Cannibalism: A Perfectly Natural History(8)

I wondered whether H. G. Wells knew about their existence when he wrote The Time Machine in 1895. In Wells’s classic novel, the Time Traveler encounters two human species: the child-sized and docile Eloi, and the brutish Morlocks, who raise the Eloi in order to feed upon them. Wells explained the Morlocks’ cannibalistic behavior by suggesting that they were once members of a worker class, toiling underground for lazy, upper-class surface-dwellers. The Time Traveler speculates that a food shortage (i.e., an environmental change) forced the subterraneans to alter their diets—at first rats, but ultimately something a bit larger. Eventually this behavior resulted in a race of hulking cannibals, feeding on the surface-dwellers, whose own evolutionary path would produce the sheeplike Eloi, pampered, well-fed, and eventually slaughtered for food.



Although the Eloi-Morlock relationship was clearly meant to serve as a cautionary tale of the horrors of class distinction, H.G. Wells imagined a biological phenomenon remarkably similar to what researchers like David Pfennig and his colleagues are working on today.

Many scientists now believe that phenotypic plasticity offers the perfect building blocks for the type of evolutionary change described by Wells over a century ago. These building blocks could be novel traits like the tiger salamander’s kin-chomping jaws or the spadefoot tadpole’s serrated beak—each having originated as an environment-dependent alternative to an already established ancestral trait (in this case, normal jaws). What these scientists hypothesize goes far beyond the realm of cannibalism and into the very mechanisms of evolution itself. Their claim is that the appearance of new traits in a population, generally regarded as a first step toward the evolution of new species, can occur by means other than the accumulation of micromutations (i.e., small-scale or highly localized mutations), the classic mechanism by which new traits, and eventually new species, are thought to appear. Some researchers now believe that given generations, novel traits originating as examples of phenotypic plasticity have the potential to produce separate species.

This idea originated with the German-American geneticist, Richard Goldschmidt (1878–1958), infamous for his stance that micromutations accumulating over long periods of time were inadequate to explain the evolution of different species. He proposed two additional mechanisms, the first: speciation by macromutations (i.e., those causing a profound effect on the organism), which eventually led to the derision associated with his name and the legacy-destroying label of “non-Darwinian.” Less well known is Goldschmidt’s suggestion (quite correct, it appears) that mutations can result in major changes during early development, and that these can lead to large effects in the adult phenotype. This hypothesis and the related concept of developmental plasticity (i.e., adaptability) are two of the key principles of the modern field of evolutionary developmental biology (a.k.a. evo devo). Goldschmidt’s contribution, though, is generally ignored. Along with the even earlier evolutionary biologist Jean-Baptiste Lamarck (the giraffe neck guy whose story is discussed in an upcoming chapter), these scientists are rarely celebrated for what they got right, and are, instead, derided for what they got wrong.

Okay, so now that I’d captured and examined cannibalistic tadpole morphs and heard all about their outsized salamander cousins, it was time to look into other examples of cannibalism in nature and to determine why these creatures were eating each other. I decided that the best way to cover and divvy up the material was to look at what I considered to be the most dramatic examples of Gary Polis’s cannibalism-related generalizations. Admittedly, some of what I uncovered was hard to categorize, thus leading me to the realization that cannibalism can extend far beyond the realm of generalization. I also learned that normal behavior or not, sometimes cannibalism in the animal kingdom can get downright weird.



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4 A lengthy intestine is a hallmark of many herbivores, since longer guts translate to longer passage times for the food moving through them—allowing for additional chemical digestion and more absorption of nutrients. In many animals, though (including all vertebrates and even inverts like termites), the digestive tract cannot digest cellulose, the polysaccharide that makes up plant cell walls. The problem is solved by the presence of endosymbiotic bacteria or protozoans (“gut flora”) that produce cellulases—enzymes capable of digesting polysaccharides. In “foregut fermenters” like cows, a multi-chambered stomach serves as a homestead for the enzyme-generating microbial horde, while in “hindgut fermenters” like horses, a pouchlike section of the intestine called the cecum houses the endosymbionts.



5 Tragically, Dr. Polis drowned when his research vessel sank during a storm in the Sea of Cortez in 2000, an accident that also claimed the lives of a graduate student and three Japanese ecologists.



6 Toads and frogs belong to the amphibian order Anura (from the Greek for “no tail”). Most anurans lay their eggs in fresh water, with hatchlings undergoing complete metamorphosis from gill-bearing tadpoles to lung-breathing juveniles.



7 Urodela (Greek for “conspicuous tail”) is the order containing approximately 655 salamanders, lizard-shaped amphibians generally found in moist terrestrial environments.





2: Go on, Eat the Kids


3rd Fisherman: I marvel how the fishes live in the sea.

1st Fisherman: Why, as men do a-land; the great ones eat up the little ones.