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This can be due to disaster or misfortune, but it can equally well be due to success. If a group becomes too large, there may no longer be enough food in the local area to keep it healthy, or parasites or disease may spread too rapidly within it. Or it may simply become too large for dominant individuals to control. In fission-fusion societies, this may lead to a temporary break-up into local subgroups that otherwise remain in contact. Sometimes, however, the pressure is too great and the only solution is for a new group to form.
Perhaps surprisingly, such permanent break-ups are most often due to competition between the females, not the males. This is because primate groups, like those of most social mammals, are matrilineal. Males leave home on reaching adulthood, but females stay with their mothers so that, regardless of the dominance of any incoming males, it's their lineage that remains at the core of the group.
In primates that live in small groups, such as lemurs, there is generally a fight between female relatives over resources, and the loser leaves to establish her own family. For species that prefer larger groups, however, things cannot be so simple. If we'd expect even smaller groups of that species to have multiple females, then there has to be a social break, with at least some females having to decide on their new allegiance - they can't head out on their own, but they can't all stay where they are, either.
In rhesus monkeys, for example, what seems to happen is that, as the group gets larger, some members spend more time hanging out with their best friends. These bonds are developed through social grooming - removing parasites and so on from each other's fur - and, if you can't groom everyone, you'll end up with preferred partners. As the group grows, such cliques become more significant and well-defined, until eventually the group splits along these pre-established lines. Basically, everyone gets to stay with whichever new band they think are the cool kids.
None of this is to say that males are never responsible for group break-ups, especially in more "patriarchal" species. In langurs and gorillas, for example, where there is typically only one dominant male in each group, if a younger individual becomes old enough and can't displace the existing patriarch, he takes a small group of females and leaves to create his own harem. He might ensure that they are all close relatives, or might not have any choice in the matter, but often, he acts despotically, forcibly breaking up pre-existing social ties to get what he wants.
Nor is this rigidly tied to every species. Formosan macaques (Macaca cyclopis), for example, can have either female or male-led break-ups, although the former are apparently more common.
Olive baboons (Papio anubis) have some of the most complex social structures of any non-human primate. One of six currently recognised species of baboon, they are the most widely distributed, living in savannah and woodland habitats from the southern edge of the Sahara to the northern edge of the dense jungles of the Congo, across the width of Africa from Mali and Guinea in the west to Ethiopia and Kenya in the east. Troops, which are based on female kinship, can be large, with some having over a hundred members.
Laikipia, in central Kenya, is a savannah region including some wildlife conservation areas. Baboon troops have been studied in the area since 1971, building up information on their behaviour and maternal relationships. (Paternal relationships are less important in baboons, as they are in most primates and, in any event, paternity is harder to test for). In recent years, an invasive cactus has begun growing in the area, proving so tasty and nutritious that the baboon population in the area has increased, leading to the classic situation where the existing troops are becoming too large to manage.
Between 2011 and 2016, two troops lived in the area, including one that had been in existence since at least the start of the studies forty years prior. Following the fission-fusion pattern, there was some movement of individuals between the two groups starting in May 2014. It began with a single male baboon, whom the researchers named 'Yohan' moving from the larger, older troop to the smaller one. He stayed there for three months and then moved back.
In January 2015, he returned to the smaller troop, this time bringing three females with him. One of the females stayed, probably because her mother already lived in the smaller troop, but the other two went back home in June. Then, in October, they came back again, accompanied by two other adult females, their young children, and two subadult females. A month later, another adult female headed across to join them, making the two troops comparable in size.
Finally, in June 2016, Yohan took all of these females with him and left to found his own brand-new troop. This allowed the researchers to look back through their data to see what had precipitated the foundation process and how and why the female baboons had decided to join Yohan rather than stay where they were.
As expected, when they did this, the researchers were able to show that the females joining the new troop had groomed one another more often than they had groomed other baboons in their own troop. Even when they lived with the smaller troop before the split, they remained somewhat apart from them, socialising less than might be expected. They were also more likely to be closely related to one another, perhaps explaining why they socialised more in the first place. In this respect, leaving to form a new troop was less of a stress than it might have been.
But the male, Yohan, was also a major factor. He had sired many of his companions' infants and, leaving aside the possibility that they just happened to like him (for whatever that means when you're a baboon), this would have given the females concrete reasons to stay as his partners. Male olive baboons, unlike those of some closely related species, are unlikely to kill the children of their rivals. However, they do back up the mothers of their children when the latter are picked on by other members of their troop, protect them from predators, and generally do everything they can to help them. It's to their benefit, after all; young female baboons who have good relations with their fathers can live for up to four years longer than those who do not - and that means more grandchildren for the father.
Thus, when Yohan left the original troop, the females who had given birth to his children faced a tough decision. If they stayed behind, they and their children would lose his protection, but, if they left, they would be breaking long-term ties with their female friends and relatives. The fact that the females that ended up staying with him were related (on their mothers' sides) and were already more friendly with each other than with other troop-mates probably made this an easier choice than it might have been.
Thus, while we don't know what triggered Yohan's decision to strike out on his own in the first place, we can see how he persuaded so many females to come with him and join in his new venture.
It's also interesting to note that the split wasn't as total as the researchers had initially supposed. Certainly, the new troop lived apart from the old one, but they were not entirely isolated. They remained relatively close by, and occasionally met up with and groomed members of their former troop. Perhaps they had formed at least some friendships during their stay, or their prior moves had made them less xenophobic - or they were just that way to start with. It may have helped that, in this particular area, there was plenty of food to go around, but not many sleeping sites, so differing troops would often end up close together without having to compete.
Regardless, at least in this case, splitting off to stay with a favoured partner and form a new troop didn't mean that the baboons had to totally abandon all of their former companions.
[Photo by "Zenith 4237", from Wikimedia Commons.]
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