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Abstract
Rhinos are the only modern perissodactyls that possess cranial weapons similar to the horns, antlers and ossicones of modern ruminants. Yet, unlike ruminants, there is no clear relationship between sexual dimorphism and sociality. It is possible to extend the study of the coevolution of sociality and sexual dimorphism into extinct rhinos by examining the demographic patterns in large fossil assemblages. An assemblage of the North American early Miocene (∼22 million years ago) rhino,Menoceras arikarense, from Agate Springs National Monument, Nebraska, exhibits dimorphism in incisor size and nasal bone size, but there is no detectible dimorphism in body size. The degree of dimorphism of the nasal horn is greater than the degree of sexual dimorphism of any living rhino and more like that of modern horned ruminants. The greater degree of sexual dimorphism inMenoceras horns may relate to its relatively small body size and suggests that the horn had a more sex-specific function. It could be hypothesized thatMenoceras evolved a more gregarious type of sociality in which a fewer number of males were capable of monopolizing a larger number of females. Demographic patterns in theMenoceras assemblage indicate that males suffered from a localized risk of elevated mortality at an age equivalent to the years of early adulthood. This mortality pattern is typical of living rhinos and indicates that young males were susceptible to the aggressive behaviors of dominant individuals in areas conducive to fossilization (e.g., ponds, lakes, rivers).Menoceras mortality patterns do not suggest a type of sociality different from modern rhinos although a group forming type of sociality remains possible. Among both living and extinct rhinos, the severity of socially mediated mortality seems unrelated to the degree of sexual dimorphism. Thus, sexual dimorphism in rhinos is not consistent with traditional theories about the co-evolution of sexual dimorphism and sociality.
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Acknowledgements
The ideas presented here benefited greatly from earlier discussions with John Eisenberg, Dave Webb, Jay O’Sullivan, and Dennis Ruez. I thank Jin Meng and Malcolm McKenna for access to fossil collections and Susan Bell and Robert Evander for their assistance with the vertebrate paleontology database and archives at the American Museum of Natural History. Florent Rivals, Tom Rothwell, and two anonymous reviewers provided valuable comments on the manuscript.
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Department of Anatomy, New York College of Osteopathic Medicine, Northern Boulevard, Old Westbury, NY, 11568-3808, USA
Matthew C. Mihlbachler
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Correspondence toMatthew C. Mihlbachler.
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Mihlbachler, M.C. Sexual Dimorphism and Mortality Bias in a Small Miocene North American Rhino,Menoceras arikarense: Insights into the Coevolution of Sexual Dimorphism and Sociality in Rhinos.J Mammal Evol14, 217–238 (2007). https://doi.org/10.1007/s10914-007-9048-4
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