Female bonobos (but not female chimps) help their sons get mates

May 21, 2019 • 1:45 pm

I’ll put this up quickly as I’m getting ready to leave. This multiauthored paper from Current Biology (click on screenshot below; access free with legal Unpaywall app, pdf here and reference at bottom) documents that if you’re a male bonobo (Pan paniscus), having your mom around at mating time gives you a serious reproductive advantage over other males. In contrast, mothers don’t enhance the reproductive success of their sons in bonobos’ closest relatives: chimpanzees (Pan troglodytes). In both species, males remain in a group when mature (they’re “philopatric”) while the females disperse.

Using genetic tests, as well as knowledge of whether a mother was part of a group when the male was siring offspring, the authors determined, for four wild bonobo and six wild chimp colonies of various sizes (see figure below), what the likelihood was for a male to have offspring with and without his mom around. As you can see from the data below, there was a substantial advantage for bonobo males when mom was there: such males had a more than three-fold likelihood of having offspring than when mom wasn’t part of their group. And you can see that there’s no significant difference for chimps. The effect of females on the reproductive success of their daughters wasn’t measured, surely because female bonobos and chimps leave the group when they grow up and thus can’t be found.

Figure 1 (from paper): Observed average likelihood of a male to sire offspring in the presence and absence of their mothers in the group. Bonobos are represented in black and chimpanzees in grey. Circle sizes represent the number of offspring. The generally higher likelihood of a male to sire a given offspring in bonobos is due to the smaller number of males in the group compared to chimpanzees.

How does this work?  From the paper:

. . . a large body of evidence suggests that bonobo mothers also behave in ways that potentially increase the paternity success of their sons. For example, bonobo mothers frequently bring their sons into close spatial proximity with estrous females, protect their sons’ mating attempts from interference by other males, interfere in the mating attempts of other males, and form coalitions with their sons to help them acquire and maintain high dominance rank.

Why not in chimps? This is probably due to the different social structure of the species: in bonobos females are more dominant and thus have the ability to control their offspring, fight off unrelated competitors, and form coalitions. As the authors say:

Such maternal behavior is more likely to be effective in bonobos, where the sexes are co-dominant and the highest ranks are consistently occupied by females, than in chimpanzees, where all adult males are dominant over all females.

Certainly it would help a female chimp’s reproductive success if she could do what bonobo moms do, but they simply don’t have the social status to permit such behavior. The genes behind their social structure have forestalled the evolution of genes for maternal helping of sons’ reproduction.

What is the evolutionary advantage of such behavior? That’s pretty clear: a female’s grandchild carries a quarter of her genes. Any gene that makes a female help her sons reproduce will be represented in more copies than alternative genes that don’t produce such helping behavior. This is just a form of kin selection: an extended form of parental care that involves not just tending your young, but helping them mate.

An unanswered question.  Having your own offspring takes a lot of effort and is costly in terms of time and physiological condition, shortening your lifespan compared to not having offspring. But if you could somehow, as a female bonobo, cease reproducing, you might be able to live a lot longer and, by helping sons have offspring, pass on even more of your genes than if you kept reproducing yourself. If this tradeoff worked, you’d expect females to stop reproducing, going through a menopause but then living a long time and helping their sons find mates.

But this isn’t the case: bonobos and chimps, like nearly every other mammal, keep reproducing until they die. As far as I remember, and I may be wrong, only humans and orcas have a well recognized menopause in females.

Why don’t bonobos have it, too? Well, the tradeoff I mentioned above may not work, or perhaps there are simply no genes around that can produce menopause. Since there’s nearly always genetic variation for every trait, I suspect that the former answer is more likely: you don’t leave more genes by ceasing reproduction and helping your sons have grandchildren for you. But that’s just a guess.

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Surbeck, M., C. Boesch, C. Crockford, M. E. Thompson, T. Furuichi, B. Fruth, G. Hohmann, S. Ishizuka, Z. Machanda, M. N. Muller, A. Pusey, T. Sakamaki, N. Tokuyama, K. Walker, R. Wrangham, E. Wroblewski, K. Zuberbühler, L. Vigilant, and K. Langergraber. 2019. Males with a mother living in their group have higher paternity success in bonobos but not chimpanzees. Current Biology 29:R354-R355.

35 thoughts on “Female bonobos (but not female chimps) help their sons get mates

  1. “If this tradeoff worked, you’d expect females to stop reproducing, going through a menopause but then living a long time and helping their sons find mates.”

    That sounds a bit like the behavior of another well known primate species, especially if also applied to grandsons.

    1. That assumes that the other primate species’ mothers consider any of the available female candidates good enough for their sons/grandsons, of course…!

  2. “nearly every other mammal, keep reproducing until they die”

    I didn’t know. So, now my question is, do human females increase there own genetic influence by ceasing reproduction? Do they help there offspring enough to offset the benefits lost? Why is this the case with humans and orcas but not many others? Why are humans different?

    1. Here is a link to a Nature paper using human demographic data that gets at the grandmother hypothesis: https://www.nature.com/articles/nature02367
      In summary, the authors found that having a living, menopausal mother increased reproductive success of both sons and daughters. They reproduced earlier and had shorter interbirth intervals.
      I think that the grandmother hypothesis tends to consider why human females live on past reproductive senescence, as opposed to considering a tradeoff between continuing to reproduce vs grandmothering.

      1. This seems like a reasonable hypothesis. I suppose, though, that modern grandmothers live longer than our ancestors. Abbreviated lives must reduce the effectiveness of grandmothering.

    1. No, because if they were Jewish mothers, they’d be sending their sons to medical school first.

      That reminds me of a joke:

      A man walks into shul with a dog. The shammas runs up to him and says, “Pardon me, this is a House of Worship! You can’t bring your dog in here.”

      “What do you mean?” says the man, “This is a Jewish dog. Look!”

      And the shammas looks carefully and sees that in the same way that a St. Bernard carries a brandy barrel around its neck, this dog has a tallis bag around his.

      “Spot,” says the man, “daven!”.

      “Woof!” says the dog, stands up on his hind legs, opens the tallis bag, takes out a kipa, and puts it on his head. “Woof!” says the dog again, opens the tallis bag, takes out a tallis and puts it around his neck. “Woof, woof! says the dog, takes out a siddur and starts to daven.

      “That’s fantastic,” says the shammas, “absolutely amazing, incredible! You should take him to Hollywood, get him on television, get him in the movies; you could make a million dollars off of him!”

      “Oy!” says the man, “You talk to him. He wants to be a doctor.”

  3. Interesting! As it happens, I am reading (or rather, listening to) Frans de Waal’s new book “Mama’s Last Hug” and he mentions there how bonobo females help their sons in the group. Maybe not specifically for getting mates, but for their standing in the group.

  4. I believe two other species of toothed whales – Belugas and Narwhals- also undergo menopause. I think the idea is that older females, with their wisdom of years, are better at keeping their kin alive (and reproducing) than if they themselves also reproduce. As an older human, I can attest to the creeping decrepitude of olde age and can see how any value I might impart to my kin in terms of reproductive success might be more effective if I didn’t also have to bear and raise young.

  5. Fascinating. I’ve known one or two males in my life with Bonobo-like mothers.

    “As far as I remember, and I may be wrong, only humans and orcas have a well recognized menopause in females.

    In humans I wonder if this is an artifact of our rapid cultural evolution rather than biological evolution?

    1. I was thinking about the era in which we evolved menopause. If it was after we separated from chimps (and bonobos) our lifespan would have been shorter than now. That leaves less time for grandmothering to be effective. Perhaps women did not often reach the age of menopause in those early times.

  6. I believe you see this benefit in humans as well but it is sister brother thing. If you have an older sister, about a year will do, it can be very beneficial to the brother, increasing girl friends and all that. Just take my word for it.

  7. This clearly doesn’t work for human males in their 30’s. There was a distinct disadvantage to having mom around when I moved back in for a bit… granted my success wasn’t that high before that, not has it improved since.

    1. Indeed, few things jeopardize a relationship as much as having a member of the previous generation around.

      So, if I learn from good bonobo moms as well as from human experience, I must not stay right with my sons’ girlfriends when/if they have any, but step back to a safe distance, out of sight, and circle around to guard against any encroaching other males!

  8. Is this like your mom telling you to go dance with her friends’ daughters at a wedding reception when you’re an adolescent?

  9. Great post. It was easily understandable for layperson, taught me something I didn’t know and left me with a question that I’ll enjoy looking into, “How much energy do Orca grandmothers expend in ensuring the reproductive success of their grandchildren?”

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