Sexual Role-Reversals

Research Blogging / by Dave Munger /

Male pipefish get “pregnant,” and Atlantic slippersnails change sexes as they grow. Researchers are now uncovering how and why these bizarre sex strategies occur.

A story about pipefish reproduction has recently received a lot of attention. “Some pipefish were excellent dads and others were deadbeats,” one article claimed. “The key factor for this attitude,” explained another, was “how the male feels about the mother.” Really? Do pipefish actually act like squabbling divorcees?

Not exactly, says the evolutionary biologist and prolific Germany-based blogger who writes as “GrrlScientist.” “There were a few reports that were so grossly misleading that they made me groan out loud,” she said. Fortunately, GrrlScientist gets the story straight on her blog (The story was also covered quite well by Ed Yong and Brian Switek). Pipefish have a fascinating reproductive system that merits attention in its own right. Like their close relatives, seahorses, it’s male pipefish that carry the fertilized eggs until they hatch. When pipefish mate, females place between 5 and 40 eggs inside a special pouch in the male’s body. After the male fertilizes the eggs, he provides nourishment for them for about two weeks until they mature.

In a study published in Nature two weeks ago, Kimberly Paczolt and Adam Jones took a closer look at the role of male pipefish in reproduction. It had long been known that male pipefish prefer to mate with large females. What Paczolt and Jones wanted to know is how the size of the female affects male behavior during “pregnancy.” So they mated males with two successive females (carrying each resulting batch of eggs to maturity) and analyzed the second brood. When the second female was smaller than the first, not only were fewer eggs deposited compared to larger second females, but a smaller portion of the eggs resulted in viable offspring.

This suggests not that some males were deadbeats, but that all males manage how they feed the embryos they carry so as to favor the eggs of larger females. What’s interesting about the study isn’t the dynamics of sexual attraction in pipefish, but how male pipefish can influence the maturation of the eggs even after they are fertilized. In addition, Switek says that the males can actually receive nourishment from the eggs. It may be that when they carry broods from smaller females, they are using these resources to build their strength for the future when a hardier mate is available.

But pipefish aren’t the only gender-bending creatures in the sea. While you may have heard that some mollusks are hermaphroditic, Eric Heupel, a graduate student at the University of Connecticut, writes about a very unusual mollusk that actually changes gender as it grows. Atlantic slippersnails can move slowly as juveniles, but as adults, they form stationary stacks. The first slippersnail in a stack is always female. Young males crawl on top, eventually building a tower as tall as 20 individuals. As they mature, they change from male to female. Heupel says this is probably because larger individuals are needed to reliably produce eggs, while the resources necessary to produce sperm are easily garnered by smaller slippersnails. Since females are conveniently nearby, the males can reach them by extending a long penis to fertilize the eggs and initiate the reproduction cycle.

Even more bizarre than the slippersnail is the sexual behavior of bedbugs, described in an entertaining post by “Cheshire,” an entomology undergraduate at Iowa State University. Male bedbugs don’t impregnate females via the reproductive tract, which is only used for depositing eggs. Instead the, process is terrifying: Males use their genitalia to pierce females directly through the chest. In fact, males will often attempt to mate with other males, although these attempts are usually aborted quite quickly. So how do bedbugs select mates, and what determines whether they finish the job or stop short? Camilla Ryne believed it might have to do with the alarm pheromones bedbugs use to alert each other to danger. She disabled the pheromone glands in some males and then introduced other males to their enclosure. The new males mated just as if they were females. Then she tried stimulating females to release the alarm pheromones while males were mating with them. The males backed off nearly as quickly as they did with untreated males! Without the alarm pheromone, bedbugs simply can’t identify the sex of their potential mates, and basically will mate with any bedbug that looks to be well-fed. Ryne’s work was published in Animal Behavior.

Researchers are constantly learning more about the reproductive strategies of animals and other organisms. Sometimes the mainstream media gets these surprising stories wrong or overhypes the results. I’ve found that bloggers often do a better job of putting exciting new findings in context. To find out more, try searching for reproduction.

Originally published March 31, 2010

Tags genetics research sex

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