While a sex change may entail complicated reconstructive surgery in humans, it takes species like clownfish little effort.
Erem Kazancioglu, a Ph.D. student, and his advisor Susan Alonzo, an assistant professor of ecology and evolutionary biology, are investigating hermaphroditic and separate-sex mating strategies to determine why sequential hermaphroditism does not occur more often — especially when the biological benefits outweigh the costs. The ongoing study, which began in March 2008, will be published in the March issue of the American Naturalist.
Sequential hermaphroditism refers to an animal that is born one sex, but switches sexes midway through its life.
The study takes a new approach from previous research into sequential hermaphroditism, which has almost exclusively asked: “Why have some species evolved to change sex?” Kazancioglu and Alonzo, instead, are interested in a different question: “Why haven’t more?”
“If it is evolutionarily favored, and the costs aren’t extremely large, why is this phenomenon so rare?” Alonzo said in an interview with the News. “If the purpose of evolving is to increase a species’ survival chances, far more animals should be engaging in sequential hermaphroditism.”
To investigate the phenomenon, the research team built a ‘game’ model in which hermaphrodites and separate sex ‘players’ — individuals who do not change their sex — compete to produce the most number of offspring. In the situation, the latter group changes the ratio of male to female offspring they produce, while the sex change group responds by varying the age at which they change sex — which can affect their fertility.
One hypothesis the scientists are testing is whether undergoing a sex change reduces an animal’s reproductive capacities.
“A possible explanation for the rarity of sex change is that costs such as decreased reproduction due to gonadal reconstruction favor separate sexes,” according to an earlier paper on the scientists’ work. “These costs, however, have not been studied empirically or theoretically.”
Another obstacle to sequential hermaphroditism could be that sex change takes a considerable amount of time, the authors said.
“A hermaphrodite could spend 30 percent of its lifetime in the process of changing sex, and still persist in a population,” Kazancioglu said.
Sex changes may also negatively impact reproductive behaviors, such as parental care, making hermaphroditism less beneficial to a species.
The obvious answer to the question Kazancioglu and Alonzo are posing would be that the costs associated with a sex change are simply too high, Alonzo said. But there may not be a simple answer at all: In fact, the authors speculate that the costs of changing sex cannot alone explain the low frequency of hermaphroditism in nature.
Hermaphroditism occurs primarily in crustaceans, snails, and fish, Alonzo said. For instance, clownfish are born overwhelmingly male and turn female after reaching a certain size, she said. The species lives in a harem-type setting with one large female, one reproductive male and several smaller, non-reproductive males, he added.
One would think that hermaphroditism would be a survival technique employed by small, genetically simple animals, but this is not always the case. For instance, grouper — a species of fish that can weigh up to 100 kilograms — are born female, and as they grow larger, become male, Alonzo said.
“The difference between male and female anatomy is tiny compared, for instance, with the difference between a tadpole and an adult frog,” David Skelly, a professor of ecology and evolutionary biology, said. “While we may consider the idea of switching between sexes as a normal part of a life cycle to be remarkable, I think the authors have asked the right question — namely, why we don’t see this more often.”
Sequential hermaphroditism has never been observed in mammals.