Behavior can impact biogeographical patterns.
I have always been fascinated by biogeographical patterns, such as Gloger’s Rule (i.e. darker animals in more humid environments) or Bergmann’s Rule (i.e. bigger animals in colder climates). Numerous studies have reported taxa that follow these rules, but the underlying mechanisms remain unclear. Bergmann’s Rule, for example, is mostly explained by the thermoregulatory benefits of having a lower surface-to-volume ratio. In other words, bigger animals are better at conserving heat. An alternative explanation suggests that bigger animals might be more resilient to starvation in harsher climates.
One aspect that has been largely ignored in the study of Bergmann’s Rule is behavior. Particular behaviors might help species cope with cold conditions, and consequently affect their adherence (or not) to Bergmann’s Rule. In a recent Ecology and Evolution study, Mark Mainwaring and Sally Street took a closer look at the impact of nest design and migration in Western Palearctic birds. Do these traits impact the conformity to Bergmann’s Rule?
Migration and Nest Design
Overall, the researchers found support for Bergmann’s Rule in Western Palearctic birds. The species’ body mass increased with breeding range latitude. When they added nest design and migration to the statistical models, some interesting patterns emerged. Whether a group of species followed Bergmann’s Rule (i.e. a correlation between body size and climate) depended on their migratory behavior and their nest design. Migration showed the strongest impact.
We find that body mass increases in colder temperatures only among sedentary species, while short- and long-distance migrants do not conform to Bergmann’s rule at all. These findings support the idea that migrating Western Palearctic species are completely buffered against selection for large body size in colder climates as they avoid exposure to the coldest winter temperatures at high latitudes by spending the nonbreeding season in warmer environments in southern Europe or Africa.
Within the sedentary species, nest design also played an important role. Species that build an open nest showed the strongest relationship between body mass and climate, whereas this pattern was weaker in species with semi-open or enclosed nests.
Niche Construction
Taken together, these patterns indicate that birds can not only adapt to colder climates by a large body size (and thus following Bergmann’s Rule), but also by building enclosed nests or migrating to other regions. Species that don’t migrate or that build open nests are exposed to colder conditions, resulting in natural selection for larger body size. Behavior matters in conformity to Bergmann’s Rule.
The idea that species can impact their own evolutionary trajectory through their behavior fits within “niche construction theory“. A classic example concerns the beaver’s dam which can create lakes that drastically change riparian ecosystems. The importance of niche construction in evolutionary theory is currently a matter of debate, specifically between advocates of an extended evolutionary theory and proponents of the “standard” evolutionary theory. An interesting discussion that I will leave for another blog post.
References
Mainwaring, M. C., & Street, S. E. (2021). Conformity to Bergmann’s rule in birds depends on nest design and migration. Ecology and Evolution, 11(19), 13118-13127.
Featured image: A bird nest © Bugaga | Wikimedia Commons
Avian Hybrids 