“To sleep—perchance to dream ...” – Shakespeare (Hamlet)

Winter is an excellent time to enjoy desert landscapes. Cool temperatures provide ideal conditions for long comfortable hikes. However, it is not my favorite time. As hot as deserts can be during the summer, the cold winter temperatures also limit who is awake and can be seen. Every day and every season desert creatures unconsciously calculate their energy budgets, and then become active or not based on that computation.

When the temperatures are too hot, food is often scarce, and those temperatures limit how much time they can forage before their core temperatures reach lethal levels. During the winter, depending upon how much rain has fallen, food may or may not be available. But even if food is abundant, ectotherms (“cold blooded” creatures like lizards and snakes) need external heat to digest food. If the lizard, tortoise, or snake eats when it is too cold, that food sits undigested in their stomachs. That food then ferments and becomes toxic to the reptile. Depending on what species and what season, extended periods of inactivity and lowered metabolic rates have been variously labeled as hibernation, aestivation, brumation, or torpor. Each of these terms have similar meanings and involve a lowered metabolic activity and an extended period of “sleep.” However, they are not synonyms.

We have all heard of hibernation and have some understanding of what it means. It is that deep, months-long winter sleep when an animal’s body temperature lowers. We think of bears and squirrels being archetypal hibernators. But not all bears or squirrels hibernate. Those living in milder climates can be active all year-round. Of the two, when they do hibernate, its squirrels that go into such a deep sleep that they cannot be roused until spring. Bears on the other hand will wake up periodically during the winter, and if prodded, or if their den becomes no longer suitable perhaps due to collapsing or flooding, they will, if necessary, move to a new location. For a while, citing that difference, scientists argued that squirrels were the true hibernators, and that what bears did was something different. Apparently, the current accepted belief is that bears do indeed hibernate. During those periods of deep sleep bears’ and squirrels’ metabolisms keep running at a lower rate, selectively accessing the energy provided by stored fat, but not muscle. When they awake in the spring, they are skinny, but have the same muscle mass as they did when they began their sleep. If it was otherwise, if they also lost muscle mass, the bears and squirrels would wake up less able to avoid dangers, less able to climb trees and find food. Most warm desert (Colorado Desert) rodents, kangaroo rats and desert pocket mice do not hibernate. We can find their fresh tracks on sand dunes at any time of year. Still, our smallest rodent, the little pocket mouse (local subspecies is the Palm Springs pocket mouse) does seem to disappear in the winter and so may hibernate. For very small nocturnal mammals, including most bats, even in the relatively mild Colorado Desert, winter temperatures are too cold and food is not abundant enough for their energy budget calculations to allow activity.

So far, we only know of one desert bird that hibernates, the common poorwill. Edmund Jaeger was first to observe a hibernating poorwill. He found the bird ensconced in a poorwill-sized divot in a granite boulder in the Chuckwalla Mountains just south of Desert Center, and then published his observation in 1949. Other birds, smaller species that opt not to migrate, are known to use “short-term” hibernation, referred to as “torpor.” Torpor typically lasts just over night, or perhaps for the duration of a winter storm. In the desert, year-round-resident Costas hummingbirds are an example of a species employing torpor to manage their winter energy budget. It is that short duration that distinguishes it from true hibernation. To sustain their high energy lifestyle, many other birds, including orioles, tanagers, and flycatchers migrate south before the onset of winter, enjoying the mild tropical climate of Central and South America. Still, some birds are year-round residents, including verdins, gnatcatchers, Say’s and black phoebes (flycatchers) and some migrate to our deserts (white-crowned sparrows and phainopeplas).

Most desert reptiles also employ a form of hibernation to survive winter climates, although it differs from the hibernation used by mammals and common poorwills. The differences are that rather than maintaining a lower-level metabolic rate, being ectotherms their body temperatures during winter dormancy are the same as the surrounding environment, even if that environment reaches freezing temperatures. Another difference is that reptiles must go into dormancy with an empty stomach to eliminate any possibility of that food fermenting and killing the reptile. By sustaining a lower but well above freezing metabolic “furnace” all winter long, mammals are allowed “one last hearty meal” as they enter hibernation. Due to these differences, winter dormancy by reptiles and other ectotherms is called “brumation,” although some contend it is just a reptilian form of hibernation and does not deserve a new name. Brumating reptiles typically find an underground site or a deep rock crevice to while-away the doldrums of winter, sites that offer enough thermal protection to keep temperatures from falling below freezing. In a study of short-horned lizards conducted in southern Canada, the lizards brumated underground in areas that typically accumulated deep winter-long snowpacks. The snow insulated the lizards from below freezing temperatures. Any of the lizards that brumated underground but where the winter winds blew away the snow could not endure those far below freezing temperatures and perished.

Given the importance of brumation for the winter survival of desert reptiles, side-blotched lizards are unique in that at least in the southern California deserts they can stay active all winter long. Further north, in the Great Basin Desert and the as far north as southern Canada, side-blotches do brumate. Even here in the southern deserts wintering side-blotches almost certainly go into nocturnal torpor, but as soon as the sun comes up, they are out and about hunting for small insects. Their small size allows them to warm up quickly, even on days that feel cool to our preferences.

A final hibernation category is aestivation. Aestivation is much like hibernation or brumation, but instead of occurring during the cold of winter, it occurs during the heat and aridity of summer. When it is just too hot to “come out and play,” when it is too hot to look for food or mates, some species will hunker down in their burrows. Desert tortoise are a good example of a species that undergo what is a summer brumation but to distinguish that from winter sleeps, it is referred to as aestivation. By hunkering down in their burrow through much of the summer, tortoises can keep from losing precious water and fat reserves, allowing them to emerge in much better condition when milder temperatures and rain return. Apparently, rather than a deep summer sleep, at least for tortoises, aestivation is more like a light nap. If the tortoise senses an impending summer monsoon they will emerge from their burrows and wait patiently, like only a tortoise can, at sites where standing water is likely to accumulate. Sometimes those sites are small pits that the tortoise dug the previous spring in anticipation of the monsoons to come.

Whether species hibernate, brumate, aestivate, or go into short-term torpor, generations of evolutionary wins and losses have calculated the most likely success formula for maintaining a positive energy budget, and then surviving. It is the survivors that live, breed, and populate the next generations. Finally, referring back to Shakespeare’s quote, scientists have determined that the deep winter sleep of hibernation, torpor, or brumation results in lowering their body temperature to a point where their body and brain is too cold to produce electric currents, and so precludes dreaming.

Nullius in verba                                                                                                                             

Go outside, tip your hat to a chuckwalla (and a cactus), think like a mountain, and be safe.