|Marianas flying fox,|
a related species
It's not the actual temperature of the blood that's relevant, here. The blood of a "cold-blooded" animal, such as a lizard, is, after all, pretty warm when it's basking in the sun. (Which is, in fact, why they bask in the sun in the first place). It's because of that that "warm-blooded" isn't the preferred term among scientists. Back when I was school I was told that the correct word was homoeothermic, but today, when we're talking about mammals and birds, the preferred term is endothermic. The two words don't mean quite the same thing, and it's actually possible for an animal to be one without being the other.
"Homoeothermy" means that the animal is able to keep a constant body temperature, largely regardless of what the environment around it is doing. Get too hot, and you find some way to cool down, and get too cold and you warm up somehow. There are a very small number of non-hibernating mammals that aren't homoeothermic. Naked mole rats are the clearest example; because they live in underground tunnels where the temperature doesn't change much, they've lost the ability to compensate if it ever does change. It's just not a problem they face, and it may be part of the reason why they don't have any fur - they don't need to keep their body heat in.
There are at least two ways for an animal to become homoeothermic. One of them is just to be enormous. Large objects lose heat slowly, so, perhaps with the aid of a few behavioural modifications, a really big animal can keep warm through a cold night simply because the night isn't long enough to have an effect on it. This probably won't work through a long winter, and the animal also takes a long time to heat up (though the latter might be an advantage in the noon-day sun), but, for many purposes, it may well be enough. This sort of homoeothermy isn't really relevant to mammals, but it may well have played a role in the lives of the really big dinosaurs like Diplodocus.
Those dinosaurs may (possibly) have been homoeothermic, but they probably weren't "endothermic", which is the second method. An endothermic animal is one whose body heat comes mainly from within. Basically, mammals and birds "burn" food to produce heat, and then maintain that heat at a steady temperature though sweating, panting, shivering, spraying themselves with water, or whatever else may be appropriate. It has the disadvantage that you have to burn more calories than, say, a lizard of equivalent size and shape, but so long as you can do that, you're fine. In particular, because you have a built in heating system, you can run about and stay active, even when it's cold. Having fur or feathers also helps to keep the heat in, which is at least part of the reason why we do.
Still, the smaller you are, the more of a problem maintaining your temperature is. For the reasons already mentioned, really big land mammals have it easy when it comes to maintaining their temperature - that's why elephants, rhinos, and hippos aren't furry. (It's sometimes said that hippos aren't furry because they're semi-aquatic, and aquatic mammals allegedly tend to be hairless. But this doesn't explain why the hairiest animal on the planet is the sea otter).
For smaller animals, though, it's more of an issue. There are several ways they can keep warm, starting with side-stepping the question altogether and hibernating through the winter. Other methods including shivering, fluffing up your fur, burning extra calories, or just staying out of the cold (say, in a burrow). And there's also huddling, in which a number of small animals bunch up close together so that, thermodynamically speaking, they become like one big animal and keep all the heat in.
We see this behaviour in many small animals, such as rodents and songbirds. If the weather's cold enough, even larger animals need to do it. Emperor penguins being a well-known example.
Bats generally deal with cold weather by entering a state of near-hibernation, or "torpor", in which they slow down their bodily functions for perhaps as little as a few hours at a time. In colder climates, of course, they frequently enter true hibernation for the duration of winter. However, not all bats can do this, and this is particularly true of the very largest bats, the flying foxes. Found mostly in tropical climes, at least some of those in cooler areas huddle together to keep warm.
Or do they? Although the response of flying foxes to cold has been studied in the laboratory before, and they've certainly been seen huddling, a recently published study appears to be the first to really look at what they're doing in the wild. The researchers chose to look at Bonin flying foxes (Pteropus pselaphon), which live about as far from the equator as any flying foxes ever do, on the Ogasawara Islands, a tiny group of humid, subtropical isles south of Japan.
The islands are so small that they can support very few bats. Indeed, there are thought to be only 250 or so adults alive today, and their population is declining as their native forest is cut down. As a result, the IUCN has given the bats its highest possible rating of "Critically Endangered", meaning that they could go extinct at any time.
The first thing to test when checking on how these rare animals cuddle together for comfort, is to see whether it really does have anything to do with the weather, or whether it's just something they like doing anyway. Here, the results were fairly clear: the colder the weather got, the more likely the bats were to bunch up in big furry masses hanging under trees.
It really didn't take much to persuade them to do this, and a temperature below about 15°C (60°F) was enough to persuade almost all of them to join in. That's about as cold as it ever gets in Ogasawara, and while we don't have any other evidence for what a Bonin flying fox considers "cold", we know that related species are really susceptible to chilly weather. Some, indeed, show signs of discomfort when the weather gets below 31°C (88°F), and none seem to be happy below 19°C (66°F). That this is so high compared with just about any other mammal of the same size is likely due to their large, hairless wings, which radiate heat away from their bodies whether they like it or not.
Younger, non-reproductive, bats were happy to bunch up in mixed-sex groups, but the adults tended to segregate. Interestingly, while males were still more likely huddle in colder weather, their groups didn't get any bigger as the weather got colder, with rarely more than ten individuals in a bunch, and usually a lot less. There is evidently only so far that they're prepared to put up with manly cuddling. Females, and, to a lesser extent, the younger bats, were not so circumspect, with bunches of over twenty being recorded, and the groups getting larger the colder it got.
However, around two thirds of those female huddles weren't really all-female at all: they included a male. Just the one, mind you, but that number is obviously significant. It is, after all, not hard to imagine why an adult male might want to be surrounded by females, and why he'd not want to let any other males share in the fun.
These males tended to have favourite places to hang out, as the researchers tested by marking a couple of individuals, and seeing where they went. However, unlike some other flying foxes, there was no sign they were scent-marking the spot to mark it out as theirs, and attract females to it. The researchers suggest that this is classic harem defence, with the male attracting females, and driving away any rivals. (The alternative would be for him to sit somewhere that the females really need to visit, because of its food supply, or whatever, and then defend that).
By no means all other bats have harems. Indeed, given the vast array of different bat species, it's unsurprising that they show a wide range of different behaviours in this regard. Among those that do, it's quite common for established females to drive away newcomers, ensuring that the group never gets too big. But, at least among Bonin flying foxes, the females really want to cuddle up against the cold, so that's not an option for them.
The males, it would seem, are taking advantage of this fact to get a bit more than just a cuddle.
[Picture by the US Fish and Wildlife Service, in the public domain]