|Myotis sodalis, the Indiana bat|
The typical reason for this appears to be that larger males are better able to drive of rivals for their mates' affections, and are also able to demonstrate their fitness and ability to obtain food - and thus, hopefully, the quality of their genes - to those same mates. But it isn't always this way round, since there are a number of species where it is the female that is larger, perhaps since it makes it easier for them to care for and defend their young. (Indeed, outside of mammals and birds, this is by far the more common arrangement). Clearly, which influence wins out, and which sex ends up the larger, depends on the species concerned.
But is there some pattern to this, some rule of thumb we can use to predict which sex will be larger, and, if so, by how much? Working in the aftermath of World War II, German ornithologist Bernhard Rensch thought that there was. In 1950, he formally proposed what has since become known as Rensch's Rule. This states that, if the males are larger, sexual dimorphism will be most extreme in the largest species in any given group, whereas, if the females are larger, it will be most extreme in the smallest species.
Now, this is a biological rule, not at all like the sorts of laws you get in physics, and, as such, it will frequently turn out to be wrong. The real question is whether it's right more often than it isn't, not whether it applies in every single circumstance, which it almost certainly won't. Rensch does seem to have been on to something with birds, which were his speciality, and there are many examples among vertebrates, and even invertebrates, such as flower mites. Among mammals, it appears to generally hold amongst primates, ruminants, and rodents, among others.
However, it has to be said that there has been more study focussed on the first part of the rule than on the second. That is, most researchers have tended to look at species where the males are larger, confirming that the difference is more dramatic the bigger the average size of the animal is. Indeed, one of the few studies in recent years to look at the females-larger part of the rule found that the exact opposite was true, at least in sticklebacks. To be fair, the reason that most studies have looked at species where the male is larger is probably because mammals and birds do tend to get more attention than other kinds of animal, and it usually is the male that's larger in these two groups.
But not, of course, always. While there are many other examples, one particularly significant one in mammals, simply because there's so many of them, is bats. In bats, it's usually the female that's larger, so we should expect dimorphism to be more extreme in the smaller species. But is it... and, for that matter, has anyone really checked?
Well, it's not entirely been ignored, but a newly published study is probably the largest so far. This study chose to look at mouse-eared bats (Myotis spp.), using over six hundred specimens held at various museums and collections across North America. Myotis is the second-largest genus of mammalian species (beaten only by Crocidura, a genus of shrews), having over a hundred members across the globe.
While whether any given group of animals contains one genus or several is a somewhat arbitrary decision, this does at least indicate that there are an awful lot of very similar bats to look at here, giving us a large base of closely related animals from which to work. According to one estimate, the New World species all descended from a single ancestor that lived around 12 million years ago, undergoing a rapid burst of evolution shortly after that, probably as a result of natural climate change. That should be more than enough to allow Rensch's Rule to properly settle in, while the animals being closely related means that there shouldn't be too many other confounding factors.
If there's a pattern here, in other words, it should be easy enough to find it.
But they didn't. Sure, the females were generally bigger than the males, especially when it came to wing size, but how much larger than the males they were didn't vary significantly based on the average size of the species. Smaller species of Myotis are not noticeably more sexually dimorphic than the big ones, as the second half of Rensch's Rule says they should be.
This could just be something unique to mouse-eared bats. This study looked at 36 species of New World Myotis, which may sound like a lot, but is only a tiny fraction of the more than 1,200 species of bat worldwide. On the other hand, there's no particular reason to see why they should be different, since, as bats go, they're fairly "normal" looking... but still, it's at least possible that there's something unusual about them that we haven't thought of.
But assuming that that's not the case, why might these bats not follow a rule that so many other animals apparently do? The implication is that, while there is some reason for the females to be bigger than the males, there's no particular reason for the males to be small, and no reason for some females to want to be bigger than others. Where males are larger, this is often driven by competition between them, and the larger they are, the better for them. But in this case, the females get to a certain size relative to the males and then... well, there's apparently no reason for them to get any larger, so they just stop.
Which leaves us with the question as to why female bats are larger than males in the first place. What do they gain by it? It's got to be something that applies to bats and not to most other mammals (or birds), and has to be something that doesn't involve competition between females to be the biggest of the big.
And, here, the study provides support for a suggestion that dates back to at least the 1970s: the Big Mother hypothesis. That's because the main way in which the female bats were larger than the males was in their wing size. Indeed, the rest of their bodies were actually pretty much the same size as those of males, and it was just their wings that were larger. So why would a female bat need larger wings?
Because, at least some of the time, she's going to be heavily pregnant. Come to that, even after she's given birth, for a time at least, she's probably going to be carrying her baby about with her. All of this presents what we might reasonably describe as "aerodynamic challenges". Bigger wings are surely going to help with that, but only up to a point.
And this, as so often, is the problem with biological rules; they are riddled with caveats and exceptions. Biology might, in the long run, be constrained by physics, but it doesn't have to behave like physics.
[Photo by the US Fish and Wildlife Service, in the public domain.]