Sometimes this is due to a simple imbalance in numbers. In species where males monopolise multiple females, a typical herd, while predominantly composed of females, will also have a single dominant male, forcing any subordinate males to live elsewhere. But this is not the case in deer, because, while stags do indeed mate with as many does as they can get away with, they don't live with them outside the rut. Nor are they alone, since other herd animals, such as goats and antelope, often do the same.
There are many possible explanations for why they might do this, although it must come down in some way to them having different requirements. Perhaps females seek out safer, but less resource-rich habitats, in order to protect their young. Or perhaps they are looking for slightly different kinds of food than the males, which want to bulk up their muscle, rather than producing milk. This might be especially true of deer, where the stags are much larger than the does. Furthermore, the fact that stags spend so much more time eating than does do may just make it difficult for the group to stay together when they can't synchronise their daily activities.
While the causes of this behaviour are still debated (there are likely to be many, and for them to apply differently to different species) most studies have focussed on ruminants, the group to which deer, goats, and antelope all belong. But this does not mean that they are the only mammals to behave in this way.
While there are other examples, some species of bat also seem to segregate themselves by sex. That's because, in the world's temperate zones, female bats are often found living at lower elevations than males of the same species. The same does not appear to be true in the tropics, but there's clearly something keeping females down in the lowlands, while allowing - and perhaps even encouraging - males to roost at higher altitudes.
In North America, at least seventeen species of bat behave in this manner, with it being more obvious, as one might expect, in mountainous areas where the steep slope means that low and high elevations aren't all that far apart as the bat flies. Obviously, this segregation doesn't happen during the mating season and a further clue as to what's happening is that it only seems to apply to pregnant or nursing bats, with younger, non-reproductive females being found alongside the males at higher elevations.
The bats in question are all insect-eating species, and it seems likely that insects are more numerous at lower elevations, where there is more vegetation for them to eat. It's also, of course, warmer, and presumably more comfortable in the lowlands than it is higher up a mountainside, so the usual explanation is that this is a better place for raising young and gaining the necessary nutrition to help with pregnancy or lactation.
Males and younger females, meanwhile, can afford to live at higher elevations because bats have the ability to lower their metabolic activity by partially hibernating. Thus, they need less food, and can better survive the rigours of colder, thinner air in a way that pregnant females cannot. By moving uphill outside of the breeding season, they may face less competition for food, and be less likely to face predators themselves - a pay-off the females can't afford to make. There is also some evidence that, in at least some species, the females actively drive males away from the maternity roosts, again, probably in order to reduce competition for resources.
But not always.
The accurately-named big brown bat (Eptesicus fuscus) is one of the most common species of bat in North America. Across much of western North America, from Texas to British Columbia, big brown bats have been observed to follow the rule that males spend the summer months higher up in the mountains than females rearing their young.
Clearly, they can't do this everywhere; there are, for example, big brown bats in Florida, where flying up the side of a mountain simply isn't an option. But, even here, males spend the summer either alone, or in very small groups that keep well away from the maternity roosts. So, regardless of the local topography, sexual segregation seems to be the rule.
The thing is, it doesn't seem to be a very strict rule.
In places as far apart as Kentucky and Ontario, male big brown bats have sometimes been spotted, in small numbers, infiltrating maternity roosts at times of the year outside of the breeding season. But, if they normally stay part, living in single sex groups, why do a minority of males break the rule? What are they getting out of doing something that, presumably, normally isn't worth it?
Perhaps the most obvious explanation is that they're there for sex. True, this isn't the regular mating season, but perhaps they're trying to get in early - something that wouldn't work if too many of them did it, but a small number might be able to find the odd female willing to mate early (perhaps if she has lost her pup).
This sort of breach of the usual patterns of the breeding season is, in fact, known in some other bat species. For instance, the little brown bat (Myotis lucifugus) has been observed to sometimes start mating in the late summer, before the more usual mating season begins. For that matter, over on the other side of the Atlantic, Daubenton's bat (Myotis daubentonii) is known to actually do so at maternity roosts, exactly as the big brown bat is suspected of doing here.
Now, these two species are far more closely related to each other than they are to big brown bats (despite the similarity in common name of one of them), which belong to an entirely different subfamily. But at least it shows it's possible, and there are some indications that this sort of thing might not be restricted to one single group.
It would be possible to watch bats inside their roosts and see what they're doing, but an easier option is to humanely capture them as they fly out, examine them, and then let them go. The thing is, when we do this with female big brown bats leaving their maternity roosts, we don't see any evidence of them having recently mated, and, indeed, there's not much evidence that the adult males living with them are in any condition to do so anyway. (Bats, like most other non-human mammals, are only fertile at certain times of the year, with their testicles visibly shrinking when they aren't needed).
This may not completely rule out the possibility that it's happening, at least on a small scale, but it does make it less likely as an explanation for what the males are doing there. So, if that isn't their motive, what is?
We're left with at least two possibilities, and it's entirely plausible that both are correct. For, instance, we can note that many of the males captured leaving such roosts are too young to be sexually mature. They're old enough that they have to have been born in the previous year, but it's entirely possible that they are (non-reproductive) teenagers that just haven't left home yet.
A small number are older, but the use of ring-tagging over many years shows that these are mostly bats that were born in that particular maternity roost and that, while they may have hibernated elsewhere through the winter, just keep coming back to the same place in the summer. But even these individuals don't seem to do so every day, apparently sleeping in smaller and more typical roosts most of the time. It may be that they can avoid annoying the adult females by using parts of the roost that the latter don't favour due to their slightly different living requirements.
So it may not be anything to do with mating, but just kids trying to sleep over at their parents' place...
[Photo by the US Fish and Wildlife Service, in the public domain.]
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