Stabbing Slinkers: Tusk Evolution Among Artiodactyls

Abstract

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.

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Acknowledgments

We thank James Dines and David Janiger at the Los Angeles County Museum of Natural History, and curators at the American Museum of Natural History and the National Museum of Natural History at the Smithsonian Institution for access to their collections and support. We thank members of the Stankowich Lab at California State University Long Beach and two anonymous reviewers for comments on previous versions of this manuscript.

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Correspondence to Theodore Stankowich.

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This research was supported by funds from California State University Long Beach, College of Natural Sciences and Mathematics. For this type of study formal consent is not required. This article does not contain any studies with human participants or animals performed by any of the authors; all subjects were previously collected specimens deposited in museum collections.

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The authors declare that they have no conflict of interest.

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Significance Statement

While the function and evolution of tusks in elephants, walruses, and even narwhals have received a great deal of scientific and public attention, we know little about what drives the evolution and maintenance of tusks in several groups of artiodactyls (e.g., pigs, muntjac, musk deer). Most male artiodactyls have some sort of sexual weapon (e.g., antlers, horns, tusks), but we don’t know what ecological factors promote the evolution of tusks in some and cranial weapons in others. Using a comparative approach, we show that living a slinking, solitary lifestyle in dense, closed habitats where long range communication during sexual combat is not possible favors the evolution of sharp, dagger-like tusks for combat during territorial disputes. We discuss the sexual benefits of tusks over antlers considering species ecology.

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Cabrera, D., Stankowich, T. Stabbing Slinkers: Tusk Evolution Among Artiodactyls. J Mammal Evol 27, 265–272 (2020). https://doi.org/10.1007/s10914-018-9453-x

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Keywords

  • Tusks
  • Canines
  • Weapons
  • Defense
  • Deer