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I Ate My Twin in the Womb

30 March 2017

As a child, the highlight of every trip to the Melbourne Aquarium was the ‘oceanarium’ – a 2.2 million litre tank encircling the viewing space. It was a magical experience being immersed in the watery world of its marine inhabitants, with bright coral climbing up the sides, stingrays gliding above and fish of every hue swimming by.

In particular, I remember watching scuba divers feed the grey nurse sharks. Despite their ferocious appearance, the sharks obediently accepted fish from the hands of the divers. Far from being the vicious man-eater that its menacing appearance suggested, the information board assured me that the grey nurse shark is actually a gentle mermaid with a placid nature. In fact, no one has ever been killed by a grey nurse shark and they will not attack a human unless provoked.

But if you’re a shark embryo trying to survive until birth, it’s a different story. Remember Lilly Okanakurama, the quiet one in Pitch Perfect with the creepy whispered confessions? The one who murmured ‘I ate my twin in the womb’? Grey nurse sharks are the aquatic equivalent, literally murdering and eating each other while still in their mother’s uterus. They must fight to the death just to be born – and they don’t just eat one twin, they eat almost all of their siblings before birth. If you thought that time you cut the hair off all your sister’s Barbies was vicious, think again.

A grey nurse shark’s uterus is a bloody battleground that turns sibling against sibling until only two remain, through an extreme form of intra-uterine cannibalism known as adelphophagy (literally, ‘eating one’s brother’). When a female becomes pregnant, she initially carries up to fifty eggs.

These eggs hatch into embryos that swim around the uterus feeding on the yolk sac left behind. But when this precious source of nutrients runs out, things turn sinister, and the Hunger Games begin. Using their sharp little teeth, the siblings murder and eat each other in the womb, until only two remain. Like Katniss and Peeta, these two reach a truce, and spend the rest of their gestation eating unfertilised eggs. After a year of pregnancy, the mother gives birth to two ‘pups’ that are large enough to be independent of their mother.

So why does nature bother with this vicious and wasteful process? The answer may be more than simple sibling rivalry, according to scientists who analysed the DNA of embryos from fifteen sharks at various stages of pregnancy. They found that, while the initial embryos often had a range of fathers, the two pups that survived until birth were likely to have the same father. This suggests that the embryos’ cannibalism may act as further competition between the males. By fathering stronger pups, males can assert genetic dominance and defeat their rivals long after mating has occurred. Another advantage may be that the female doesn’t need to be too choosy about who she mates with, as she knows only the strongest will survive. This is sexual selection at its most violent.

So next time you see eager divers entranced by a seemingly docile grey nurse shark, remember that shark’s violent beginnings – it isn’t as innocent as it seems. And cut your brother some slack next time he hogs the TV – if you were a shark it could be much worse.