As such, we'd typically expect a genus to contain only a small number of species - it is, after all, the smallest standard grouping of species that there is. And, at least for mammals, this is typically the case. Many genera, in fact, have only one known species, or at least only one that isn't extinct (as is the case for our own genus, Homo, for example). But there are some exceptions, cases where there are so many incredibly similar species that we just have to lump them all together.
The single largest mammal genus, as commonly defined today, is Crocidura, which represents nearly half of every species of shrew we know about. But even the second largest, Myotis, is a whopper.
The word 'Myotis' literally means "mouse-eared", and so these creatures are commonly referred to as "mouse-eared bats". They are found on every continent, bar Antarctica, and just about every habitat where there are insects to eat and places to roost. The exact number of species is unclear, and constantly changing, especially as we keep discovering new ones - but it's certainly over a hundred.
Perhaps unsurprisingly, there have long been attempts to find some pattern within all of these species that might allow us to arrange them a little better, and hopefully also tell us something about why so many seemingly similar animals can live alongside one another. The problem, of course, was that, apparently because they have changed relatively little since they first evolved, they wouldn't have been placed together in the first place if it wasn't really hard to tell them apart.
Back in the 1940s, a review of the then known species of Europe and Asia proposed grouping them into seven subgenera on the basis of subtle physical distinctions. By the 1970s, four of these subgenera had been ditched, but it still seemed that we could at least divide them into three broad types.
Then, of course, we managed to do the genetic studies, and found that none of the three sub-groups actually existed, in an evolutionary sense. Instead, for the most part, Myotis bats that live together on the same continent are more likely to be closely related to each other than to those that don't. (Even then, there are exceptions, presumably because bats can fly from one continent to another if they really, really want to).
But, in a way, this might actually be helpful. After all, the three physical subtypes do actually exist, and if they don't represent direct evolutionary relationships, that means that they have evolved separately on each continent, quite possibly more than once each. Now, we're talking pretty subtle distinctions here, since the bats are still all Myotis, but these adaptations must be useful to have been evolved so many times, and the most obvious way that that might be the case is if they relate to feeding. If each of these three kinds of bats eats different food, it would help to explain why there are quite so many species living, in many cases, in exactly the same area without driving each other to extinction.
Of course, we already know, in general terms, what mouse-eared bats eat: insects. But insect species are even more numerous and varied than mammal species are, and there's no guarantee that, in the wild, all the bat species are eating exactly the same kind, even allowing for their various geographic localities. Checking the gut contents and dung of over 100 different species of bat to compare them is clearly a ridiculously large task, but what we can do is examine all the studies of individual bat species already conducted over the decades (which comes to around 22% of the total known species) and see if they have found any differences.
When we do, what we find is that there are three distinct types of diet found among mouse-eared bats, and these correlate reasonably well with the three physical subtypes identified back in the 1970s. The matches aren't perfect, likely perhaps because some species are pretty flexible in their diets, changing their exact habits to suit particular circumstances. But, in general, it fits with previous ideas of how those subtle physical differences relate to how differing bat species catch their prey.
For example, one group fed primarily on fast-flying insects, such as wasps and dragonflies. All but one of the species with this diet had a physical form with longer snouts, teeth more adapted to stabbing than slicing, longer legs, shorter tails, and smaller wings than the average Myotis. Bats with this physical form have been described as "trawlers", using their feet to catch insects in flight, often over water. (The one exception lives in Madagascar, where the choice of insects, and indeed, the nature of its competition, may be different than it is for similar bats elsewhere).
The second type of diet favoured soft-bodied, slow-flying insects, such as moths and mayflies. Bats that preferred this food typically have short snouts, small teeth, and weak jaw muscles, all of which makes sense if you're mainly eating soft food. They also have wide wing membranes between their legs and their tail, anchored to their ankles by bony spurs. They use these to literally scoop insects out of the air before eating them - which would explain why they go for the slower, less agile, ones.
The third type of diet consisted almost entirely of large, hard-bodied, crunchy insects, such as beetles and grasshoppers, and also included a number of flightless non-insect arthropods, such as spiders and centipedes. Bats with this diet were physically larger than the other kinds, lived only in Eurasia, and all had long jaws with relatively powerful muscles and slicing teeth, wide wings and small feet. These are "gleaning" bats, slow, but highly manoeuvrable in the air, and that feed by plucking prey off of leaves or other surfaces.
They weren't the only mouse-eared bats in the review to feed like this, since some American species do, too, while apparently favouring smaller, softer prey, but even they had the "large body / strong jaws" physique that would suggest they could eat crunchier prey if they wanted to. (The bat in the photo at the top of this article belongs to one of these species). Of course, it's a lot easier to eat soft prey using strong jaws than it is to eat hard prey with weak ones, so a lot may depend on what food happens to be around at the time.
Back in the 1970s, we thought that these three body types had something to do with how the mouse-eared bats had evolved as a group. They aren't - not directly, anyway - but if one bat eats beetles while another eats moths, even very close relatives can live in the same area without getting in each other's way.
[Photo by Al Hicks of the US Fish and Wildlife Service, in the public domain.]