Sunday, 29 June 2014

The Family Lives of Japanese Badgers

As I mentioned previously as part of my overview of the weasel family, we used to think that the "common", or Eurasian, badger inhabited much of that continent, with individuals found from Britain to Japan. We now know that that's not the case; there are at least three different species of badger within this area, albeit very closely related.

In Britain and Ireland, we think of badgers as living in communal setts, with multiple different family groups sharing the same network of tunnels and chambers, that may have built up over generations. Yet, while this is true in the British Isles, it isn't true anywhere else, even when we are looking at the exact same species where it lives on continental Europe. Here, as with most other members of the weasel family, European badgers live alone when they aren't raising their young.

The reason for this is thought to be a combination of Britain's climate and a diet rich in earthworms.The frequent rains in the British Isles are good for earthworms, and there are so many available to eat that the badger population in the islands is unusually high, something that they deal with by crowding together in larger groups. Elsewhere in Europe, there are less earthworms, and the badgers have a broader diet, but one that requires them to defend their territory from others of their own species if they don't want to starve.

At the opposite end of Eurasia, however, we have the Japanese badger (Meles anakuma).  These, like continental European badgers, live relatively solitary lives. Yet, like British badgers, they mainly eat earthworms. Granted, they aren't the same kind of earthworm as those in Britain, but most people would probably be hard-pressed to tell the difference. Japan is also hardly lacking in rain, especially during the typhoon season (Tokyo, for example, is wetter than even notoriously rainy parts of Britain, such as Stornoway). The population density of Japan is not too far off that of the UK, either - at least compared with much of the rest of Europe.

So it may be useful to compare the lifestyles of these two very closely related animals, to see how they differ. A recently published study used radio-tracking and genetic analysis to see how families of Japanese badgers interact with one another, and how they occupy the space available to them.

The study examined around twenty badgers living in forested land close to Hinode, a suburb on the western edge of the great Tokyo metropolis. In this area, the badgers feed on earthworms supplemented with persimmon and the local relatives of raspberries. Much as in Britain, the badgers also share the woodland with red foxes - which, in this case, really are the same species as the European kind.

Being solitary animals, each badger should have an exclusive home range - an area through which it regularly travels, and which it does not share with its rivals. The size of a home range depends, not just on the animal species we're talking about, but on the local climate and other conditions. An animal does not want to have to defend any more territory than it needs to, but, on the other hand, there does have to be enough food in it to support it comfortably. So harsher, more marginal, territory tends to lead to larger home ranges.

The thing is, the amount of food an area can support depends not just on where it is, but what time of year we're talking about. The "constant territory size hypothesis" suggests two different tactics for coping with this. Which, perhaps unsurprisingly, are that you either change the size of your home range with the seasons, or that you don't - in the latter case, you obviously need to need to consider the worst case scenario when deciding how big your territory needs to be.

It turns out that the female badgers followed the latter tactic. They occupied ranges with an average of 15 hectares (37 acres) each, with only around 2% overlap with their neighbours. This presumably reflects what the land can supply in winter, as well as taking account of the need for females to provide for their young as well as for themselves.

The males, on the other hand, were quite different. Their territories were much larger, with an average 33 hectares (80 acres) in winter, but also much more variable, doubling in size during the summer. We can only guess as to why the home ranges are so much larger - certainly, male Japanese badgers are larger than females (and to a greater extent than is true for the European species), but they aren't that much larger. It might simply be that they have more freedom to roam, being able to sleep where they want, rather than having to stay close to the setts where the females raise their young.

It's unlikely that their territory becomes larger in summer due to lack of food - if anything, the opposite should be true. However, it's not just food that the males want in summer, but also sex. During the winter, and even during the summer when they're too young to breed, males, just like females, avoid the territories of others of their kind. Indeed, they leave home after as little as a year, slowly wandering further and further away, and seeing their mother less and less until they eventually find somewhere to settle down. Perhaps they just want to avoid the aggravation of competing for food. (In later life, this might even include their own cubs, who they clearly wouldn't want to starve).

In the summer, though, the territories expand and start to overlap with those of females as they wander in search of breeding opportunities. Not, it has to be said, that they spend any more time with their partners than the bare minimum, but they do at least need to find them in the first place. Indeed, their "partners" aren't anything long-term either, because, unfettered by the need to stay close to a breeding sett, their territories move over the course of several years, changing the particular females that they are interacting with.

Females, on the other hand, stay with their mothers for rather longer - two or three years is perfectly normal. But they can't breed while she's around, and wouldn't have the resources to raise their cubs if they did. So, eventually, they have to leave. They don't, generally, travel as far as the males when they do this, and most manage to find a territory abandoned due to a recent death. Sometimes, however, they inherited the territory of their own mother, the same sort of behaviour that likely played a part in the development of multi-generational setts among their British kin.

Clearly, despite their similarities and close relationship, there are differences between European and Japanese badgers. The latter, for example, enter a partial state of torpor in the winter, which reduces the amount of food they need at such times. But, in general, they face similar pressures, and those pressures can change based on more than just where they happen to live.

[Photo by "Alpsdake", from Wikimedia Commons]

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