Sunday, 9 April 2017
Obviously, reproductive anatomy is an important field if we want to really understand how an animal functions and behaves. Testicular size, for example, can tell us about its mating strategies. This is because the rule is not simply "the bigger the animal, the bigger its gonads". In order to gather a harem of receptive females around itself, a male has not only attract them to itself with suitably impressive antlers (or whatever) it also has to fight off rivals, and it's going to have to be big and muscular to do that. But if the females are sexually promiscuous, that's pointless. Instead, what you really need is to produce so much sperm that yours swamps that of your rivals. So, in those species, the males tend to be smaller, but their testicles larger (proportionately speaking). Alternatively, if your species is monogamous then neither of these things are much of a concern.
So male reproductive anatomy can tell us quite a bit. But you won't typically find, in most descriptions, is quite so much information on the female reproductive tract. There's probably some information on the shape of the uterus, which can relate to things like litter size, and maybe for a few other features besides, but it tends to be rather less than you'll find for the males.
There are a number of reasons why this might be so. One could be that, historically, scientists have tended to be men, who may be focussing on what they see as important. On the other hand, this is less true than it used to be, and there are considerably more female biologists (relatively speaking) than there are, say, female physicists or chemists. So it may also be relevant that, unexamined biases aside, it is rather easier to study male genitalia than female.
For one thing, they're mostly on the outside, where they are easier to get a look at. There is also, in most non-primate mammal species, a bone inside the penis, which is the sort of thing you can keep and preserve in your museum collection without having to worry about the difficulty, and distorting effects, of pickling it. (Mind you, there is often one in the clitoris too, although, for obvious reasons, it's usually a lot smaller and more delicate).
While it's probably fair to say that the female genitalia of mammals vary less than those of males, that isn't to say that they're all the same. After all, both sexes are evolving together, and the precise shape of the penis wouldn't vary as much as it does between species if it didn't matter at all to the females. There's also the point that the female genitalia of mammals are used for more than the mating act itself - if nothing else, mothers also have to give birth through them.
The vagina is, strictly speaking, a structure unique to mammals. Birds, reptiles, and even egg-laying mammals do have a structure called a vagina, which is the last bit through which the egg passes, but it's not really the same thing, functionally speaking. Marsupials have a vagina that's so complicated there's effectively two or three of it (one for each head of the penis, and one to give birth through, if you're wondering). In placental mammals, though, it's basically just a muscular tube.
We've known for a long time that dolphins and porpoises have unusual folds of tissue in their vaginal walls, sometimes making the internal structure quite complex. In fact, while we haven't had the chance to examine every single species, this seems to be true of all the various kinds of whale, too. The shape, size, and structure of these folds varies between species. For example some porpoises are said to have two folds, one at the front and one at the back, so that the vagina has an S-shape in cross-section, while common dolphins (Delphinus delphis) just have the one, so that the cross-section is more of a U-shape.
It's obviously reasonable to ask why this should be; the folds are presumably doing something, but what? In all honesty, despite having known about them for absolutely ages (the link in the previous paragraph is to a paper published in 1932), we're still not that much closer to an answer. One place to start is to get a good overview of just how much variation we're talking about here, and whether there's any obvious pattern to it.
Which brings me to what is probably the largest comparative study of cetacean vaginas ever conducted, which was published just over a week ago. The authors studied a total of 59 animals, all of which had either stranded, or died of natural causes and been washed up ashore. Most of the animals were dolphins, belonging to eight different species, plus a number of harbour porpoises (Phocoena phocoena). However, the sample did include some larger cetaceans. Of those, two were killer whales (Orcinus orca), and two were pilot whales (Globicephala spp.), which are basically just really large members of the dolphin family. However, no less than eight species of "true" whale were examined, with the largest being a minke whale (Balaenoptera acutorostrata).
Perhaps the first thing to say about the findings is that they show a remarkable diversity; it's certainly not true to say that the vagina is simply a hollow tube in cetaceans. The number, position, and size of folds varies considerably between species, and to some extent, between individuals of the same species. Some species have just the one, others have several, with five or six not being unusual, and one harbour porpoise having thirteen. Despite these variations, however, there were some common features - the folds tended to point towards the rear, and were usually found further in, towards the uterine end of the vagina.
A second finding was the the vaginas in these animals were broadly in proportion to the rest of the body. That is, if the cetacean was twice the size, the uterus was about as long. Which may not sound very surprising, but it's frequently not the case with, say, penises. Moreover, and perhaps more significantly, the same could not be said for the folds themselves. Yes, as a general rule, the bigger the whale, the bigger the folds, but the increase wasn't in proportion to that in the rest of the body. This suggests that the folds are doing something specific, and only need to be a certain size to do that, rather than just getting bigger because the whale is. It's also possible that they aren't doing exactly the same thing in every species, which would help explain the variety.
There are also some clues as to what that something might be, at least in general terms.
Perhaps the most popular theory up until now has been that the folds serve to stop seawater getting into the reproductive tract. Since seawater would kill sperm, this is really something that cetaceans would want to do, so it does make a lot of sense. In fact, freshwater would be just as lethal, and it's perhaps worth noting that there is one (and only one) other known mammal to have vaginal folds: the hippopotamus (Hippopotamus amphibius). Since hippos mate underwater, this would fit with the theory (so far as I know, nobody has checked to see if it's also true of pygmy hippos, which are a different closely related, species).
However, there are reasons to suppose that this isn't the full explanation, and, indeed, might be wrong altogether. Perhaps the first thing to point out here is that these folds certainly don't exist in seals, sea lions, sea otters, or manatees. So, if that really is what they're for, there's clearly at least one other way of achieving the same objective. As for hippos... well, it's been known since the early 1990s that hippos are, in fact, the closest living relatives of cetaceans. Which sort of make sense when you think about it (large, almost entirely hairless mammals that spend a lot of time in the water) but was kind of a surprise at the time. At any rate, if they're related, then this isn't a case of parallel evolution, and there may be something else that the two groups of animal have in common, besides the obvious.
That's circumstantial, but a stronger argument could be that, if the folds really do protect from seawater, you'd expect the folds to be largest where the vagina was shortest, since distance alone should be something of a protection against leakage in those animals with a particularly long vagina. But they're not; if anything, it's quite the reverse.
What does that leave us with? It could be that the folds are not to do with mating, as such, at all. They might help to retain sperm, something that would be more important in cetaceans than in other mammals, because, unusually, their semen does not coagulate, instead remaining fluid. (This, in turn, is because, for whatever reason, they have no seminal vesicles).
Another possibility is that, rather then keep water out, they help keep sperm out of the uterus, giving the female some sort of unconscious control over which male gets her pregnant. It could even be something that occurs much later on, such as helping to keep the womb of pregnant whales sealed against external pressure when they are diving at great depth. Not that that would explain the hippos, of course, but something can have more than one function...
Frankly, for all we know, they could just be sexually stimulating the males to make them more likely to ejaculate. At the end of the day, it's still a mystery, and we need more of an examination of dolphin gynaecology to find the answer. Because not everything is about males.
[Photo from the National Oceanic and Atmospheric Administration, in the public domain.]