Sunday, 22 September 2013

Why Prairie Dogs Sleep Around

For the white-tailed prairie dog,
one male is quite enough, thank you
[Today is World Rhino Day. For past posts at Synapsida on the subject of rhinos, see here.]

The general pattern for mammals is that males seek to mate with as many females as possible, while females prefer to keep a single partner, at least in any one mating season. This is because females can't be pregnant with more than one litter at a time, so they may as well pick the fittest males around and get them to father all their children. Males, on the other hand, have no such investment, so the more females they mate with, the more children they have, and the better their genes survive.

But this is really a gross oversimplification. It's the basis of the polygynous mating system, where a powerful male drives off rivals and attracts a harem, or acquires multiple mates by some other means. This sort of thing is most apparent in animals like seals and deer, but it's by no means universal. For one thing, many mammals are more or less monogamous. This typically occurs where the male has to help to look after his children if they're to survive, thus putting him under the same pressure as the females.

But what are we to make of polyandry, where one female mates with multiple males in the same breeding season? It's surprisingly common, so there must be some reason for it. One, of course, is that they might not get much choice in the matter - if there's lots of randy males around, then it may be easier to give in than to put up a fight. It's hardly acceptable behaviour for humans, but, then, animals aren't human.

But there are a number of other possibilities, reasons why a female might actually want to mate with more than one male. It might, for instance, increase her chances of getting pregnant, if she can't really be sure how fertile any given male is. But perhaps she's up to something a lot more subtle. Might, for example, the sperm from different males be fighting it out after the fact, so that she ends up being fertilised only by the best?

Prairie dogs (Cynomys spp) are ground-dwelling members of the squirrel family. They're often confused with gophers, which do look rather similar, but while the latter are rodents, they aren't squirrels, so the two aren't all that closely related. The reason that they're useful animals to look at when it comes to determining who mates with whom is that their entire annual breeding season lasts about six hours. So long as you get the right day, therefore, you only have to watch your female prairie dog for about that length of time to get a count of exactly how many males she has mated with.

For the last ten years or so, John Hoogland has been watching prairie dogs have sex, and evaluating what strategies work for then, and what don't. Well, all right, not literally watching them, because they mate in their burrows. Which has the advantage that you at least know where to look, without chasing them about all over the place, but the obvious disadvantage that you can't see them. Still, if a fertile male and female gopher sniff and lick each other's genitals, make mating calls, and then disappear into the female's burrow for half an hour or more, it's not too hard to guess what they've been doing down there.

A recent report summarises his latest findings about prairie dog polyandry, and whether or not there's a good reason for the females to engage in it, or it's just down to the males being determined.

There are five different species of prairie dog, all of them native to North America. One, an endangered species, lives only in Mexico, but the long-term study was able to look at all four of the others. The best known is probably the black-tailed prairie dog (Cynomys ludovicianus), which is found across the Great Plains from Montana to Texas. The other three all live further west, in various different regions from Wyoming down to Arizona.

Despite being so closely related to one another, it turns out that there are differences between the mating behaviours of the species. The black-tailed and white-tailed (C. leucurus) species don't practice polyandry much, with about two thirds of the females mating with just one male during their six hour fertile period. This turns out not to be very hard from them to achieve - they just stay down their burrows after they've mated the first time. Males only look for partners above ground, so this hiding strategy works quite well.

The other two species don't, with most females mating with two or three males over the course of the day. But there's no obvious reason why they couldn't hide if they wanted to, and no reason why it shouldn't work as well for them as for their relatives. That they don't do this suggests that they actively want to mate with more than one male. Which, in turn, means that they must be getting something out of it.

Indeed, it seems that the most polyandrous species, Gunnison's prairie dogs (C. gunnisoni), are both more likely to become pregnant when they have multiple partners, and have bigger litters when they do. Utah prairie dogs (C. parvidens) aren't far behind, and, neither, come to that, are the white-tailed species, even though it's a rarity for them. Without detailed paternity testing it's hard to know for sure why this happens, but it could be that some males are less fertile than others, either because there's something wrong with them, or just because they'd already visited too many females that day by the time they visited that one. Another possibility for the larger litters is that the female actually became pregnant by more than one of her partners, and some members of the litter have one father, and some another - it may sound odd, but it does happen.

However, it's not merely the number of children that you have that determines your reproductive success and your ability to pass your genes on. Grandchildren are also important. The Tudor dynasty in England didn't die out because Henry VIII didn't have enough children - he had at least seven - but because none of them had children of their own (and neither did his only brother). It's no good having even a large number of children, if none of them reproduce themselves.

With wild animals, that's most likely to happen if they die young, before reaching sexual maturity. So we can look at that, too, and when we do, it turns out that this is also true of all of the species bar the black-tailed one: if they have more sexual partners, their children are more likely to survive for at least nine months after weaning and reach their own breeding season.

Which makes you wonder why they don't all do it - especially the white-tailed species, who generally avoid it. Well, there's also a downside; having children is exhausting work. With the white-tailed and Gunnison's species, at least, females with more sexual partners were less likely to survive for another year, and it's likely precisely because they put so much effort into looking after larger litters, and ensuring they all grew up.

Evidently, different species have weighed up the costs and benefits of this behaviour in different ways, perhaps due to subtle differences that aren't apparent at first sight. But, in a way, that's the point. Black-tailed prairie dogs are widespread, and it would have been perfectly reasonable to look at them alone, and conclude that we have absolutely no clue why some of them are polyandrous. (We still don't; it could just be an evolutionary relic of their past). Because, for them, the most common species, it seems to make no difference.

But by looking at the other three species, all seemingly near-identical, we can see that there really is a difference, and that some of them have taken advantage of that. It's a lot easier to just look at one thing, but sometimes it can be useful to cast the net a bit wider.

Whether you're a researcher or a female prairie dog desperate for children.

[Picture by "Devonpike", from Wikimedia Commons]

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