Two Sand Tiger Shark embryos from one uterus. The big one was in the process of eating the small one when they were sampled. (Handout/D. Abercrombie)

It’s a tough world from the moment of conception for a sand tiger shark. When a female gets pregnant, it’s usually with multiple offspring of several different male sharks. As soon as the fetuses are old enough, they begin a cannibalistic battle for primacy in utero, with only one surviving.

Now scientists have concluded that this is not just a response to crowded conditions but represents an evolutionary strategy that allows the most aggressive male sharks to father the successful baby and thereby outcompete sexual rivals.

“For most species, we think of sexual selection as ending when males fertilize eggs, because once the male’s fertilized eggs he’s won, there will be some genetic representation in the next generation,” said Stony Brook University marine biology professor Demian Chapman, lead author of the study published online Wednesday in Biology Letters. “This is demonstrating that embryonic cannibalism is actually whittling down the number of males producing offspring.”

It’s still unclear whether the evolutionary strategy works because the most-aggressive fathers get their offspring growing in utero before others, thus giving them a developmental advantage in the cannibalistic battles to come, or if they produce offspring that gestate more quickly.

Female sand tiger sharks have two uteri and carry hundreds of eggs. During their fertile periods, they can mate with many male sand tiger sharks. But each time they give birth after a 12-month pregnancy, they produce just two offspring — one from each uterus.

By the early 1980s, scientists had figured out that sand tiger hatchlings start to consume others in the womb, along with unfertilized eggs, starting around five months into their gestation. But their mating behavior remained “a black box,” in Chapman’s words, since they were hard to observe in the wild and scientists couldn’t prove they mated with multiple partners.

Some of the sand tiger shark’s close relatives, including great white and mako sharks, consume unfertilized eggs while in utero, too, but they do not eat their hatchling brothers and sisters.

Over the course of four years, Chapman and his six colleagues collected 15 pregnant sand tiger sharks that had died after being caught in nets set off Richards Bay, South Africa. By performing genetic tests on the embryos in different states of development, they were able to determine that while the majority of the females had mated with multiple males, in 60 percent of the cases they were carrying only babies from the same father, suggesting that all other male shark offspring had already been killed.

The adult male who emerged victorious did so because his hatchlings — what Chapman described as “well-armed, active hunting embryos” with big eyes and teeth — had consumed all of the eggs inseminated by other males. Genetic monogamy had won the day.

University of North Florida biology professor Jim Gelsleichter, an expert in shark reproduction who was not involved with the research, said it “just opens up so many questions” because now scientists must figure out whether sand tiger fathers end up winning the evolutionary battle because they inseminate their partner first or because they produce the fastest-developing embryo.

“It really illustrates this evolutionary arms race these males and females have in terms of sexual selection,” Gelsleichter said.

When held in captivity, he added, male sand tigers demonstrate “a dominance hierarchy” in which sharks with higher levels of testosterone fend off other males from mating with their chosen female. But since these animals are hard to observe in the wild, he added, scientists must discover whether it is this behavior or something about the embryo’s genetic material that allows them to succeed in the womb.

“It’s not necessarily first-come, first-serve,” Gelsleichter said.

Once a mother sand tiger gives birth, her two babies are each more than three feet long, meaning that they are larger than baby whale sharks and almost as big as great white sharks at birth. Chapman described this process, in which mothers have allowed their offspring to feed on all their siblings, as “sort of the ultimate in parental care” since it clearly equips them to be aggressive enough to survive in the wild.

“That’s great for them, so when they’re born, they’re bigger than any other fish,” he said.

Like many other shark mothers, female sand tigers abandon their young after giving birth. Given the fact that their babies are nearly half their size and are experienced killers, Chapman said the mothers “probably run a mile.”