Out of Lockdown

Many mammal species are, by nature, solitary, spending most of their adult lives apart from others of their kind except when mating or raising young. But others are inherently social, living in herds, packs, or simply in mated pairs that live together for a long time even when they don't have children. For such animals, being separated from others is an abnormal situation that can be stressful.

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.

Sharing Your Burrow

One of the key features of animal behaviour is sociality - to what extent the animal associates with others of its kind. Many mammals are solitary, meeting up to breed, but otherwise spending their adult lives alone, except when mothers are raising their young. That so many aren't is probably partly due to that period of long parental care. Mammals are defined by their ability to produce milk, which necessarily implies some degree of mother/child bonding, and it may well not take too many behavioural modifications to get from there to just not leaving home at adulthood.

Social behaviour has both benefits and drawbacks. On the plus side, pack hunting makes it easier to take down larger and otherwise unavailable prey, if you're a predator. If you're not, there's safety in numbers, and the more of you there are, the easier it is to spread out the duties of looking out for threats. On the downside, large numbers do make you rather more obvious, and if you're all after the exact same kind of food, there'd better be a lot of it about or some of you will go hungry.

Friends and Family Among the Degus

A great many mammals are solitary. They spend most of their adult lives more or less alone, only meeting up with others of their kind in order to mate. Apart from a mother with her young, the extent of their social lives outside the mating season is just driving off rivals. But, of course, there are a great many that are sociable amongst themselves, forming herds, packs, or other associations. For a herbivore this often provides safety in numbers, while a pack-hunting predator may have the ability to take down larger prey than it otherwise could.

How does social living get started, in evolutionary terms? Perhaps the simplest way is that children simply fail to leave their mother, creating a fairly permanent family group. According to one theory, such groups are likely to become particularly stable if there are not enough resources around (for whatever reason) to allow the children to wander off and have young of their own. In these situations, the theory proposes that the older children hang around in order to help their close kin, such as younger siblings, and thus have at least some chance of passing their genes on to the next generation.