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