Sunday 5 March 2023

Friendship and Fission-Fusion

Mammal species have a wide range od different social systems into which they organise themselves. Many are essentially solitary outside the breeding season, others form long-lasting pair bonds, and others live in larger associations which may themselves have varying different structures. Among the latter, one common pattern is that of the fission-fusion society.

Here, rather than having a long-lasting association, perhaps bonded by ties of family, the animals live in groups but the membership of that group is not constant. New animals are constantly wandering in, while others break off and leave for other groups. On a larger scale, it may be that the individual fission-fusion groups - the actual bands of animals you would see travelling together - are themselves gathered into larger social networks that may have a relatively consistent structure. That is, the new animals joining the group aren't random; they're individuals already known to the group, and rival social networks may exist nearby that do not mix their members.

Such societies are perhaps best known amongst primates. For example, several species of spider monkey live in this way, and the flexibility it gives them may help them to adapt after unexpected disasters, such as a hurricane. It's also the arrangement seen in chimpanzees and orangutans and, arguably, it's a description of how our own species operates and can be a good fit for how some hunter-gatherer communities operate (here, with the "tribe" taking the place of the larger social network and the membership of the smaller groups and hunting parties within it fluctuating over time).

While primates, especially chimpanzees, may provide some of the most extreme examples of the fission-fusion society, they are found amongst many other mammal species, too. Such societies are also seen in elephants, zebras and wild donkeys, hyenas, cetaceans, and, among smaller animals, bats

The way in which such temporary groups can form and break up can vary depending on what it is that draws disparate individuals together. Animals may prefer to associate with others of their own age, or prefer their relatives, or, as is particularly common, based on sex. (The latter, for instance, being echoed in human tribes, where hunting bands are typically all-male).

It's perhaps easier to see why females might want to associate together to the exclusion of males than the reverse. For them, this can provide assistance with child-rearing, increase access to resources while pregnant or nursing, or just deter unwanted attention from males. While males are more likely to be competing with each other for mating opportunities all-male groups are seen in some animals, such as lions, especially if they aren't yet old enough to attract a mate. Here the group can still provide protection from predators, and even if, as in the case of the lions, there isn't much to worry about there, there may be increased foraging/hunting opportunities or a better chance in fights with rivals.

Understanding how and why such social patterns form is not only useful for the general study of animal behaviour but can help with conservation. For example, it may help us to model how diseases would spread through a population and how easy it will be to maintain genetic diversity in the face of human disturbance.

One species that has been the subject of such study is the common bottlenose dolphin (Tursiops truncatus) - the sort of dolphin you're probably most familiar with from dolphinaria. Here, female dolphins gather together when nursing their young, creating nursery groups similar to those seen, for example, in giraffes. For the first few months of its life, therefore, a young bottlenose dolphin is associating with whomever its mother is associating with.

Bottlenose dolphins are not weaned until at least the age of two, if not later, but even before this they will start to venture away from their mother, making new contacts with other dolphins and presumably developing their social skills in preparation for independence. Once they are weaned, but before they reach sexual maturity, they often form mixed-sex associations with other juveniles. These not only provide for further social development but can result in long-lasting friendships that survive through their fission-fusion society, so that they meet up with the same individuals repeatedly even if they don't stay together permanently.

A recent study confirmed this for a population of bottlenose dolphins living in an estuarine area on the east coast of Florida, a population under particular threat from human activity. However, they were also able to trace it back rather further, showing that bonds of friendship formed while the dolphin calf was still with its mother tended to continue into adolescence, and thus presumably into full adulthood. Many of these bonds were, as one might expect, with the calf's mother and any siblings they might have that were already associating with her. Indeed, they were able to follow one such association that first formed when one of the pair was two years old (thus, before weaning) to the age of nine (close to adulthood) when the study ended.

We don't know the sex of this particular pairing, but male-male bonds are more common amongst bottlenose dolphins than the reverse. These are often between brothers - and, because there's only one birth at a time, and females wait for years between pregnancies, these would necessarily be of different ages. Nonetheless, female associations do occur, and these seem to be particularly common between mothers and daughters. This, of course, may affect the eventual formation of nursery groups, if relatives (even if they're only sisters) are more likely to meet up again to do so.

The upshot of this is that, even within fission-fusion societies, marked as they are by constantly changing membership of separate groups, certain bonds of affiliation remain, with animals having other individuals that they prefer to hang out with. These bonds can form way back in infancy, and remain with the animal for life, no matter how much it may choose to wander.

[Photo by Gregory 'Slobirdr' Smith, from Wikimedia Commons.]

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