When people or animals are thrust into threatening situations such as combat or attack by a predator, stress hormones are released to help prepare the organism to defend itself or to rapidly escape from danger — the so-called fight-or-flight response.
Now U-M researchers have demonstrated for the first time that stress hormones are also responsible for altering the body shape of developing animals, in this case the humble tadpole, so they are better equipped to survive predator attacks.
Through a series of experiments conducted at field sites and in the laboratory, researchers found that prolonged exposure to a stress hormone enabled tadpoles to increase the size of their tails, which improved their ability to avoid lethal predator attacks.
“This is the first clear demonstration that a stress hormone produced by the animal can actually cause a morphological change, a change in body shape, that improves their survival in the presence of lethal predators. It’s a survival response,” said Robert Denver, a professor of molecular, cellular and developmental biology and of ecology and evolutionary biology.
The team’s surprising findings are detailed in a paper published online March 5 in the journal Proceedings of the Royal Society B. The first author of the paper is Jessica Middlemis Maher, a former U-M doctoral student, now at Michigan State University, who conducted the work for her dissertation.
Scientists have long known that environmental changes can prompt animals and plants to alter their morphology and physiology, as well as the timing of developmental events. For example, tadpoles can accelerate metamorphosis into frogs in response to a drying pond, a high density of predators or a lack of food.
The term “phenotypic plasticity” is used to describe modifications by animals and plants in response to a changing environment.
“There’s been a lot of interest in phenotypic plasticity among developmental biologists and evolutionary ecologists for more than 70 years, but there’s been relatively little focus on the mechanisms by which the environmental signal is translated into a functional response,” Denver said.
“We’ve known, for example, that tadpoles can change their body shape in response to predation risk. But until now, nobody knew the basic physiological mechanisms mediating that response. That’s what’s novel about this study.”
The study involved wood frog tadpoles and the stress hormone corticosterone, which is similar to the human stress hormone cortisol. Tadpoles were collected from ponds at U-M’s E.S. George Reserve in Pinckney, northwest of Ann Arbor.
The third author of the Proceedings of the Royal Society B paper is Earl Werner, director of the E.S. George Reserve and a professor in the Department of Ecology and Evolutionary Biology.
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