Females Rise in Baboon Ranks With Help From Moms and Sisters

Susan Alberts
Janet Fang 31 Jul 2014, 23:44

For female baboons, maintaining close ties with mom and having supportive sisters can boost social ranking, according to a new study published in Animal Behaviour

Previous work have suggested that social rank among female baboons is determined not by size or strength, but by nepotism: The higher the mother is ranked, the higher her daughter will be ranked. That's why researchers tend to believe that dominance rank in these baboons is determined at birth. “Daughters of high-ranking females generally mature more quickly, produce more offspring, and have better access to food and mates,” Susan Alberts of Duke University says in a news release. “It’s like being born with a silver spoon in your mouth.”

But baboons may have more social mobility than scientists previously thought. 

A team led by Duke University’s Amanda Lea looked at how maternal and paternal relatives could influence a female’s rank as she grows up. They studied a population of yellow baboons (Papio cynocephalus) in Kenya, living near Mount Kilimanjaro, that has been under close observation since 1971. 

To determine dominance rankings, they analyzed interactions between pairs of females from nine social groups using data spanning from 1977 to 2010. A higher-ranked female would harass those lower in the ranks, reminding them of her right to first dibs on food or nice resting spots. When she bares her teeth, lunges forward, or slaps the ground, the subordinate female cowers and grimaces in fear, then averts her gaze or just slinks away.

The team then compared each female’s social ranking at sexual maturity (or menarche, her age at her first period) with her ranking when she became a mother for the first time. This allowed them to track social mobility across early adulthood.

By the time they became mothers, 22 percent of the females either climbed up or fell down the baboon social ladder. The team found strong evidence that the support of mothers and maternal sisters were behind these changes.

Females who had slipped in the ranks had mothers who spent less time with them while they were growing up. This could happen if their group split, or if the mothers died before their daughters reached adulthood. The team saw this happen with a young female named Abby, whose mother died when she was two; Abby never achieved her expected rank. Mothers intervene on behalf of their daughters in any playground spats over food or mates, helping to ensure their daughters’ place in the pecking order, Alberts explains.

The team also found that females with more sisters were more likely to reach their expected rank. The reason: Sisters gang up on unrelated rivals. Older sisters help out their younger sisters when it comes to out-competing females from other families. However, there’s a double-edged sword here: The more sisters a female has, the less likely they’ll surrender their own position to the younger ones.

Males, however, mate with multiple partners, so their allegiances are murkier. They found no evidence that male or paternal relatives affect female dominance rank.

[Via Duke University]

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