Although not quite as infrequently practiced as it is among the invertebrates, parental care occurs in only around 20 percent of the 420 families of bony fishes (a group composed of nearly all living species except sharks and their flattened relatives, the skates and rays). The primary reason for this trend can be explained by the fact that the natural world is full of tradeoffs. Here the tradeoff works like this: Since females expend a tremendous amount of energy producing eggs (sometimes millions of them), they can’t afford to expend much energy caring for them or their young when they hatch. For this reason, the eggs and fry of most fish species exist in dangerous environments inhabited by a long list of potential predators, including conspecifics. But even in the 90 or so piscine families where parental care does occur, filial cannibalism is an extremely common practice, and here the primary reason has to do with who is doing the babysitting.
Among the land-dwelling vertebrates, females are the principal caregivers, while males take on support roles or simply make themselves scarce. In bony fishes that guard their own eggs, though, it’s usually the males who are involved, undertaking these chores at nests otherwise known as oviposition sites. These can range from slight depressions in the substrate, to rocks, plants, and other materials to which the sticky eggs (generally numbering in the hundreds) adhere in discrete clumps. The male guardians often wind up consuming some of the eggs (partial filial cannibalism), and sometimes all of them (total filial cannibalism).
One reason that male fish engage in this seemingly counterproductive behavior is that generally, they have much less invested in the brood than do females. It is less costly to produce a cloud of sperm than it is to produce, carry around, and distribute an abdomen full of eggs. Furthermore, with their ability to search for food seriously constrained by caregiving duties, males are forced to undertake at least some degree of fasting. This practice decreases their overall physical condition and thus the likelihood of future reproductive success. By consuming a portion of their own brood, males can increase the chances that they’ll survive and produce additional offspring. New eggs are consumed more often than older eggs because there has been less parental investment in maintaining them.
In some examples, though, the loss of eggs from an oviposition site is not the fault of a hungry male guardian. Unrelated conspecific males regularly raid nests in order to consume or steal eggs. Egg theft can be explained by the preference for some females to spawn at sites already containing eggs, even if they’re not hers. In these instances, once a female deposits her own clutch, the male will selectively eat the eggs he previously stole and deposited there.
While we’re on the topic of parental care in fishes, mouthbrooding cichlids deserve a brief mention, if only because they serve to strengthen the often tenuous link between a mouthful of kids and lunch. Mouthbrooding occurs in at least nine piscine families, most famously in the freshwater Cichlidae. Cichlids, especially the African varieties, are extremely popular with aquarium keepers, as well as connoisseurs of tilapia—the Spam of gourmet fish. With more than 1,300 species, cichlids have evolved extremely specialized lifestyles that serve to reduce competition with related species living in the same area.
Mouthbrooding is a common form of behavior in cichlids. Typically, it refers to post-spawning behavior in which parents (usually females) hold their brood of fertilized eggs inside their mouths until they hatch and sometimes even after that. This provides the eggs and fry with a haven from predators, a point commonly portrayed in crowd-pleasing nature videos that depict young fish darting back into their parent’s mouth at the first sign of danger. Conspicuously missing from these lighthearted reports is the fact that parents holding a mouthful of eggs usually eat a considerable portion of them, and sometimes the entire brood. Also destined for the digital equivalent of the cutting room floor are shots showing male cichlids fertilizing the eggs in the females’ mouths, always a difficult topic to explain during family TV time.
Mouthbrooders practice filial cannibalism primarily because, as we all know, eating a regular meal is next to impossible while carrying around a mouthful of eggs. Cichlids and other mouthbrooders get around this vexing problem in the simplest way possible: cannibalism. Interestingly, scientists had thought that for the first few days after spawning, female mouthbrooders selectively consumed only unfertilized eggs from their broods. When researchers set out to determine just how mothers were able to distinguish between fertilized and unfertilized eggs, they were surprised to find that 15 percent of the consumed eggs were actually fertile. We now know that, mistakes aside, once all of the unfertilized eggs have been eaten, hungry mothers continue to consume small quantities of their own fertilized eggs. And should the brood reach about 20 percent of its original number, many mouthbrooders will write off the entire batch, and eat them all. As with similar examples of total filial cannibalism, this usually occurs when the cost of caring for the brood becomes higher than the benefit of producing a less-than-normal number of offspring. Rather than investing in a smaller brood, it becomes more advantageous for the female to recover some energy by consuming her remaining young and then moving on to find a new mate.