Sunday, 24 July 2011

Should I Stay or Should I Go?

Although they are hardly alone in this, mammals are noted for their care of their offspring. Yet many of them are solitary animals, and the young leave home as soon as they are able to survive on their own, so that the only groups seen are when a mother is caring for her children. But, of course, many mammals live in herds, packs, or other groups, with at least some individuals staying with their parents once they grow up.

But, even in herds, some animals do leave home, to establish or join new groups elsewhere. How do they make this decision as to whether to leave or stay? For many, its a fairly simple rule: the males leave, and the females don't. That means that a herd or other group is dominated by a core of females descended from a single matriarch, while the males have generally come in from outside. Initially, in most cases, the males wander about looking for a new group, perhaps together with one or two other young males in a similar situation, until they eventually find someone that will take them in. This ensures that they don't end up mating with their own female relatives, and keeps the gene pool as wide as possible.

This is one reason why its often more useful to trace the past movements of an animal population by examining its mitochondrial DNA (which is inherited from the mother) than by doing the same for the Y chromosome (which is inherited from the father). Males wander all over the place, and the picture would get blurred and confused quite quickly. But what of animals where both males and females may either stay close to their parents, or choose to leave - how do they make the decision?

The red fox (Vulpes vulpes) is an animal familiar to many people across the northern hemisphere. It's often the case that very similar animals from North America and Europe will be referred to by the same name, although they are actually separate species - this is true, for example, of otters, bison, and badgers. The red fox is an exception; it really is the same species on both continents. Fossil records show that they first appeared in Asia, and it seems that they crossed the Bering land bridge to reach America when sea levels were lower during the last Ice Age.

In fact, the term "fox" is a fairly vague one, biologically speaking. It's generally used for any small member of the dog family that, well, looks like a fox. Most of these animals are, indeed, genuinely related to the red fox, and form a natural group within the canines, but some of them - specifically the "foxes" of South America - are actually closer to wolves. The red fox, however, is definitely a fox. Indeed, as the type species of its genus and tribe, it is, in a sense, the very definition of what it means to be fox-like.

Red fox,    Kit fox,     
  etc.        etc.        Grey fox,
   ^           ^            etc.
   |           |             ^ 
   |           |             |      
   -------------             |       Bat-eared fox    
         |                   |             |
         |                   |             |
         ---------------------             |         "Wolf-like"
                  |                        |            Dogs
                  |                        |              |
                  --------------------------              |
                              |                           |
                      (first "true" foxes)                |
                              |                           |
                                     (first canines)

The red fox is an adaptable animal, that will live anywhere north of the tropics, other than the coldest of wastelands or the driest of deserts. In particular, it has successfully colonised urban environments, and many foxes live on the fringes of human society. Even in Britain, which lacks such things as raccoons, I find that foxes go through my rubbish bags, and I can often hear them at night, or see them on the streets in the early morning, even though I live in quite a large town.

For the last thirty years, there has been an ongoing study of urban foxes in the English city of Bristol. Helen Whiteside and colleagues of the University of Bristol, used that data to examine how foxes in the city are related to each other, and what they do when they reach adulthood. Red foxes are reasonably sociable animals, and form small packs with a dominant breeding male and female, and a number of younger individuals. The younger foxes do get a chance to breed, but they have less opportunity than the dominant pair, so the question arises of why they bother doing it.

Why, in other words, don't they all just leave - or at least all the males - as soon as they get the chance? What convinces at least some of them to stay at home? At least part of the answer seems to be that they help to look after the young of close relatives within the pack, giving the pack as a whole a better chance at survival. But that really only makes sense if you know that the cubs really are your close relatives, and has to be offset against the fact that you want to find a partner - ideally one that isn't a close relative - that can give you cubs of your own.

So it makes sense that whether a young fox will stay with its parents or strike out on its own will depend on the role its parents have within the pack. Surprisingly, though, it turns out that it doesn't really matter who the father is. That may be because of the urban environment, where there's more than enough food lying about in rubbish bags and the like for access to that not be an issue. And if younger males within a pack slink off every now and then to find a mate from a neighbouring group, the dominant male in their native group probably doesn't care much whether they're around or not; either way, they aren't competing with him.

But it turns out that who your mother is does make a difference: male cubs born to dominant mothers within the pack are much more likely to leave. That's probably because, if they stay, since the dominant female does most of the mating, if they stay around, they'll end up mating with her, which clearly isn't a good idea. But, if the young male's mother is subordinate, she mates less often, and that's less of a problem - the male might as well stay where he is rather than take the risk of finding somewhere new.

But, for young females, exactly the opposite is true. They stay if their mother is dominant. That means that most future cubs will be their close relatives, giving them a reason to help them out. They have little reason to compete with their own mother, and can, in fact, wait until she dies and become the dominant female themselves. If the mother of a young female is herself subordinate though, the young fox leaves. Whether she does that voluntarily, or she whether she gets chased away is unclear, and it may be a bit of both, but, either way, the dominant female has little to gain from having too many unrelated potential rivals hanging about. Her own daughters are one thing, but anyone else is a possible risk.

All of this sounds quite reasonable, and its perhaps surprising that nobody has really shown it to be true before. The fact that an animal's choice to leave home is influenced by its mother, but not its father is, perhaps not so obvious, and shows why its worth checking this sort of thing from time to time. That males and females pick opposite strategies may also show us how this sort of group living got started and that, as so often, it's your mother that's really important.

[Picture from Wikimedia Commons. Cladogram adapted from Bardelben et al. 2005]

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