A strange group of molluscs, crustaceans, worms and fish have found a way to switch sexes to meet their mating needs. Why don’t more of us follow suit?
A pair of Yale scientists used mathematical models to tackle that question — and reached a surprising conclusion that challenges conventional wisdom on the topic.
Many bluehead wrasse, for example, are born female. Some of the largest of the fish eventually turn into males and maintain a “harem,” ensuring mating success.
That may sound bizarre, but Yale evolutionary biologists have studied organisms that can change sex (technically called “sequential hermaphroditism”) and this month published their puzzling results. Their paper is in the March issue of The American Naturalist. (Read it here.)
Using mathematical modeling they discovered that the ability to change sex is usually at an evolutionary advantage, prompting the question: Why aren’t more animals sex-changers?
Erem Kazancioglu (pictured above), a graduate student at Yale and lead author, and Suzanne H. Alonzo, assistant professor of ecology and evolutionary biology at the university, speculate that behavioral patterns may be part of the answer.
For instance, parental care may trump the advantage afforded by switching sexes. Kazancioglu and Alonzo are conducting further research to figure out why more organisms lack the sex-change option.
For the record, the hermaphroditic species actually do transform from one sex to the other. For example, a newly minted male fish not only looks the part, his gonads and all other reproductive organs, also change, Kazancioglu said.
Fish are the only backboned creatures known to change sex.
Some worms, crustaceans, and other animals also apparently evolved similar systems independently. Oddly, there are some closely related species in which some can change and others can’t.
As far as humans are concerned, you’re normally either an XX (female) or XY (male). The sex-changing organisms must have great reproductive plasticity to pull off such a major change, he said.
While it remains unclear why more animals are not capable of changing sex, those few who can are very sensitive to their immediate environments such as relative size or social status of males and females, he said.
On the other hand, behavioral changes can stem from hormonal changes, too, making sex-changing mating systems difficult to tease apart.
“As an evolutionary biologist I started thinking about the theory behind all of this,” said Kazancioglu. “If it’s advantageous, why aren’t more species able to change sex?”
To answer that question, Kazancioglu and Alonzo performed a mathematical analysis based on game theory. They developed models to see what would happen in populations of sex changers and non-sex changers. They put individuals that can and cannot change sex into the same population and let the computer run. The strategy that confers a greater advantage to individuals would spread in the population.
In the models, the pair varied the “cost” of switching sexes. “Cost” refers to the loss of energy or reproductive time while undergoing a sex change. For example, if changing sexes took up 90 percent of an organism’s lifetime, it might not be advantageous to make the switch.
What the researchers discovered poked holes in a conventional theory about why more species don’t change sex. Before Kazancioglu and Alonzo’s study, conventional thinking was that sex change is so rare because the costs are too high to make switching worth the sacrifice. The pair found that that theory fell flat.
“The costs had to be really large to favor a single sex system,” Kazancioglu said. “Even with costs, changing sex is better.”
The early and prevailing theory, proposed in 1969, is known as the “size-advantage hypothesis,” which reasons that the ability to change sex would be an evolutionary advantage if the sexes differ in the relationship between size or age, and reproductive fitness.
That’s well and good to the Yale biologists, but does not explain why sequential hermaphroditism is not more widely spread.
After grinding the numbers through some mean equations, they found that sex changing is almost always a better path if size enhances the chances of reproduction.
In fact, “the costs have to be large” before non-sex changing becomes an advantage, the pair wrote.
Besides being an intriguing biological conundrum, the research might lead to better understanding of a phenomenon that is quite important for managing fisheries, Kazancioglu said.
Their research continues.