Cannibalism: A Perfectly Natural History(23)
“Well there you go,” I responded with a laugh. “That word does set something off in us.”
“Right,” Rogers agreed. “Before I knew it, USA Today was talking about dinosaurs, chianti, and fava beans.”
7: File Under: Weird
Cannibalism is found in over 1,500 species. Anthropophagusphobia (fear of cannibals) is found in only one. Which seems unnatural now?
— Author unknown
Is eating one’s own fingernails or mucus an example of auto-cannibalism? And what about breast-feeding? Is this type of parental care actually a form of cannibalism? Raymond Rogers considers scavenging the body of a conspecific dinosaur a form of cannibalism. Mark Norell, not so much. All are examples of a gray area between what most people consider cannibalism and other forms of behavior.
Like breast-feeding, the following example is a form of parental care, but one that extends further into the realm of cannibalism-related behavior. It occurs in the caecilians, a small order of not-very-obvious amphibians, whose legless bodies often get them mistaken for worms or snakes. Caecilians inhabit tropical regions of Central and South America, Africa, and Southern Asia—a neat trick that definitely lends support to the theory of continental drift. Although some caecilians are aquatic, it is not believed that their ancestors were strong enough swimmers to cross the Atlantic Ocean. Instead, prehistoric caecilians were likely separated when the current continents of South America and Africa split apart between 100 and 130 million years ago.
Caecilians also serve as great examples of convergent evolution, in which unrelated organisms each evolve similar anatomical, physiological, or behavioral characteristics, because they inhabit similar environments. As a result of their subterranean lifestyles, caecilians share a number of anatomical similarities with moles and mole rats. In each, the eyes are either set deeply into the skulls or are covered by a thick layer of skin, and as a consequence they are nearly blind.
Caecilians also possess a pair of short “tentacles” located between their nostrils and eyes. These chemosensors enable the subterraneans to “taste” their environments without opening their mouths, as they burrow through the soil or leaf litter in search of insects and small vertebrates. Similar types of sensory structures can be seen in other burrowing creatures, most notably the aptly named star-nosed mole (Condylura cristata).
As a group, caecilians exhibit a fair degree of reproductive diversity (which will become an important aspect of their cannibal-related behavior). Approximately half of the 170 species are oviparous (egg layers), and hatchlings either resemble miniature versions of their parents or pass through a brief larval stage. Other species are viviparous, giving birth to tiny, helpless young.
All caecilians do share one characteristic unique to the amphibians: internal fertilization, and during this process, sperm is deposited into the female’s cloaca with the aid of a penis-like structure called a phallodeum. For the orifice-challenged, a reminder that in many vertebrates (like amphibians, birds, and reptiles), the cloaca is a single opening shared by the intestinal, reproductive, and urinary tracts.
But as interesting as the concept of legless caecilians wielding their penises underground might be (admittedly, it disturbed some of my older Italian relatives until I explained the spelling differences), information about caecilian cannibalism began emerging from Marvalee Wake’s lab at the University of California, Berkeley. The herpetologist extraordinaire was looking at fetal and newborn individuals from several viviparous species and began investigating the function of their peculiar-looking baby teeth (better known to scientist-types as deciduous dentition).
While some of the teeth were spoon-shaped, others were pronged or resembled grappling hooks, but none of them resembled adult teeth. Wake also performed a microscopic comparison of caecilian oviducts. She observed that in pregnant individuals, the inner (i.e., epithelial) lining of the oviduct was thicker and had a proliferation of glands, which she referred to as “secretory beds.” These glands released a substance that fellow researcher H.W. Parker had previously labeled “uterine milk.” Parker described the goo, which he believed the fetuses were ingesting, as “a thick white creamy material, consisting mainly of an emulsion of fat droplets, together with disorganized cellular material.” He also thought that the caecilians’ fetal teeth were only used after birth, as a way to scrape algae from rocks and leaves. Wake, however, had her doubts, especially since she noticed that these teeth were resorbed before birth or shortly after.
Pressing on with her study, Wake saw something odd. In sections of oviduct adjacent to early-term fetuses, the epithelial lining was intact and crowded with glands. However, in females carrying late-term fetuses, the lining of the oviduct was completely missing in the areas adjacent to the fetuses, although it was intact in regions well away from the action. Wake proposed that fetal caecilians used their teeth before birth to scrape fat-rich secretions and cellular material from the lining of their mother’s oviduct. Although this behavior couldn’t be seen directly, she had gathered circumstantial evidence in the form of differences in the oviduct between early-term and late-term individuals. After an analysis of fetal stomach contents revealed cellular material, Wake had enough evidence to conclude that caecilian parental care extended beyond the production of uterine milk and into the realm of cannibalism. Unborn caecilians were eating the lining of their mothers’ reproductive tracts.