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Sexual Dimorphism and Inter-Generic Variation in Proboscidean Tusks: Multivariate Assessment of American Mastodons (Mammut americanum) and Extant African Elephants

Abstract

Characteristics of social structure, mating strategies, and parental investment can be inferred for mammalian species based on degree of sexual dimorphism, especially when males are substantially larger than females. American mastodons (Mammut americanum) and extant African elephants (Loxodonta africana, Loxodonta cyclotis) both exhibit marked dimorphism in tusk size. To evaluate the hypothesis that this dimorphism might be indicative of similar, and possibly conserved, behavioral patterns in each lineage, we undertook a detailed evaluation of the pattern of tusk growth in these two genera. Separate discriminant function analyses (DFA) of 21 adult mastodon tusks of inferred sex and 48 adult elephant tusks of known sex show that patterns of ontogenetic change in tusk circumference, regardless of genus, effectively discriminate between sexes. Canonical variates analysis (CVA) of tusks from male and female mastodons and male and female elephants shows that male tusks in both genera are larger than female tusks across all measurements, especially for maximum tusk circumference and pulp cavity depth. CVA’s emphasis of inter-group differences in tusk morphology also shows that mastodon tusks are more robust than elephant tusks. Overall, this study illustrates that there is a characteristic male and a characteristic female tusk form shared by elephants and mastodons. This shows that elephants and mastodons display a shared syndrome of traits beyond sex-linked differences in tusk size, supporting the hypothesis that mastodons exhibited behaviors similar to those we observe today in African elephants.

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Acknowledgments

We thank W. Stanley and J. Phelps at the Field Museum of Natural History, Chicago, IL, L. Gordon and D. Schmidt at the National Museum of Natural History, Washington, D.C., and A. Fleming and E. Westwig at the American Museum of Natural History, New York, NY, for access to collections and assistance moving and measuring tusks; J. Johnson for his invaluable assistance during the data-collection portion of this project; C. Badgley, R. Bebej, P. Gingerich,. D. Fox, and one anonymous reviewer for comments and suggestions that improved this manuscript greatly; and A. Rountrey, W. Sanders, A. Wood, and M. Zelditch for contributions throughout this study. Funding was provided by a Scott Turner Award from the University of Michigan, Department of Earth and Environmental Sciences (KMS) and National Science Foundation grants EAR 9628063 and 054095 (DCF).

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Correspondence to Kathlyn M. Smith.

Appendices

Appendix I

Table 5 Measurements (cm) used in this study. Abbreviations as in Fig. 2. The dashed line separates mastodons (above) from African elephants (below)

Appendix II

Table 6 Mastodons included in this study and the methods used to evaluate the sex of each (modified from Smith and Fisher 2011: table 1). Sex-1 lists sex assignment based on prior work utilizing non-tusk dental evidence of age (D), associated skeletal remains (S), or position within the univariate distributions (U) in Smith and Fisher (2011: fig. 1). Sex-2 lists sex assignments determined from a multivariate analysis in Smith and Fisher (2011). Age is the number of annual increments in tusk dentin on the tusk exterior (E) or on a longitudinal section (L)

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Smith, K.M., Fisher, D.C. Sexual Dimorphism and Inter-Generic Variation in Proboscidean Tusks: Multivariate Assessment of American Mastodons (Mammut americanum) and Extant African Elephants. J Mammal Evol 20, 337–355 (2013). https://doi.org/10.1007/s10914-013-9225-6

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Keywords

  • Tusk
  • Sexual dimorphism
  • Morphology
  • Mammut americanum
  • Loxodonta
  • Behavior