Tiny bat bodies provide the perfect balance between flight costs and heat dissipation

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Many species of mammals living in cold climates tend to have large bodies and short limbs to reduce heat loss – a general pattern known as Bergmann’s rule. However, bats are the exception to the rule, displaying small body sizes in both hot and cold regions.

A McGill-led team of researchers is shedding light on this long-running debate over bat body size and focusing on why bats appear inconsistent with ecogeographical patterns found in other mammals. Their findings offer a new method for studying complex macroecology across bat species.

The (energy) cost of the flight

“Measuring energy requirements for temperature regulation and locomotion in wild animals is a challenge. In bats, this is compounded by the fact that some key characteristics such as metabolic rate and body temperature vary dramatically when individuals are captured to make these measurements,” says Juan G. Rubalcaba, Marie Skłodowska-Curie Postdoctoral Fellow in McGill’s Department of Biology.

“As a result, we know little about the energetic processes driving the evolution of morphology, foraging strategies, and other traits that make bats a unique group of mammals.”

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

The team performed an analysis of the wing surface-to-mass ratio in nearly 300 species of bats and their results confirmed the model’s prediction that body shape evolves towards an optimal ratio. High surface areas relative to body mass increase heat dissipation rates and thus the cost of maintaining optimal body temperature, while high body mass increases the cost of flight.

Small, but mighty

Additionally, research found that the selection force towards the optimal ratio was relatively high in species from cold climates, where thermoregulation costs are higher. These results suggest that selective forces that act to reduce the energy costs of flight prevent cold-region bat species from having large body sizes to help maintain body heat.

“Understanding the factors and processes driving the evolution of organisms is key to predicting their responses to environmental changes,” says Dr. Rubalcaba, lead author of the study. “We hope that our new approach for calculating the energy requirements of bats in relation to climatic conditions can be applied to study the physiological impacts of climate change on all species of bats.”

The research has been published in Proceedings of the National Academy of Sciences.


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More information:
Juan G. Rubalcaba et al, Physical Constraints on Thermoregulation and Morphological Evolution of the Flight Drive in Bats, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2103745119

Provided by McGill University


Quote: Tiny bat bodies strike the perfect balance between flight costs and heat dissipation (2022, July 21) Retrieved July 21, 2022 from https://phys.org/news/2022-07-tiny- bodies-flight-dissipation.html

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