Males prefer older females! (??)

Male Chimpanzees Prefer Mating with Old Females

In a recent paper, (Current Biology, November, 2006) the authors show that in chimpanzees, males prefer older females as mates. They explain this as an effect of promiscuity- and lack of pair bond. This is in conflict with multiple studies of diverse taxa that show males generally prefer young(er) females mates.

According to the authors, the primary benefit of youth is future reproductive value- and benefit that is not applicable if your current partner is not likely to be your future partner, as is that case in promiscuous animals. While I agree that FRV is one benefit, it is not the only, and probably not the most important.

For example, another important quality of a prospective mate is current FECUNDITY, which decrease with age. Forget about the performance of your mate in the future, diminishing returns theory seems to argue that one might accept lower probability of future reproductive success for increased probability of current success. Male chimp preference for old-bag female chimps is perplexing.

They contrast this to human monogamy- and our preference for youth.. But wait...Humans aren't monogamous by nature, but instead "forced" into it by culture. Think about this...

1. We see evidence of extra pair mating ALL THE TIME. I forget where I read it, but a recent study estimated the rate of EPC to be greater than 20% in Great Britain.
2. STD's. Think of the huge problem that is HIV and other STD's. STD's are not compatible with a scenario of strict genetic monogamy. Rapid spread of sexually transmitted diseases is a characteristic of a non-monogamous mating system.
3. Evidence from reproductive morphology and sperm competition.

Assuming for now that humans aren't monogamous (and at best practice serial monogamy). Why then, don't humans prefer older female mates?

A potential solution to the old-bag chimp problem:

Maybe what drives chimpanzees to prefer older females is experience. Baby chimp survival is tightly correlated with parental care- i.e. only offspring with good mothers survive to weaning. What if male chimps prefer older females because the on average- older mothers are better mothers- and are more likely to wean babies... HUMAN NOTE: Although there is an effect of experience, new human mothers are (thankfully) 99.9% as likely to rear offspring to weaning as are experienced mothers.

In addition to these arguments- don't chimps have a dominance hierarchy- where offspring inherit their mothers rank? Choosing an old mate might enhance your likelihood of having a high ranking (and thus high fitness) offspring? HUMAN NOTE: There is not a dominance hierarchy in human societies (generally speaking)

The authors use the difference in age preferences to argue that human preference for youth is a derived character, but I don't think that's the case... A more parsimonious explanation is, given the general preference for youth, that the change happened in the chimp lineage after divergence between the 2 lineages occurred.

Abstract:

Cross-cultural studies indicate that women's sexual attractiveness generally peaks before motherhood and declines with age [1]. Cues of female youth are thought to be attractive because humans maintain long-term pair bonds, making reproductive value (i.e. future reproductive potential) particularly important to males [2, 3]. Menopause is believed to exaggerate this preference for youth by limiting women's future fertility [1, 4]. This theory predicts that in species lacking long-term pair bonds and menopause, males should not exhibit a preference for young mates. We tested this prediction by studying male preferences in our closest living relative, the chimpanzee (Pan troglodytes). We show that despite their promiscuous mating system, chimpanzee males, like humans, prefer some females over others. However, in contrast to humans, chimpanzee males prefer older, not younger, females. These data robustly discriminate patterns of male mate choice between humans and chimpanzees. Given that the human lineage evolved from a chimpanzee-like ancestor, they indicate that male preference for youth is a derived human feature, likely adapted from a tendency to form unusually long term mating bonds.

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