Cannibalism occurs in every class of vertebrates, from fish to mammals. For researchers, factors like relatively larger body size and longer lifespans have made these backboned cannibals easier to study than invertebrates. As a result, previously unknown examples of this behavior are being revealed on an increasingly regular basis. Additionally, factors related to the increased size and longevity of vertebrates have facilitated the ability of scientists to determine and track kin relationships (have you tried tagging a mosquito?), leading to a greater understanding of the complexities of cannibalism-related behaviors. One such result has been the classification of distinct forms of cannibalism, such as filial cannibalism (eating one’s own offspring) and heterocannibalism (eating unrelated conspecifics), both of which have become vital to the concept of cannibalism as normal behavior.
In mammals, filial cannibalism has been reported in rodents (like voles, mice, and wood rats), and lagomorphs (rabbits and their relatives), as well as shrews, moles, and hedgehogs (a.k.a. The Mammals Formerly Known as Insectivores).8 These mammal moms sometimes eat their young to reduce litter size during periods when food is scarce. Cannibalism also occurs when litter size exceeds the number of available teats or when pups are deformed, weak, or dead.
In the fishes, by far the largest of the traditional vertebrate classes, individuals in every aquatic environment and at every developmental stage are ambushed, chased, snapped up, and gulped down on a scale unseen in terrestrial vertebrates. One reason that cannibalism occurs so frequently in fish may be the fact that the group as a whole has more in common with the invertebrates (where cannibalism is often the rule and not the exception) than do the other vertebrate classes (reptiles, birds, and mammals). Another way to consider this is to think of the class Pisces as a mosaic—composed of a suite of more recently evolved, vertebrate traits (like a vertebral column and larger brain) but still retaining some invertebrate characteristics. Here it’s the production of high numbers of tiny offspring with less parental care, as well as a proclivity for consuming both eggs and young—even one’s own.
At its most extreme, reproductive success in many fish species depends on a romantic-sounding technique known as broadcast spawning, during which females can release millions of eggs, while males simultaneously release clouds of sperm (milt). The end result is that some of the eggs get fertilized. Conceptually, given our own reproductive behavior, one might be misled into thinking that broadcast spawning is an inefficient mating technique. The bottom line, though, is that it works, as do similar variations on this theme employed by many amphibians and invertebrate species alike. Such reproductive strategies are successful because the vast number of eggs released offsets the low probability that any single egg will develop into a mature individual. Along those lines, scientists estimate that for every million eggs produced by an Atlantic cod (Gadus morhua), approximately one egg will result in an adult fish. Partially compensating for these lottery-like odds is the fact that each female produces between four and ten million eggs in a single spawning. On a related note, while this is a remarkable number of potential offspring for most vertebrates, it’s something akin to sexual dysfunction in the ocean sunfish (Mola mola), a strange-looking, open-ocean species that can broadcast 300 million eggs in a single spawning—a vertebrate record.
But it’s not just the abundance of eggs and young that makes fish such a popular menu item for members of their own species. Many terrestrial vertebrates produce few or even a single offspring, and most of these newly born or newly hatched individuals already exhibit considerable body size. In many fish species, the extraordinary number of eggs produced imposes a limit on their size, and so a full-grown cod might be a million times (or six orders of magnitude) larger than its own eggs. This fact goes a long way to explain why the majority of them exhibit about as much individual recognition of their offspring as humans do for a handful of raisins. Fish eggs, larvae, and fry (i.e., young fish) are vast in number, minute in size, and high in nutritional value. This makes them an abundant, nonthreatening, and easily collected food source. It’s also why ichthyologists consider the absence of cannibalism in fishes, rather than its presence, to be the exceptional case.