Understanding the Ecological Trap Threatening Winter-Ready Species

Even as autumn days grow shorter, a warming climate means Ohio winters aren't as cold as they were just a few decades ago, according to National Oceanic and Atmospheric Administration data.

This increasing mismatch between hours of daylight and temperature could spell trouble for animals that thrive by relying on seasonal cues to prepare their bodies for winter, according to new research from Case Western Reserve University.

Scientists studying gray tree frogs-a species that survives freezing temperatures by storing "antifreeze" compounds in their bodies-have discovered that these amphibians naturally begin to prepare for winter by responding to shorter days rather than colder temperatures.

This means they may be allocating precious energy resources toward winter preparation they don't actually need. The broader implication is that species with overwintering strategies cued by day length may prepare for winter too soon and respond poorly to climate change."

Michael Benard, professor and chair of the biology department, Case Western Reserve University

The findings were published online in "Early View" in September in the Journal of Animal Ecology.

The research was the result of the BioScience Alliance, an internationally recognized program in biological research and student training involving Case Western Reserve, Cleveland Metroparks Zoo and Holden Arboretum.

Manipulating Nature's Clock

Gray tree frogs survive winter by freezing solid and thawing when the weather warms up. (They do this by storing glycogen in their livers, later converted to glycerol that is pumped throughout their bodies to prevent cells from bursting in freezing temperatures.)

To understand how frogs are cued to store glycogen, researchers manipulated how much daylight tadpoles and later young frogs experienced. They simulated days getting longer (between April and June), days getting shorter (between August and November) and normal day length (July to September) as a control group. All the frogs were kept at the same temperature.

The results were dramatic: Frogs exposed to a simulated fall stored as much as 14 times the amount of glycogen compared to other frogs. Since glycogen is stored in the liver, their livers increased to three to four times the size of frogs in other groups. These winter-prepped frogs were also smaller overall and grew more slowly.

'Ecological Trap' in the Making

Species that adapt their behavior according to day length to prepare for colder temperatures may find themselves caught in what scientists call an "ecological trap" as the climate warms. This ecological trap could affect many critical behaviors, including breeding cycles and migration patterns.

"There was a significant reduction in growth; they're not eating as much, and what they are eating they are allocating toward glycogen storage in the liver as opposed to bone or muscle growth," said Troy Neptune, who led the research as part of his doctorate at Case Western Reserve and will continue his research on light and biology on a Fulbright Fellowship at the Doñana Biological Station in Spain.

"We haven't seen a population crash or anything with gray tree frogs, and they are widely distributed in the U.S.," he said. "But for other species that may be more geographically constrained, this kind of mismatch in the timing of behavior related to the environment could be catastrophic."

Innovative Research Methods

The research was conducted using a unique combination of facilities at Case Western Reserve and the zoo. To manipulate daylight, researchers covered and uncovered outdoor pools with light-blocking covers at the University Farm Biology Research Field Station in Hunting Valley, Ohio. When tadpoles metamorphosed into frogs, they were brought into laboratory settings where lights were automatically controlled.

"This work is a great example of how our unique collaboration is leveraging our collective resources to help solve some of the most pressing conservation challenges," said Diana Koester, adjunct assistant professor of biology at Case Western Reserve and a curator of research at the Cleveland Metroparks Zoo. Koester oversaw research conducted at the zoo, where Neptune used special techniques normally used by scientists to assess zoo animal health and well-being to measure glycogen levels in the frogs' livers.