By and large, this is quite correct, with the alternatives being particularly rare among typical quadrupedal mammals. Doubtless that's because, for many quadrupeds, the alternatives would be particularly awkward, but that's not necessarily the only reason. It has been proposed, for instance, that in insects this position is advantageous when at least one - and ideally both - of the sexual partners still want to be able to look about them for predators or similar threats. If so, there's no reason that that couldn't also be true for at least some mammals.
Having said that, there are exceptions: mammals for which the ventro-ventral position is common, or even standard.
A quick word on terminology here, for those who might not be familiar with the terms. "Dorsum" refers to the back of an animal, which is to say, in vertebrates, the side that the backbone runs down. The opposite side is the "venter". The reason that we use these terms rather than any more obvious ones is to retain consistency when we're talking about different animals, and to remove any possible ambiguity. In quadrupeds, the venter is the underside, but in humans, it's the front, and, if we want to use the same term for both, "underside" is clearly not going to cut it. While "back" is perhaps less ambiguous in this context, it could still be confused for "hind quarters" on a quadruped, which, again, is not what we mean by "dorsum".
So, the ventro-ventral position is one in which the partners' bellies are facing towards one another. Among mammals, this is most common in aquatic animals. It's particularly so in whales, where the presence of the tail would make it difficult to bring the sexual organs into contact any other way, but has also been observed in both seals and dugongs. Among land-dwelling mammals, it's really only common among apes, including, of course, our own species.
But, as it turns out, there are some other exceptions.
It probably won't surprise you to discover that sexual positions in non-human mammals is not a subject that has exactly received a huge amount of attention in the scientific literature. Sure, we'll study when they do it, how often they do it, how long they do it for, and even the precise shape of the genitalia involved, but not, typically, what position they do it in. No doubt this is because, compared with, say, insects, there really isn't that much variety to be studied.
Four-striped grass mice (Rhabdomys spp.) are a group of moderately sized mice found across almost the whole of southern Africa, from the Cape to the edge of the Congo rainforest. During the 20th century, they were thought to belong to a single species, but more recent evidence suggests that there are at least two species, and probably rather more than that. Considering the similarity between these species, and the possibility that they might, if given the chance, cross-breed, researchers decided to determine just how well the mice themselves could tell the difference. Would they, in other words, behave differently to members of the distinct, but very closely related, species, than they did to members of their own species?
The answer to that was, "yes, but not by much". Specifically, for our purposes today, while their partner being a different (but very similar) species discouraged them a little, they still attempted to mate with one another. But what surprised the researchers was that, in about two-thirds of cases, whether they were the same species or not, the mice mated in the ventro-ventral position, with the female lying on her back.
The mice were mating in, roughly speaking, the missionary position.
But why? There's clearly no physical reason why this should be preferred to the alternative, since the mice can, and do, use both positions. Once they had examined the relevant video recordings, however, the scientists realised that there was a difference in how the mice initially behaved towards one another, that seemed to influence which sexual position they would eventually pick.
It turned out that when the females were proactive in initiating mating, whether by grooming their prospective partners (which is almost foreplay, if you're a mouse), or by more overt means of solicitation, ventro-ventral copulation was the more likely outcome. Although it's a small sample, and one has to be careful drawing too many conclusions from the experiment, it would appear that, while the males don't really care about position, the females prefer the ventro-ventral one.
Figuring why she'd prefer it that way takes us, necessarily, further into speculation. We'd certainly need further experiments to decide what sort of benefit she is seeking by reversing the usual mammalian mating position. But the scientists conducting the study do have one possible explanation, and it's one we don't often see in zoology.
Perhaps she just really liked it.
In apes, the ventro-ventral position is thought to result in greater clitoral stimulation, which can have certain consequences in human females. We have no clue as to whether that's also true in mice, although the female does have the requisite anatomical equipment. A few years ago, however, a study in rats showed that clitoral stimulation of females in heat resulted in a boost to the levels of the hormones melanocortin and oxytocin.
These hormones have two functions relevant to what's going on here. Firstly, they control muscular action in the vagina, which may help to transport sperm, making pregnancy more likely. If true, this present a sound evolutionary reason for the process (although, to be fair, it would advantage males, too). Secondly, in humans, and probably in other apes, as well, it generates the sensation of orgasm.
Now, okay, we're talking mice here. Who knows exactly how they experience the world, and what sensations they feel? We don't even know exactly how these sorts of thing evolved in our own species... although we can be fairly confident that motivation to engage in such activity is part of the "why". So we can't really say that female four-striped mice prefer the ventro-ventral position because it gives them greater sexual pleasure - even if the purpose of that pleasure is to encourage them to do something that increases their chances of pregnancy, or of having healthy pups.
But can we definitively say that they don't?
[Photo by C.R.Selvakumar, from Wikimedia Commons.]
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