Mice No Longer Out in the Cold
Research Models
Christina Winnicker

Mice No Longer Out in the Cold

Give a child a Rubik’s Cube and motor and math skills develop. Give the same Rubik’s Cube to a rat, and well…nothing happens. It’s totally irrelevant to their development. This is the idea behind behaviorally relevant enrichment.

“Enriching” an environment means different things for different animals. Mind-bending puzzles and brain games work for humans–because we’re visual and tactile–but rodents need different stimuli. Enrichments have to be relevant to the behavior of the species. That is, they need to take into account an animal’s sense organs, behavioral tendencies and natural environmental preferences.

For rodents, we ask three questions with regards to enrichment: 1) what makes sense for the animal (i.e. what’s natural), 2) what supports production and research efforts, and 3) what’s actually feasible?

Behavioral Thermoregulation
Temperature for rodent housing is usually controlled at around 24°C. For a mouse, this is slightly more chilly-than-normal. They generally prefer a temperature around 30°C, or 80°F. With that being said, mothers with litters, pregnant females, and group-housed mice actually prefer cooler temperatures.

Like all warm-blooded animals, rodents produce their own heat. Because of their large body surface to body mass ratio, they lose heat to the environment easily. To combat this, rodents normally change their behavior and build nests to retain heat. This is called behavioral thermoregulation. Without a nest, mice consume more food and use the energy to stay warm. With a nest, that food and that energy could be used elsewhere.

Going All Natural
We thought: let’s just add nesting material to cages. Because rodents build elaborate nests in the wild, it would mimic their natural environment. If the mice were to build nests out of this material they might then be able to behaviorally thermoregulate and thus combat heat loss due to a cold environment. And maybe these “energy savings” could be used elsewhere.

So, that’s what we did. In two different studies, we put nesting material in the cages of two nude strains of mice and three commonly used strains of mice, respectively. The nesting material is fibrous, structurally sound, autoclavable, can be irradiated, and is relatively inexpensive. We gave this material to mice over a six-month period and looked at the number of pups produced and food consumed.

What we found was staggering. Not only did breeding groups produce more pups–an average increase of 8-10 pups over a six-month breeding lifespan–they consumed much less food per gram of pups produced: 27% less! For example, over a six-month breeding period, our Balb/c Nude (CAnN.Cg-Foxnnu/Crl) mice produced an average of 16 pups without nesting material and 34 with nesting material per breeding cage (2 females & 1 male).

Less is More
Overall, we found that nests lessen the impact of cool animal housing temperatures on mice, and the energy conserved by providing nesting material was reallocated from heat generation to improved breeding performance.

We’re just now starting to really understand the behavioral repertoire of rodents. The idea of combining animal welfare, science and production is relatively new and this study represents the first in a series of investigations to look at enrichments more critically.

So, what does this all mean? Well, often in transgenic research we get poor breeding numbers. With this kind of innovation, we may not need as many breeding groups. Thus, it is a refinement that could reduce the number of animals needed for research. More broadly, behaviorally relevant enrichments produce a more complex and fully developed animal. This gives us the opportunity to do better science.