compete for the same resources. Other problems, which might be less obvious, include the fact that infections and parasites are more likely to spread amongst a group of similar animals that remain in close contact. All of these are good reasons for solitary living, as seen in a great many mammal species.
Despite which, of course, there are many mammals that do live communally. For these animals, whatever costs there may be are obviously outweighed by the benefits, of which there are several. While a large group of animals may be easier to see, it's also easier for them to keep a lookout, since not everyone needs to be actively scanning the horizon (or whatever) all of the time. They can also share in communal tasks, such as child-rearing, putting less strain on the individuals. Predators that hunt in packs can take down prey far too large for any one of them to kill on their own. Even huddling together against the cold can be a worthwhile benefit.
Sociability is common in primates and is something that has obviously been very useful for humans. In our case, we can take further advantage of our group-living through the use of language, making it easier for us to cooperate on a large scale in all manner of things that we couldn't otherwise do. The real kicker here is that we can pass on information from generation to generation, and actually build on what has gone before without having to start again from scratch.
Other animals don't really have language in this sense, but that doesn't mean that they don't communicate, and it's plausible that improved communication could also be a benefit to group-living mammals. They may not be able to go from taming fire to inventing the internal combustion engine, but there is surely useful information they can impart to one another.
Arguably the most sociable mammals other than humans are bats. Bat colonies can rival human cities for population figures, with vast swarms of the animals flying out every night to feed. Many of the advantages to group living listed above are likely also relevant to bats, and there's the additional factor that caves aren't all that common. While this obviously doesn't affect those bats that roost under tree canopies or the like, the cave-dwelling sort are probably forced to live together if they want to find anywhere suitable at all. Indeed, even tree-dwelling bats may face similar pressures if their roost requirements are narrow enough - if not quite on the same scale.
But do bats also benefit from communicating useful information to one another? If so, is this something that has developed along with sociability or just an inevitable consequence of it? The most likely sort of information that a bat might wish to impart to its fellows is where good sources of food can be found. This applies equally to bats looking for trees that are currently in fruit and to those that want to find temporary aggregations of insects. (If the food source is more permanent, communication is less relevant, since the individual bats will likely figure it out for themselves before too long anyway).
Certainly, we can observe that, where suitable food is temporarily abundant, it's not uncommon to see large numbers of bats feeding together. The question is how they knew about it. After all, this doesn't absolutely require any communication at all; they might simply have all independently found the food source using whatever usual methods they might have. In fact, even if they do communicate, it would be surprising if this didn't happen from time to time anyway.
Even if they aren't independently stumbling across the same food source, this doesn't necessarily imply any active communication. Perhaps they happen to notice that several members of their own species are flying about in a particular area, and go over to see what all the fuss is about. Except that, in the case of bats, this probably isn't a visual thing... bats' night vision is better than often assumed, but it's not ideal at a distance.
As insect-eating bats home in on their target, many of them emit a particular series of echolocation calls that narrow down its exact location. This so-called "feeding buzz" consists of a short series of very rapid broadband calls, ideal for tracking a specific, fast-moving, aerial target. Crucially, since they don't make this sound unless they have already found something worth chasing, it's possible for other bats to "listen in" and head towards a site where they can hear feeding buzzes, rather than the differing types of call that they use while still searching.
Just how far away a bat has to be in order to be able to hear its fellows targetting insects is the sort of thing that varies between species. In the lesser bulldog bat (Noctilio albiventris) it's about 40 metres, while, in the velvety free-tailed bat (Molossus molossus) it's closer to 55 metres. How do we know this? Because scientists have set up loudspeakers emitting fake feeding buzzes, and watched what nearby bats do in response. And what they do is head over to check it out.
To be fair, this isn't universal. It has been observed in several different species (some at ranges of up to 150 metres) but some don't seem to pay any attention. And, given that there are well over a thousand different species of bats, it's unsurprising that we've only checked a small number of them. On the other hand, at least some bats will pay attention to calls even if they aren't made by their own species. This has been observed, for example, in two species of mouse-eared bat (Myotis spp.) that tend to have similar feeding preferences.
For that matter, one group of researchers, perhaps for reasons best known to themselves, played a recording of a feeding buzz produced by spotted bats (Euderma maculatum) backwards... the nearby bats fled the scene.
But what about fruit bats? While they often use echolocation to navigate, avoid obstacles, and so on, they don't need to make a feeding buzz to focus in on immobile fruit. However, they may still impart information after returning to the roost, for the simple reason that the smell of the fruit may still be on their breath, or perhaps their fur. Seba's short-tailed bats (Carollia perspicillata), for example, have been shown to sniff out other bats that have recently had a good meal, and follow them on their next outing to see where it came from.
Insect-eating bats can do this too; evening bats (Nycticeius humeralis) are apparently able to detect whether their fellows have eaten well, possibly by sniffing their urine. This sort of thing allows bats to pool their information, even if they aren't sending out "deliberate" signals. Interestingly, a study with tent-making bats (Uroderma bilobatum) showed that they pay more attention to these signals if they are from a bat they don't know than from one they are familiar with - perhaps because the newcomer is more likely to know something they don't.
When it comes to deliberate signalling to help out one's fellows, however, the evidence is rather lacking. The methods mentioned above are unavoidable consequences of either hunting or feeding, something fellow bats make use of regardless of whether the bat sending the signal wants them to or not. While there has been some suggestion that the sorts of social contact calls bats make amongst one another in the roost may also be used to call foraging bats over to a location, the reverse seems to be more common.
Pipistrelles (Pipistrellus spp.), for instance, have been shown to send out warning calls telling other bats to leave them alone (presumably under an implied threat of violence if anyone tries to steal their food). Mexican free-tailed bats (Tadarida brasiliensis) go one further: they send out jamming signals that interfere with the echolocation calls of other members of their own species, causing them to miss their target.
Because, while cooperation is all very well, sometimes you can have too much of a good thing.
[Photo by the US Fish and Wildlife Service; in the public domain.]