Cannibalism: A Perfectly Natural History(19)
As a vertebrate zoologist, I was interested in determining whether or not a transition in polar bear diets had actually taken place. And if it had, I wondered whether we were involved.
Polar bears (Ursus maritimus, Latin for “marine bear”) are among the world’s largest carnivores, a diverse mammalian order whose members include cats (felids), dogs (canids), raccoons (procyonids), and weasels and their relatives (mustelids). They are, of course, famous for their meat-eating diets and many of them share a characteristic known as carnassial teeth (or carnassials). In the majority of mammal species, when the jaw closes, the premolar and molar teeth on the upper jaw fit snugly into those on the lower jaw. This facilitates the crushing of food items before they’re swallowed. In most carnivore species, though, when the jaws close, the last upper premolar and the first lower molar on each side shear past each other like blades, effectively slicing large pieces of meat into smaller pieces that can be readily swallowed. Carnassial dentition was lost in most bears as they evolved more omnivorous feeding habits. Here, the hard-to-digest plant material required a mash-up by more traditional molars, thus increasing its surface area and allowing for more efficient breakdown by enzymes like cellulase. In polar bears, however, fully functional carnassials have apparently re-evolved—a reflection of the species’ strict meat-eating diet, which consists primarily of ringed seals (Pusa hispida) and bearded seals (Erignathus barbatus).
Secondarily evolved traits like carnassial teeth in polar bears are common in nature. For example, having inherited the ability to swim from their fishy ancestors, many ancient vertebrates lost the ability (and related features like fins) as they became more and more adapted to terrestrial lifestyles. Swimming re-evolved in some lineages, leading to creatures like seals and whales, whose fins are actually modified terrestrial limbs.
Cannibalism has been recorded in at least 14 species of carnivores. In pumas (Puma concolor), lynx (Lynx lynx), leopards (Panthera pardus), and sea lions (Phocarctos hookeri), it appears to occur for many of the usual reasons, including stress (due to lack of food), elimination of rivals, and increased mating opportunities.
Heterocannibalism, in this case, eating the cubs that another male sired, is clearly a reproductive strategy in male lions (Panthera leo) after taking over a pride. Through the practice of infanticide, the incoming males terminate the maternal investment in unrelated cubs. A lioness with cubs will not come into heat for a year and a half after giving birth, but similar to what has been observed in other mammals, a lioness that loses her cubs becomes sexually receptive almost immediately.
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.