Inside its mother’s womb, an unborn sand tiger shark is busy devouring its brothers and sisters. It’s just 10 centimetres long but it already has well-developed eyes and a set of sharp teeth, which it turns against its smaller siblings. By the time the pregnant female gives birth, it only has two babies left—one from each of its two wombs. These survivors have already eaten all the others. They’re the bloody victors of a pre-birth battle.

The arched back, upturned snout and protruding teeth of a sand tiger shark give it a particularly brutish look. Its reproductive habits don’t help. After sex, any fertilised eggs settle in one of the female’s two uteri. Like a quarter of shark species, the sand tiger never lays these eggs. Instead, they hatch inside her once they reach a certain size.

Timing matters. The first embryo to emerge in each uterus—the ‘hatchling’—always cannibalises its younger siblings. It’s so voracious that at least one scientist has been bitten by a sand tiger pup while unwisely sticking a finger in a pregnant female’s uterus.

The cannibal not only nourishes itself on its siblings’ bodies, but also gains sole access to the nutritious supply of unfertilised eggs that its mother provides. On this rich diet of yolk and flesh, the hatchling grows at a tremendous pace. When it is eventually born, it’s already a metre in length—that’s big enough to protect it from many predators. “Only really big sharks eat baby sand tigers,” says Demian Chapman from Stony Brook University in New York.

Chapman studies the mating behaviour of sharks, and was fascinated by what the sand tiger’s bizarre practices mean for males. If a female mates with many males, her litter could initially include pups from many fathers. But it’s entirely possible that the pups of some males are cannibalised by the pups of others, before they’re even born! So if you gave sand tiger pups a paternity test, what would you find?

Chapman studied the bodies of 15 pregnant sand tiger sharks that died near South Africa’s beaches after getting caught in protective nets. (The nets were put up after a series of shark attacks in the 1960s.) He took tissue samples from mothers and embryos, and analysed microsatellites—short, repetitive pieces of DNA that are used in family tests—at 10 places in their genomes.

View Images A sand tiger shark hatchling next to an embryo (the smaller one on the top left). Credit: D.ABERCROMBIE.

The results showed that females regularly mate with at least two partners. In cases where the hatchlings hadn’t finished devouring their siblings, the embryos were fathered by two or more different males. And when only the hatchlings remained, they were often half-siblings rather than full ones.

Chapman’s results showed that, yes, some males manage to mate with females but never actually contribute to the next generation of sharks. Their young are devoured in the womb.

This is yet another reminder that sex is just one step towards actual reproduction. In many animals, females exert a surprising amount of choice over who fathers their young. Even after sex, females can store the sperm of different partners in separate compartments and determine which ones get to fertilise her eggs.

For males, this means that sexual competition continues after sex. It’s not just about finding mates, but about ensuring that your sperm fertilises her eggs. This leads to fierce “sperm competitions” and bizarre adaptations, where males scrape away the sperm of past mates, guard or plug females so they can no longer accept partners, “traumatically inseminate” her through her back, or even poison partners with toxic sperm to limit future sexual encounters.

The sand tiger shark’s “embryonic cannibalism” takes these competitions to a whole new level. Even if a male successfully fertilises a female’s eggs, there’s still no guarantee that his offspring will actually be born. This may explain why, in captivity at least, male sand tigers often guard the females they mate with. You’d also expect natural selection to also favour males whose offspring grow particularly quickly, so they become the cannibal hatchlings rather than the devoured runts. This may explain why sand tiger embryos have evolved to have such unusually well-developed eyes and teeth.

Females, meanwhile, may not need to bother with carefully choosing their mates. By having sex with several males, and just letting their offspring duke it out inside her, she ensures that the she gives birth to the offspring of the fittest partners.

Reference: Chapman, Wintner, Abercrombie, Ashe, Bernard, Shivji & Feldheim. 2013 The behavioural and genetic mating system of the sand tiger shark, Carcharias taurus, an intrauterine cannibal. Biol Lett http://dx.doi.org/10.1098/rsbl.2013.0003