ENVIRONMENT
SIBLING RIVALRY
How sand tiger shark mothers grow cannibals
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A MEAL BEFORE BIRTH
It’s one thing to be a fearsome predator in the wild; it’s another thing to hone your hunting skills before you’ve even been born. For sand tiger sharks (Carcharias taurus), their first taste of flesh occurs within their mother’s womb – and their siblings are on the menu. During breeding season, a female sand tiger shark will typically produce a dozen or so encapsulated embryos across two uteri. One by one, shark hatchlings emerge from their capsules within either the right or left uterus. However, it’s the first ones to hatch that get to survive. Once free from their capsules, the first hatchlings, typically around ten centimetres long, embark on a spree of devouring all the other shark embryos in each uterus until there are only two pups left. With their appetites not yet satisfied, the two triumphant shark pups will turn to unfertilised eggs for food. Sand tiger shark pups are born after a gestation period of between eight and nine months and emerge as sharks up to 105 metres in length. This juvenile form of cannibalism is known as intrauterine cannibalisation and occurs in several shark species.