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Separate vacations and other sexual differences

Three recent papers in Proceedings of the Royal Society discuss differences between males and females or, in one case, among males.

The costs of risky male behaviour: sex differences in seasonal survival in a small sexually monomorphic primate? by Cornelia Kraus and others, is based on a 10-year study of differences between male and female behavior in grey mouse lemurs. During the breeding season, males had lower survival than females, despite any possible risks associated with pregnancy or raising young. The higher risk for males apparently resulted from their tendency to travel more, looking for females.

The sexes also differ in winter behavior: females hibernate, while males remain active. Is there something about female physiology that makes hibernation healthier for them than it would be for males? Maybe, but there was no difference in winter survival between the sexes, which don’t differ much in size in this lemur species. The authors suggest that hibernation might have longer-term benefits in females, such as increased lifespan, whereas males need to stay active to bulk up in preparation for the breeding season.

This paper reminded me of an earlier paper on albatrosses, in which "in each pair, the male spent the winter just north of the pack ice in Antarctic waters whereas the female stayed south of Madagascar." It’s not hard to understand why males and females might differ in various ways (size, color, etc.) but differences in behavior outside of the breeding season are more interesting.

The second paper addresses an old argument between Charles Darwin and Alfred Russel Wallace, who developed similar explanations of evolution by natural selection at about the same time.

Darwin thought that differences between male and female butterflies resulted from sexual selection: evolution of males, driven by female preferences. Wallace thought that female evolution was more important. They could evolve to be more cryptic (better camouflaged) or they could evolve to mimic a poisonous or distasteful species avoided by predators. In a paper titled “Mimetic butterflies support Wallace’s model of sexual dimorphism,? Krushnamegh Kunte analyzed the family tree of a large group of related butterfly species. Males of related species resembled each other, but females sometimes resembled other species avoided by predators, although there were some exceptions. This pattern is more consistent with Wallace’s hypothesis than Darwin’s. I heard a seminar by Steve Pruett Jones last week on fairy wrens; there, too, differences between the sexes have sometimes resulted from female rather than male evolution.

In the third paper, “Genetic variation in threshold reaction norms for alternative reproductive tactics in male Atlantic salmon, Salmo salar?, Jacinthe Piche and others consider differences among males. Male salmon that spend years in the ocean grow to > 1 kg, whereas other males become sexually mature as much smaller “parr? (10-150 g) without leaving the river. Parr fare poorly in direct competition with much larger males, but they can sneak in and fertilize some eggs. (This reminds of “sneaker male? lizards, which resemble females enough that other males may fail to drive them away.)

The authors compared different salmon populations. In a common environment, larger males from each population were more likely to become sexually mature – smaller males would presumably migrate out to sea and return when they were much larger – but there were differences among populations. This suggests that the threshold for sexual maturity depends on genes which differed among populations. Genetic differences are the raw material for evolution. So if ocean conditions become more dangerous, e.g., due to fishing, genes for males maturing sexually in rivers at a smaller size would become more common. In some fish species, individuals can change to the opposite sex, depending on size and other factors. The threshold conditions for sex change in fish could presumably evolve similarly.

Comments

It sounds like both sides are arguing the same thing: sometimes males and females are subject to different evolutionary pressures.

Wallace emphasized pressure on females, Darwin (at that point) emphasized pressure on males, but obviously both occur, and result in sex differences. The makes sexual variation different from species variation is that of the former, the only ones that survive mating are those which succeed in getting linked to sex traits.

You are probably right that Darwin and Wallace would have agreed that selection could differ between males and females. They appear to have disagreed, however, on which sex had (usually?) changed from the ancestral state. This could presumably differ among species, with Darwin (males changed) and Wallace (females changed) each being right in some cases.

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