Sunday, 20 July 2014

Learning To Be a Good Mother (If You're a Squirrel)

Like birds, but unlike many other kinds of animal, mammals spend a lot of time raising and protecting their young. It's therefore obviously important that mother mammals have some kind of instinctive understanding of what to do to look after their children. And instinct does, indeed, play a big role in maternal care - even among humans, we talk about a mother's "instinctive" desire to protect and nurture her children.

Among humans, though, it isn't all instinct; we have many ways of learning even something as basic as this. But how true is this of other mammals? In fact, there is solid evidence that other animals get better with practice. Instinct may be important, but animals are capable of learning from experience, and a second-time mother generally has a better idea of what she's doing than one who's new to the whole thing. Indeed, not all wild animals are equally good mothers. There is strong evidence, for instance, that animals that suffered the equivalent of child abuse when young grow up to be poor parents themselves.

There are, of course, a number of differences between humans and other mammals when it comes to child rearing, and not all of them purely cognitive. For example, human children take an astonishingly long time to grow up. Human mothers who have a second child therefore commonly still have the first one still living with them at the time. For most other mammals, this usually isn't the case: the first litter will have left home before the second one is born. Sometimes only just before, it's true, so the mother is pregnant while she's still raising the earlier litter, but she doesn't actually have to raise two litters of different ages at the same time.

So, by the time a mother mammal has her second litter she has, barring accident, already had the full experience of raising one or more children to the age of independence. Which gives us the opportunity to really see just how much she has learned from the experience, and what the relative roles of learning and instinct are.

Belding's ground squirrel (Urocitellus beldingi) is one of many species of ground-dwelling squirrel in North America. They live at high altitude, typically above the tree line, in the mountains of California, Nevada, Oregon, and Idaho, and, to some extent, in neighbouring states. They spend eight or more months of the year hibernating, which gives them rather a lot to do in the three or four months of the year that they're actually awake.

One of the very first things they do after waking in the spring is have sex. But, like prairie dogs (which are also, essentially, ground squirrels), they don't spend too long over it - their breeding season lasts just one day, and then they're celibate for the rest of the year. The day in question is typically less than a week after they've emerged from their burrows, and is followed by a 25 day pregnancy. One month after that, the young are ready to be weaned, and are already starting to leave the burrow in which they were born. Which means, of course, that mothers have exactly one litter every year (barring accident, of course).

Belding's ground squirrels are territorial animals, so the mother defends her patch of ground from all comers, in order to protect her young. That doesn't just mean predators, for another difference between humans and many other mammals is the danger of infanticide - that other females may try to kill your young in order to try and give their own children an advantage in life. (For many animals, it's the males that are the bigger threat - not so here, presumably because they don't know which pups are theirs). At any rate, this territoriality gives us one way to measure how mothers change their behaviour in the light of experience with previous litters. How nervous are they, for example, and do they get any better at figuring out what's a threat and what isn't?

By its nature, this sort of study takes years. You have to spend many hours each day, over the appropriate period, watching mother squirrels to see what they do, recording how much time they spend on guard, how often they get aggressive with intruders, and so on. And, since you need to know which ones are first-time mothers, you have to have at least watched them in previous years, and to be able to identify which ones are which. Since you're trying to estimate how they normally guard territories, you really need to do this in the wild, and not just watch them in a laboratory somewhere.

One researcher has recently published the results of doing just that. Compared with un-mated females, those with young did, as expected, become more aggressive in defence of their territories, and spent more time looking out for threats. This was particularly true while they were first establishing those territories, and marking them off from those of their neighbours. Things calmed down a bit after that, only to escalate again once the young began to venture outside the burrow, exposing themselves to danger, but before they were old enough to leave altogether.

Moreover, second-time mothers were more aggressive, and more vigilant, than those giving birth to their first litter. Not only were they more likely to challenge intruders, but they were more likely to become violent when they did, escalating from chattering and twitching their tail in a menacing manner to chasing, and, if that failed, physical violence. At an extreme, if the intruder really wouldn't back down after a few quick nips, they began to wrestle with their opponents, chest-to-chest, all the while biting at their head and neck. That is, to borrow a phrase, one angry mother.

It could still be that this is, at least in part, a matter of instinct rather than experience. An older mother, for example, might have less to lose, and we could argue that evolution has geared up her hormones to promote riskier behaviour. In a second part of the experiment, however, mothers with children were exposed to a male stranger locked in a cage. Being in a cage, he was harmless to their young, but would they know that?

Now, you'll probably have noticed that I said above that males are less of a risk than rival females, so this is, perhaps, not an ideal test. But males aren't entirely harmless, especially if they're too young to have mated (unlike females, they don't reach sexual maturity for a couple of years), and so "know" that any young they come across can't possibly be theirs. More importantly, since this was being done with short-term captives from the wild, any female the researchers locked up would be forcibly parted from her young for the duration - not really a good idea if you have their welfare in mind. So males it was.

And the thing is, the experienced females were less aggressive towards caged males than first-time mums. Given that, overall, they're the opposite, this would suggest that they are learning to pick the battles that are worth fighting. They have, it would seem, learned from past experience that, if an intruder can't get at your children, there isn't much point worrying about him. Much better to spend that time and energy caring for your young directly.

In such ways, it seems, ground squirrels learn, over the years, to be better mothers.

Synapsida will take a break next week, but will be back 3rd August

[Photo by Yathin S Krishnappa.]

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