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STEVE INSKEEP, HOST:

This next report may make some men say: I'm glad I'm not a mollusk. And in case you've forgotten what a mollusk is, Monty Python has this reminder.

(SOUNDBITE OF TV SHOW, "MONTY PYTHON'S FLYING CIRCUS")

JOHN CLEESE: (as character) The mollusk is a soft-bodied, unsegmented invertebrate animal usually protected by a large shell.

INSKEEP: Now, it's not just that few men would care to think of themselves as soft-bodied and unsegmented. No. It has to do with the rather unusual behavior some male mollusks exhibit, although I'm afraid there's some technical stuff we have to get through before we get to the interesting part.

(SOUNDBITE OF TV SHOW, "MONTY PYTHON'S FLYING CIRCUS")

GRAHAM CHAPMAN: (as character) We know that.

CLEESE: (as character) However, what is more interesting is the, uh, mollusks', uh, sex life...

INSKEEP: There we go. I think this is where we ask NPR science correspondent Joe Palca to take over.

JOE PALCA, BYLINE: First, let me be clear. We're not talking about all mollusks, just a marine snail about two inches long called solenosteira macrospira. These snails live off the coast of Baja, California, and during the mating season, the beach is awash with male and female snails in connubial bliss. But after the deed is done, something very strange happens. In most snail species, females will deposit the eggs her partner has fertilized in the sand or attaches them to a rock.

But when it comes to solenosteira macrospira, the female deposits the eggs into papery capsules and attaches them to the male's shell like so many Post-It notes, only these don't come off. And it's the male who has to lug around these developing snails for months. Now, it's not unheard of in the animal kingdom for males to bear the brunt of early child rearing.

RICHARD GROSBERG: Sea horses, for example. Male sea horses take care of the offspring.

PALCA: Richard Grosberg is an evolutionary biologist at the University of California, Davis.

GROSBERG: Male water bugs also carry eggs and babies on their back. There are toads that do the same thing as well.

PALCA: But Grosberg says in the case of these sea horses and water bugs and toads, the offspring males are caring for are their own. Grosberg began to wonder if the same were true about these snails. Not an unreasonable question. It's quite a Bacchanalian scene on the beach at mating time.

GROSBERG: There are thousands and thousands of males and female snails, all mixing and changing partners, presumably, so we wondered whether or not the males that were caring for the babies were actually the fathers of the babies that they were caring for.

PALCA: So Grosberg and his colleague Stephanie Kamel brought the males back to the lab and did paternity tests on these itty bitty snails the males were schlepping around. And lo and behold...

STEPHANIE KAMEL: We found that in fact very few of the offspring that the male was carrying around were his.

PALCA: In other words, says Kamel, not only was the male snail doing all the heavy lifting...

KAMEL: He was taking care of babies from, basically, 25 other guys.

PALCA: Kamel and Grosberg report their findings in the journal Ecology Letters. From a strictly male chauvinist pig point of view, this behavior is odd in the extreme. But Grosberg says it's also odd biologically speaking because normally parents don't put themselves out for the offspring of others.

GROSBERG: In purely selfish terms, the smart thing to do, if snails were smart, would be to only care for their own offspring.

PALCA: But biologist Suzanne Alonzo of Yale University sees this snail sex saga somewhat differently.

SUZANNE ALONZO: I find the results striking, but I actually don't find them surprising. I think they actually make a lot of sense.

PALCA: By unselfishly carrying around eggs of others, a male snail is showing a female snail that he's a caring father, and that makes females eager to mate with him.

ALONZO: So as long as he mates with her and sires a bunch of offspring, it doesn't really matter if they're on his back or someone else's back.

PALCA: Yeah, he's carrying around some other snail's kids, but some other snail is carrying around his, a kind of molluscan version of all for one and one for all. Joe Palca, NPR News, Washington.

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