Intrauterine cannibalism, where sharks consume siblings or unfertilized eggs in the womb, represents a reproductive strategy employed by certain shark species. This natural process maximizes the survival chances of offspring even before they enter the marine environment.
Understanding Intrauterine Cannibalism
Intrauterine cannibalism refers to developing shark embryos consuming other embryos or unfertilized eggs within the mother’s uterus. Pups use these internal resources as a food source, ensuring their growth and development. This strategy highlights sibling competition that begins long before birth. This unique form of prenatal development results in fewer, but more robust, offspring.
Different Forms of Intrauterine Feeding
Intrauterine feeding occurs in two primary forms: oophagy and adelphophagy. Oophagy, or “egg-eating,” involves embryos feeding on a continuous supply of unfertilized eggs produced by the mother. These eggs provide a rich nutrient source, allowing embryos to grow significantly before birth. This is common among all Lamniformes, including mackerel sharks.
Adelphophagy, also known as embryophagy, is a more extreme form where stronger embryos consume weaker siblings or less developed embryos. This competitive process ensures only the most dominant individuals survive. The first embryo to hatch often develops teeth early, actively hunting siblings and eliminating competition.
The Survival Advantage of This Behavior
These reproductive strategies provide a significant evolutionary advantage. Intrauterine feeding results in fewer, larger, and more robust offspring, increasing their survival chances upon birth. Greater size at birth allows these pups to better compete for food and avoid predators in the open ocean.
Resource limitation within the maternal environment drives this behavior. By concentrating available nutrients into a select few, the mother shark ensures her offspring are well-equipped for independent life from the moment they are born.
Shark Species Exhibiting This Trait
Several shark species exhibit intrauterine feeding. The Sand Tiger Shark (Carcharias taurus) is a prominent example, known for both oophagy and adelphophagy. A female Sand Tiger Shark typically has two uteri; from dozens of fertilized eggs, only one pup usually survives in each. The first embryo to hatch within each uterus consumes its siblings and unfertilized eggs, resulting in large, powerful newborns up to three feet long at birth.
Other Lamniformes species, including the Porbeagle (Lamna nasus), Shortfin Mako (Isurus oxyrinchus), and Thresher sharks (Alopias species), also exhibit oophagy. The Tawny Nurse Shark (Nebrius ferrugineus) also practices this behavior. Approximately 14 shark species are thought to exhibit some form of intrauterine cannibalism.