Sexual dimorphism in the fossil ground sloth Lestodon armatus (Xenarthra, Folivora)

ABSTRACT

Mylodontidae (Mammalia, Xenarthra) is a family of ground sloths widely distributed in the South American fossil record, with members also present in Central and North America. Within the Mylodontidae, Lestodon armatus is the largest species, with an estimated body mass of more than three tonnes. This work focuses on the enlarged lower caniniforms of L. armatus as possibly exaggerated sexually dimorphic structures. Lower caniniforms from the late Pleistocene of Argentina, Uruguay, and Bolivia were studied using specimens from seven palaeontological collections. The possible sexual dimorphism in the caniniforms and its implications regarding the existence of sexual selection was assessed through morphometric analyses. The results support the existence of sexual dimorphism in L. armatus. Sexual dimorphism in an exaggerated structure in a large mammal suggests the existence of sexual selection, via competition between males or female mate choice, resulting in the evolution of the dimorphic structure. In L. armatus, the enlarged caniniforms would correspond to males and could have functioned as armaments in intraspecific fights or ornaments for sexual display. Based on observations in extant mammals, a polygynous mating system is proposed as highly probable in L. armatus, although the existence or composition of social groups cannot be certainly determined.

KEYWORDS:

• South America
• Pleistocene
• Sexual Selection
• Intraspecific Variability
• Allometry
• Sex Estimation

Acknowledgments

We are grateful to Alejandro Kramarz (Museo Argentino de Ciencias Naturales ‘Bernardino Rivadavia’, Argentina), Marcelo Reguero (Museo de La Plata, Argentina), Andrés Rinderknecht (Museo Nacional de Historia Natural, Uruguay), and the personnel of the museums of Colonia del Sacramento, Uruguay, for access to the collections under their care. Two anonymous reviewers made constructive comments that helped to improve the manuscript.

Data Availability Statement

All the data required to reproduce the analyses reported in this work is available in the main text.

Disclosure of potential conflicts of interest

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Agencia Nacional de Investigación e Innovación (ANII), grants FCE_1_2014_1_104572 to RAF and POS_FCE_2015_1_1005196 to LV, and a NatGeo Grant (Nº 178431) to RAF