Animals with higher body temperatures more likely to become herbivores, study finds

A University of Arizona study has found a surprising relationship between an animal’s body temperature and its likelihood of evolving into a herbivore. The study, published in the journal Global ecology and biogeographyoffers new insights into the evolution of plant-based diets in tetrapods, which include terrestrial vertebrates (amphibians, birds, reptiles, and mammals). These findings could reshape scientists’ understanding of the evolution of animal diets.

The study, which analyzed data from 1,712 species, found a consistent trend: animals with higher body temperatures are more likely to become herbivores. This relationship holds true for all major groups of land vertebrates.

The relationship between body temperature and herbivory is related to the unique digestive challenges posed by a plant-based diet, said study lead author John Wiens, a professor of ecology and evolutionary biology at the University of Alberta.

Higher body temperatures may be needed to support gut bacteria that break down cellulose, the main component of plant cell walls. The relationship between an animal’s body temperature and its gut microbiome could be key to understanding why some species are better equipped to adopt and maintain a herbivorous diet, Wiens said.

“We were particularly interested in body temperature because it is one of the most widespread and comparable factors that may determine diet across different groups of animals,” said Kristen Saban, the study’s lead author. Saban was a second-year student at the University of Alberta specializing in ecology and evolutionary biology when she began the study.

The research team conducted extensive analyses, examining a variety of other factors that could influence diet change, including body size and diurnal or nocturnal activity patterns. Body temperature ultimately emerged as the most crucial factor in predicting how a herbivore’s diet would change.

“We found that body temperature is sort of a necessary condition for the evolution of a herbivorous diet. We haven’t really seen herbivores that didn’t have a high body temperature. Typically, the body temperature was above 30 degrees Celsius,” Saban said.

Some previous studies have looked at how a herbivorous diet might have influenced the evolution of body temperature in animals, Saban said. This study looked at it the other way around, trying to find the factors that led to the evolution of herbivory.

The researchers compiled relevant data from dozens of previous studies and conducted their analyses. The study also sheds light on the evolutionary timeline of herbivory. Contrary to what one might expect given the current prevalence of herbivorous animals, herbivory appears to be a relatively recent diet, Wiens said. Tetrapods are ancestral carnivores. Lizards, for example, had meat as a major part of their diet, while iguanas evolved to be herbivores. The herbivorous groups that exist today are all less than 110 million years old, he said, although tetrapods are 350 million years old.

The discovery coincides with the spread of flowering plants, angiosperms, which became dominant about 110 million years ago and now make up about 90 percent of all plant species. Overall, these findings open up new avenues for future research, especially since older herbivorous lineages, such as plant-eating dinosaurs, have not persisted to the present day, Wiens said.

The study also found that herbivory is an evolutionarily unstable trait. The researchers observed many instances of species switching from herbivory to carnivory. Wiens said that Latin American singing mice, for example, evolved from their herbivorous ancestors to feed primarily on insects. These reversals were just as common as the origins of herbivory. The reason for this instability remains unknown, Saban said.

“Once a species evolves, it doesn’t necessarily stay around for very long,” Wiens said. “It’s possible that some of the groups that are herbivores today will become animal-eating species again.”