With a new semester beginning and the festivities of the holidays behind us, oftentimes all we want to do is crawl back in bed and hide until winter is over. But for a lot of other critters, instead of hunkering down in a heated home, they have to find some other way to last the season.
The first image that springs to mind is the iconic bear tucked away in its den, sleeping until winter is over. But — as Jim Staples, a biology professor at Western University explains — there’s a lot more to it.
“I’m interested in how animals in general can survive stressful environmental conditions,” Staples says.
There are a lot of misconceptions about hibernation. Contrary to popular belief, most small mammals don’t hibernate. There’s even debate in the scientific community if bears are actually “hibernating” when in their dens. Finally, and perhaps most surprisingly, hibernation is not sleep. Hibernation isn’t constant over the entire winter, and mammals will periodically arouse to do various things — such as sleep!
“The EEG patterns suggest that they’re not sleeping when they hibernate,” Staples explains.
EEG readings show a marked difference between hibernating and what appears to be sleeping — the latter of which displays REM-like patterns, a phase of sleep associated with high levels of brain activity and memory development.
All this is to say that, if you strap machinery onto a hibernating mammal to measure the activity in its brain, you won't get a pattern that is indicative of REM sleep. REM sleep is significant because it is a very deep sleep necessary for a healthy brain.
In a general sense, hibernation is when an animal reduces its metabolic demand and body temperature to survive stressful environment conditions, like cold weather and low food availability.
“It's cold in the winter, there's not much food, so how do you survive? You reduce your metabolic demand, and that means you don't have to eat as much, and you can survive on the body fat you have,” explains Staples.
Metabolic demand relates closely to energy demand and reducing it means the animal can run on less fuel. Even if animals pack on the pounds before winter, this reduction in metabolic demand has to be quite dramatic.
“We usually measure metabolism by the rate of oxygen consumption.… Normally, when a squirrel is awake, its breathing is at about 50 times per minute and its heart is beating 300 times per minute,” Staples says. “When they hibernate, they breathe once every two minutes and their heart rate goes down to three beats per minute.”
But it’s not just mammals that have developed means to survive the winter — insects have a similar method. But to complicate things, insects are ectotherms — cold-blooded — meaning they cannot produce their own body heat. Jacqueline Lebenzon, a doctorate student at Western in the department of biology, studies how insects survive the cold weather.
In mammals, this is called hibernation, but in insects, this process is called diapause. Because there’s no consensus on whether these processes are identical, scientists are reluctant to call them the same thing.
“[There are] two main ways [to survive the winter]: you can become really [freeze] tolerant and survive the really low temperatures the winter has to offer, because that’s really the main issue, and then you can also enter diapause, which kind of helps you deal with the low resource availability issue,” Lebenzon explains. “Diapause is … shutting yourself down so you don’t have to deal with those conditions.”
Lebenzon also discusses the importance of freeze tolerance.
“Some insects can become freeze tolerant, so they can survive internal ice formation.… They can make a bunch of antifreeze molecules to put in their blood … and that prevents them from freezing,” she says.
But insects can't just hop right in to winter — prepping takes time.
"In the summer, if you were to take a cricket outside and freeze it, it’ll die," she explains. "But if you take that same species from London in the winter … and freeze it, then it’ll thaw and keep hopping and survive."
Meanwhile, the insects that aren’t diapausing migrate to warmer temperatures. Lots of invasive insects move indoors, where warm homes provide what amounts to a constant summer for these critters.
Broadly, the solutions are similar: store excess energy and avoid the cold temperatures. In both cases, behaviour, physiology and biochemistry have become well-adapted for the cold so that the animal might survive.
People like Staples and Lebenzon are conducting the research needed to help animals (and humans) in the face of climate change. Changing temperatures can harm hibernation in a lot of ways. For insects, the effects are more obvious.
“The first thing that’s going to mess things up is those insects that respond to [day length]. Because, regardless of what’s going on outside, they’ll enter diapause when the day lengths are short.… When they come out of diapause, let’s say we get a freezing bout in March and April, and they can’t survive that,” Lebenzon says.
For mammals, as Staples explains, the effects can be a little subtler.
“It might mean they would’ve lost time that they could’ve had more babies.… If climate change is going to reduce their food supply and they can’t put on enough fat, then they’re going to die. It’s already estimated that in one species of ground squirrel, 25 per cent of them do not survive the winter.”
This inconspicuous phenomenon seems to have a lot of wide-reaching implications, and not just for the bear in its den. Lebenzon stresses the ecological importance of insects — even if you don’t particularly care for creepy crawlies. Lebenzon explains that, if the insects threatened are pollinators, there could be catastrophic effects on crop growth.
“The take-home message is that they’re not just bugs: they’re the most abundant animal in the animal kingdom. They make up over 75 per cent of all species. Millions have yet to be discovered. Even if it’s just one or a couple species that emerge [from diapause] early, it can have cascading effects on the larger world's ecosystem.”