Why would these animals kill their siblings?

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0:00It’s common for siblings to fight – for food, for attention, or to establish

0:04dominance. In many cases, parents step in to make sure the rivalry doesn’t go too far – not just out

0:05of, you know, love and whatnot, but because the more babies you have, the more genes you pass on.

0:05But the Sand Tiger Shark takes sibling rivalry to an extreme. A mama shark

0:10can incubate as many as 20 eggs at a time in her uterus, so theoretically,

0:14she could birth 20 babies at once, but all those babies would be way too small to thrive:

0:19they couldn't hunt or escape predators. So instead, female sand tiger sharks have

0:23evolved reproductive systems that trap their babies inside after hatching, forcing them to

0:28snack on their own siblings to survive. After several months, the sole survivor is born:

0:34it’s now a meter long, with a full mouth of razor-teeth – and ready to take on the open ocean.