Secrets of the Mound-building Mouse

In temperate climates, such as we have in Europe, and in the northern parts of North America, winter can be a difficult time for animals. The weather is cold, there is often snow on the ground, and food is in short supply. Some animals hibernate through the winter, while others struggle on through the harsh weather. Smaller mammals, such as rodents, often store dry food in caches that they can return to when there is nothing fresh available. Squirrels, for example, hide nuts to bide them through hard times. But some rodents go to greater lengths.

Out of the over six hundred members of the mouse family, the steppe mouse (Mus spicilegus) is one of the three closest species to the familiar house mouse (Mus musculus). It is found from the easternmost border of Austria, through Hungary, Romania, and Serbia, and out into the steppe lands of Ukraine and western Russia. They are found primarily in and around agricultural land and orchards, and rather less in the wild grasslands where they presumably originated. Indeed, while the species seems to have diverged from the house mouse around three million years ago, long before modern humans were around, today it seems to rely upon us to create the unnatural habitats in which it thrives best. In this respect, it resembles the house mouse, which only rarely lives outdoors.

As a close relative, it also looks very similar to the house mouse, and the two are very hard to tell apart. So similar in fact, that this created a problem with the scientific name for the animal. The steppe mouse was first formally described by Alexander von Nordmann in 1840, under the name of Mus hortulanus. One of the key things you have to do when describing a new species is find a specimen of your animal, and use that as the holotype - the individual against which all other members of the species will be measured. When somebody else finds a new specimen, in theory what they do is check to make sure it belongs to the same species as the holotype specimen, and, if it is, they know what they've found. Nordmann already knew what a steppe mouse was, so he went out and found one to use as his holotype, wrote it up, and gave it its scientific name.

Only, unfortunately for him, it later turned out that he'd picked up a house mouse by mistake. His scientific name was associated with a specimen of the wrong animal, and had to be stricken from the record. A newer description, from 1882, was used instead, and that author's choice of name, Mus spicilegus, became the official one.

The steppe mouse is also called the "mound-building mouse", and that's because of an unusual habit that is quite different from those of its closest relatives. In the autumn, several mice gather together to collect seeds and other plant materials, and deposit them in a large mound that they then cover with soil. The mounds can be large - a foot or so high, and four or five feet across. Deep in the soil underneath the mound, the mice dig burrows centred around one or two nests, and there they spend the winter.

The burrows are shared between a group of mice. They aren't necessarily siblings, but more of an extended family, with related mothers, but unrelated fathers - effectively cousins related through their aunts. How many mice share each mound is not entirely clear, and may well vary from place to place. Estimates have proved difficult, because whenever researchers try to dig up the mounds to count the mice underneath, the mice tend to run away (who'd have thought?) A combination of having a few exit tunnels some distance away from the mound itself, and the habit of dashing into the tunnels of other nearby mounds while researchers are still chasing them does not help matters. But somewhere between six and twelve seems about typical.

Its natural to assume that the mounds are food caches to allow the mice to survive through the winter, so that they can emerge refreshed in the spring, and start breeding again. But just because they are giant larders doesn't necessarily mean they can't have any other function. Indeed, evolution often works like this, when a structure or habit evolved for one purpose turns out to be useful for something else as well. In this case, for example, it has been suggested that the mounds may also help protect the mice, whether from bad weather or from potential predators, or both.

Péter Szenczi and colleagues of Eötvös University in Budapest wanted to know how effective the mounds were as a shelter from the weather (since, after all, the mice are already digging burrows underground), and how useful they are as a winter food source. They looked at a large number of mounds in two different areas in Hungary, measuring them, analysing what they were made of, and checking to see how good they were at keeping the mice alive through the winter.

Their first finding is, perhaps, something of a surprise. The mounds are made of seeds and other parts from a number of different plants, depending on what was available nearby, with the main components (other than soil) being either barnyard grass or goosefoot, and the remainder being various other, mostly grassy, plants. But when they looked at the animals' dung pellets, right through the winter, that wasn't what they'd been eating. Instead, they preferred amaranth, ragweed, and cocksfoot.

The "obvious" explanation for the mounds, one you'll frequently see online or in books whenever the steppe mouse is discussed, apparently isn't true. The mice are not eating the stored seeds. That their burrows don't extend into the mounds, but remain quite separate in the soil underneath, one or two feet underground, also supports this idea. That's not to say that, if the winter got really bad, they wouldn't eat the seeds in the mounds at all - it would presumably be better than starving - but it doesn't seem to be something they normally do.

So, if the mounds aren't food caches, as most people have generally assumed, what are they for? Why bother to store a huge pile of food and then not eat any of it? Temperature probes and soil analysis provided what seems to be the answer: the soil below the mounds is warmer than that around them, even allowing for the height of the mounds themselves. Moreover, it's also drier, which helps to keep it more comfortable. By digging their burrows under a pile of insulating vegetation that soaks up rain and snow, the mice can huddle in cosy nests, protected from the worst of the winter weather.

It seems to work, too, because when the researchers counted the numbers of mice they found under mounds at the beginning and end of the winter, they came to about the same value. In other words, pretty much all the mice that went into shelter in the autumn were still alive in the spring. Whether the mounds also help protect against predators that might want to dig the mice up is less clear, and we can't really tell how much difference they make without disrupting them to see what happens, but that they go to the trouble of collecting around 140,000 cm³ (five cubic feet) of soil and plant matter would suggest its probably significant.

So, yes, rodents often do cache food to bide them through the winter. But that isn't necessarily their only reason for collecting plants. Because sometimes, the "obvious" answer isn't the right one.

[Image from the Carpathian Basin Digital Collection. Cladogram adapted from Lundrigan et al. 2002.]