A Long Winter's Nap
Some Texas animals unplug when the weather turns cold.
When winter comes around, some animals go underground. While humans can keep warm with a blanket and hot chocolate, some Texas wildlife must take more drastic measures to survive.
Texas has a few true hibernating animals (rare because of the mild climate), and it has several species — mammals, reptiles and even one bird — that go into states of inactive torpor when it gets cold.
“Universally, hibernation is about two things: buying time during a period of resource scarcity and escaping harsh weather conditions,” says Texas Parks and Wildlife Department mammalogist Jonah Evans. “In Texas, we sometimes have harsh weather conditions, but we don’t get socked in three months with snow like some other areas.”
When insects go away in winter, certain animals have a choice.
“For animals that depend on insects for food, there are two basic ways to handle the winter: hibernate, or migrate to where more insects can be found,” Evans says. “Insectivorous bats do both. Species such as Brazilian free-tailed bats migrate, while cave myotis and tri-colored bats hibernate.”
Torpor, dormancy and brumation
Torpor is a temporary state, lasting hours, days or weeks, deployed as a way to save energy or deal with harsh conditions. Some animals may slow their bodily systems and go into torpor just for a night or go into torpor during times of drought.
Hibernation typically is a seasonal state of long-term torpor — consecutive, multiday bouts of torpor. In both cases, animals find a safe place to hang out and lower their metabolism.
“There’s a slowing of all systems,” Evans says. “Heart rates slow way down. Respiratory rates slow way down. Body temperature goes way down. The digestive system basically shuts off. All the systems just slow way, way down.”
In Texas, some bats and ground squirrels hibernate. Bears, typically associated with hibernation, don’t hibernate but go into a type of torpor. Reptiles such as snakes, alligators and turtles also go into a state of dormancy.
“They call it brumation in reptiles,” says TPWD wildlife biologist Nathan Rains. “They’re cold-blooded. Their metabolisms just slow down with the environment.”
During hibernation, mammals experience occasional periods of arousal when they may eat, drink or even mate.
“Hibernation is a dynamic period,” says TPWD bat specialist Nathan Fuller, who got his Ph.D. in bat hibernation. “A lot of people think of it as a time when animals go to sleep and wake up six months later. Bats will be down in hibernation for two weeks or so at a time. Some kind of trigger causes them to arouse from their hibernation and do some stuff — they go to the bathroom, fly around a little bit. This cycle can go on for months. That whole process is called hibernation.”
Before hibernation, mammals search for suitable hibernacula (overwintering sites) and bulk up on food.
During the fall “swarming” before hibernation, little brown bats visit caves and fly in and out of the caves from dusk till dawn as they explore. They eat a lot of bugs, and they mate. When they start hibernation, they find a humid spot in the cave and settle down.
Hibernating little brown bats can reduce their metabolic rate to about 1 percent of the normal rate.
“Their metabolic rate is basically nothing,” Fuller says.
Heart rates for the bats — normally around 210 beats per minute at rest and 1,365 beats per minute in flight — fall to 20 beats per minute. They commonly go for 45 minutes without taking a breath. Their body temperature matches the surrounding temperature.
Unfortunately for bats, hibernation can now be a time that proves fatal.
Hibernating bats, including those in Texas, are facing the deadly threat of white-nose syndrome. The syndrome has caused millions of deaths in bats across North America. The fungus that causes white-nose grows on hibernating bats, acting as a chronic disturbance and possibly causing dehydration.
“The leading hypothesis of why they suffer from greater periodic arousal frequency has to do with water balance,” Fuller says. “The fungus starts to grow on their wings and put holes in their wings, and that increases evaporative water loss. They lose water and become dehydrated, and they have to arouse to drink water. There are other ideas out there. Maybe they are just irritated by the fungus and want to groom it off.”
Bats are weakened by disrupting their hibernation cycles and using critical body reserves to combat the fungus. Even before white-nose syndrome, a bat’s arousal cycle was where it spent most of its energy. One arousal cycle costs as much energy as 67 days of torpor.
“When white-nose syndrome starts taking effect, the period between arousals starts to shrink,” Fuller says. “Instead of arousing every 14 days, they start to arouse every three to five days. They’re using many more times the amount of energy they should be using. They can’t make it to the end of hibernation, and that’s when you start seeing dead bats.”
Russell A. Graves
Several species of bats hibernate in Texas, including the big brown bat, tri-colored bat, cave myotis and southeastern myotis. Hibernation can last for months. Bats spend time before hibernation packing on the calories, but it’s a balancing act — their need to fly limits how much they can bulk up.
Bats need roosts that are cool and remain at a constant temperature, such as caves. Bats mate in the fall before hibernation, and the females store the sperm until spring.
Russell A. Graves
Bears go into a period of dormancy for three to four months but do not exhibit the characteristics of true hibernation; their temperature does not drop markedly and their heartbeats and respiratory rates are only moderately reduced.
“Sometimes they just curl up on the ground in the thicket during a cold spell,” Evans says.
They may awaken and move around at times. Pregnant females may wake up to give birth and then go back into torpor.
Several species of ground squirrels hibernate, including the 13-lined ground squirrel, Rio Grande ground squirrel and rock squirrel, and some of these squirrels are the longest sleepers around. The 13-lined ground squirrel has been found to hibernate for 240 days of the year.
Bella Lataste | Ouh La La Photography
Alligators slow their metabolic rate and stop eating. Brumation typically lasts from October to March. Some alligators create mud holes for warmth and shelter. Others reside underwater, surfacing occasionally to breathe.
Rick & Nora Bowers | KAC Productions
The common poorwill, found in the western half of the state, is the only Texas bird known to go into torpor. The other birds in its family of nightjars migrate each year, but the common poorwill sticks around for the winter. The lowest naturally occurring body temperatures in birds have been recorded in poorwills in torpor.
John C. Abbott & Kendra K. Abbott | Abbott Nature Photography
Snakes find a crevice or burrow to crawl into and escape the elements for a few months. They become lethargic as temperatures drop. They may share dens with other snakes. On warm days, snakes may come out of their den to bask in the sun.
Russell Roe is the managing editor of Texas Parks & Wildlife magazine.
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