Sibling Rivalry Spurs Sand Tiger Shark Embryos To Eat Each Other

A recent scientific study shows that sand tiger shark babies eat their litter mates in the womb in the attempt to be the embryo that is ultimately born. Melissa Block speaks with study author Demian Chapman, an assistant professor at Stoney Brook University in the School of Marine and Atmospheric Science, on why survival of the fittest in sand tiger sharks starts before birth.

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MELISSA BLOCK, HOST:

Beware the embryo of a sand tiger shark, especially if you're another embryo. That's because survival of the fittest starts inside the womb, before a shark is even born. Scientists have known for years that unborn sand tiger sharks devour their womb-mates until there are just two left. Now, new research has figured out why.

Here to tell us more about the ultimate sibling rivalry is Demian Chapman. He's a marine biologist at Stony Brook University in New York and he led the research. Welcome to the program.

DEMIAN CHAPMAN: Thanks very much.

BLOCK: Well, let's go back to the beginning here. Female sand tiger sharks, if I understand this right, mate with multiple males, right? So it's the offspring of a bunch of male sharks who are battling it out in-utero?

CHAPMAN: Yeah, that's exactly right. Most large sharks actually, the females mate with multiple males. But typically they have a litter, say, a dozen or so and it's what we call multiple-paternity. There's multiple fathers in there and, of course, all of those fathers are actually genetically represented in the next generation.

BLOCK: But not so with sand tiger sharks, what happens with them?

CHAPMAN: Well, what happens with sand tiger sharks is actually the embryo that reaches a certain size first turns around and consumes all of its other offspring. They actually have two uteri and the biggest one in each uterus actually consumes all of the other ones in there. And as a result, the female only has two pups that are produced from each pregnancy.

BLOCK: One from each womb. Well, how did you figure out who was emerging triumphant after the pregnancy?

CHAPMAN: We used DNA fingerprinting just like you might see on the "Maury Povich Show," or something like that, to actually figure out the relationships of the embryos. What we found is that by the time that cannibalism was finished, it was actually much more common for only the two remaining pups to be full siblings, which means they had the same father. So, oftentimes, what was happening was one of the fathers was actually being excluded by this cannibalism.

BLOCK: But it would be safe to assume that that one was what - more aggressive, bigger, stronger?

CHAPMAN: The thing that's most likely is that the female actually ovulates over a fairly long period, so all of the embryos are actually at different stages of development. One of the things that's important is the male that mates with the female first has a big advantage, as far as having the embryo that reaches the size where it starts to kill and eat its siblings.

And one thing that's very interesting about sand tigers in captivity, when mating is being observed the males actually form a dominance hierarchy. And the dominant male actually guards the female and prevents other males from mating with her. And that might be a strategy to actually ensure that he's the one, the dominant male is the one to fertilize those earliest eggs which are then going to become the cannibals.

BLOCK: Do you see this, this embryonic cannibalism in any other sharks or is it just the sand tiger sharks where this happens?

CHAPMAN: As far as we know, it's just the sand tiger that does this.

BLOCK: You know, I've read the story and I assumed it was not true. But maybe it actually is true that a shark embryo bit the finger of a researcher when he went into dissect the mother's uterus.

CHAPMAN: Yeah, that's a story that all of us shark biologists have probably heard at one stage or another. I think - I believe it was in the 1960s, a pretty famous shark biologist was dissecting a female. And he reached in and apparently the cannibal embryo bit his finger. So as far as I know that's the only time I can think of an unborn animal biting a human.

BLOCK: Well, Demian Chapman, thanks for talking to us about your research on the embryonic cannibalization among sand tiger sharks.

CHAPMAN: Oh, thanks very much.

BLOCK: Demian Chapman is an assistant professor at Stony Brook University in the School of Marine and Atmospheric Science.

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