We’ve probably all heard the baby shark, mommy shark song by now, but did you know that shark reproduction is notoriously difficult to study? Sexual reproduction varies substantially between genera.
For example, female sand tiger sharks exhibit a remarkable reproductive mode that combines elements of both oviparity and viviparity; this hybrid form of reproduction is called “ovoviviparity.”
Females give birth to live young that have been nourished by egg yolks inside the uterine cavity. There is no nutritive connection between mother and babies once the fertilized eggs are laid within her own body. This is quite different from the direct nutritive connections (placenta) that occur in purely viviparous species.
Female sand tiger sharks have a pair of uteri. Embryos travel to one of the females’ uteri. They remain here for a further period of gestation after being fertilized.
Only the Strongest Baby Sharks Survive
The first embryo in each uterus to reach 2.1 to 2.3 inches in length hatches. They then begin a period of exponential growth while systematically attacking, killing, and cannibalizing all siblings in that uterine environment. Once the hatchling reaches a size of approximately 4 inches in length and has ingested all the siblings have been ingested and the hatchling has reached a size of approximately 4 inches in length, it will begin to feast on the unfertilized ova that have accumulated in the mother’s uterus.
Imagine all this action taking place within one’s body while simply carrying on with the day-to-day tasks of life. A veritable war is taking place within mommy shark’s uteri. She only needs to wait for the fittest and strongest offspring to emerge at birth.
They will be approximately 37.5 to 50 inches at birth and will have few predators for the rest of their lives.
What this means for sand tiger shark moms is that the strongest, fittest juveniles will triumph from a war that takes place before her babies are even independent.
Who’s Your Daddy Shark?
Moms can seek sperm donations from a variety of potential fathers. Only the sperm of the very fittest males will be the successful sire of her offspring.
In this way, mommy shark can be behaviourally polygynous but functionally monogamous from each uterus.
This post is an adapted excerpt from my book “Wild Moms: Motherhood in the Animal Kingdom“