This leads to the concept of sexual selection, where the evolutionary demands on the male may be different than those on the female, leading to different behaviours and appearances. There are often said to be two main driving forces behind such sexual selection. The first is the need for males to compete with one another for access to females, resulting in larger, more heavily built males, often with natural weapons of some kind, such as horns or tusks. The second is driven by the females, when they choose a mate based on some particular feature, leading the males to emphasise that feature in response. (This is, perhaps, most obvious in birds).
The two drives do not need to be in opposition, and may well complement each other. For instance, in deer, antlers can evolve as a means to help stags compete with one another when gathering harems. But the fact that they are, in every other respect, a complete waste of nutrients means that any stag with large antlers is likely to be physically fit, a signal that females will find sexually attractive. So the antlers are large both because the male needs them that way to fight with, and because the females like males with large antlers and voluntarily choose to mate with those that have them.
However, while it wasn't really studied much before the 1990s, there may be a third influence that can also affect sexual selection, and that is the desire of males in some species to use coercion against females to get what they want. This can be difficult to define or identify when we're dealing with nonhuman species, but one scheme considers there to be three basic forms of sexual coercion in mammals... and they're depressingly familiar from human societies.
Firstly, there is intimidation, where the male uses threats to prevent the female from leaving him and mating with someone else. This may use similar methods to that which males use to fight each other, but whereas there, the objective is to drive a rival away, here it's to do the opposite, keeping the female close by. Secondly, there is harassment, which, in this context, means pestering the female sufficiently often that it's less effort to just mate with him and get it over with.
It's worth noting that these kinds of coercion aren't without costs, and these can apply to both sexes. For instance, if you're guarding a potential mate to stop her meeting other males, that may not only restrict her movement and opportunities to find food, but it can restrict your own as well, something that has been shown, for example, in baboons.
And, thirdly, there is the use of direct physical violence, of the sort that would be considered sexual assault in humans. This is the most problematic, since, from an evolutionary point of view, the last thing you want to do is harm the female sufficiently that she has subsequent difficulty giving birth or raising her young, let alone accidentally kill her. (Obviously, this is bad for the female, but if the male wants lots of offspring, it's bad for him, too... just less directly).
Given all this, one would expect that the principle of sexual selection means that female animals would have found ways to minimise this problem. So what do they do, and how effective are such strategies?
When the specific model described above was proposed in the early '90s, the basic idea wasn't new, but the number of studies looking into it in detail was not great, making it hard to see if there were any patterns. 30 years on, there have been many more, and a recent review has gathered them together, to see how widespread the phenomenon is, and, where it does occur, what females do in response.
Of course, such a review still has the limitation that, if nobody has ever done a study on (say) sexual harassment in hyraxes, then we won't know whether or not it occurs. But, with that in mind, it turns out that there is little evidence for sexual aggression in many groups of mammal, including the biggest two, rodents and bats. Which, since rodents, in particular, are fairly well-studied animals, that may be significant - although not, of course, conclusive.
On the other hand, there are still a number of mammal groups where it clearly does occur. For instance, while it doesn't seem to be widespread among marsupials, it has been reported in some species of kangaroo and wallaby. Overall, though, eight families of mammal show up in the data as including species that are particularly likely to engage in sexual harassment of one sort or another.
Three of these are hoofed mammals: the deer, horse, and cattle families (the latter including sheep, goats, and antelopes). Here, we are mainly talking about sexual harassment, with the male constantly chasing the female about until she gives in.
Something similar is seen in the right whale and dolphin families. An extreme example is seen in the case of bottlenose dolphins, where males not only gang up on females to pursue them as a mob, but can become physically violent, biting females that fail to cooperate.
The sea lion family is perhaps the one with the greatest level of sexual aggression, which is mainly carried out by frustrated males that have failed to build their own harem. It's notable that in sea lions, as in deer and antelope, males are physically much larger and better armed than females, largely because these are also the families where males use aggression against each other to determine dominance.
The remaining two families where sexual coercion has been commonly observed are the Old World monkeys (where it has been especially studied in baboons) and... well, the great ape family. Which is the one we belong to.
Indeed, it's in this mammal family that the review was able to find the only study that showed that, at least some of the time, sexual coercion works as a tactic - at least, from the male's perspective. Specifically, a study published in 2014 showed that male chimpanzees that regularly beat up and traumatise females father more children than those which do not, despite their violent assaults not being overtly sexual in themselves.
But males don't get it all their own way; there are a number of things females could do in response. Simply avoiding males is perhaps the most obvious option, and many species of antelope and whale do segregate into single-sex groups for most of the year. But this doesn't really work come the mating season, so as a means of avoiding sexual harassment specifically, it's probably not very effective.
By far the most common methods instead turn out to be the females ganging up into large groups or selecting a dominant and sexually attractive male to protect them from unwanted attention. This has the additional advantage that not only is the dominant male expending his energy fighting off rivals rather than harassing his own harem, but that he has to spread any efforts he does make in that direction out among a larger field. Even if he's using such tactics more often in total, each individual female experiences them less often.
Of course, the male benefits from females doing this as well, since he has a readily available harem that hangs around so he can protect them. It has even been suggested that this may be part of how extreme polygyny evolves in the first place, since both sexes can benefit from it.
On the other hand, monogamy can itself be a means of avoiding sexual coercion. Monogamous species tend to have the lowest levels of such activity, and there is a greater tendency for the sexes to be, if not identical in outward appearance, at least physically matched. Males don't need to compete amongst themselves, and there's no point in trying to use dodgy tactics to acquire additional mates.
That most mammal species don't - so far as we can tell - employ sexual coercion to any great extent indicates that it clearly isn't necessary, even if it works for a few. Humans, as a sentient species, have no excuse at all. We are not chimpanzees.
[Photo by "Caelio" from Wikimedia Commons.]