# Why do shark embryos eat one another?

Tia Ghose: "[they] cannibalize their littermates in the womb, with the largest embryo eating all but one of its siblings. Now, researchers know why."

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1. oasisob1 says:

We should isolate the gene and inject it into law students.

1. awjt says:

2. max00 says:

I don’t think you thought this thru very well.

3. Funny you say that. I was working for a county law library the summer before I left for law school. One of the regulars mentioned being reminded I was heading there while seeing a documentary the night before, about sharks so vicious that they cannibalize each other in the womb.

She wasn’t wrong.

2. TooGoodToCheck says:

The data, as it was reported in the linked article, sounds really iffy.  Because the fact that 5 embryos are more likely to have different fathers than 2 embryos is not actually much of a finding.

Like, if you flip 5 coins, they will not often land with all the same side facing up, whereas if you flip two coins, they’ll have the same side facing up half the time, but that does not necessarily represent an evolutionary strategy.

I imagine that anyone taking the time to cut open 15 sharks has taken the time to do a decent statistical analysis on their results, but that analysis totally failed to make it into the article.  There is really no sense of what is the probability that their findings could be pure chance.

1. Stepan Riha says:

There is a pretty good sense that this isn’t pure chance.

They were saying that early litters with 5-7 pups had 2+ fathers and late litters with 2 pups “more often than not” shared a father.

Assuming 6 pups by 2 fathers (3 pups each), the random probability of 2 pups sharing a dad is 6/15 = 0.4 or 40%
Assuming 6 pups by 3 fathers (2 pups each) the random probability of 2 pups sharing a dad is 3/15 = 0.2 or 20%

3.  Maybe I missed it in the article, but why does the largest stop eating at “all but one of its siblings?”

“Those litters with five to seven embryos had at least two fathers
(embryos from other fathers may have already disappeared), while the
litters with just two sharks more often had just one father.”

So in those 5-to-7 embryo cases, it seems likely that there would be some eating of full siblings, in order to pare it down to two. Then there are the two-embryo cases, which more often- but not always- had just one father.  In those less often cases, two-father pairs just decided to stop eating each other? What am I missing here?

1. Tribune says:

Dessert

2.  One shark, two uteruses, or uteri?. It’s in the text of another article: Shark Dads Lose Babies to Unborn Cannibal Siblings – Phenomena: Not Exactly Rocket Science

4. fredh says:

Ah, well that destroys my theory that it was the shark equivalent of siblings kicking each other in the back seat of the station wagon because someone crossed over to “their side.”

5. dragonfrog says:

Just the fact that sharks have wombs is news to me – I guess there are egg-laying mammals, so why not?

1. AnthonyC says:

Yup, sharks birth live young. Crazy, huh?

1. GawainLavers says:

Depends on the shark, plenty lay eggs.  Many non-mammals (various sharks, snakes, lizards, etc.) are Oviviparous.  Mammals are defined by their mammaries: specialized organs for feeding offspring without having to secure and carry food to them.  Some lay eggs (monotremes), some are essentially oviviparous (marsupials) with a secondary “womb”, but most are placental, which is actually different from merely hatching out eggs internally.

6. awjt says:

Maybe shark…  _tastes good_.

1. GawainLavers says:

The fins actually taste like crap.  That’s the infuriating thing: it’s not a “delicacy” in terms of being “delicious”.  It’s  a “delicacy” in terms of demonstrating conspicuous, wasteful consumption.

7. Post-conception sperm competition?

8. OldBrownSquirrel says:

“A possibility is that embryos from the first male to fertilize the female simply get biggest first, devouring their littermates.”

Occam’s Razor.  Female shark gets laid.  Female shark gets knocked up.  Female shark *keeps producing eggs*, because sharks don’t have placentas, and it’s the best way to feed the baby sharks between hatching and emergence, and the greater size provides them with protection against predation.  *Time passes*.  Lady shark gets laid and knocked up *again*, since she’s still producing eggs.  Some of the eggs get fertilized, but they’re developmentally behind their older half-sibs, and they get gobbled up, just as they would have been if they hadn’t been fertilized.  I’m not convinced eating half-sibs is as much a tactic as it is an accident.  Devoured full sibs would mostly be runts.

9. 0ffh says:

Because those that did survived better than those that didn’t

10. SamSam says:

The strategy could also help females select good mates. Shark mating involves violent biting, so intrauterine cannibalism may allow females to avoid resisting and avoid being “too choosy” about mating, while still ensuring that a high-quality male sires her offspring, Gelsleicther said.

This explanation isn’t necessary. If I, as a male, have a new variation that gives my offspring greater chance of surviving in the womb than another male’s offspring, my genes are going to spread into the population. There doesn’t need to be any benefit to the female.

The same goes for the “sperm wars” hypothesis, which is that human male sperm may compete with each other, and particularly with the sperm of other males, before fertilization. There is probably no benefit of this to the human race as a whole, but to the males with genes that improve this variation, their genes get passed on more, so it ends up spreading into the population.

1. ocker3 says:

It’s a benefit to the female shark, who lets the embryos fight rather than having to fight as hard to keep other sharks from mating with her once she’s already mated. Not having to heal all of those wounds and/or risk death is an evolutionary advantage.

11. I am tempted to say “it’s because they’re sharks, duh.” This is why I’m not a marine biologist.

12. oldtaku says:

BECAUSE THEY CAN.

Man, if I could go back and…  oh right, none of my siblings are same litter or even twins.