Tucson Standard

A University of Arizona study has uncovered a relationship between an animal's body temperature and its likelihood of evolving into an herbivore. The study, published Monday in the journal Global Ecology and Biogeography, offers insights into the evolution of plant-based diets across tetrapods, which include amphibians, birds, reptiles, and mammals. The findings could reshape scientists' understanding of the evolution of animal diets.

The study analyzed data from 1,712 species and found a consistent pattern: animals with higher body temperatures are more likely to evolve into herbivores. This relationship holds true across major land vertebrate groups.

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

"Body temperature may be necessary to support the gut bacteria that break down cellulose, the primary component of plant cell walls," Wiens explained. "The relationship between an animal's body temperature and its gut microbiome could be key to understanding why certain species are better equipped to adopt and maintain herbivorous diets."

"We were specifically interested in body temperature because it is one of the most widespread and comparable factors that might determine diet across different groups of animals," said Kristen Saban, the study's lead author. Saban was a sophomore majoring in ecology and evolutionary biology when she began the study.

The research team conducted extensive analyses examining various other factors that might influence diet evolution, including body size and activity patterns. Body temperature emerged as the most crucial factor in predicting an herbivorous diet.

"What we found was that body temperature is somewhat of a requirement to evolve an herbivorous diet. We really didn't see any herbivores that don't have a high body temperature. Typically, the body temperature was more than 86 degrees Fahrenheit," Saban said.

Some previous studies have looked at how an herbivorous diet may have influenced the evolution of body temperature in animals. This study examined what factors drove the evolution of herbivory.

Researchers compiled relevant data from dozens of previous studies for their analyses. The study also sheds light on the evolutionary timeline of herbivory. Contrary to expectations given today's prevalence of plant-eating animals, herbivory appears relatively recent. Tetrapods are ancestrally carnivorous; for example, lizards had meat as a significant part of their diet while iguanas evolved to be herbivores. Herbivorous groups existing now are all less than 110 million years old despite tetrapods being 350 million years old.

This timing coincides with flowering plants—angiosperms—becoming dominant around 110 million years ago and constituting about 90% of all plant species today. Overall, these findings open new avenues for future research, especially considering older herbivorous lineages like dinosaurs haven't persisted to modern times.

The study also revealed that herbivory is unstable in evolutionary terms. Researchers observed numerous instances where species reverted from herbivory back to carnivory. For example, singing mice from Latin America evolved from their herbivorous ancestors to feed mostly on insects. These reversals were as common as origins of herbivory; reasons for this instability remain unknown.

"Once it evolves, it doesn't necessarily stick around very long," Wiens said. "It's possible that some groups that are herbivores today will evolve species that feed on animals again."