Among living carnivorans, the degree of sexual dimorphism in canine tooth size is correlated with breeding system. Monogamous pair-bonding species and those that form multi-male, multi-female groups tend to be less dimorphic than uni-male, group living species. This correspondence between social behavior and dental dimensions suggests that the inference of sexual dimorphism in extinct carnivorans could shed light on their breeding structure. In this paper, we estimate level of sexual dimorphism in skull length, canine tooth size, and lower molar length for two extinct species, the dire wolf, Canis dirus, and the sabertooth cat, Smilodon fatalis. Three methods are employed to estimate sexual dimorphism: extrapolation from coefficients of variation, division of the sample about the mean, and finite mixture analysis. Results indicate that dire wolves were similar to most canids in their low level of sexual dimorphism, suggesting a pair-bonded breeding structure. Smilodon fatalis appears to have been significantly less dimorphic than living or fossil lions and more comparable to solitary living felids in canine and skull size dimorphism. Thus it seems unlikely that S. fatalis had a polygynous breeding structure like lions in which males compete intensely for access to females. Instead, if S. fatalis lived in groups, these would have been composed of a monogamous pair and their offspring from current and perhaps previous years.
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Vol. 22 • No. 1