Cold hamsters: wild species boosts immunity for winter.

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. They must survive on hoarded food and what they can glean from the snowscape. In general, limited food and extreme cold can make it harder for an animal to fend off disease and recover from injury.

In their PNAS report, the researchers gave the first results in hamsters of a technique that tags immune-cell types with glowing, color-coded antibodies. Short-day hamsters had more of some major immune cells than long-day animals did.

Neuroendocrinologist Rae Silver of Barnard College in New York City especially praises this part of the project. "The beauty of this paper is that they've taken very, very good measurements to document the phenomenon," she says.

The second part of the hamster study builds on work by coauthor Firdaus S. Dhabhar of Ohio State University in Columbus. He showed that animals suffering psychological stress alone--without any wounds--move immune cells from the bloodstream to their skin and lymph nodes that drain the skin.

In the current study, when researchers distressed the animals by confining them in a short tube for 2 hours, the short-day hamsters sent more immune calls to their skin than the long-day hamsters did.

Bruce McEwen of Rockefeller University in New York comments that the experiments on a seasonal stress response "open a new arena of study." The work "raises many interesting questions," he says, such as, "Does seasonality of the immune system pertain to humans?"