Combat weaponry, including elaborate horns and antlers and complex dentition, evolved independently several times among mammals. While it is evident that tusk and tusk-like dentition have emerged primarily among males for intrasexual combat, it is unclear what ecological factors favor the retention or re-evolution of tusks. We investigated patterns of tusk evolution in artiodactyls while exploring specific ecological factors that might favor their use over other cranial weapons (e.g., antlers, horns). We show that among males, small (<15 kg), solitary species tend to retain well-developed canines, and more solitary species live in more closed habitats. These results suggest that tusks are a better weapon option for smaller, slinking artiodactyls in forested environments with low visibility, whereas larger taxa living in more open environment can bear the cost of elaborate headgear and are better served by communicating across distances an honest signal of fighting ability. Small species in dense habitats may also be more likely to be ambushed by predators and have a need to defend themselves; small, slicing daggers may be a better defensive weapon and allow more maneuverability and faster escape than cumbersome headgear in densely vegetated habitats.

中文翻译：

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刺伤者：偶蹄类动物的象牙进化

战斗武器，包括精致的角和鹿角以及复杂的牙列，在哺乳动物中独立进化了几次。虽然很明显，象牙和象牙一样的牙列主要出现在雄性之间进行性交，但尚不清楚哪些生态因素有利于象牙的保留或再进化。我们调查了偶蹄类动物的象牙进化模式，同时探索了可能有利于它们使用其他颅骨武器（例如，鹿角、角）的特定生态因素。我们表明，在雄性中，体型较小（<15 kg）的独居物种倾向于保留发育良好的犬科动物，而更多的独居物种生活在更封闭的栖息地中。这些结果表明，在能见度低的森林环境中，象牙是较小的、狡猾的偶蹄动物的更好武器选择，而生活在更开放环境中的较大类群可以承受精心制作的头饰的成本，并且通过远距离交流可以更好地提供战斗能力的诚实信号。密集栖息地中的小型物种也可能更容易遭到捕食者的伏击并需要自卫；在植被茂密的栖息地，小型、切割匕首可能是一种更好的防御武器，并且比笨重的头盔具有更高的机动性和更快的逃生速度。