Hibernation patterns in mammals: a role for bacterial growth?

Summary1 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 °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.