In polygynous systems, one male mates with multiple females, to maximise the number of offspring he can sire. In the polyandrous system, it's the other way round, with a single female mating with multiple males (some mole rats do this, but it's rare in mammals). The final option is a promiscuous system, where both sexes have multiple partners.
While monogamy and promiscuity are often found among mammals, it's polygyny that's the most common mating system. Since the way that sex is determined in mammals typically means that males and females are more or less equally common, at least at birth, this means that a lot of males will lose out, and there is intense competition for mates. Males may develop special features for fighting one another, such as horns, although the details vary between groups.
One thing is a well-established trend across all kinds of mammal, however: the more polygynous the species, the larger the males will be in comparison to the females. The most extreme examples are among seals and sea lions; in the latter, males are typically around three times the size of females when fully grown. But, of course, we also see it among such animals as deer and kangaroos, and many more besides.
The standard explanation for how this has arisen is fairly straightforward. Females put far more effort into raising young than males do, and this is especially true among mammals because of the demands of pregnancy and, in particular, providing milk. Males can instead put their resources into ensuring that they have more mates in the first place, and thus, more offspring. The best way to do that is to win fights with other males, and a powerful muscular body helps do that. Stronger males have more offspring, but there's only a limited amount of food around, so there's no point making the females larger as well. So male offspring inherit increased strength from their fathers, sire more children as a result, and so on until an effective balance is reached.
The thing is, there may be rather more to it than that.
This is not to say that it's fundamentally wrong, just that the reality may be complex than the simple scheme would suggest. For instance, one would assume that the more sexual partners a male animal has, the more children he will father. That seems logical enough, and there has to be some truth to it or polygyny would never evolve in the first place, but it's not a simple one-to-one relationship.
This is because, especially since the advent of the polymerase chain reaction, which makes easier to run paternity tests on animals, it has become clear that female mammals are often far less faithful to their partners than we had previously thought. Which means that, if we want to assess how successful a given mating system is, we can't just count the number of females that a particular male associates with, but the number of children he's actually fathering. In other words, competition between males may be less extreme than we think if the "weaker" males are still managing to have children without the powerful ones noticing.
It turns out, that, once you allow for this, things become rather less clearcut than when you just consider the number of females the male controls. For instance, a study on nine species of seal in 2014 showed could not find any correlation between the size of the dominant males and the number of children they would sire. (Even so, there was such a correlation for the two species of elephant seal, so it's not as if this never happens).
So, if males being larger doesn't mean that they have any more actual children (because it just encourages smaller males to cheat and sneak in when they're not looking) why is it that males are so often larger than females in polygynous mammals?
There have been a number of possible explanations put forward for this, including that it's just a side-effect of the general trend of animals getting larger over evolutionary time, or that it has to do with the different energy requirements of animals that need to tend their young compared with those that can just walk away. The details are unclear, but whatever the cause, there is evidence that, in seals at least, the males became larger than the females before they became polygynous, and fought with one another to build up harems.
Once they did that, the older theory can kick in, and the difference in size increases, since it's still useful for dominant males to be larger and to fight off their rivals. But, even so, if larger male size evolved first, we'd expect it to be found amongst monogamous and promiscuous species, too, which it typically isn't. So is there some reason why species with larger males are more likely to develop polygyny than those who never evolved down that path?
Well, yes: sexual harassment.
Okay, so "coercive mating behaviour" tends to be how it's phrased by biologists, but, in human terms, we're talking about sexual harassment. Or, in some cases, sexual assault.
It stands to reason that being larger and more muscular makes it easier to get away with this sort of thing, and mammals don't have our sense of morality. It's not just physically imposing male seals that go around harassing females for sexual favours; it's also been observed in reindeer, bottlenose dolphins, and certain primates, among others. This, as one might expect, is not the sort of thing females want to put up with, especially when they already have young to raise. Even if they aren't physically injured, it can prevent them from foraging for food or whatever else it might be that they're wanting to do.
At least one way to minimise this is to benefit from safety in numbers. Female red deer congregate in large groups during the breeding season, female sea lions in small groups are less able to protect their young from rampaging males than those in large ones, female antelope that leave the breeding group to wander off on their own are more likely to be harassed by passing males than those that stay behind. And so on.
Once the females are all grouped together in one place, polygyny becomes a natural response; it's possible for one male to monopolise as many females as he can get away with - and, yes, fight off his competitors. But an interesting side effect of this is that, by being in large numbers, the females don't necessarily have to submit to the dominant male as much as they might if they were on their own. Assuming that a smaller male can sneak into the group, the female can choose to mate with him instead, and it's evident that this does indeed happen.
For the species as a whole, though, the males get larger. And, while the larger males may not get everything their own way, they still get the bulk of the benefits, and no longer need to find other ways to maximise their reproductive success. Fighting, rather than the evolutionary tactics seen in promiscuous species, becomes the main focus of their development, and they can't afford to spend resources on both.
Which is probably why the most muscular and aggressive male seals also have the smallest genitals.
[Photo by Nestor Galina, from Wikimedia Commons.]
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