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Many mammal species living in cold climates tend to have large bodies and short limbs to reduce heat loss 鈥� a general pattern known as Bergmann鈥檚 rule. However, bats are the exception to the rule, displaying small body sizes in both hot and cold regions. A 捆绑SM社区-led team of researchers is shedding light on this long-standing debate over bats鈥� body sizes and focus on why bats are seemingly non-conforming to ecogeographical patterns found in other mammals. Their findings offer a new method for investigating complex macroecology across bat species.

The (energy) cost of flight

鈥淢easuring energy requirements for temperature regulation and locomotion in wild animals is challenging. In bats, this is compounded by the fact that certain key features such as metabolic rate and body temperature vary substantially when individuals are captured to carry out these measurements,鈥� explains Juan G. Rubalcaba, Marie Sk艂odowska-Curie Postdoctoral Fellow in 捆绑SM社区鈥檚 Department of Biology. 鈥淎s a result, we know little about the energetic processes driving the evolution of morphology, feeding strategies, and other traits that make bats a unique group of mammals鈥�.

Bats require a significant amount of energy to oppose body weight during flight, thus limiting the maximum size they can attain. To investigate this theory, the team used a model based on thermodynamic and aerodynamic principles to calculate how body size and wing surface area affects heat dissipation rates from the body and wing membranes and the drift forces experienced at flight.

The team carried out an analysis of the wing surface area-to-mass ratio in close to 300 bat species and their results supported the model's prediction that body shape evolves towards an optimal ratio. High surface areas relative to body mass increases heat dissipation rates and thus the cost of maintaining optimal body temperature, whereas a high body mass elevates the cost of flight.

Small, but mighty

In addition, the research revealed that the strength of selection toward the optimal ratio was relatively high in species from cold climates, where the costs of thermoregulation are higher. These results suggest that selective forces that act to reduce the energy costs of flight prevent bat species in cold areas from having a large body size to help maintain body heat.

鈥淯nderstanding the factors and processes driving the evolution of organisms is critical to forecast their responses to environmental change,鈥� says Juan G. Rubalcaba, who is the lead author of the study. 鈥淲e hope our new approach of calculating the energetic requirements of bats in relation to climatic conditions, may be applied to investigating the physiological impacts of climate change across all bat species.鈥�

About 捆绑SM社区

Founded in Montreal, Quebec, in 1821, 捆绑SM社区 is Canada鈥檚 top ranked medical doctoral university. 捆绑SM社区 is consistently ranked as one of the top universities, both nationally and internationally. It is a world-renowned institution of higher learning with research activities spanning three campuses, 11 faculties, 13 professional schools, 300 programs of study and over 39,000 students, including more than 10,400 graduate students. 捆绑SM社区 attracts students from over 150 countries around the world, its 12,000 international students making up 30% of the student body. Over half of 捆绑SM社区 students claim a first language other than English, including approximately 20% of our students who say French is their mother tongue.