Hamsters that have to survive winter outdoors in Siberia rev up their immune systems when days grow short, according to new lab tests. They even pump up their response to psychological stress.

In the same genus as pet-store standards, the wild Phodopus sungorus survives winter temperatures that plunge to --40 [degrees] C, says Staci D. Bilbo of Johns Hopkins University in Baltimore. She and her colleagues have wondered whether a dwindling day length cues the animals to prepare for trouble. Previous work with other animals showed that seasons trigger changes in immune responses.

Bilbo and her colleagues now offer two reports of lab tests with artificial lighting that suggest that, yes, Siberian hamsters prime their immune systems. Animals kept under 10 hours of artificial daylight recovered from fevers faster than hamsters under 14-hour regimens did, the researchers say in the March 7 Proceedings of the Royal Society of London B.

Hamsters living under a 9-hour-daylight regimen also have greater numbers of certain immune cells than hamsters experiencing 15-hour days do, say Bilbo and her team in the March 19 Proceedings of the National Academy of Sciences (PNAS).

Also, the short-day hamsters mobilize a stronger immune-readiness response to psychological stresses. "That's the first time that's been shown" for any animal, says Bilbo.

In winter, Siberian hamsters slump into a few hours of morning torpor with a drop in metabolic rate, but they don't get the benefit of full hibernation. …