Animals with high body temperatures often become herbivores

A recent study led by the University of Arizona has discovered a clear link between the body temperature of animals and their likelihood of evolving into herbivores. 

The research offers insights into the evolution of dietary habits among tetrapods, which include land dwelling vertebrates such as amphibians, birds, reptiles and mammals. The surprising results have the potential to significantly challenge existing beliefs about how animal diets evolve over time.

Body temperature and herbivory

By analyzing data from 1,712 species, the experts discovered a consistent trend: animals with higher body temperatures are more prone to evolving into herbivores. This trend was observed across all major groups of land vertebrates.

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

Why certain species are herbivores

Wiens suggested that higher body temperatures might be essential for supporting the gut bacteria that break down cellulose, the primary component of plant cell walls. 

According to Wiens, the connection between an animal’s body temperature and its gut microbiome could be essential to understanding why certain species are more capable of adopting and sustaining 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,” added 

lead author Kristen Saban, who began the research as a sophomore majoring in ecology and evolutionary biology at the University of Arizona.

Factors that influence diet evolution 

The research team also considered other factors that could influence diet evolution, such as body size and the animal’s activity patterns, whether diurnal or nocturnal. Despite these considerations, body temperature emerged as the most significant predictor of 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.

While previous research has examined how an herbivorous diet might affect the evolution of body temperature, this study took a different approach by investigating what factors influence the evolution of herbivory.

Evolution of herbivores

The research also provides insights into the evolutionary timeline of herbivory. Although plant-eating animals are now common, Wiens pointed out that herbivory appears to be a relatively recent development in evolutionary history. 

Tetrapods originally had carnivorous diets; for example, lizards were traditionally meat-eaters, whereas iguanas have evolved to become herbivores. Notably, the herbivorous groups that exist today have all emerged within the last 110 million years, despite tetrapods having existed for 350 million years.

This timeline coincides with the rise of flowering plants (known as angiosperms), which became dominant around 110 million years ago and now constitute 90% of all plant species.

Instability of herbivory over time 

Another intriguing discovery from the study is the evolutionary instability of herbivory. The researchers observed that many species reverted from herbivory back to carnivory. 

Wiens cited Latin American singing mice as an example, noting that these animals evolved from herbivorous ancestors but now primarily consume insects. According to the study, these reversals were as frequent as the initial shifts to herbivory. In Saban’s view the reasons behind this instability remain unclear.

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

Body temperature and herbivory evolution

“Using phylogenetic logistic regression, we support the hypothesis that higher body temperature helps explain the evolution of herbivory across tetrapods and within birds, mammals, lepidosaurs and turtles,” wrote the study authors. 

“Phylogenetic path analyses suggest that body temperature generally drives the evolution of herbivory, and not vice versa. Our analyses also suggest that body temperature is more important for the evolution of herbivory than large body size or diurnal diel activity, which are both significant predictors of herbivory in some cases.”

The study is published in the journal Global Ecology and Biogeography.

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