Sand tiger sharks have a tough go of life even before they're born, according to a new study of in utero activity among the species.
Female sand tiger sharks usually become pregnant with more than one male offspring. In their study, the researchers found that once the fetuses reach a certain age they begin to cannibalize each other in a battle of the fittest. The researchers found that the sand tiger shark fetuses not only behave this way due to their crowded environment, but also as part of an evolutionary process where only the most aggressive male sharks survive.
"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," study lead author and marine biology professor at Stony Brook University Demian Chapman said. "This is demonstrating that embryonic cannibalism is actually whittling down the number of males producing offspring."
The researchers came to their conclusion by studying sand tiger shark embryos at different stages of gestation. They discovered that it was more likely that the embryos had just one father the further along the pregnancy had gone. Among 15 female sharks studied, 10 carried only two embryos. The other five were at an earlier gestation stage and had between five and seven embryos. The researchers then determined paternity through DNA analysis. They found that those sharks with five to seven embryos had at least two fathers, while those with only two embryos mostly just had one.
The results suggested that one shark fetus tended to consume fetuses from other fathers, rather than its own siblings. The study is significant in revealing that female tiger sharks do in fact mate with more than one partner.
It's still unknown how exactly one shark embryo becomes dominant over the others. One simple possibility is that embryos from the first male to fertilize the female grow larger first, thereafter consuming their siblings. This shark sibling rivalry could also hold implications for mate selection, given that shark mating itself involves violent biting.
The findings of the study are published in the journal Biology Letters.
