Deer Mice

Just like us, PEI’s wildlife has been busy getting ready for winter. And just like us, our neighbours of other species use various strategies to do this. Some head south and avoid it altogether. Others burrow into ground, mud, or leaf litter and hibernate. Still others – like these Deer Mice (Peromyscus maniculatus) – use a combination of strategies.

Photo: Deer Mice are regular autumn vistors to my rural PEI mailbox.

Deer Mice are common and native to the Island. Unlike non-native House Mice, Deer Mice are two-toned: white underneath from chin to tail (including their feet) and grey-brown above. They’re also different from Meadow Voles (aka Field Mice) in having slimmer bodies, longer tails, pointier noses, longer whiskers, and larger ears and eyes. All in all, they’re very attractive mice!

If you’ve never seen a Deer Mouse, you’re not alone. Unlike Meadow Voles that share our grassy habitats and are often seen during the day, Deer Mice prefer wooded areas and are more nocturnal. As we’re settling in for the night, they’re setting out on a mission to avoid being eaten and find a meal. (Seeds, berries, nuts, fungi, insects, eggs, and even nestling birds are all on the menu). By daybreak, they’re back at home under a fallen log or in a tree cavity – yet another example of the importance of standing and fallen dead trees.

Spring and summer are times for breeding and family life, but in late fall and winter Deer Mice gather together in larger groups. Our habitats can provide useful overwintering shelters and it’s not unusual to find Deer Mice nests in older homes, cottages, garages, barns or even machinery that’s been parked for awhile. (Almost every year, a group makes a cozy nest of dry leaves and grass in my mailbox, as shown in the photo). This communal living results in huddling together, which keeps everyone warm.

In the coldest months, Deer Mice enter a state called torpor. Similar to (but not as extreme as) hibernation, torpor is a temporary slowing of metabolism and lowering of body temperature to conserve energy. Deer Mice stay in torpor from early morning to late afternoon, waking in the evening to feed. While they may venture out of the nest in search of food, they also stockpile large stores of seeds nearby to get through the winter.

In the most extreme cold, Deer Mice produce more red blood cells. Those extra cells increase oxygen circulation and give them an extra boost of energy to shiver, which raises body temperature without depleting their fat reserves. These five strategies – warm nests, huddling, food stockpiles, torpor, and shivering – help Deer Mice survive an Island winter.

While Deer Mice are well-adapted to our cold season, not all make it through. Survival rates are influenced by how much food they were able to store, the late summer and fall weather, and the severity and length of winter. It’s rare for more than half to survive and in some years only about one in five will make it to spring.

Deer Mice make up for high mortality with high reproduction, with each female producing multiple litters a year and young females able to breed when they’re just a month old.  It may seem harsh to us, but high reproduction and high mortality is a common and effective strategy in nature.

As we move out of fall and into winter (yes, I said it!) I’ll be bringing back my Wildlife Tracks and Sign series. Deer Mice are among the many animals whose tracks we find in the snow on PEI untamed!