Just Cold Enough

Northern meadow jumping mouse

Hibernation is one of the most effective means that mammals can employ to withstand harsh winter temperatures. Many mammals, especially larger ones, use other means, such as growing a winter coat, caching food, or simply moving somewhere warmer, but, if the goal is to reduce energy requirements when you can't get out to feed, hibernation is the most effective means of doing so.

During hibernation, bodily metabolism slows right down, so that an animal may need as little as 1% of its usual calorie supply to stay healthy. This has negative consequences, so that the animal does need to wake at intervals to stave them off, and how often this happens varies from species to species. In this respect, true hibernation can be distinguished from shorter, often daily, bouts of torpor by the fact that each "sleep" can last for weeks or even months. 

If you're going to hibernate, however, you need somewhere safe to do it. Bats use caves that predators are unlikely to to enter, clinging to the ceiling out of reach even if they do, but most other animals use a burrow or similar small shelter. The place that an animal uses to hibernate is called a hibernaculum and, since they are going to spend a long time there with little, if any, ability to move elsewhere, it's important for them to pick one that's just right.

One of the most important features of a hibernaculum is the temperature it is likely to maintain through the winter. The whole point is to stay warm, and to do so while using as little of your own body heat as possible. If a hibernaculum is too cold, the animal will be forced to burn its fat reserves to stay alive, which is the last thing you want to do when you can't get out to feed. 

Thus, a good hibernaculum not only has to be safe from predators or other threats, but also have the right degree of insulation. There has been a good body of research on how snow cover can affect the insulation properties of hibernacula for animals such as Arctic ground squirrels. This will likely be significant if climate change alters the amount of snow available but it's likely not the only factor, since the depth below the soil is likely also relevant, especially where snow is less common. 

The southern meadow jumping mouse (Zapus luteus) lives in southern Colorado, central New Mexico, and eastern Arizona. It was considered a subspecies of the more widespread "meadow jumping mouse" (Zapus hudsonius, now the "northern meadow jumping mouse") until 2017 and, since being split off, may potentially count as an endangered species. 

Jumping mice are not members of the mouse family proper, although they do look rather similar. Instead, they belong to their own family, the Zapodidae, which was itself only raised to family status in 2013, following recognition of how different it was from the jerboas and birch mice that it had previously been included with. Even before that, however, it was recognised as more distant from the true mice and rats than, say, hamsters, voles, and blind mole-rats due, in part, to the presence of two extra sets of molars in the jaw. In addition, like the jerboas, they have elongated hind legs adapted for jumping and long tails for balance.

The southern meadow jumping mouse hibernates for eight to nine months each year, spending most of its life in a state of torpor. However, because it has only recently been recognised as a species in its own right, we don't know a lot about its hibernacula, or, indeed, its resting places more generally. Studies on the northern and western (Z. princeps) meadow jumping mice show that they dig their own burrows, rather than using those prepared by other species, or, for that matter, sheltering in natural crevices or fallen logs. We do know something of how they select their desired habitat, but, prior to last year, the only study to look at this included a description of just one hibernaculum.

That's a good start, but it's only a start. Which is why a new study has taken a wider view, acknowledging that the species is threatened, not least because it relies on rivers and streams, and the few that exist in the American Southwest are mostly being developed for ranching (and, to a lesser extent, urban and recreation areas, plus some threats from forest fires and general drought). 

The study looked at two populations of jumping mice, in Colorado and Arizona. Both lived in high-altitude river valleys dominated by willows and grasses and surrounded by higher forested slopes with juniper, pine, fir, and spruce trees. It consisted of humanely trapping mice, luring them in with oats and crushed peanuts, then fitting them with microchip tags and tiny radio collars before releasing them. They could then track them to their dens, following them daily through the winter to see when and where they hibernated, monitoring the properties of their hibernacula from November to June - two weeks after the start and end of the maximum hibernation period.

In total, the researchers captured 28 mice, but sometimes the collars fell off, or the battery went flat, or the animals just disappeared, leaving a total of 11 hibernacula. While two were on flat terrain, all of the others were on north-facing slopes. All were in vegetated areas with at least 30% tree canopy cover, and were, on average, 44 metres (145 feet) from the nearest river or irrigation ditch. 

They consisted of single tunnels with a low slope running to a sleeping chamber 8 cm (3 inches) across at about 30 cm (12 inches) below ground level and without any food caches or side tunnels. The chamber was lined with insulating material, although what the mice used depended on what was available. Where there were oak trees, for example, the mice used fallen leaves from them, while, in areas dominated by conifers, they relied instead on locally gathered moss.

The depth of the tunnels is probably sufficient to ensure that temperatures remained stable throughout the winter, while the north-facing slopes accumulate more snow which may also help in that regard. What was notable, however, was that the hibernacula were colder than randomly selected sites nearby at the same soil depth. This suggests that the mice are searching for cold places - the shading tree cover, for example, may help with this, although detailed soil conditions may also be relevant.

Since the ideal temperature for a hibernaculum is one that matches the body temperature reached during torpor, jumping mice must chill themselves unusually far down. In fact, the average temperature in the sleeping chambers was just 1ºC (34ºF). Analysis of soil at different depths around the burrows showed that, at 10 cm below the ground, temperatures got much colder in December and January, commonly reaching -5ºC (23ºF), so at the coldest times of the year, the depth helps to insulate them, while in the spring it can get noticeably warmer, which is also undesirable before the animals wake up. Placing temperature probes deeper down, to 50 cm, made little further difference, so the mice are likely picking depths (and possibly soil conditions) to get everything as stable as they can without wasting effort by  digging too far.

Since this is something of a preliminary study, we don't know how low the body temperature of a jumping mouse gets when it hibernates, but this would suggest it's probably around 1ºC. Although this isn't uniquely low for a rodent, it's lower than we would expect for ones that live and can't huddle together, with only a couple of species of ground squirrel known to regularly tolerate conditions colder than this.

All of this aids our understanding of what habitat conditions these animals need, and may go some way to explaining why their numbers are declining. They don't just dig anywhere, or even in just any location near water, but they carefully select places with just the right conditions to keep the temperature at just the level of chill that they prefer.

[Photo by Ryan Hodnett, from Wikimedia Commons.]