Sharks do eat other sharks, a practice known as cannibalism. This intraspecies predation is a recognized part of their existence and manifests in two distinct ways. One form is an external, opportunistic event driven by hunger or vulnerability in the ocean environment. The other is a mandatory, internal process that occurs during the earliest stages of development, long before birth.
Observed Cannibalism in Juvenile and Adult Sharks
Cannibalism among free-swimming sharks is primarily opportunistic predation, generally involving a significant size disparity between the aggressor and the victim. Large, established species frequently prey upon smaller, younger members of their own kind, or even smaller adult sharks of different species. This behavior is most often observed when a shark is injured, weakened, or during periods of resource scarcity.
Many of the ocean’s largest predators, including Great White Sharks, Bull Sharks, and Tiger Sharks, exhibit this tendency. For a massive predator, a smaller conspecific represents a high-calorie meal that is easier to subdue than healthy prey. The energy gained provides a substantial nutritional boost, making it a viable strategy when other food sources are limited.
This external predation is size-dependent, meaning the largest individuals actively regulate the population structure by consuming the smallest. Juveniles are particularly vulnerable to this predation after leaving nursery grounds. This behavior can also escalate into feeding frenzies, where the scent of blood from an injured shark draws multiple predators.
This size-structured predation is a self-regulating mechanism that ensures only the fastest-growing and most robust individuals survive to maturity. This natural selection process helps maintain the overall health and dominance of the remaining adult population. The consumption of a conspecific is simply a matter of energy optimization for a top-tier predator.
The Unique Strategy of Intrauterine Cannibalism
While external predation is opportunistic, a specialized and mandatory form of cannibalism occurs inside the mother’s uterus in several shark families. This reproductive strategy ensures that the few pups born are large, strong, and immediately capable of surviving as miniature predators. This internal feeding takes two primary forms: oophagy and adelphophagy.
Oophagy, meaning “egg-eating,” is a common form where the developing embryo consumes a continuous supply of unfertilized eggs produced by the mother. Species like the Mako and Great White sharks utilize this method, allowing the embryo to grow significantly larger and more robust than if it relied solely on its initial yolk sac. The mother provides a rich, ongoing food source for her developing young.
Adelphophagy, or “eating one’s brother,” is the most extreme version of this strategy, famously exhibited by the Sand Tiger Shark (Carcharias taurus). Female Sand Tiger Sharks possess two uteri, and multiple embryos begin to develop in each. The first embryo in each uterus to hatch and develop teeth begins to actively attack and consume its smaller, developing siblings.
The resulting pups are born at an immense size, often measuring over three feet long, which reduces their risk of predation upon entering the ocean. After consuming all their siblings, the two surviving embryos, one from each uterus, sustain themselves by switching to oophagy for the remainder of the gestation period. This intense competition results in one of the lowest reproductive rates among sharks, yielding only two highly developed pups per litter.
Ecological Drivers and Population Dynamics
The underlying ecological factors that drive both forms of shark cannibalism are resource limitation and density-dependent pressures. In the ocean environment, cannibalism acts as a natural control on population size, ensuring that the number of sharks does not exceed the capacity of the local ecosystem to support them. When a local shark population density rises, encounters between large and small individuals increase, leading to a higher rate of predation on the young.
Cannibalism is a negative feedback loop for population growth, especially in areas where traditional prey species are scarce. When food is limited, the threshold for a large shark to attack a smaller conspecific decreases, making a vulnerable member of its own species an easy and energy-rich meal. This behavior directly thins the ranks of the least competitive individuals, which helps to stabilize the food supply for the survivors.
At the population level, the practice ensures that the strongest genetic lines and most aggressive individuals are the ones that survive to reproduce. By eliminating a large number of offspring and weaker juveniles, the remaining sharks have reduced competition for prey and better access to territory. This self-regulation mechanism is an effective method of population management in a resource-constrained world.