Cannibalism

by Bill Schutt

  Interestingly, females of a similarly social predator, also found on the African savannah, possess a potent defense against infanticide and cannibalism. This adaptation has also enabled females of this species to become the dominant clan members. How this phenomenon works is fascinating, although it requires a brief review of development genetics. An additional bonus for this approximately 60-second commitment will be an answer to one life’s great mysteries, namely, “Why do men have nipples (or penises, for that matter)?”

  During early embryological development, mammal embryos are genderless. At a certain point, tiny buds of tissue grow into precursors of the penis and mammary glands. Sex determination is based on the embryo having one of two combinations of the X and Y sex chromosomes. These combinations, XY or XX, act like two versions of a blueprint. The XY blueprint results in the production of the male hormone testosterone, a chemical messenger that stimulates the growth of the penis bud into a penis. Since testosterone prevents the further development of the mammary glands, this explains why males still have the nipples they grew as genderless embryos—but don’t produce milk. Alternately, having two X chromosomes results in the production of estrogens, the primary female sex hormones, and these stimulate the production of mammary glands. The female hormones also put a halt to the growth of the penis bud, leaving behind the clitoris, a tiny, erectile structure, which unlike the penis, is not involved in urination.

  In the spotted hyena (Crocuta crocuta), the developmental scenario described above has been flipped on its ear. Scientists believe that at some point in their evolutionary past, a genetic mutation initiated the production of higher levels of the male sex hormone testosterone in female spotted hyenas. As a result, bulked up, hyper-aggressive females dominate every interaction with their male counterparts, with males even coming up short in the external genitalia department. Female hyenas develop a remarkably elongated clitoris, which resembles a longer version of the penis. Additionally, the normally liplike vulva is fused closed, thus enabling females to urinate through their pseudopenises (or pseudopenes), the tips of which are also penetrated by the Real McCoy during copulation. Completing the he-man look, the sealed-up vulva forms a matching pseudoscrotum, within which deposits of fat stand in for a functioning pair of testicles.

  The female hyena’s uniquely shaped external sex organs actually gave rise to a myth that these mammals are hermaphrodites. Although this is definitely not the case, the birth process is an extremely painful and dangerous experience for first-timers, and by now you may have guessed the reason. Large, full-term hyena fetuses must pass through the clitoris, which, if things proceed smoothly, causes it to tear open. Reportedly, stillbirths and instances of maternal mortality during delivery are high, but after the successful birth of the first litter, the clitoris never fully closes again, making subsequent births somewhat easier. So, while some aspects of this adaptation sound counterproductive, the fact remains that Crocuta crocuta is the most successful mammalian predator in all of Africa. One reason may be related to the fact that, unlike in lions, there is little danger that males will attempt to kill and consume unrelated cubs. Females, on the other hand, have been known to do just that.

  All right now, what about those polar bears?

  In 2009, mainstream media outlets began reporting that polar bears were undergoing a serious change in dietary habits. The take-home message was that global warming had reduced the Arctic sea ice, thus resulting in shorter hunting seasons for the bears and fewer seal kills. As a consequence, the stressed-out bears were starving and resorting to cannibalism in order to survive. The problem with most of these stories was that the authors left out a rather important fact—and it was one that researchers have known for decades.

  According to wildlife biologist Mitchell Taylor, “Polar bears will readily eat other polar bears when they can do so without excessive risk of injury.” In fact, males of most North American bear species will kill and eat conspecific cubs pretty much whenever they can get their paws on them. Researchers believe that infanticide during the breeding season may provide males with “a reproductive opportunity as well as a nutritional reward” since like the previously described lionesses, female polar bears will come into estrus more quickly if their offspring have been killed. Because of this, cannibalism has been, and continues to be, one of the greatest contributors to bear cub mortality, especially just after leaving the maternity den. The threat from adult males is one of the key reasons that mother bears are so protective of their cubs and also explains why females give males such a wide berth when selecting maternity den sites.

  Recently, another probable cause of polar bear cannibalism was added to the mix. Because of incomplete reporting by the media, though, and a tendency to stress sensationalism over detail, the result has been a cannibalism-themed fiasco.

  The mess came about soon after the 2006 publication of a paper by Arctic researcher Stephen Amstrup. He and his coworkers were clearly alarmed by three incidents of cannibalism by polar bears in the southern Beaufort Sea, which occurred during a two-and-a-half-month period. Two of the incidents involved the death and partial consumption of adult female bears. In one, the female’s body was found inside a maternity den that collapsed during an attack by a predatory male bear. In the second case, the female polar bear was killed on the sea ice, presumably not long after emerging from its den with a cub. In the third case, a one-year-old male was killed and partially consumed by an adult male. According to Amstrup and his colleagues, these attacks were unique because they had taken place in areas not generally frequented by male polar bears. Each year, once the Arctic sea ice melts and polar bears are forced onto the land, males are usually found near the coast while females and their cubs venture farther inland, and away from the males.

  In the cases documented by Amstrup, the researchers concluded that “the underlying causes for our cannibalism observations are not known.” They suggested that the incidents could have been “chance observations of previously unobserved rare events, or even a single rogue bear that adopted a [hunting] strategy including cannibalism.”

  What got the media machine cranking, though, was the researchers’ hypothesis that these attacks and subsequent cannibalism might have resulted from male polar bears being “the first population segment to show adverse effects of the large ice retreats of recent years. . . . We hypothesize that nutritional stresses related to the longer ice-free seasons that have occurred in the Beaufort Sea in recent years may have led to the cannibalism incidents we observed in 2004.”

  The problem was not in the presentation of Amstrup’s hypothesis, but the fact that many of the media reports that followed neglected to mention that cannibalism in polar bears was already known to be a naturally occurring event, with the first published report surfacing in 1897. By leaving out this vital fact, those working to publicize the effects of global climate change suddenly found themselves on the wrong end of some serious butt-kicking from climate change deniers. These zealots were quick to point out that cannibalism was quite common in polar bears and that the attempt to link polar bear cannibalism to what they referred to as the “Global Warming Hoax” was just another instance in which scientists were flat-out lying to the public. In reality, modern researchers have been reporting on non-climate-change-related infanticide and cannibalism in polar bears for decades, a point Amstrup and his coauthors also discussed their paper, and a point neglected in most of the media coverage.

  Ultimately, though, the authors of the sensationalized headlines ignored that information. Instead they cobbled together their stories from non-scientific sources, including a short article by another non-scientist. This one warned of “GRAPHIC PHOTOS” and opened with the line, “Cannibalism is not part of the polar bears’ M.O.” As a result, a valid scientific hypothesis—Global climate change has led to a reduction in Arctic sea ice, and this may be causing increased incidences of cannibalism in polar bears—now takes a back seat to a distorted take on the subject as well as a decep
tive but well executed argument by climate change deniers.

  This would be my first experience with cannibalism-related sensationalism, but it would definitely not be the last.

  6: Dinosaur Cannibals?

  Personally, I suspect that a whole pack of full-grown T. rex would have a very hard time finding enough to eat.

  — Paleontologist Nicholas Longrich, Discovery News, October 15, 2010

  While we’re on the topic of large, meat-eating animals embroiled in cannibalism-related controversies, I thought this would be the perfect time to bring up the topic of cannibalism in dinosaurs—or the lack thereof.

  Coelophysis bauri was one of the earliest dinosaurs—a carnivorous and remarkably birdlike biped that lived approximately 200 million years ago across what is now the southwestern United States. A fast runner, it stood about a meter tall at the hips and had an overall length of about three meters from snout to tail. Equipped with a mouthful of recurved and bladelike teeth, Coelophysis was thought to feed on smaller animals like lizards.

  In 1947, a team from the American Museum of Natural History (AMNH) working at Ghost Ranch in New Mexico unearthed a huge bone bed composed of hundreds of Coelophysis skeletons. After examining the fossils, famed AMNH paleontologist Edwin Colbert claimed that the abdominal cavities of some of the specimens contained the bones of smaller individuals of the same species. Thus was born the “cannibal-Coelophysis hypothesis” and the subsequent portrayal of Coelophysis and other dinosaurs as cannibals. Reminiscent of the misconceptions concerning male-munching black widow spiders, the depiction of dinosaurs as cannibals remained unchallenged for decades.

  In 2005, another group of researchers from the AMNH set out to determine whether or not claims of dinosaur cannibalism could be supported. Led by paleontologists Sterling Nesbitt and Mark Norell, they performed detailed morphological and histological analyses of the bones (something Colbert did not do). Soon enough, the scientists uncovered a slight problem—not only were the bones in question not from juvenile specimens of Coelophysis, they weren’t even dinosaur bones. Instead, the fragments recovered from the abdominal cavities of the two relevant Coelophysis specimens belonged to crocodylomorphs, a group that includes crocodiles and their extinct relatives—but not dinosaurs.

  Investigating further, Nesbitt and Norell determined that another example of reputed dinosaur cannibalism was also problematic. In a much-publicized case, paleontologist Aase Jacobsen reported in 1998 that bite marks on the bones of tyrannosaurs from the Dinosaur Park Formation in Alberta, Canada, were consistent with bite marks from conspecifics. The AMNH researchers, however, called this cannibalism claim into question by pointing out that there were actually two tyrannosaur genera, Gorgosaurus and Daspletosaurus, living at the site. Since even by its loosest definition, cannibalism requires that the participants be conspecifics, the potential that the predation took place between separate species led Nesbitt and Norell to raise a warning flag. They concluded that while cannibalism “may be expected in non-avian dinosaurs,” it was “not as prevalent as was once supposed.”

  In the limited realm of dinosaur cannibalism, an additional piece of evidence came from paleontologist Nicholas Longrich and several high profile co-authors in 2010. The researchers discovered bite marks on four museum specimens of Tyrannosaurus rex. They reasoned that since T. rex was the only large predator alive at that time, the score marks and gouges on the skeletons must have been made by conspecifics, and that the most likely scenario was that the marks had been made during scavenging of carcasses. Longrich and his coworkers concluded that their results provided solid evidence that “cannibalism seems to have been a surprisingly common behavior in Tyrannosaurus, and this behavior may have been relatively common in carnivorous dinosaurs.”

  I interviewed Mark Norell on a beautiful mid-September afternoon at the American Museum of Natural History. His lab is a dinosaur lover’s dream—a remarkable, fossil-filled space that opens onto one of the museum’s famous turrets. The view from Norell’s high-ceilinged office was nothing short of spectacular, and as we talked it was difficult not to look over his shoulder at the wide swath of Central Park below us.

  “I think there’s very little evidence at all for dinosaur cannibalism,” Norell told me. “Although a lot of it really depends on what you’d call cannibalism. If a tyrannosaur dies and another tyrannosaur comes along and eats it, is that cannibalism? Or is that just scavenging a dead carcass? I have a picture around here someplace of a camel eating a dead camel that was lying there. Is that cannibalism?”

  I told him that I didn’t consider hunting and killing to be prerequisites for cannibalism and that, from what I’d learned, scavenging your own species was cannibalism. I used the example of besieged cities, where the victims of starvation or exposure were consumed, sometimes by their own relatives.

  As scientists began to study cannibalism in nature, some of the behaviors they observed expanded on the definition of cannibalism coined by Elgar and Crespi in the early 1990s. The requirement that the cannibal kill the conspecific before eating it was dropped (enabling certain forms of scavenging to be considered cannibalism), as was the necessity that the cannibal consume the entire victim. As a result, some forms of behavior now straddle a line between cannibalism and something else—in this case, scavenging.

  But even allowing for a broad definition of cannibalism, in the case of dinosaurs, Norell had some major problems with the evidence that Longrich had presented. For example, he said that the gouges on Longrich’s T. rex bones could have been inflicted by conspecifics fighting each other, not necessarily eating each other.

  According to Norell, the only compelling evidence for dinosaur cannibalism appeared to have occurred in the late Cretaceous theropod Majungasaurus crenatissimus, an example uncovered by geologist Raymond Rogers from multiple bone beds from a Madagascan rock formation thought to be between 70.6 and 65.5 million years of age.

  In a 2014 phone conversation, I asked Rogers how he had come to the conclusion that Majungasaurus was a cannibal. He explained that he’d been looking at dinosaur bones for decades and that specimens from a particular region in Madagascar (dated to approximately 70 million years ago) had a remarkably high number of teeth marks.

  “Quickly enough, you start thinking about who could have made these bite marks, and [in this instance] there are very few candidates.” According to Rogers, there were only a few large carnivores living at the site, which they named MAD05-42. One was a crocodile, “which would have made no comparable traces on the bones” and the other a small theropod dinosaur, “which had tiny little teeth.”

  “And then you’ve got Majungasaurus, which had large teeth. When you look at the scoring patterns on [Majungasaurus] bones you can match them to the spacing of Majungasaurus teeth and their actual denticle patterns.”

  “So there’s no potential that you might be missing another large predator—something you just haven’t dug up yet?” I asked.

  “Right, I don’t think we’re missing anything. And if we are missing something it would have to be big, and arguably it would have to be rare. But the bite marks are anything but rare. So . . . whatever it is, it would have to be really big, really cryptic, really rare, and it would have to bite everything, which doesn’t make any sense. Basically, there are tons of bite marks that match the teeth of Majungasaurus and nothing else matches those traces.”

  I pressed on. “So how do you know that these tooth marks on Majungasaurus bones weren’t made during combat between conspecifics?”

  Rogers said he had good evidence to prove that hadn’t been the case. Elements of the vertebral column showed evidence of having been scraped and possibly gnawed. This type of “late stage scavenging” took place after the limb muscles and the guts had been consumed, and when the scavenger had to work hard to obtain any further nutrition from the carcass.

  “There’s no evidence, whatsoever, for killing or jostling with conspecifics. There’s evidence for feeding. And the evidenc
e we have for feeding is consistent with some pretty focused effort. These bones were scraped.”

  It was beginning to sound like Majungasaurus had been following two of Gary Polis’s cannibalism-related generalizations: first, that incidents of the behavior increased during stressful environmental conditions; and second, that the absence of alternative food sources often led to cannibalism. So I asked Rogers if he believed that stressful environmental conditions could have driven Majungasaurus to scavenge conspecifics.

  “It kind of looks that way,” he replied. “The overall reconstruction that we put together is pretty well supported. It seems like these ancient ecological systems were devastated again and again and a lot of things died. I think that if you’re a large meat-eating dinosaur, you’ll capitalize on whatever there is to feed upon, and I have no doubts that a creature like Majungasaurus would take the resources that were available.”

  Finally, I asked the paleontologist how prevalent he thought cannibalism had been among the dinosaurs.

  “I doubt that it was uncommon,” Rogers replied. “What’s uncommon is evidence. Because I think that more often than not, animals can probably get their meals without heavily working bone, and when you’re dealing with fossils . . . that’s all you got. If it’s not written on the bone, you’re not going to get the story. So . . . I don’t think cannibalism was unique to any particular theropod group. When you drive around Montana today and see ground squirrels eating ground squirrels and you read papers about dogs eating dogs, and lizards eating lizards . . . it’s pervasive. I have no doubts that cannibalism was widely practiced by dinosaurs. The fact that there have only been two cases of dinosaur cannibalism . . . that’s just an artifact of paleontology and the [scarcity of the] fossil record.”