Tree frogs freeze themselves for a winter that never comes

Autumn in Ohio still brings earlier sunsets, but winters aren’t as biting as they were a few decades ago. That growing disconnect – steady day-length cues and milder cold – could spell trouble for wildlife, like frogs, that time key life processes by light rather than temperature.

New research from Case Western Reserve University shows gray tree frogs begin gearing up for winter based on day length alone, not falling temperatures. In a warming climate, that means they may pour precious energy into winter preparation they no longer need – and may never get back.

Frogs built to survive solid ice

Gray tree frogs are champions of the deep freeze. As winter approaches, they stockpile glycogen in the liver, then convert it to glycerol – an “antifreeze” that protects their cells when the frogs literally freeze solid. Come spring, they thaw and hop off as if nothing happened.

The new study reveals the trigger for this remarkable makeover isn’t cold air – it’s shorter days. Because the length of daylight doesn’t change with climate warming, the frogs’ biological clock can push them into winter mode even when temperatures stay relatively mild.

Study senior author Michael Benard is a professor of biology at Case Western Reserve University.

“This means they may be allocating precious energy resources toward winter preparation they don’t actually need,” said Benard. “Species with overwintering strategies cued by day length may prepare for winter too soon and respond poorly to climate change.”

When sunlight sets the clock

To pinpoint the cue, researchers manipulated the daily light exposure of tadpoles and newly metamorphosed frogs while keeping temperatures constant.

One group experienced a “lengthening day” schedule (mimicking April–June), another a “shortening day” schedule (August–November), and a control group the relatively steady photoperiod of midsummer (July–September). Everything else – water, food, temperature – stayed the same.

The results were striking: frogs exposed to the simulated fall photoperiod hoarded up to 14 times more glycogen than the other groups.

Because glycogen is stored in the liver, their livers ballooned to three to four times the size of their peers. The trade-off was immediate and costly: these winter-primed frogs grew more slowly and ended up smaller overall.

Frogs frozen by false alarms

That energy reallocation – away from growth and toward antifreeze stockpiles – makes perfect sense when bitter cold is guaranteed.

But if winters keep warming, frogs cued by day length alone could be walking into an “ecological trap”: making the right decision for the wrong environment. They’ll eat less, invest in liver stores, and sacrifice muscle and bone growth for a freeze that may never come.

“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 lead author Troy Neptune, who conducted the work as a doctoral student at Case Western Reserve.

Gray tree frogs are widespread and not crashing, the team notes. But for species with restricted ranges – or tighter energy budgets – the same mismatch could be devastating, derailing breeding cycles, migration timing, and survival in ways that ripple through entire ecosystems.

From ponds to lab lights

The project leveraged an unusual blend of outdoor and indoor facilities through the BioScience Alliance, a research and training partnership among Case Western Reserve University, Cleveland Metroparks Zoo, and Holden Arboretum.

At the University Farm Biology Research Field Station, the team used light-blocking covers on outdoor pools to dial day length up or down without changing temperature. Once tadpoles became froglets, they moved indoors to precisely controlled light regimes.

Cleveland Metroparks Zoo collaborators applied clinical techniques typically used to monitor zoo animal health to quantify liver glycogen, giving the study a high-resolution look at the frogs’ internal fuel economy.

“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, an adjunct assistant professor of biology at Case Western Reserve University and a curator of research at the zoo.

Frog clocks won’t change

Photoperiod is among the most reliable signals on Earth: it’s fixed by planetary tilt and orbit, immune to year-to-year weather swings and long-term climate trends.

Evolution has wired many organisms to use day length to schedule costly transitions – migration, breeding, diapause, hibernation – because it’s so dependable.

The trouble begins when the environmental conditions those schedules were designed to anticipate (like deep cold) shift out of sync. Climate warming breaks the link between the cue and the consequence; the calendar stays the same even as winter grows shorter and gentler.

Broader warning beyond frogs

The study’s message reaches far beyond a single amphibian. Any species that primes for winter based on day length could face similar inefficiencies – birds that molt too early, insects that enter diapause unnecessarily, mammals that fatten for a freeze that never arrives.

Each case represents an energetic gamble; in lean years, those gambles can determine who survives to reproduce.

For wildlife managers and conservation planners, the implications are clear. It’s not enough to track temperatures alone. Understanding – and where possible, mitigating – cue–environment mismatches will be key to helping species navigate rapid climate change.

That could mean protecting high-quality foraging habitat in late summer and fall to offset energy trade-offs, or preserving and restoring microhabitats that still deliver the cold snaps some species need to synchronize life stages.

The future of frog time

The team’s approach – combining controlled photoperiod experiments with clinically precise metabolic measures – opens the door to testing additional species and life stages.

It also points toward practical monitoring: if livers are swelling and growth stalling under warm autumns, managers may have an early warning that photoperiod-driven traps are forming on the landscape.

The sky’s clock won’t change. Our climate already has. Studies like this one show what happens when those clocks fall out of sync – and how urgently we need to help wildlife keep time.

The study is published in the Journal of Animal Ecology.

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