In the case of humans, of course, much of our social knowledge is built up as children, learning social rules through observation and experience as well as through more explicit instruction. And, while out ability to use language to impart detailed information is something that's essentially unique to us, the need for a suitable environment to fine tune social behaviour isn't something that just arose out of nowhere. Raising an animal in isolation from others of its species might not be as cruel as trying to do the same to a human child, but that's not to say that it wouldn't have some effect on them.
In particular, this can be viewed in terms of an animal's "social competence", the ability to respond appropriately to the social cues of others, depending on the context of the situation. When, for example, should one react aggressively to another animal? If this is entirely hard-wired, then the response to any given social cue will always be the same, but, in reality, previous social experience and upbringing clearly also play a role.
This isn't restricted to mammals, either. For example, it is well known that the ability of songbirds to sing is related to their experience of listening to others. Those that are raised without hearing regular birdsong - whether because of isolation, or because of deafness - do still produce songs that are appropriate for their species, but they lack the ability to create the richness and complexity of conventional birdsong. Indeed, if they can hear songs from a recording, but not socially interact with the bird producing it, that, too, affects their ability to create original compositions. Perhaps more surprisingly, similar effects on behaviour have even been seen in lizards and in some insects, such as cockroaches.
In the case of the lizards and cockroaches, of course, this has nothing to do with song. Even mammals don't really have anything that's quite equivalent to birdsong, although the complexity of, say, gibbon calls, is at least in the same general area. But vocal communication is a key part of many mammal species' lives, and it's plausible that that, too, might be affected by social rearing - albeit probably not in the same way that it is in songbirds.
The classic animal for studying this sort of thing is the everyday house mouse (Mus musculus), familiar to us from homes across the world, and widely used in laboratories for a range of purposes. House mice are social animals, so it's no surprise to discover that rearing them in isolation affects their behaviour. Most obviously, mice that lack any social learning tend to be more aggressive than mice reared with others, perhaps finding it harder to interpret social cues of submission or friendliness, and reacting to any stranger as something unwanted. But it has been known since at least the 1970s that mice reared in isolation exhibit a much wider range of changes to their behaviour and to their physiology.
Mice, like many other rodents, produce ultrasonic song-like squeaks, too high-pitched for humans to hear, that are a key part of their social interactions. And we know, from past studies, that lonely adult mice call more frequently, when encountering a stranger, than do mice that regularly meet others of their kind. We'd reasonably expect the same thing to be true for mice that have been reared alone since infancy, but how might this be related to their other behavioural changes?
Mice are dependant on their mothers for the first three weeks of life, so we can't truly raise them in isolation for their entire lives, even if we wanted to, but a recent study took male mice at around the age of weaning, and raised them either alone, or in groups of four. They then placed them with other male mice and watched them for fifteen minutes, recording their calls, in order to see how their responses differed. (The researchers note that, had the answer been "one of them violently attacks the other", they would have quickly separated them, but fortunately this turned out not to be necessary).
As expected, the mice reared in isolation called more often, and for longer, than those that had not been, and they also spent more time investigating the stranger by, for example, sniffing them. However, a new discovery was that the previously isolated mice emitted ultrasonic squeaks that were lower in pitch than those typically emitted by their socially experienced peers. This is interesting, because such "low" pitched ultrasonic calls are typically associated with courtship - the sort of thing we'd expect male mice to do when meeting females, but not, as in this case, other males.
Presumably, the lack of social experience on the part of the isolated mice is in some way changing the calls they chose to make when meeting other males. It could simply be that lower pitched noises are the sort of thing they do when they're excited, but it could also be that they're having some difficulty working out that the stranger is, in fact, a fellow male. After all, female mice do find lower pitched calls sexy, so it's surely no coincidence that males make them in mixed-sex encounters. Not only that, but the individually reared mice, when placed with a stranger raised in a group, often tried to mount the interloper, placing their paws on his back and making thrusting motions with their hips.
From a human perspective, this would certainly seem to suggest sexual interest in the stranger. But that's not necessarily the case here. In some animals, mice included, it has been suggested that this sort of apparently homosexual activity is really just a display of dominance of one male over another. If so, this would explain why the individually reared mice tried to mount their socially raised peers, but did not do the same when meeting other mice which had been raised as they themselves were. In other words, their problem is not that they aren't sure what fellow males are supposed to smell like, but that they interpret the less aggressive social cues of group-reared mice as being a sign of weakness and submission.
Without social experience of other mice, those reared alone have difficulty making sense of how other mice are supposed to act, and are more likely to respond to even harmless social cues with aggression. This is seen with other animals too; rhesus monkeys reared in social isolation sometimes react badly to being groomed by strangers - something that should be entirely harmless, and indeed, pleasurable. To them, gestures of friendship are seen as potentially aggressive, perhaps an invasion of their personal space, because they don't know how else to interpret them. While there are other possible explanations on the part of the mice (i.e. it really could be a sex thing), this could well be the same kind of confusion.
Either way, it seems to show that even in mice, understanding social norms is something that has to be learned, and that doesn't come automatically. For we humans, it's a longer, and more complex process, influenced no doubt by the specific culture in which we happen to have been raised, but even young mice need positive social experiences in order to develop into normal members of their "community".
[Photo by "4028mdk09", from Wikimedia Commons.]
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