Nature Note: American Bullfrogs Take A Nap

American bullfrogs (Lithobates catesbeianus) are deeply associated with spring and summer–their deep “jug-o-rum” serenade evokes images of warm nights: fireflies, and leafed-out trees. But what are they up to during cooler times? As with all living things in the Lake Champlain region, bullfrogs need to adapt to the winter season, and as aquatic amphibians, many do this in the lake itself.

Like other frogs, bullfrogs are true cold-blooded amphibians, which means that they need to regulate their body temperature through their environment–seeking sun when they’re too cold, and shade when they’re too hot. This is not as easy in the winter, so bullfrogs opt for hibernation. They do this underwater in shallower sections of the lakebed where temperatures are more stable and remain above freezing.

American bullfrogs are quiet during the winter, not attracting our attention the way they do in the warmer seasons.

Unlike other animals hibernating at the bottom of the lake, bullfrogs and other aquatic frogs nestle into an open cave of lakebed materials they build around October, just above the mud. They require direct skin contact with the water because they engage in cutaneous respiration–in other words, they breathe through their skin. Burying themselves completely would cause suffocation. Bullfrogs and other amphibians have extremely thin skin that needs to  be kept moist, which allows for oxygen to be transported into the bloodstream while underwater without the use of gills. This adaptation is a tradeoff–amphibians are highly sensitive to deteriorated water quality because of this direct interface. 

They remain mud-bound all winter, occasionally swimming slowly about the lakebed. This approach requires little energy–bullfrogs will survive off of internal stores from the summer and fall all the way until spring. Stirring from hibernation too early can be fatal–in periods of early-season warming followed by cold snaps, a pattern that may increase with climate change as springtime weather patterns become more volatile.

Bullfrogs have their starring role in the chorus of life during spring and summer nights, but don’t forget that these animals are around us all year long! Keep your ears tuned for the males’ distinct deep-throated mating call starting in May–they are among the last of the frogs in the region to emerge from hibernation and start mating.