Do Green Tree Frogs Hibernate? A Detailed Look At Their Winter Behavior

If you live in an area with cold winters, you may have wondered what happens to your local green tree frogs when temperatures start to drop. Do these noisy amphibians hole up somewhere and sleep away the winter like bears? Or do they stick around and endure the chill?

The answer is complicated, but here’s a quick summary: green tree frogs don’t truly hibernate, but they do go through periods of dormancy called brumation to survive the colder months.

In this comprehensive guide, we’ll take an in-depth look at brumation in green tree frogs, examining how it differs from true hibernation, what environmental cues trigger it, where the frogs go to brumate, and how their metabolism and other bodily functions change during this dormant period.

We’ll also discuss the risks brumation poses for green tree frogs and what you can do to support local populations in your area.

What Is Brumation and How Is It Different from Hibernation?

Brumation Is Not the Same as True Hibernation

Brumation refers to the winter dormancy period exhibited by some cold-blooded animals like reptiles and amphibians, including green tree frogs. It is similar to hibernation in mammals, but there are some key differences.

While hibernating mammals truly sleep through the winter, reptiles and amphibians in brumation remain alert enough to emerge for food or water if needed.

During brumation, green tree frogs experience a drastic slowdown in metabolic activity. Their heart rate, breathing rate, and other bodily functions decrease to conserve energy. However, their bodies do not drop to extremely low temperatures like those of hibernating mammals.

Key Differences Between Brumation and Hibernation

Here is a comparison of some of the main differences between brumation in reptiles/amphibians and true hibernation in mammals:

As the table shows, the key thing distinguishing brumation from true hibernation is that the decrease in physiological activity during brumation is not as extreme. Reptiles and amphibians keep their bodies ready to become active if conditions call for it.

What Triggers Brumation in Green Tree Frogs?

As temperatures drop and the days shorten in late fall and early winter, the bright green tree frogs that enlivened the warm summer months begin to slow down. Going into a winter resting state called brumation allows these frogs to conserve their energy and resources during the cold months when food is scarce and temperatures too low for activity.

Shortening Day Length

The green tree frog’s brumation cycle is heavily influenced by the shortening periods of daylight. As the fall months progress, frogs perceive less sunlight exposure and their pineal gland begins to secrete more of the hormone melatonin.

Elevated melatonin levels signal it is time for the frogs to prepare for winter dormancy by slowing their metabolism and other physiological processes.

Cooling Temperatures

In addition to shortening days, the steady decline in ambient temperatures from September through November further cues the tree frogs to begin brumation. Studies show sustained cold between 40-60°F triggers hormonal signals for the frogs to start decreasing activity and food intake, and seek protected shelter for the winter rest period.

Limited Food Availability

The drop in temperature limits cold-blooded frogs’ ability to catch insect prey, depleting their main summertime food source. This scarcity of prey animals to sustain the high-metabolism tree frogs further convinces them to begin energy-conserving brumation.

By November, most small insects have died off or become dormant themselves, leaving frogs little choice but to follow suit and minimize nutritional needs until spring brings warmer weather and renewed food supplies.

By responding to all three influences – shortening days, cooling weather, and scarce food sources – bright green tree frogs demonstrate an impressive ability to adapt to seasonal changes. Their brumation behaviors allow survival through harsh winters until spring brings a renewal of ideal warm, prey-rich conditions.

Where Do Green Tree Frogs Go to Brumate?

When winter approaches and temperatures start to drop, green tree frogs (Hyla cinerea) prepare for brumation by seeking out shelter in hollow logs, tree cavities, leaf litter, and even underground burrows.

These sheltered spots protect them from the elements while allowing their bodies to enter a dormant state similar to hibernation.

Hollow Logs and Tree Cavities

The most popular overwintering spots for green tree frogs are hollow logs and holes or cavities in trees. These enclosed spaces maintain higher temperatures than the surrounding environment, allowing the frogs to safely lower their body temperatures and metabolism for a few months. According to one study, over 60% of brumating tree frogs were found in tree cavities or hollow logs.

The small openings of these hideaways provide insulation while also preventing the frogs from completely freezing. Inside, the frogs remain motionless to conserve energy. Their heart rate, breathing rate, and other bodily functions slow dramatically until warm spring rains rouse them once more.

Leaf Litter

In the absence of hollow logs or tree cavities, leaf litter on the forest floor is another popular refuge. Layers of decomposing leaves provide insulation from frigid air and ground temperatures. The frogs burrow under the leaves by wriggling their small bodies into the piles of detritus.

One advantage of leaf litter over other brumation spots is humidity. The decomposing plant matter releases moisture, preventing the frogs’ skin from drying out during dormancy. The leaf layers also preclude freezing temperatures from penetrating too deeply.

Herpetologists have tracked the winter movements of radio-tagged green tree frogs taking cover more than 30 cm deep in leaf litter.

Burrows

While less common, green tree frogs also use burrows for brumation on occasion. Typically, they occupy burrows and tunnels built by small mammals like mice, voles, and chipmunks. These borrowings below the frost line guard against freezing and are somewhat humid due to being underground.

Not all frogs live near suitable logs, tree hollows, or deep leaf litter for winter shelter. Without better options, borrowing underground quarters from rodents can mean the difference between survival and death.

After all, as cold-blooded amphibians, prolonged freezing temperatures could prove lethal.

Regardless of whether green tree frogs brumate in hollow logs, leaf litter, or underground burrows, the shelters all allow them to safely lower their body temperature and enter dormancy. Come spring, they will emerge from these hideouts to begin mating and lay eggs in temporary woodland ponds across eastern North America.

Metabolic and Physiological Changes During Brumation

When the weather turns cold and food becomes scarce, green tree frogs enter a hibernation-like state called brumation to conserve energy. This involves some key physiological changes.

Lower Body Temperature

One of the most notable changes is a drop in body temperature. While normal body temperature for Hyla cinerea is about 77-86°F, during brumation it can fall as low as 35-45°F. This greatly slows down metabolic processes and reduces energy requirements.

Slowed Heart Rate and Breathing

Related to the temperature drop, heart rate and breathing also decrease dramatically. Heart rate may decline by 50% or more compared to normal active periods. Slowed cardiorespiratory function allows the frogs to get by on less oxygen and food.

Suppressed Immune System

Unfortunately, being cold-blooded and inactive leaves brumating frogs more vulnerable to pathogens. Studies show declines in lymphocyte counts and certain antibodies during brumation. This suppressed immune function persists even after emerging in spring until nutrients from food can restore immunity.

In essence, brumation induces a sort of metabolic hibernation, allowing green tree frogs to ride out harsh weather and food scarcity. By strongly limiting activity, movement, and essential body processes, they’re able to conserve precious energy reserves.

Risks and Threats to Brumating Green Tree Frogs

Predation

Green tree frogs have vivid green skin that allows them to blend into foliage and avoid predators. However, their bright color makes them more visible against snow or bare ground in the winter (Alford & Richards 1999).

Larger frogs may burrow into soil or leaf litter, but smaller individuals often brumate out in the open, exposed to predators like birds, snakes, and small mammals (potentially increasing mortality risk during winter months).

Freezing

As ectotherms, green tree frogs cannot self-regulate their body temperature and are vulnerable to freezing when ambient temperatures drop. However, these frogs have adaptations to survive sub-zero conditions.

Glycerol levels in their blood and tissues increase as temperatures decline, acting as an organic antifreeze. This allows vital organs to keep functioning even if up to 65% of their body water turns to ice (Irwin & Lee 2003).

Mortality still occurs if freezing exceeds thresholds frogs can tolerate. One study found only 58% of brumating green tree frogs survived experimental freezing at -4°C for 48 hours. Frost damage to skin and organs can be fatal post-thaw.

And repeated freeze-thaw episodes deplete energy reserves needed to survive winter.

Fungal Infections

The frog-killing chytrid fungus (Batrachochytrium dendrobatidis) poses a substantial threat to brumating amphibians with suppressed immune function. Spores embed in skin cells and rupture cell walls to release zoospores that reinfect other areas.

This disrupts cutaneous respiration, electrolyte transport, and osmoregulation capacity (Voyles et al. 2009).

In green tree frogs specifically, chytridiomycosis outbreaks during winter can result in mortality rates exceeding 90%. The fungus thrives in cool environments, and dormant frogs already stressed by freezing temps and fasting are highly susceptible (Berger et al. 2004).

Vigilance for clinical signs of infection is critical.

Supporting Local Green Tree Frog Populations

Provide Sheltered Habitat Areas

One of the best ways to support local green tree frog populations is by providing suitable habitat areas for them to live and brumate in. Frog-friendly gardens with plenty of native plants, fallen logs, and leaf litter give frogs the food, shelter, and brumation sites they need.

Water features like ponds or bog gardens are especially attractive to frogs. Make sure to include areas that are protected from cold winds and temperature fluctuations – places like under dense shrubs, in log or rock piles, or underground burrows.

Providing good habitat helps sustain frogs through winter and allows populations to thrive.

Avoid Disturbing Known Brumation Sites

It’s important not to disturb areas where frogs are known to brumate in the winter. Brumating frogs are vulnerable because their metabolism and body functions slow down dramatically. If a brumating frog is exposed or awakened prematurely, it can use up crucial energy reserves and may not survive.

Clearly mark and avoid disturbing potential frog brumation zones like log and rock piles, burrows, pond bottoms, and dense vegetation. Educate children and others about the importance of leaving brumating frogs undisturbed. Limit winter yard cleanup and landscaping work in key frog habitat areas.

Being careful not to disrupt brumating frogs will give local populations a better chance of emerging successfully in the spring.

Monitor for Signs of Mass Mortality

Keep an eye out for signs of mass frog mortality during winter. Sudden cold snaps or stretches of unusually cold weather can sometimes cause large numbers of brumating frogs to freeze to death. Other issues like disease, pollution, or loss of habitat can also lead to die-offs.

Monitoring local frog populations and habitat areas through the winter can help detect deadly problems early. If you see evidence of mass mortality, try to safely collect and dispose of dead frogs to avoid further disease spread.

Reporting winter die-offs to local wildlife agencies can also prompt investigations into causes and help develop conservation actions. Staying vigilant helps maintain healthy green tree frog populations that will thrive into the future.

Conclusion

While green tree frogs don’t truly hibernate through the winter, their dormant brumation period serves much the same purpose – helping them conserve energy and survive until spring returns. Understanding brumation and supporting favorable habitat can help these charming amphibians continue thriving, even in cold climates.

If you hear that familiar ‘ribbit’ on the first warm night of spring, you’ll know your local green tree frogs have weathered another winter thanks to their amazing physiological adaptations!