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.