It's a shark-eat-shark world: literally! It never ceases to amaze people that one of the ocean's top predators is fine taking a chunk out of their own species (or a different species of shark). Shark scientists can begrudgingly tell you how a bigger shark ate a smaller one from their line (having experienced that myself, it is always fascinating to pull up) and your average person has captured this predation on video via their smartphone countless of times. But did you know that shark-on-shark eating happens inside the mother shark as well?
Sharks live a tough life. Their parents are not in the picture at all, receiving no parental care once they hit the salty water of the ocean (some even earlier, if their mother deposited their egg case somewhere safe). Although they are feared by most oceanic animals when they grow up, they are fair game when they are freshly born and so little. Meaning baby sharks (called 'pups') must constantly avoid becoming dinner for a bigger animal- including other sharks!
That's if they even make it to the ocean at all. Even in the womb, they are not safe from their own flesh and blood: their siblings. The "Story of Life" BBC nature documentary starring Sir David Attenborough depicts this very type of cannibalism inside a sand tiger shark (Carcharias taurus). Also called grey nurse sharks, spotted ragged-tooth sharks, or tiger sharks (not to be confused with Galeocerdo cuvier) these sharks have a bloody war going inside their uteruses. Yes, plural. Female sand tiger sharks have two uteri! Attenborough says it best: "Inside each female, infant teeth are being put to good use, as the female's two largest unborn pups slowly eat their siblings. It ensures only the strongest and largest babies survive." This macabre event was accidentally discovered in 1948 when a scientist poking around one of the uteri of a sand tiger shark was suddenly bit by one pup on the hand.
While cannibalism within the womb is not common in the animal kingdom, cannibalism after birth happens in many animals such as cane toads, rabbits, salamanders, and polar bears. This sort of cannibalism is unusual in sharks, and it gets weirder: once the sand tiger sharks are finishing eating their brothers and sisters, they turn to their mother's unfertilized eggs. This practice is called oophagy (sometimes referred to as 'ovophagy') and literally means "egg eating." Relatives of the sand tigers have pups in utero that also consume unfertilized eggs (e.g. great white sharks, thresher sharks, porbeagle sharks, and mako sharks).
However, if that wasn't bizzare enough, the inner workings of a mother shark just keeps getting weirder: in 1993, footage shot for a Discovery Channel program showed embryos inside a sand tiger shark moving from one uterus to another! Scientists just recently found this same migration in another species. Using a special ultrasound device, scientists from Okinawa Churaumi Aquarium in Motobu, Japan, were able to see unborn pups of captive tawny nurse sharks (Nebrius ferrugineus) not only swim around their own uterus but move to the other one. "Our data shows frequent embryonic migration between the right and left uteri, which is contradictory to the 'sedentary' mammalian fetus," the researchers state in their report published this month. The data ranged from embryos switching uteruses three times to 24 migrations throughout the shark's pregnancy.
Although the first observation of this migration was done back in 1993, it was seen while an invasive surgical procedure was being carried out, and many wondered if this behavior was natural and would 'occur under less stressful conditions.' It seems it does! And the researchers of this latest paper think this behavior may be due to how tawny nurse and sand tiger sharks feed their developing pups. "It seems likely that in this mode of reproduction, the active swimming ability of the embryo may allow it to effectively search and capture nutritive eggs in the uterine environment," the scientist state.
So why do these siblings eat one another? It isn't because they are running out of room in there- it all comes down to who their daddy is. Female sand tiger sharks, like many animals, end up mating with multiple males. Females tend to choose which male(s) to mate with and they either store the sperm for later or make sure to fertilize their eggs with what they've got. That means if they mated with multiple males, the babies in the same womb may have different fathers. But while the female may choose who she mates with, it doesn't mean the genes of that male will go the distance and successfully produce a pup that leaves the uterus.
Authors of a 2013 study constructed microsatellite DNA profiles of 15 female sand tiger sharks and their offspring in South Africa between 2007 to 2012. By comparing the embryo genetics, they were able to see how many males were able to successfully fertilize eggs. Nine of the females (60 percent) had multiple mates, but what was surprising was that 60 percent of the embryos that hatched first and grew shared the same father.
Male sand tiger sharks are interesting in that they stay close to the female they just copulated with and guard them against other males. They also "produce a conspicuously large amount of sperm compared to other sharks" according to the Smithsonian. Are they hoping that their sperm will fertilize an egg and successfully implant into the uterus, giving them a head start to grow up big and strong and eat the others? Possibly. “This competition can play an important and probably under-appreciated role in determining male fitness,” the scientists say in their research.
Shark reproduction is still a rather large mystery. But uncovering secrets like these make sharks just that much cooler.
