This week’s column, by popular request, will be on cannibalism (the very fact that cannibalism is a frequently requested topic should speak volumes about the sort of company I have).
The subject at hand arose from a discussion of tunicates, sponge-like sessile sea vases that are nonetheless basal chordates and therefore closer to us than the great majority of other animals — if the animal kingdom had a civil war, I’d trust them to be on our side. Though the adults are little more than filter-feeding bags of slimy jelly, hints of their true affinity are found in their larvae, tadpole-like animals that crawl to and fro in search of a settling place. A far cry from the sessile adult, the larva is equipped with sensory organs (an eye and a balance-sensing structure called a statolith), a notochord (a protein-based structure that appears during the embryonic development of all chordates, including us) and a nervous system, which are doubtless of profound importance while you’re looking for a place to settle down and try to emulate a sponge, but equally worthless once you’ve succeeded in doing so. So the tunicate, having situated itself, promptly resorbs all these structures, including most of the nervous system, back into its body during metamorphosis, effectively digesting its own brain. Two different variations on this life cycle exist: salps and their allies do not cement themselves to one location and instead float on the open ocean like jellyfish, sometimes forming massive colonies that include thousands of individuals joined in a single chain, while larvaceans exhibit larval characteristics even in their adult lives and enhance their filter-feeding capacity with a massive mucus “house” that guides particulate food into their gullets.
I am not quite convinced, though, that tunicate metamorphosis qualifies as cannibalism — a clearer example is provided by a number of sharks. First discovered when a researcher inspecting a sand tiger shark uterus got bitten by the developing embryos inside (which frankly is the sort of event that would be more at home in an “Alien” movie), adelphophagy (brother-eating) refers to the process by which the largest and most mature embryos devour their siblings within the mother’s uterus, leaving only the fittest shark pups to be born. A similar process is seen in starfish of the genus Parvulastra (which I believe I have mentioned before), where developing starfish compete for survival by sticking as close as they can to their parent’s gonadal walls and holding their stomachs against each other — any juvenile that fails to back into a secure wall and keep its guard up at all times is digested by its siblings. One author refers to this competition as a “stomach duel” — clearly a serious contender in any “scientific phrases that shouldn’t exist” list, alongside “cloacal gestation” (also exhibited by sharks) and “intrauterine leech infection” (three guesses as to what animal this was found in — hint: it is a really bad idea to read the literature on sharks, since it includes descriptions of things like copepods the size of your thumb that anchor themselves in shark eyes and graze on the corneal surface — also, sharks do get cancer, so they aren’t safe in that regard either).
Mating-related incidences of cannibalism are among the most infamous in nature (though one is advised to note that some such events may occur only because the participating animals are placed in an alien environment and therefore often famished), and it is well known that certain spider males, such as those of redbacks, willingly somersault into the jaws of females in order to increase their chances of passing on their genes. My favorite example, though, is that of sagebrush crickets, whose females are known, uncharacteristically, to court males (this is unusual in the animal kingdom, as females generally invest more in their offspring and therefore must be picky in their choice of a mate in order to get the best out of that investment). The reason, it turns out, is that the male cricket also devotes a substantial amount of energy to the well-being of its progeny, albeit in an unusual manner: the female feeds on the fleshy wings of the male during mating, providing her with enough nutrition to keep her eggs nourished. And finally, there are animals where the offspring eat their parents, behavior again found in spiders (there also exists a slightly less extreme form where the spiderlings are initially content to merely feed on the blood of their mother, whom they eat only after she is completely drained).
Human cannibalism is a whole different ball game, and admittedly not one I am very experienced in — though it is taboo for a good reason, as there’s no better way to catch every prion disease in the book. Those interested may take a look at the history of kuru, the prion-based laughing sickness that established itself among Papuan tribes only because of their cannibalistic customs.