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Sex bias in lizards

When humans can control the sex of their offspring, they usually choose to have more sons than daughters. This collective behavior results in a shortage of females, so females are more likely to marry than males are. Parents who chose to have only sons are therefore less likely to have grandchildren. This week's paper shows that lizards are more sophisticated.

"Cryptic sex-ration bias provides indirect genetic benefits despite sexual conflict" was published in Science by Robert Cox and Ryan Calsbeek.

In many species, females prefer to mate with larger males. In species where this is true, males are often larger than would be optimal (in terms of food needs, ability to squeeze through tight spaces, etc.) in the absence of this preference.

To the extent that size is inherited, the sons of females who mate with larger males will be more likely to find mates, because... females often prefer to mate with larger males. But what about their daughters? They may also inherit genes for large size, so will they have the disadvantages of large size, without the male-mating-success advantage? If so, that cost could counter-balance the offspring-fitness benefit of mating with larger males.

One possible solution for this problem would be for females lucky enough to mate with large males to produce mostly male offspring, while those who mate with smaller males produce mostly female offspring. But is this possible?

It is for anole lizards. They have about 20% more sons than daughters, after mating with a large male, but the reverse if their mate was small.

This would make evolutionary sense if sons of large males were larger and therefore more fit, while daughters of large males were larger and therefore less fit. In field tests, however, only the first of these was true. So, by producing relatively more sons when their mate was large, they increase the fitness of their offspring. But why bias sex ratio the other way, when their mate is small?

The paper doesn't seem to answer this question. Maybe their genes reason (to use what Richard Dawkins has called a "harmless anthropomorphism") as follows: "Some lizards mated with larger males. They will have mostly sons. So if I make this female have mostly daughters, they are sure to find mates, whereas sons might not. So I'm more likely to make it into the next generation by biasing reproduction in favor of daughters than if I contribute to the male surplus."

If only humans were so smart.

For those who may be interested, I have finally sent a draft of my book, "Darwinian agriculture: where does nature's wisdom lie?" off to Princeton University Press. They will be sending it to two scientists for review, after which I assume I will have some revision to do. Also, my student Ryoko Oono has a paper accepted for publication in New Phytologist about the evolutionary history of an interesting interaction between legume plants and the symbiotic rhizobial bacteria that live in their root nodules and, to varying extents, provide their plant hosts with nitrogen. I'll write more about that when it comes out.

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