If the cultural phenomenon known as the Snuggie has taught us anything, it’s that humans are masters at controlling their environment.

Other animals control their environments, too, to a greater or lesser extent. Mice, for instance, build nests to stay warm—in the wild, at least. In laboratory “mouse houses,” mice are often housed in relatively chilly rooms (20–26°C), yet are provided little material with which to build a cozy pied-à-terre.

The solution isn’t as simple as raising the thermostat, however. Mice tend to become more aggressive as the temperature rises. And females overall prefer slightly warmer conditions than males, though lactation, a highly thermogenic process, may alter that trend.

The next best solution: allow the mice to build themselves a nest. But, how much nesting material should be provided per cage? Given that some mouse facilities house thousands of animals, that’s not a trivial question.

So, Gaskill, then a graduate student at Purdue University, with Joseph Garner (now Associate Professor of Comparative Medicine at Stanford University), developed some guidelines that could help lab managers out.

Gaskill designed a behavioral “titration experiment,” which essentially asks animals to balance two competing needs, such as food and temperature. In this case, the experiment asked mice to choose between a cool cage with nesting material and a warmer cage without nesting material. the results were published in PloS One (1).

One cage, the nesting cage, was set at 20°C with 0–10 grams of nesting material. The other cage, the temperature cage, was set between 20°C and 35°C. Using a connecting tube, the mice could freely move back and forth between the cages, both of which included food and water. Then, Gaskill monitored their behavior.

“The idea is, how much nesting material do I have to give you for you to prefer to stay in a cold cage with nesting material, rather than go on holiday in a warmer cage without it,” says Garner.

Unsurprisingly, the overall rate of nest-building varied depending on the temperature of the cage and the amount of nest-building material in the nesting cage. Yet there were some subtle differences.

For instance, Gaskill observed a difference in predilection for one cage versus the other between males and females. Gaskill also found that different mouse strains adopted different strategies for “behavioral thermoregulation,” a reminder that all mice are not alike.

Perhaps most surprisingly, Gaskill says, the animals sometimes “carried-over” nesting material from one cage to the other, a behavior she had not previously seen before. In other words, rather than making a kind of binary choice—nest or heat—they took their nesting material with them to the warmer chamber.

The animals, Garner explains, “didn’t like the choice that we forced them to make. And so a lot of them decided that they would [essentially say]: ‘thanks very much for the nesting material, we’re going to take it to this nice, warm, cozy temperature.’”

Obviously, mouse facilities do not normally house mice under the conditions used in this study, so what can facility managers do to make their charges more comfortable?

Gaskill recommends providing each cage with between 6-10 grams of nesting material, and let the animals make themselves comfortable. She estimates that 8 grams of Enviro-dri for six months costs about $0.62 per cage.

References

1. Gaskill, B. N., C. J. Gordon, E. A. Pajor, J. R. Lucas, J. K. Davis, and J. P. Garner. 2012. Heat or insulation: Behavioral titration of mouse preference for warmth or access to a nest. PLoS ONE 7(3):e32799+.