Where are Pocket Mice during Winter?

Infrared light helps researchers observe an active Pacific pocket mouse in our breeding facility.

Infrared light helps researchers observe an active Pacific pocket mouse in our breeding facility.

What do pocket mice have in common with bears? Aside from both being mammals, probably not much, you might think. But while many of us are familiar with the idea that bears hibernate during the winter, few of us expect tiny mammals—mice!—to be hibernating also. But that is precisely what Pacific pocket mice and many of their small relatives are doing right now.

Pocket mice build burrows underground. While there is still a lot we need to learn about their burrow structure and how they use them, similar species have multiple chambers (like rooms) with tunnels between them. They probably go more than a foot deep, and the mice move up and down to regulate their temperature. When the sun is out, the chambers closest to the surface will be the warmest. When it is cold, or if there is frost or snow on the ground, further down is better insulated. The mice probably also bring bedding in, like grasses or leaves, to line the sleeping chamber and keep them warm.

Here on the coast of Southern California, we don’t usually need to worry about it getting too cold, even in winter! But for Pacific pocket mice, winter isn’t just about cooler temperatures. These animals almost exclusively eat seeds, and plants typically don’t produce seeds during the winter. When there isn’t much food available, animals have a few choices. They can move to find food somewhere else (by migrating), they can store up fat and live off their reserves for a while (penguins, for example, do this when they are breeding and waiting for their chicks to hatch), or they can reduce the amount of energy they expend and go into torpor, which is like being in deep sleep. Not only does this save the energy it takes to go out and look for food, but animals can slow down their breathing and heart rate and really minimize their energy usage. They still be use their fat reserves, but at a much slower rate than if they were up and active.

A wild Pacific pocket mouse filled his cheek pouches with seeds to take back to his burrow this past summer.

A wild Pacific pocket mouse filled his cheek pouches with seeds to take back to his burrow this past summer.

Pocket mice can be torpid for days at a time. But they have another trick to survive the food-scarce winters. They cache, or store, seeds in their burrow. Every few days they wake up, eat some of their seeds, and then go back to sleep. This means they don’t have to live off their fat stores alone, and they still use minimal energy because the seeds are stored in their burrow with them. It also means they have to invest extra energy into caching seeds during the summer and fall, instead of eating everything they find right away. Storing food also means an animal doesn’t have to come out of its burrow for months at a time!

The Pacific pocket mice living in managed care as part of the San Diego Zoo Institute for Conservation Research’s breeding program have a little more luxury. They are still torpid for periods of time, but they are fed regularly during the winter to make sure everyone has enough food to survive. This means they can be a little more active than their counterparts in the wild, but it is still a quiet time in the breeding facility! In the next few weeks, the animals in the wild will start coming out, and our pocket mice will be getting ready for the next breeding season!

Did you know that some mice hibernate? Sometimes I wish I were a bear (or a pocket mouse) so I could hibernate all winter, too!

Rachel Chock is a graduate student and volunteer with San Diego Zoo Global’s Pacific pocket mouse project. Read her previous post, What’s in Your Backyard?

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