Sand Tiger Shark Embryos Consume Siblings In Womb, Preserve Best Genes: A Study

Sand tiger sharks bring a whole new meaning to “sibling rivalry,” what with the first embryos in each of the paired uteri consuming all of their smaller siblings during gestation - a practice that enables them to reach such a large size at birth that they face few predators.

Adding to the seemingly Jerry Springer-like drama, is the fact that because female sand tiger sharks mate with multiple males, it’s thus possible for litters to initially have multiple sires.

However, this practice of embryonic cannibalism (EC) may not just be the result of overcrowding in mom’s womb, but, according to a study published in the journal Biology Letters, may represent an evolutionary strategy to ensure a genetically superior future generation.

As the researchers report, in analyzing shark embryos at various stages of gestation, they found that the later it was in the pregnancy, the more likely the remaining shark embryos came from the same father.

The result, the scientists explain in their study, is genetic monogamy despite the animal's behavioral practice of polyandry.

In all, the researchers were able to capture an analyze 15 litters (mothers and embryos) from 2007 to 2012 in addition to 21 adult males and females captured in order to estimate population allele frequencies.

Five of the 15 litters captured were collected prior to the completion of EC and boasted six to nine embryos each. Each litter showed evidence of the female having matted with at least two males each.

Ultimately, the researchers write, “Female sand tigers make an extremely large investment in two offspring per gestation."  For this reason, they argue, “EC may increase the probability that compatible and/or competitively superior males (in terms of behavioral dominance, sperm competition or producing competitive embryos) sire the hatchlings, even if females are unable to control mating frequency or outwardly assess mate quality.”

Although rare, the competition, the researchers write, “can play an important and probably underappreciated role in determining male fitness.”