Small British Mammals: The Smallest Mice

The smallest species of mouse native to Britain, and, indeed, one of the smallest species of mice anywhere in the world, is the harvest mouse (Micromys minutus). With a typical adult weight of around 7 grams (0.25 oz.) or so, it's only around a quarter of the weight of a typical house mouse, and is known by terms translating as "dwarf mouse" in a number of European languages, including both German and Greek.

Harvest mice are widespread, being found across much of Europe, and in a band across Asia that reaches as far as Japan and Taiwan in the east. A 2009 study from Vietnam suggested that the harvest mice there might be a separate species, but it was small scale and not yet fully accepted. Certainly, it's an unusually hot climate for harvest mice which, in Europe, are found only in northern Spain and Italy, only sporadically in Greece, and not in Portugal at all. On the other hand, while they are reasonably common in central and southern Finland and southern Scotland, they are rare in Sweden and absent from Norway. (They also don't reach Ireland, but the sea is the likely barrier there, rather than the climate).

Even aside from the smaller size, harvest mice are easily distinguished from the other mice native to Britain, with their more rounded faces, smaller ears, and reddish-brown colouration. As the English name of the animal indicates, they are often found in crop fields, retreating to hedgerows, or even small burrows, during the winter. Their native habitat was most likely reed beds around wetlands and stream banks, since they do show a preference for wetter environments. At least in modern times, however, they are more likely to be found in wild or semi-wild areas with long grass, rather than in heavily cultivated fields. (Except for rice fields; they love those). They can even be found in areas where long grass penetrates into relatively urban environments, such as orchards, waste ground, or overgrown churchyards.

Harvest mice are omnivores, but the bulk of their diet consists of plant matter such as seeds, berries, and blades of grass. As with other small mammals, they have a high metabolic rate to compensate for bodily heat loss, and have to consume a third of their body weight in food every day to survive. Winter is a particularly difficult time for them, especially since they do not hibernate.

They are primarily nocturnal and, despite their food requirements, don't require a particularly large territory to sustain themselves - assuming a suitable habitat, they can spend almost all their lives in a 400 m² (0.1 acre) area and feel perfectly happy. Of course, those lives aren't terribly long, either; even the oldest don't survive through more than one winter, and they are regularly eaten by predators. One study from Poland, for instance, showed that they were the second most common food item in the diet of the local owls.

Perhaps the best-known feature of the harvest mouse, however, is its habit of building small spherical nests from woven grass. These are constructed on tall plant stems, typically around 50 cm (19 inches) above the ground - although this may depend on such things as the likelihood of seasonal flooding.

It's because of this that harvest mice prefer reed beds and plentiful long grass. As might be expected, they are expert climbers, although better suited for narrow stems than, say, tree trunks. Their toes end in thick pads, while the soles of their feet have warty lumps that can aid their grip on narrow structures. While they are not quite as sophisticated as primates (and possibly don't need to be, because they're so much smaller) they also have semi-prehensile tails and relatively inflexible, but still opposable, thumbs.

The nests are only used for breeding, which takes place mostly in the autumn, although any time of year other than winter will do. The mice live alone, and there seems to be relatively little competition between the males for access to females, something supported by the fact that they're around the same size. If anything, the decision as to who to mate with lies mainly with the female, and she apparently prefers males she has met previously to strangers from further away. Litters average four or five young, although the infant mortality rate is high, especially towards the end of the year. Since they are fully grown and sexually mature at just six or seven weeks old, not only will a female breed a couple of times a year if she can, but she could have great-grandchildren within twelve months.

Unusually, harvest mouse mothers have been observed to feed their young on regurgitated food, as well as with milk - something that's otherwise normally only seen in dogs.

Harvest mice have no close living relatives - the so-called "harvest mice" of North America (Reithrodontomys spp.) aren't even members of the true mouse family and are more closely related to voles. They do, however, have a surprisingly long fossil history, with species identifiable as harvest mice being found as far back as the Late Miocene around 6 million years ago.

Harvest mice regularly vanished from parts of their range as the Ice Ages came and went, probably retreating to refuges in central Asia. Unusually for something so widespread, genetic studies have shown that the last ancestor of all living harvest mice probably lived no more than 80,000 years ago. This suggests a rapid expansion across Eurasia during and after the Last Ice Age, perhaps reaching Britain and Japan when the seas were lower. Up until this time, different species inhabited Europe and China, with some evidence suggesting that it was the latter that were the most likely ancestors of the modern sort.

But, even if they have no close living relatives, there must still be something that they're related to. The earliest known species of harvest mouse (M. chalceus) was, if anything, even smaller than the living one, so we might expect that the best place to look for their modern relatives would be among other kinds of unusually small mouse. But that, as it turns out, would be wrong.

Genetic evidence suggests that the split between harvest mice and all other living species took place 8 or 9 million years ago. This is a relatively early split within the 'true' mice; field mice, for example, only date back around 6 million years. In all the time since, the specific group that they seem to have split away from evolved down quite a different path. And those animals are now amongst the largest of all members of the mouse family.

So large, in fact, that we don't call them 'mice' in English: we call them 'rats'. And it is to them that we will turn next month...

[Photos by "Llez" and "Hajotthu", from Wikimedia Commons.]