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Bigger males, or smaller females?

As you may have noticed, males and females look different in many species. In “brood parasite? cuckoos, those members of the cuckoo family that lay their eggs in nests of other "host" species, males are mostly bigger and more colorful than females. Did males become bigger and more colorful over the course of evolution? This could be due to sexual selection, based on female choice or conflict between males. Or, did females become smaller and less colorful? That could be due to coevolution with the host species. Less colorful females are less likely to be noticed hanging around host nests, and smaller females may lay smaller eggs that are harder for hosts to tell from their own eggs.

This week’s paper is “The evolution of sexual dimorphism in parasitic cuckoos: sexual selection or coevolution?? by O. Kruger and colleagues at the University of Cambridge and Boston University, published online in Proceedings of the Royal Society.

Kruger and colleagues looked at all 141 cuckoo species. Some care for their own young; others are parasitic. Two authors independently scored each species for male versus female differences in appearance. Wing length was used to see which sex was bigger in each species.

They then compared these data to the cuckoo phylogeny (family tree). It was already known that brood parasitism has evolved three times. Further evolution within the three parasitic branches led to a total of 59 parasitic species, almost half of all cuckoo species.

They used a computer program that combines family trees with data on living species to infer the mostly likely traits of ancestral species at each branch of the tree. This showed how traits changed along each branch.

In cuckoos that raise their own young, 71% of species have females larger than males -- eggs cost more than sperm -- whereas 84% of parasitic species have males larger than females. Compared to their nonparasitic ancestors, two branches showed a change towards males being larger than females. In the third branch, males were already bigger, but this difference increased. Statistical analyses linked to the family tree suggest that these trends were mainly due to a decrease in size for both sexes, with females decreasing more than males.

Evolution of color differences was less consistent. In two of three parasitic branches, plumage changed from “showy? to “cryptic? in both sexes, but the third branch became more showy. The showy species trick hosts into chasing them, then sneak back to the nest and lay eggs.

Coevolution of traits linked to parasitism was a better explanation for evolutionary trends in sex differences than was sexual selection based on female preference or competition among males.

But is there still some role for sexual selection? If females that mate with smaller males have smaller daughters, and those daughters are less likely to have their eggs detected and thrown out of host nests, have females evolved a preference for smaller mates? If so, how does female preference interact with conflict between males differing in size? As we scientists like to say, more research is needed.

Other interesting papers in the same issue:

Although some female preferences based on male appearance may be arbitrary, another paper shows that colorful sexual ornaments in stickleback fish are a good predictor of whether the male has enough carotenoids for a long and healthy life, a trait he might pass to his sons. "Why do men marry and why do they stray?" looks at the timing of male infidelity in one foraging and farming culture with <10% polygamy and concludes that men marry to maximize resource provisioning to their children, not to monopolize "access to women's fertility." Meanwhile, back at Meercat Manor, subordinate males, who we thought were stuck home baby-sitting, may father up to 25% of babies in nearby groups. Are fundamentalists right that evolution leads to sex? It certainly did here, but it's risky to generalize from only one planet.


"It was already known that brood parasitism has evolved three times".

I don't have access to this journal, so I wasn't able to read the references in Kruger's paper. Would you mind giving me the reference to the convergent evolution of brood parasitism? I'd love to read how that work was done. My own research was so closely focused on molecular evolution that I neglected to read much on macroevolution and I'm gradually putting that right!

Thanks in advance

I think this is the one you want: Sorenson, M. D. & Payne, R. B. 2002 Molecular genetic
perspectives on avian brood parasitism. Integr. Comp. Biol.
142, 388–400. (doi:10.1093/icb/42.2.388)
Often, the authors of a paper will send you a PDF if you email them with "reprint request" in subject line.

Excellent, thanks!

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