My personal favorite example of piscine cannibalism is yet another instance in which immature animals are the ones getting consumed. But in sand tiger sharks (Carcharias taurus), the individuals doing the cannibalizing haven’t even been born yet.
Sand tigers, like hammerheads (Sphyrna zygaena) and blue sharks (Prionace glauca), do not deposit their eggs into the environment. Instead the eggs and young develop inside the females’ oviducts, a developmental strategy known as histotrophic viviparity. Scientists who first looked at late-term sand tiger embryos in 1948 noticed that these specimens were anatomically well developed, with a mouthful of sharp teeth—a point (or several) driven home when one researcher was bitten on the hand while probing the oviduct of a pregnant specimen. Strangely, these late-term embryos also had swollen bellies, which were initially thought to be yolk sacs—a form of stored food. This was puzzling, though, since most of the nutrient-rich yolk should have been used up by this late stage of development. Further investigation showed that the abdominal bumps weren’t yolk sacs at all, they were stomachs full of smaller sharks! These embryos (averaging 19 in number) had fallen victim to the ultimate in sibling rivalry—a form of in utero cannibalism known as adelphophagy (from the Ancient Greek for “brother eating”), or sibling cannibalism.
This behavior is possible because sand tiger shark oviducts contain embryos at different developmental stages (a characteristic that also evolved in birds). Once the largest of the shark embryos run through their own yolky food supply, they begin consuming eggs. And when the eggs are gone, the ravenous fetal sharks begin consuming their smaller siblings. Ultimately, only two pups remain, one in each oviduct. According to renowned ichthyologist Stewart Springer, the selective advantage for the young sharks may extend beyond the obvious nutritional reward.
Springer, the first to study the behavior, believed that the surviving pups were born “experienced young,” having already killed for survival even before their birth. He hypothesized that this form of sibling cannibalism might afford the young sand tigers a competitive advantage during interactions with other predatory species also looking for meal.
Although the sand tiger is the only species known to consume embryos in utero, several other sharks exhibit a form of oophagy, in which the unborn residents of the oviduct feed on a steady supply of unfertilized eggs. Additionally, a form of adelphophagy occurs in some bony fishes (superclass Osteichthyes) in which broods mature at different rates. Once again, in these species it’s the older members of the brood that cannibalize their smaller siblings.
Cannibalism of the young also occurs in many species of snakes, lizards, and crocodilians, where, for example, it accounts for significant juvenile mortality in the American alligator (Alligator mississippiensis). Although reptiles do not transition through larval stages like most fish and amphibians, the smallest and most defenseless individuals, namely eggs, neonates, and juveniles, run the greatest risk of being eaten by conspecifics.
Cannibalism is relatively rare among birds, a fact that may be related to one aspect of their specialized anatomy—their beaks. These keratinous structures are responsible for the designation of most bird species as “gape-limited predators.” In other words, their lack of teeth limits them to consuming prey small enough to be swallowed whole. Existing under this anatomical constraint, when cannibalism does occur in birds it’s generally the eggs and young that are consumed.
According to Cornell ornithologist Walter Koenig, “Since brood reduction is widespread in birds, it’s likely that sibling cannibalism would be a lot more widespread than it is if birds had beaks that were capable of tearing dying offspring to pieces, or opening their gape wide enough to swallow them whole.”