In the vast arsenal of animal weaponry, the most exaggerated, elaborate and diverse devices such as tusks, claws and antlers have not been shaped by a need to fend off fierce predators. Rather, these impressive forms are driven by sex.
“Everybody understands at a gut level that it’s usually males that have flashy displays or weapons like tusks and antlers,” says Doug Emlen, an animal weapon expert at the University of Montana in Missoula. Biologists say that these fantastic shapes — from the giant curved tusks of woolly mammoths to the nightmarish jaws of stag beetles — evolved to ward off competition from rival males and to impress females.
Examples of such sexually selected weapons abound throughout the animal kingdom in insects, fish, crustaceans, reptiles and mammals as varied as narwhal, rhinoceros and moose. Even extinct species such as trilobites and dinosaurs sported elaborate projections. The number and variety of examples argue that evolution has turned to weaponry time and again in the race to reproduce successfully.
It’s such a common theme that Emlen had to persuade his editors to include seven detailed, full-page line drawings in a survey of nature’s weapons that he wrote for the 2008 Annual Review of Ecology, Evolution, and Systematics, featuring more than 280 examples of fantastical spikes, horns, antlers, pincers, tusks, claws, extended jaws, saws and spears. The illustration above offers a taste.
Scientists still debate the degree to which female choice plays a role in shaping the weapons’ flair and are still trying to figure out what factors drive the diversity of weapon forms seen among even closely related species. But it’s clear that the wild array of weapons evolved to aid successful mating.
