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.
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.
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.