Androgynous Snails

Androgynous Snails

Females of one species of marine snail conceal their gender to avoid excessive mating.


Androgynous snails. I'm Bob Hirshon and this is Science Update.

To fend off relentless sexual advances, some female sea snails actually hide their gender. This according to researchers at the
University of Gothenburg in Sweden. Marine ecologist Kerstin Johannesson explains that male snails find females by tracking feminine chemicals in their mucus trails. But in one kind of periwinkle snail, the females confuse the males by leaving gender-neutral trails.


That's of course a problem for them, so they quite often end up trying to mate other males.

Johannesson notes that this species lives in dense communities, and mates once or twice a day all year round. And every time they mate, they're exposed to predators. She suspects that if the females didn't keep a low profile, they'd be approached far more often, and incur more risk without any biological benefit. I'm Bob Hirshon, for AAAS, the science society.

Making Sense of the Research

Depending on the species, males and females may face different evolutionary pressures for mating. This can result in the development of different physical traits, different behaviors, and/or different strategies for choosing mates. It's long been assumed, for example, that males benefit from seeking out as many mating opportunities as possible, because they can father an unlimited number of offspring, whereas females benefit more from a strategy of choosing high-quality mates and investing care in the offspring they do have. This assumption has been challenged recently, and there are many exceptions to the pattern, but it's still a useful concept to start from in this case.

Here, the mating strategies of male and female periwinkle snails (from the species Littorina saxatilis) differs even more sharply: Not only do males seek out more mating opportunities than the females, but the females have developed a way to actually reduce the number of males that approach them. Unlike similar snails, which leave chemical signals in their mucus trails that identify them as male or female, the female periwinkle snails leave gender-neutral trails. 

Looking at this alone, one might suppose that this particular snail doesn't rely on mucus trails at all to find mates. But the fact that the male periwinkle snail still follows the trails, and frequently tries to mate with other males, suggests that the female L. saxatilis snails started disguising themselves relatively recently in their evolutionary history. Moreover, the researchers tested the male L. saxatilis on the mucus trails of closely related periwinkle species, and found that they could sniff out those females just fine.

Why would the female snails want to avoid the males? The answer, according to the researchers, can be found in L. saxatilis' ecosystem. These snails live on rocky shorelines, in extremely dense populations. In fact, the communities are so crowded that the snails hardly need help finding one another. Even without leaving feminine mucus trails, female L. saxatilis snails still end up mating about 20 times per day. That's more than enough to maximize their reproductive success; more mating opportunities wouldn't give the females any additional benefit in that regard.

It would, however, come with a cost. Whenever periwinkle snails mate, they stack on top of one another, increasing the risk that they'll get knocked into the sea or noticed by a predator. For the males, it's worth risking life and limb to mate as much as possible, because every new mating opportunity increases its reproductive success. Females, on the other hand, don't get any additional benefit after a certain number of matings—so beyond that point, every additional pairing is all risk and no reward. Or to put it another way, mating 40 times a day would be twice as risky as mating 20 times a day, but to a female, it wouldn't result in any additional offspring. In fact, females that leave sexually ambiguous trails are more likely to survive and reproduce again in the future, which could explain why this trait was gradually favored over time in L. saxatilis populations. 

Now try and answer these questions:

  1. How do mucus trails play a role in periwinkle snail mating?
  2. How do L. saxatilis mucus trails differ from those of other periwinkle snails?
  3. How did the researchers eliminate the following hypotheses: a) Male L. saxatilis snails have poor tracking abilities. b) Male L. saxatilis snails don't use mucus trails to look for mates.
  4. How does the density of L. saxatilis fit into the story? Why might female snails in less dense populations benefit by leaving clear trails, even if mating poses a risk?

You may want to check out the October 1, 2010, Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: how dolphins breathe, sea snails that hide their gender, the genetics and epigenetics of ant colonies, and looking mad-cow disease in the eye.

Going Further

For Educators

The animal kingdom is full of interesting mating rituals and behaviors; students can learn about several others in the Science NetLinks Science Update lessons below.

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