One thing we often see when animals that have been forcefully separated are reunited is that they interact even more than usual, rebounding after their isolation. For instance, male rats kept apart from their friends for a week greet one another with increased petting, social grooming, and anus-sniffing, compared with how they behave on normal days. It probably doesn't come as much of a surprise that something similar is known to be true of humans that have experienced temporary social isolation.
Although usually without the anus-sniffing, at least in my experience.
There are multiple different reasons why this might make sense, and in many cases, it's likely that more than one of them applies. Social animals kept apart from one another are missing a whole range of different activities and experiences that they would normally have, and it can be hard to tease apart which - if any - is more important to this rebound in activity than the others. Perhaps it's just simple stress reduction, or perhaps there's something more specific that renewed greetings allow them to do.
For instance, female hyenas, which normally live in packs, greet one another after separation. Watching how they do this, and with which other individuals shows that, for them, the most important factor seems to be renewing friendly, social, bonds with their hunting partners rather than, say, rebuilding their dominance hierarchy and deciding who is boss. Similarly, among howler monkeys, it seems to be more about reducing tension between equally dominant males than renewing a conflict.
On the other hand, there are a few exceptions. A 1999 study showed that lion tamarins, a kind of monkey related to marmosets) are actually less likely to act in a friendly manner when reunited after a time apart. Perhaps they are in a huff about being left alone... but, at any rate, this seems to be unusual among mammal species.
Some of this is doubtless about renewing bonds with family members, or, in monogamous species, with a long-term partner. In the latter case, in particular, it's surely important to be able to rebuild trust in one another, and to ensure that nothing has changed in your relationship since you last met. To try and tease these reasons apart from more general ones, it's been suggested that it might be helpful to look at a species that, while social, is neither monogamous, nor particularly cares to hang out with its own family.
Such an animal is the common degu (Octodon degus), a South American rodent which, as I noted in a post back in 2016, would rather live with unrelated friends than it would with its siblings or parents. Members of a group dig communal burrows, look out for predators, and even raise each others' children, but they seem generally friendly with one another rather than being dedicated to specific existing bonds. For them, being together with a stranger is seemingly just as good as being with an animal they've previously met; it's just the sociality that they miss when apart.
Being the size of a small rat, degus are easy to keep in captivity, and, indeed, are sometimes kept as pets. This makes it easy to observe how they behave after being placed in individual cages, where they can't physically interact, even if they can see and hear other degus being kept in the same room. While this may have some differences from how they would behave in the wild, it does allow more detailed and controlled experiments than if one went out into the scrublands of Chile to watch them.
So what happens when we try this?
The first result is one that isn't very surprising: degus do, indeed, socialise more following a 24-hour period of isolation than they do if they are separated for, say, 15 minutes. The sort of behaviour they engage in when being reunited with their former partners is much the same as that seen in rats - grooming each other, nuzzling face-to-face, attempting to climb over one another, and sniffing each other's rears. It's worth noting that, while they are both rodents, degus are not that closely related to rats, instead belonging to the guinea pig group - but these sorts of behaviours are fairly common greeting styles in a number of mammals.
Longer separation also led to the animals making more calls when they were re-united. Whether these are specifically 'greeting' calls intended to indicate friendliness or the like is harder to say. Degus do have quite a surprising range of vocalisations, with at least 15 different call types being recognised, but the researchers in this case were unable to find any difference in the sounds made after the 24-hour separation than after the brief ones. They just made them more often.
(As an aside, I'll note that the paper in question does indicate one possible, specific, call type that increased in frequency after the long separation. But, given the small sample size and the number of possible combinations of sound being analysed, it strikes me that this could well be a coincidence. Even if you insist on being 95% sure that a given finding isn't due to chance, if you check 20 different things then, on average, you're going to be wrong about one of them. In fairness, the researchers seem aware of this and don't sound terribly convinced themselves).
What turns out to be interesting, though, is that the degus were even more friendly if, instead of being reunited with another animal they already knew, they were instead paired with a stranger after being isolated. Not only did they engage in more friendly activity, and for a longer period of time, there were also some differences in how they acted. Specifically, they spent proportionately more time sniffing each other's rears, presumably to get better acquainted and make it easier to identify each other in future.
The sorts of sounds they made were, in this case, also notably different. They were certainly more vocal when meeting a stranger than when greeting an old friend, but proportionately fewer of these sounds were classified as "chitters" - as opposed to whistles, chirps, squeals, and so on. This may indicate that the chittering sound is specifically a greeting between familiar animals, and used to reinforce an existing social bond. With a stranger, things are less certain, and the degu may wish to spend more time scoping them out and seeing how they behave before committing to a closer relationship.
Again, this fits with some of what we know from other rodents. If rats are prevented from making the usual friendly/playful greetings to others of their kind, then any meetings rapidly turn into aggressive fights, with one rat seemingly furious about the ominous silence of the other.
One simpler explanation for increased social behaviour after separation is that it's a response to the stress of isolation. There's probably some truth to this, and the study mentioned above also showed increased vocalisation and social activity following general stress rather than prolonged separation. But, again, the activity was different, not lasting for as long and with less of the chittering sounds. The authors point out that, in the wild, stress is most often due to nearby predators, making it a really bad time to make a lot of noise but, either way, it's a different response.
Degus, like humans, want social interactions with others, and the actions they take on meeting others after being forced to spend time apart not only allow them to reestablish social bonds that will be important going forward, but likely just make them feel better.
[Photo by Arjan Haverkamp, from Wikimedia Commons.]