A recent study led by researchers from The University of Texas at Austin has unveiled new insights into the intricate evolutionary relationship between temperatures and changes in the size of animal body parts. Mainly focusing on birds, the research provides crucial information on how animals may adapt to the rapid temperature rise brought about by global climate change.

Published in Nature Communications, the study examined nearly 7,000 species of terrestrial nonmigratory birds, which constitute approximately two-thirds of all bird species. The researchers explored how bird morphology evolved in response to gradients in ambient temperature and other selective agents.

The study’s findings confirmed two well-established biological rules. Bergmann’s rule indicates that animals in colder climates tend to have larger bodies to retain heat. In contrast, Allen’s rule states that animals in colder climates have smaller extremities, such as limbs, ears, and beaks, which aid in shedding heat. However, changing body and extremity sizes can present challenges for animals. For instance, reduced body size may hinder birds from hunting the same food as their ancestors, while alterations in bill size could impact their foraging efficiency and mating calls.

Interestingly, the research revealed that some bird families deviate from these rules. The study proposed that these exceptions are due to a combination of complementary changes in body and extremity size. By adopting such an approach, lineages can respond to thermal gradients with more minor changes in each trait than those predicted by Bergmann’s or Allen’s rule alone. This “complementarity” in the two rules was found to be present in most bird families.

Moreover, the study identified that bird families showing significant gradients in either bill or body size tend to be more speciose, widely distributed, or ecologically constrained. These findings validate Bergmann’s and Allen’s rules and emphasize that body and bill size are convenient proxies for the essential quantity of interest—the surface-to-volume ratio.

In conclusion, this research sheds light on how birds adapt to changes in temperature, contributing to our understanding of the subtle but crucial evolutionary mechanisms occurring amidst climate change.

References

• Baldwin, J. W., Garcia-Porta, J., & Botero, C. A. (2023). Complementarity in Allen’s and Bergmann’s rules among birds. Nature Communications, 14(1), 4240. https://doi.org/10.1038/s41467-023-39954-9
• Robards-Forbes, E. (2023, July 19). Relationships between temperature and animals’ sizes has been clarified. UT News. https://news.utexas.edu/2023/07/19/relationships-between-temperature-and-animals-sizes-has-been-clarified/