increase partial appendage lengths (i.e., foot and snout) to adapt to the specific environment (e.g. longer period of snow cover, up to 2573 m) rather than on a general shortening of appendages to cope with colder conditions.

Spatial and temporal variation in morphology in Australian whistlers and shrike-thrushes: is climate change causing larger appendages?

Allen’s rule is an ecogeographical pattern whereby the size of appendages of animals increases relative to body size in warmer climates in order to facilitate heat exchange and thermoregulation. Allen’s rule predicts that one consequence of a warming climate would be an increase in the relative size of appendages, and evidence from other bird species suggests that this might be occurring. Using measurements from museum specimens, we determined whether spatio-temporal variation in bills and legs of Australian Pachycephalidae species exhibits within-species trends consistent with Allen’s rule and increases in temperature attributable to climatic warming. We conducted regression model analyses relating appendage size to spatio-temporal variables, while controlling for body size. The relative bill size in four of the eight species was negatively associated with latitude. Tarsus length showed no significant trends consistent with Allen’s rule. No significant increases in appendage size were found over time. Although bill size in some species was positively correlated with warmer temperatures, the evidence was not substantial enough to suggest a morphological response to climatic warming. This study suggests that climate change is not currently driving adaptive change towards larger appendages in these species. We suggest that other adaptive mechanisms might be taking place.

Bergmann’s rule and Allen’s rule in two passerine birds in China

Background Animals that live at higher latitudes/elevations would have a larger body size (Bergmann’s rule) and a smaller appendage size (Allen’s rule) for thermoregulatory reasons. According to the heat conservation hypothesis, large body size and small appendage size help animals retain heat in the cold, while small body size and large appendage size help them dissipate heat in the warm. For animals living in seasonal climates, the need for conserving heat in the winter may tradeoff with the need for dissipating heat in the summer. In this study, we tested Bergmann’s rule and Allen’s rule in two widely-distributed passerine birds, the Oriental Magpie (Pica serica) and the Oriental Tit (Parus minor), across geographic and climatic gradients in China. Methods We measured body size (body mass and wing length) and appendage size (bill length and tarsus length) of 165 Oriental Magpie and 410 Oriental Tit individuals collected from Chinese mainland. We used linear mixed-effect models to assess variation patterns of body size and appendage size along geographic and climatic gradients. Results Oriental Magpies have a larger appendage size and Oriental Tits have a smaller body size in warmer environments. Appendage size in Oriental Magpies and body size in Oriental Tits of both sexes were more closely related to the climates in winter than in summer. Minimum temperature of coldest month is the most important factor related to bill length and tarsus length of male Oriental Magpies, and wing length of male and female Oriental Tits. Bill length and tarsus length in female Oriental Magpies were related to the annual mean temperature and mean temperature of coldest quarter, respectively. Conclusions In this study, Oriental Magpies and Oriental Tits followed Allen’s rule and Bergmann’ rule respectively. Temperatures in the winter, rather than temperatures in the summer, drove morphological measurements in Oriental Magpies and Oriental Tits in Chinese mainland, demonstrating that the morphological measurements reflect selection for heat conservation rather than for heat dissipation.