Hibernation patterns in mammals: a role for bacterial growth?

1. To examine the hypothesis that stimulation of immune function plays a role in periodic arousal from hibernation, bacterial growth during hibernation was estimated using a simple mathematical model of the general dynamics of bacterial abundance at body temperatures experienced during hibernation. 2. In the model, periodic arousals were important for animals infected with Salmonella at body temperatures above 7 degrees C, but not below. In contrast, periodic arousals appeared to be important at all temperatures examined when infected with several species of coliform bacteria and Pseudomonas, species that grow well at low temperatures. 3. The modelled outputs were compared with torpor patterns seen in captive European Ground Squirrels, Spermophilus citellus, under natural light and temperature conditions. We used maximum likelihood to estimate model parameters and show that the six bacterial species examined are consistent with the immune stimulation hypothesis. 4. Our analyses suggest that bacterial infection could be a selective force on torpor behaviour and warrants further experimental investigation.