The babirusa, a wild pig native to the Indonesian islands of Sulawesi, Togian, Sula, and Buru, possesses one of the most bizarre dental adaptations in the animal kingdom. Male babirusas feature upper canine teeth that do not grow into the mouth; instead, they penetrate the skin of the snout, curving upward and backward toward the forehead. If an individual lives long enough and does not wear down or snap its upper tusks through fighting or foraging, the sharp tips can eventually pierce the pig's own skull, leading to severe infection or fatal injury.
Evolutionary biologists have long pondered why natural selection would preserve such a dangerous physical trait. The leading theory suggests that these tusks are a product of runaway sexual selection. Female babirusas prefer males with prominent, well-maintained tusks, as they signal survival capability and genetic vigor.
Interestingly, despite their intimidating appearance, the fragile, curved upper tusks are rarely used as offensive weapons in territorial battles. Instead, males rely on their lower canines for slashing opponents, while the upper tusks act as a protective shield for the eyes and face against incoming strikes. This bizarre trade-off demonstrates how sexual selection can push physical traits to the absolute brink of survival viability.