One of the most dramatic sexual conflicts documented so far, that in Tanzanian cockroaches, turns out to have yet another bizarre twist. Females are most attracted to low-status males, and those couples have fewer sons than do females matched with randomly supplied mates.
What triggers this drop in sons is a scent, or pheromone, produced by the fathers, report Allen J. Moore of the University of Manchester in England and his colleagues. The effects of this perfume, which attracts females like a magnet, show up in the next generation when the number of sons is reduced by about 12 percent but the number of daughters stays the same, the researchers report in the March 7 Proceedings of the Royal Society of London B. Moore says he is unaware of another pheromone that has such an effect on the next generation.
“It’s an extremely puzzling result,” comments Locke Rowe, an evolutionary biologist at the University of Toronto. “It’s the opposite of what you’d expect. Why would she make a choice that would knock down her reproductive output?”
Moore says he became intrigued by these thumb-size roaches, Nauphoeta cinerea, because “males form dominance hierarchies like chickens.” He notes that females bear their young live and protect the fragile newborns until their cuticles harden.
Moore and his coworkers already had observed dramatic conflicts of interest between males and females. As males compete for status in intense wrestling matches, females have their feelers out for something else.
When the males chase one another and roll around, “it can be dangerous,” Moore observes. “Subordinate males mostly run away.” Researchers found that in this macho roach world, three pheromones orchestrate the males’ places in their hierarchy.
Males reeking mostly of the first two rule. When the researchers dabbed the third component onto a male, his status plunged and previously lesser males chased and pummeled him. Low-ranking males run a higher risk of having other males chase them away from females.
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Why wouldn’t the genes for such a status-reducing scent disappear from the male population? The female roaches like it, the Moore team found. The amounts of the first two high-status scents don’t seem to matter to females. Instead, the female roaches respond to males based on the amounts of the third, status-lowering chemical.
Just what’s in it for the females isn’t clear yet. “My pet theory is that females are avoiding hyper aggressive males,” says coauthor Patricia Moore.
In their latest exploration of this reproductive battleground, the researchers tested for consequences of mating choice. They put females in special chambers to see which male’s scent the females preferred. Each female was then allowed to mate with her chosen fellow or the reject. The researchers also let little groups of two females and two males sort out the matings however they chose.
When researchers compared the offspring of these matings, females who had sniffed the desirable scent during courtship had fewer sons in the first clutch, regardless of which male had ended up as her mate. When females had a second clutch of young, the effect of the pheromone on the number of sons was no longer significant.
In a second experiment, the researchers treated males with various combinations of scent and kept track of their offspring. The low-status scent alone accounted for the decrease in sons.
The Moore team says it has yet to figure out what evolutionary advantages might come with the lower son counts.
The roach system “goes against old ideas about sexual selection,” says Allen Moore. Males compete, and the usual expectation is that females prefer the winner of the competition. For example, deer with big antlers fight well, and females tend to prefer males with big antlers. The Tanzanian roaches provide one of the most clear-cut examples of females choosing the loser in a male-based competition.