Ask the Naturalist January-March 2001

Are there any resident animal species in the region that hibernate in winter, even though we don't usually have snow or freezing temperatures?

That depends on what you mean by hibernation. All but one of the Bay Area's 13 species of bats are capable of hibernating; the exception is the abundant Mexican free-tailed bat. But according to bat rehabilitator Patricia Winters, "No bat takes hibernation very seriously in this area." Although they can remain torpid for weeks when temperatures are low, they become active during warmer spells. (They don't all hole up in caves or hollow trees; the hoary bat roosts in foliage and the red bat hibernates on the ground, in leaf litter.) Bay Area reptiles show a similar pattern. Although western pond turtles may dig into soft stream-bottom mud for the winter, they've been observed out in the December sun. So have western fence lizards, which shelter in rock crevices or under bark; 27 were found in a single oak log. The fence lizard's nemesis, the Alameda whipsnake, chooses rodent burrows for its winter inactive period. Winter dormancy is not confined to mammals and reptiles. At least one bird, the poorwill, hibernates in the Mojave Desert portion of its range, although the Bay Area population migrates south instead. Anna's hummingbird—a year-round resident—can also become torpid on cold nights to conserve energy.

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