Cannibalism

by Bill Schutt

  Movement and touch related, I thought, remembering my suggestion about a possible trigger for the spadefoot cannibals. But instead of speculating about my own half-baked ideas, the conversation turned toward the pros and cons of cannibalism, especially as it pertained to consuming kin.

  One of Gary Polis’s general characteristics regarding cannibalism is that immature animals get eaten far more often than adults. Ultimately, this makes larvicide (or infanticide) the most common form of cannibalism in the animal kingdom. Intuitively, it doesn’t seem logical to eat the next generation, but the behavior can make evolutionary sense for several reasons. Young animals not only provide a valuable source of nutrition, but in most species they’re relatively defenseless. As such, they present instant nutritional benefits but little or no threat to larger members of the same species, most of which are invulnerable to attacks from immature forms.

  But beyond acquiring a meal, and as we saw with spadefoot toads, cannibalism enables individuals of some species to accelerate their developmental process, thus allowing them to quickly outgrow a stage in which they might be preyed upon or perish due to unpredictable environmental conditions. In species like the flour beetle (Tribolium castaneum), the behavior may also impart a reproductive advantage, since studies have shown that cannibalistic individuals produce more eggs than non-cannibals. Finally, many animals maintain specific territories, within which they are intolerant to the presence of conspecifics (i.e., members of the same species). According to Polis, crowding increases the frequency with which individuals violate the space of others. By reducing overcrowded conditions, cannibalism can serve to decrease the frequency of territory violations.

  There are also serious drawbacks to being a cannibal.

  In all likelihood, the most significant of these is a heightened chance of acquiring harmful parasites or diseases from a conspecific. Both parasites and pathogens are often species-specific and many of them have evolved mechanisms to defeat their host’s immune defenses. As a result, predators that consume their own kind run a greater risk of picking up a disease or a parasite than do predators that feed solely on other species. In the most famous example of cannibalism-related disease transmission, the Fore people of New Guinea were nearly driven to extinction as a result of their ritualized consumption of brains and other tissues cut from the bodies of their deceased kin, kin who had themselves been infected by kuru, an incurable and highly transmissible neurological disease. More on that topic later, but given its importance, the potential for disease transmission stands as a prime example that non-humans and humans alike share some of the negative consequences of cannibalism.

  Cannibals—whether microbes or Methodists—who eat their own relatives can also experience decreases in a measure of evolutionary success known as inclusive fitness, in which the survival of an individual’s genes, whether they’re from an offspring or a collateral relative (like a brother or cousin) is the true measure of evolutionary success. A cannibal that consumes its own offspring, siblings, or even more distant relatives, removes those genes from the population, so it reduces its own inclusive fitness. Since this is bad juju, natural selection should favor cannibals that can discriminate between kin and non-kin, primarily because eating non-relatives results in no loss of inclusive fitness. In many instances, this is exactly what happens.

  Because of the significance of inclusive fitness, it made perfect sense that David Pfennig and his colleagues had also worked on questions related to kin recognition, basically seeking to determine if some of their favorite cannibal species would avoid eating their own relatives. The researchers found that their study subjects did so by recognizing cues associated with their kin that were absent in non-kin.

  “Most examples would fall under the heading of ‘the armpit effect,’ ” Pfennig told me. “Here, an individual forms a template for what its kin smell like based on what its own smell is.” He used the example of a species of paper wasps (subfamily Polistinae) that regularly raid the nests of conspecifics to provide food for their own broods. In these species, individuals learn that “If an individual smells like your nest or burrow . . . you don’t eat them.”

  Similarly, tiger salamander larvae are more likely to eat the larvae of unrelated individuals than they are to consume relatives. Pfennig explained that he and his colleagues determined this experimentally by “preventing them from being able to smell.”

  “How did you do that?” I wondered, envisioning a team of micro-surgeons hovering over a tiny, amphibious patient. Irrigation please, Nurse. Can’t you see this patient is dehydrating?

  “By applying super glue under their nares,” he replied.

  “Oh, right,” I said with an uncomfortable laugh, before Pfennig finished up by assuring me that the condition was temporary.

  If you’re wondering whether or not spadefoot toads avoid eating their kin, Pfennig told me that omnivores school preferentially with their siblings, whereas cannibals generally associate only with non-siblings. In close encounters of the bitey kind, cannibal tadpoles release siblings unharmed and consume non-relatives. In the lab, though, apparently all bets are off if the cannibals are deprived of food and then placed in a tank with other tadpoles. In these cases, starvation becomes the great equalizer, and both kin and non-kin are eaten. As I would learn from researchers unearthing new evidence about the Donner Party, this particular aspect of cannibalism spans the entire animal kingdom.

  On the plane ride back to New York, I thought a great deal about the cannibalism I’d seen in the temporary ponds below the majestic Chiricahua Mountains, and about the tiger salamanders I’d collected at Long Island golf courses as a kid.

  Cannibal morphs.

  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.

  * * *

  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.

  — William Shakespeare, Pericles, act 2, scene 1

  Many invertebrates do not recognize individuals of their own kind as anything more than food, and so a significant amount of cannibalism takes place within invertebrate groups like mollusks (clams and their pals), insects, and arachnids (spiders and scorpions). Thousands of aquatic invertebrates like clams and corals have tiny, planktonic eggs and larvae, and these are often a major food source for the filter-feeding adults. Since the planktonic forms often belong to the same species as the adults feeding on them, by definition this makes filter-feeding a form of indiscriminate cannibalism.

  Although both fertilized and unfertilized eggs are eaten by thousands of species, the practice of consuming conspecific eggs appears to have led to the evolution of an interesting take on the concept of the “kids’ meal.” As the name implies, trophic eggs, produced by some species of spiders, lady beetles, and snails, function solely as food. These prepackaged meals often outnumber the fertilized eggs in a given clutch—a fact exemplified by the results of an observational study on the rock snail (Thais emarginata). This species commonly lays a clutch of around 500 eggs but averages only 16 egg-munching hatchlings.

  The black lace-weaver spider (Amaurobius ferox) behaves similarly: one day after their spiderlings hatch, new mothers lay a clutch of trophic eggs, which are doled out to their hungry babies. The trophic eggs last for three days, after which the spiderlings are ready for their next stage of development, but in this case, the “smaller gets eaten” rule gets turned on its head.

  Arthropods like spiders, insects, and crabs are characterized by having their skeletons on the outside of their bodies. To grow in size, they undergo a regular series of molts, during which their jointed cuticle or exoskeleton is shed and replaced by a new skeleton arising from beneath the old. After their first molt and after the trophic eggs have been consumed, black lace-weaver spiderlings are too large for their mother to care for, though they are in dire need of additional food. In an extreme act of parental care, she calls the babies to her by drumming on their web and presses her body down into the gathering crowd. The ravenous spiderlings swarm over their mother’s body. Then they eat her alive, draining her bodily fluids and leaving behind a husklike corpse.

  Insects undergoing pupation, the quiescent stage of metamorphosis associated with the production of a chrysalis or cocoon, are also vulnerable to attack from younger conspecifics. The ravenous larva of the elephant mosquito (Toxorhynchites) not only consumes conspecific pupae, but also embarks on a killing frenzy, slaying but not eating anything unlucky enough to cross its path. The reason for this butchery appears to be the elimination of any and all potential predators before the larva enters the helpless pupal stage itself.

  In some snail species, cannibalistic young transform into vegetarian adults. In one food preference test, hatchlings from an herbivorous snail fed on conspecific eggs exclusively (even when offered lettuce); four-day-old individuals ate equal amounts of eggs and lettuce; and 16-day-old individuals preferred the veggies. When snails older than four weeks of age were denied the lettuce option, they starved to death, even in the presence of eggs. The reason for this gradual transition in feeding preference appears to be that these snails, like other herbivores (from termites to cows), require a gut full of symbiotic bacteria before they can digest plant material. Since newly hatched snails have no gut bacteria, they’re compelled to consume material that can be digested, even if this turns out to be their own unhatched siblings.

  Cannibalism occurs in every class of vertebrates, from fish to mammals. For researchers, factors like relatively larger body size and longer lifespans have made these backboned cannibals easier to study than invertebrates. As a result, previously unknown examples of this behavior are being revealed on an increasingly regular basis. Additionally, factors related to the increased size and longevity of vertebrates have facilitated the ability of scientists to determine and track kin relationships (have you tried tagging a mosquito?), leading to a greater understanding of the complexities of cannibalism-related behaviors. One such result has been the classification of distinct forms of cannibalism, such as filial cannibalism (eating one’s own offspring) and heterocannibalism (eating unrelated conspecifics), both of which have become vital to the concept of cannibalism as normal behavior.

  In mammals, filial cannibalism has been reported in rodents (like voles, mice, and wood rats), and lagomorphs (rabbits and their relatives), as well as shrews, moles, and hedgehogs (a.k.a. The Mammals Formerly Known as Insectivores).8 These mammal moms sometimes eat their young to reduce litter size during periods when food is scarce. Cannibalism also occurs when litter size exceeds the number of available teats or when pups are deformed, weak, or dead.

  In the fishes, by far the largest of the traditional vertebrate classes, individuals in every aquatic environment and at every developmental stage are ambushed, chased, snapped up, and gulped down on a scale unseen in terrestrial vertebrates. One reason that c
annibalism occurs so frequently in fish may be the fact that the group as a whole has more in common with the invertebrates (where cannibalism is often the rule and not the exception) than do the other vertebrate classes (reptiles, birds, and mammals). Another way to consider this is to think of the class Pisces as a mosaic—composed of a suite of more recently evolved, vertebrate traits (like a vertebral column and larger brain) but still retaining some invertebrate characteristics. Here it’s the production of high numbers of tiny offspring with less parental care, as well as a proclivity for consuming both eggs and young—even one’s own.

  At its most extreme, reproductive success in many fish species depends on a romantic-sounding technique known as broadcast spawning, during which females can release millions of eggs, while males simultaneously release clouds of sperm (milt). The end result is that some of the eggs get fertilized. Conceptually, given our own reproductive behavior, one might be misled into thinking that broadcast spawning is an inefficient mating technique. The bottom line, though, is that it works, as do similar variations on this theme employed by many amphibians and invertebrate species alike. Such reproductive strategies are successful because the vast number of eggs released offsets the low probability that any single egg will develop into a mature individual. Along those lines, scientists estimate that for every million eggs produced by an Atlantic cod (Gadus morhua), approximately one egg will result in an adult fish. Partially compensating for these lottery-like odds is the fact that each female produces between four and ten million eggs in a single spawning. On a related note, while this is a remarkable number of potential offspring for most vertebrates, it’s something akin to sexual dysfunction in the ocean sunfish (Mola mola), a strange-looking, open-ocean species that can broadcast 300 million eggs in a single spawning—a vertebrate record.