Filial cannibalism – the consumption of one’s own offspring – isn’t a vanishingly rare phenomenon. Remember, nature is a nightmare: as noted by National Geographic, mother bears, felines, primates, canids, rodents, insects, fish, amphibians, reptiles, and birds have all been seen or otherwise implicated in killing and eating their younglings.
We can certainly add the male barred-chin blenny fish (Rhabdoblennius nitidus), which hangs around in coral reefs in the western Pacific Ocean, to that list. As spotted by New Scientist, this critter happens to eat its offspring too, but not for the reasons many thought.
Cases of filial cannibalism vary somewhat between species, but the driving factor is normally the same: resources. Extra young all need feeding, particularly if they’re not the healthiest. Take them out of the equation, and you have more resources to yourself.
As noted by the authors in the Current Biology paper, the leading hypothesis for why filial cannibalism happens is known as energy-based (EB) cannibalism. It posits that the nutritional benefits of eating the young outweigh those of making a mark on the gene pool, and for plenty of cases this holds true – but exceptions have been seen in some fishes.
They note that in some species, the male that’s required to look after the eggs as they develop decides to dine on them when the total number of eggs is small, and reproduction restarts. Although this could fit in with the EB hypothesis, the authors – led by Nagasaki University – note that this is “puzzling”, as the male can still tend to the eggs while seeking out additional females.
In the barred-chin blenny, the team suspected the number of eggs controlled something related to the mating cycle that they hadn’t properly observed yet. In order to find out, the team let the fish do their mating shenanigans, but surreptitiously controlled the number of eggs that were present post-copulation.
Turns out that it’s all about the male’s levels of androgens, a group of hormones linked to the growth and development of the reproductive system. The presence of eggs causes their androgen levels to drop precipitously, which prevents them from mating. No eggs equals more androgen, whether they are eaten by the male or they have all hatched.
The idea, then, is that these males eat small numbers of eggs not for nutrition, but so that their androgen levels spike and they can mate again, hopefully producing a larger brood. This is corroborated by the fact that they eat the eggs regardless of their overall hunger levels. They even spit some of them out after giving them a good chew.
So, it’s not cannibalism, but infanticide or, technically, embryocide, for R. nitidus. If you want a way to stand out from the young-murdering crowd, this'll do it.