Sand Tiger Sharks Eat Their Siblings While in Mom’s Uteri

Female sand tiger sharks host an epic cannibalistic cage match between their offspring, and it takes place within their own bodies long before their shark babies are even born.

Female sand tiger sharks foster a unique case of sibling rivalry before their babies are even born. They exhibit something called ovoviviparity. What appears to be a live birth is actually a two-phase process that begins with fertilized eggs that hatch into mini-adults - all within the confines of the reproductive temple. Female sand tiger sharks mate with several males during their annual reproductive phase. Different eggs can be fertilized by sperm from different sexual partners, and between six and nine encapsulated embryos eventually, settle into each of her paired uteri for further development.

The first embryo to reach a size of about two inches in length hatches from its egg capsule – and is called “the hatchling”. Having used up all of the resources from within its own egg, the hatchling is almost immediately hunts for more food sources. It proceeds to attack, kill and consume all of the siblings on its side of the uterus. As it does this, it becomes even larger, stronger – and hungrier. It then begins to eat the unfertilized eggs in mother’s reproductive tract. This phenomenon is called oophagy, or egg eating, and it’s a way for a mother to provide additional nourishment to her strongest babies.

Scientists remain uncertain as to why an embryonic shark becomes the first and last hatchling. It is known that it’s not simply just the sperm that arrives to the eggs first. Biologists believe that the strongest male sharks produce the strongest sperm. And it’s their sperm that likely fertilizes the majority of eggs, which gives these embryos a better chance of becoming the hatchling. But it’s also possible that female sand tiger sharks exhibit something called cryptic female choice, where mothers can favor the sperm of some males over others.
In the end, there is only one victor from each of mom’s uteri. By the time these babies emerge, they can be nearly four feet in length. Their size makes them much less susceptible to predatory fish and mammals that would otherwise take advantage of their vulnerable newborn status.