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)?”