Tuesday, March 19

Dispersal Patterns

In this post, I want to discuss an important aspect of primate group behavior: dispersal. Most baboons spend their entire lives as part of a single group, with one main exception. As males approach adulthood, they will disperse, leaving their troop of birth to join a new troop, where they will (usually) spend the rest of their lives.

Female dispersal is rare, for dispersal is hazardous. Leaving the safety of the troop is a problem, but leaving the safety of known territory is also problematic. It's much easier to get away from a predator when you know the location of the closest tree. So why do primates disperse at all? For the most part, dispersal is the only way genes are exchanged between groups of primates. There must be some mechanism for individuals to change groups, or else inbreeding will become a problem.

Male baboons typically leave a troop around the age of 8 or 9. Leaving at that time benefits everyone: the males are strong enough to have a decent chance of surviving on their own (barring bad luck, which happens), the male will have better chances at mating and producing healthy offspring in another troop, and the female relatives of the dispersing male improve their own fitness by encouraging him to leave the troop and reproduce elsewhere.

Females will often refuse to mate with males born in their group, even if the female and male are not directly related. The male can force himself on the female, but that kind of behavior usually isn't worth the extra energy investment. If a male forms a mating pair with a female who doesn't want to be paired with him, she will be looking for any extra-pair copulations she can find, and the male isn't going to get any sympathy from the rest of the troop.

If a male isn't strong enough by age 9? He can usually stick around for a little while longer, but chances are he won't get much stronger, and at some point, the males and females in the troop are likely to let him know that he isn't wanted any longer. In all scenarios, the male will usually become ostracized if he does not leave.

Dispersal is difficult to study. Researchers have to follow young adult males, which is not particularly easy even when they stay within a troop. They need to stay close to the males, so they don't miss when they leave the troop, and then follow them through mostly empty bush, savannah, forest, until they join another troop. Little is known about the daily life of a dispersing male. As far as I am aware, no one has made a career of following dispersing males. Much of what we know has been observed by researchers following troops, who have tried to glean as much as possible from events where new males have joined the troop from outside.

Around the Cape, dispersal is a major issue because the baboons' natural predators have been removed from the ecosystem by humans. While in the process of dispersing, many males are caught by leopards or lions or hyenas. If a male isn't part of a troop, he will more easily fall prey, particularly if he is weak compared to other dispersing males. Without any large predators around, there is a higher male population, and usually not enough space among the existing troops to sustain all of them.

In the second troop, there has been a large influx of males more recently. Bopple seems to be hanging onto his status as alpha, but there have been as many as nine males in a troop of about 50. The troop is almost constantly in chaos. Males need to be able to keep apart from one another, and with that number of males in a single troop, it is simply impossible for them to keep distance between themselves. As a result, they chase each other, they chase the females, and they fight. It isn't a pretty sight. Watching male baboons face-off against each other is exciting, but it does get old, and that sort of volatility doesn't make data collection easy.

For additional reading, and more rigor (though I don't know how outdated their findings are, check out Alberts and Altmann, 1995.

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