Why hibernating animals occasionally wake up?

One of the several hypotheses floating around over the past several years to explain the phenomenon of repeated wake-up events in hibernating animals although such events are very energy-draining, is the notion that the immune system needs to be rewarmed in order to fend off any potential bacterial invasions that may have occured while the animal was hibernating:

Now, a group of researchers provided a mathematical model that supports this hypothesis:

"A habit in some animals to periodically wake up while hibernating may be an evolutionary mechanism to fight bacterial infection, according to researchers at Penn State. The finding could offer an insight into the spread and emergence of infectious disease in wildlife, and has potential implications for human health.

Many warm-blooded animals slip into an inert sleep-like state as part of a unique strategy to get past harsh winters when food supplies are low and the need for energy to stay warm is high. The immune system is in sleep mode as well.

"The production of antibodies, and white blood cells is stopped. Basically all cell reproduction shuts off," says Angela Luis, a doctoral candidate in ecology at Penn State's Center for Infectious Disease Dynamics.

However, animals regularly snap out of their torpor, and become fully active. But such sudden breaks from slumber eat into much of the animal's stored energy reserves, and it is not fully clear why the animals need to wake up, and how often

Some scientists think the answer lies in bacterial infections that could run rampant in the face of an immune system that is essentially asleep.

"Animals cannot tell when they need to wake up, or if they are infected," says Luis. If the animals hibernate for long they risk serious infection, she says, while waking up frequently wastes precious energy, and could prove fatal as well.

In other words, animals with an optimal time of torpor will win out over others, says Luis, who presented her findings at the 91st annual meeting of the Ecological Society of America.

Luis and her colleagues used a simple mathematical model that mimicked the growth of bacteria such as E. coli and Salmonella in European ground squirrels, and how it affected their torpor patterns in relation to temperature.

Microbial growth depends on temperature. Most bacteria grow faster when it is warm and much slower when it is cold. For animals exposed to Salmonella, which multiplies rapidly in warm temperature, a regular break in hibernation would be an important adaptation to combat the germs, when experiencing a warmer winter. However, Salmonella doesn't thrive at very low temperatures, so when animals experience a particularly cold winter, these breaks wouldn't be crucial.

But if the animals were exposed to certain pathogens that thrive at low temperatures, like some E. coli, the animals would still have to regularly break their hibernation to ensure protection at all temperatures, Luis explains.

"Our model, which is confirmed by field data, shows that torpor patterns generally seen in some hibernating animals may be an evolutionary adaptation to help protect them from bacteria that grow well in low temperatures," says Luis.

The researchers suggest that an understanding of how pathogens interact with their hibernating hosts could provide valuable insight into the spread and emergence of zoonotic diseases."

This is excellent, but now it has to be confirmed with field and laboratory experiments. And alternative hypotheses have not been kicked out of the picture yet either. Oh, and by the way, in at least some species, it is the circadian clock that times these bouts of waking...