Plants and plant-eating mammals have been warring for millennia, and with every meal, animals must overcome natural toxins that plants produce as an anti-herbivore defense. However, those compounds become more toxic at warmer temperatures, and mammals – like woodrats who live on a diet heavy in creosote resin – may have to find alternative food sources by either switching diets or relocating elsewhere. The findings are published in Proceedings of the Royal Society B this week. 

Most plants produce chemicals to prevent being eaten, and over evolutionary time, herbivores have developed countermeasures to avoid, metabolize, or tolerate these plant compounds. However, recent work revealed that plant chemicals commonly ingested by mammals ranging from rabbits to free-ranging cows become more toxic at higher temperatures. 

To investigate this so-called “temperature-dependent toxicity,” a University of Utah team led by Patrice Kurnath performed experiments with desert woodrats (Neotoma lepida) collected from the Mojave Desert in southwestern Utah. These fat-hamster-sized herbivorous rodents naturally encounter toxic plant compounds in the creosote bush (Larrea tridentata), which makes up the majority of their diet. In fact, woodrats live on creosote resin amounts that kill or cause kidney cysts in lab rats.

The woodrats were placed in temperature-controlled metabolic cages (pictured to the right) where the team could measure their food and water intake as well as urine and feces production. The woodrats were fed rabbit chow treated with toxic creosote resin extracted from leaves also collected from the Mojave. The team increased the concentration of creosote up to 12 percent over 21 days at warm, cool, and room temperatures. (Any woodrat that loses 10 percent of its body weight was immediately removed from the experiment.) 

Cooler rats ate three times more food than the woodrats at room and warmer temperatures, even though they were eating the same amount of creosote. At warm temperatures, the woodrats ingested significantly less creosote resin: Their maximum dose was two-thirds that of woodrats in the cooler temperature. Furthermore, woodrats in both warm and room temperatures couldn’t persist on the same dose of creosote resin as rodents in the cool temperature. Not only does warmer temperature reduce plant toxin intake, it also reduces tolerance in the rodents. 

The team thinks that liver processing of toxins is reduced at warmer temperatures because woodrats invest more energy into regulating body temperature. "We think that specifically there is some sort of balancing act going on in the liver. As the largest visceral organ, it produces a lot of heat as a byproduct of enzymatic reactions, and that heat helps maintain high body temperatures in mammals," Kurnath explains to IFLScience. "However, our woodrats seem to be facing a choice at higher ambient temperatures: keep eating creosote, which the liver helps to detoxify, or overheat. Mechanistically it is still a black box because we are not sure if it's enzyme reactions that are changing, or gene expression of detox enzymes in the liver, or even changes in blood flow​."

But if slight increases in ambient temperature greatly reduce the ability of woodrats to consume their naturally toxic diet, that means warming temperatures will likely make it harder for any herbivore to eat plant toxins. 

^{Image in the text: Kevin Kohl/University of Utah}