Utah researchers learn how rodents manage toxic diets
Study » Species of wood rat eats fewer meals, drinks more water.
A white-throated woodrat carries a sprig of toxic juniper as it runs among rocks at night. This rat species normally eats several toxic plant species in amounts too small to cause poisoning. University of Utah researchers found that when the white-throated woodrat is given a diet with too much of one toxic plant, juniper, it limits its food intake and drinks more water so it doesn't become ill. Courtesy Michal Samuni, University of Utah
Posted 09 August 2011, by Brian Maffly, The Salt Lake Tribune (MediaNews Group), sltrib.com
Plants have evolved the ability to produce toxic compounds to ward off hungry animals, but evolution is a two-way street. A new study by the University of Utah shows that certain desert species of wood rat, which inhabit landscapes that are particularly toxic for rodents, adapt by regulating their eating patterns.
They nibble on a greater array of plants, eat smaller meals, increase time between meals, and drink more water, according to research published Tuesday in the British Ecological Society’s journal Functional Ecology.
“For decades, we have been trying to understand how herbivores deal with toxic diets,” said Denise Dearing, a U. biology professor. “This study compares wood rats that eat only a single plant — juniper — with another species that eats several kinds of plants, including a small amount of juniper. We found that the wood rat that eats many types of plants was better at limiting toxin intake than the wood rat that eats only juniper.”
The juniper-specialist in Dearing’s study was the Stephen’s wood rat or Neotoma stephensi, collected in Arizona’s Wupatki National Monument, and the “generalist” — the one with the diverse diet — was the white-throated wood rat or Neotoma albigulu, collected in Utah’s Castle Valley.
“In some habitats they are found side by side and the are the same body size so they make a great comparison,” Dearing said.
Stephen’s wood rats have powerful enzymes in their livers that allow them to metabolize the juniper toxins, while generalist species are able to metabolize only small amounts of different plant toxins.
Desert plants, which form the cornerstone of rodent diets, are chockful of poisons that disrupt the nervous system, stymie the absorption of nutrients and cause dehydration. Juniper is rich in a class of toxins called terpenes, including those in turpentine — the solvent used in oil-based paints. These toxins induce water loss, a lethal condition in arid environments.
Dearing and her colleagues fed their wood rats, 11 white-throated and seven Stephen’s wood rats, a diet of increasing concentrations of juniper over a 15-day period. They were allowed unlimited access to water. The specialist Stephen’s wood rat, as expected, stayed healthy without changing its behavior, while their white-throated cousins lost weight in response to elevated terpene levels.
“It’s important to note that in nature they are not going to have an opportunity to find fresh water. Most of the fresh water they get is from the plants they eat,” Dearing said.
The researchers documented how these rats avoided illness by cutting their food intake in half, eating 7 percent fewer meals, increasing time between meals by 10 percent, and doubling their water intake.
So how do the generalist wood rats know when to adjust their eating? Dearing does not believe they eat less in response to feeling sick.
“We think there might be something in their guts, like a poison counter, that can signal to their brain to eat less,” Dearing said. “The response to a food aversion is usually long lasting. These animals go back and eat the same thing day after day.”
Dearing’s co-authors are U. graduate student Ann-Marie Torregrossa, now a postdoctoral fellow at Florida State University, and Anthony Azzara of Bristol-Myers Squibb, the New Jersey-based pharmaceutical maker. The National Science Foundation and the American Museum of Natural History funded the research.
Copyright 2011 The Salt Lake Tribune
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