When White-footed Mice Invaded Michigan

When I'm describing a particular species of mammal on this blog, one of the first things I usually mention is where in the world it lives wild. Clearly, there are differences in the fauna of different continents or specific islands. Cougars, black bears, pronghorn antelope, and coyotes are all common enough in the US, but don't live in Europe. We have Eurasian and Iberian lynx, but not bobcats and Canada lynx. There are bison in Europe, but they're a different species than the American sort, and we only have wild raccoons because some were deliberately released in Germany in 1934. 

Clearly, the existence of the Atlantic Ocean is not to be sniffed at. However, this applies on a smaller scale, too, where some physical barrier that's some way short of an ocean, but is still significant, prevents an animal from advancing further across its home continent than it might like. 

And then there's the fact that animals have particular requirements as to the climate and vegetation of their native areas - even if the vegetation is only affecting the prey animals that they themselves need to feed on. The difference between this and the physical geography of rivers, mountains, and so on, is that it's changing over a much faster timescale, especially in recent decades. Animals may be forced to move to new areas, which can be a problem if the physical geography prevents them from doing so and can be a problem for other reasons even if it doesn't.

Life Lessons from Monogamous Mice

As a general rule, male mammals don't have much to do with raising their young. In many cases, that's because the species in question prefers the solitary life, and the two sexes only meet up to mate before parting ways again. In more social species, males and females may still live in different herds for most of the year, often because they have different feeding requirements and want to avoid competition. Even where are there mixed-sex groups, the males often spend more of their time fighting off rivals than they do actually looking after the young they sire - although, in this instance, they may be helping indirectly by protecting the herd from predators or the like.

But there are plenty of exceptions. 

Lots of Little Deer Mice

If a sexually reproducing animal is to preserve its species without population loss, each pair of parents has to produce, on average, two offspring that will themselves live long enough to reproduce. This is just basic arithmetic, but there are at least three different ways that an animal can achieve this result and which one is used varies from species to species.

One approach is to maximise the chances of each of your offspring surviving. This is called the K-selection strategy, and results in the population being as close to the maximum capacity of the local habitat as possible. (The 'K' stands for 'capacity'... in German). Such animals don't need to reproduce very often, or produce very many offspring when they do, but they have to invest a lot of resources in their survival. They tend to be relatively large animals, with few predators... elephants, primates, and whales are all good examples among mammals. Humans are a particularly extreme example, given how long it takes us to raise our children, and, as with other strong K-selectors, twins are rare in our species.

Small British Mammals: Rats!

Brown rat

The term "rat", as commonly used, has no defined biological meaning. A "rat" is simply any rodent that looks at least somewhat like a mouse, but is larger. Animals fitting this description have evolved multiple times, so that rats do not form a single evolutionary group, and not all of them are even members of the mouse family.

Nonetheless, there is a biological group that we can call, for lack of a better term, the "true rats" (technically the "Rattini") which includes the animals that most people likely think of when the term "rat" is used without further qualification. There are two species of this kind of animal in Britain, not least because both of them are found just about everywhere else as well.

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).

Small British Mammals: Field Mice

Wood mouse

While the common house mouse is likely one of the most familiar of all animals living wild in Britain, it is only one of four species of mouse native to the islands. (France and Germany, for comparison, have six each, although they aren't the same six). Of the other three, the closest relatives of the house mouse are the two species of field mouse, which are also found widely on continental Europe.

Field mice have benefited less from the presence of humans than house mice, since they tend to avoid human dwellings, and there are rather a lot of those in Europe. Given their common name, however, one might suppose that they have at least benefited from the spread of agricultural land. This, however, only true of one of the British species. This is the wood mouse (Apodemus sylvaticus), sometimes also known as the "long-tailed field mouse". In Britain, when the term "field mouse" is used without qualification, it typically means this one.

Small British Mammals: House Mice

Over the last few years, I have done a roughly annual series covering particular groups of mammals, exploring all the different species, familiar and unfamiliar, that belong to them. (If you haven't read the past ones, the link "Synapsida Series" points to the list of them; note that the individual posts appear in reverse chronological order once you select a series to view). One problem with selecting which groups to cover in a given year is that only some groups will really work, and, even then, some are clearly better than others.

One problem, for example, is that some groups are simply too large, and if you split them down into narrower groups that are more easily handled, there's no longer enough variety for it to be worth it. Such is particularly true of small mammals. There are over 700 species in the mouse family, for instance, with over 500 of those in the "murine" subfamily, which includes all of the familiar ones. Even if it would be practical to describe them all, most of the individual species haven't been studied in any detail, so you'd rapidly get a long list of "looks mouse-like, lives in XX".

Broken Bones and Missing Toes

Woodland jumping mouse

Life can be hard, especially out in the wild. Injuries can be common, and among the most painful are broken bones. As humans, we can mitigate bone fractures using splints, braces and so on, but wild animals have no such luxury. Nonetheless, bones do heal by themselves, a process that starts with the formation of a tough fibrous scar at the injury site that at least helps to keep things fixed partially in place. Over time, the scar is mineralised to form weak, but functional, bony material, and then eventually rebuilt with as much of the original structural integrity as possible.

Without splints and braces, this is likely to be an imperfect fix, even assuming that the injury doesn't prevent the animal from feeding or otherwise kill it before the process completes. If the animal does survive, the signs of the injury are always going to be there in its skeleton, and are quite likely to cause at least some ongoing problems. But, survive they do, and bone healing wouldn't have evolved in the first place if it never worked in the wild.

Miocene (Pt 6): The Coming of the Mice

Palaeotragus

The Early and Mid parts of the Miocene epoch were, for the most part, times when the world was much warmer than it is today. It wasn't a steady pattern, however, and I've already described how the fluctuations in climate, over the course of many millions of years, affected the rodents of Europe. It was a time when the most common small mammals in Europe were not mice and voles, but dormice, accompanied by early hamsters, squirrels, and the gliding eomyids.

By 10 million years ago, however, the colder, drier climate had become locked in for the long term. We know that the forests of Europe changed dramatically at this time, the old subtropical trees, such as figs and palms, being replaced by oak, alder, and elm. Likely as a result of this change in the available food supply, most of the dormice died out, leaving only a few close relatives of the relatively small number of species we have today.

Mice, Mice, and More Mice

A common feature of my blog posts, pretty much since the beginning, has been the inclusion of cladograms, tree-like diagrams that show how different species, or groups, of mammal are related to one another. Our knowledge of these relationships is constantly evolving, as we get more and more information, or find different ways of doing the necessary measurements.

In general, though, what happens is that scientists measure a number of different features from the animals that they want to study, and compare which ones have the most in common. These days, these are most likely to be physical measurements, such as fine details of the shape of the skull or teeth, if at least some of the creatures we're looking at happen to be fossils. Otherwise, it's much more likely that the things being compared are stretches of genetic code. The further apart two animals are, evolutionarily speaking, the more differences there are likely to be, and if we pick a gene that both animals possess, and that can accumulate a reasonable number of changes without stopping working altogether, we have a good basis for comparison.

News in Brief #7

Geoffroy's spider monkey

Monkey Sleep, Monkey Poo

Poo is useful stuff to zoologists: a lot of field work involves examining the stuff. It can tell you what an animal has been eating, how it behaves, and maybe something about it's genetics. And, of course, it can also tell you where it's been. Take Geoffroy's spider monkey (Ateles geoffroyi), for example.

Spider monkeys are moderately sized monkeys, noted for their prehensile tails and remarkable agility. I saw some (although not Geoffroy's species) at London Zoo on Monday, and, despite the poor weather, they really were a delight to watch, full of enthusiasm and energy. Seeing them at play, it's hard to escape the impression that they have five arms: their feet are effectively an extra pair of hands, and their tail is so flexible and has such a good grip that it might as well be a fifth one.

They live, unsurprisingly enough, up in the trees - specifically ones in Central America - sleeping in them at night and travelling between them to feed during the day. They have particular favoured trees to sleep in, and deposit a lot of dung at the base over the course of the night. Such 'latrine' use is common among many mammals, and just finding where they are provides a good means of establishing what the monkeys are up to without having to disturb them.

Mating in the Rain

Cairo spiny mouse, a related species

Humans have no defined breeding season, remaining sexually active throughout the year. This is relatively unusual among large mammal species, especially in temperate regions (although it's worth remembering that our species is originally from the tropics). That's at least partly because mating in humans also serves a social function, helping to bind individuals together. We see something very similar in bonobos, but most large mammal species do have a defined time of the year when the females come into heat, and mating begins.

The reason for this is to ensure that the young are born at an appropriate time of year, when food is most plentiful. That's particularly important in temperate regions, because there's inevitably going to be a shortage of food in the winter. However, the picture can become more blurred as we move towards the tropics, and when we look at smaller mammals.

The issue in the tropics is that there is no winter, and temperatures don't change radically throughout the year. However, due to the complex nature of climactic weather systems, even in the tropics, there is usually a rainy season and a dry season. While the jungles of Southeast Asia, for example, are never going to be as short on food in the dry season as Canada is in the depths of winter, there is still a notable difference. For example, flowers and fruit may be more available in the wet season, as may fresh growths of new plants. Even for carnivores, that's helpful, because the animals you're feeding on are plumper and more numerous when there's more tasty plants around. So, many species time their breeding so that births occur during the rainy season.

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.