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. African elephants (Loxodonta africana, Loxodonta cyclotis) exhibit marked dimorphism in tusk size and show behaviors typical of strongly dimorphic species. American mastodons (Mammut americanum) also exhibit pronounced tusk dimorphism, but mastodons and elephants diverged from a most recent common ancestor over 25 Ma, so whether the two genera exhibit similarities in behavior must be inferred. Similar behavioral traits in mastodons and elephants could be supported if patterns of tusk dimorphism are consistent across the two genera. Separate discriminant function analyses (DFA) of 21 mastodon tusks of inferred sex (assessed in independent analyses) and 45 elephant tusks of known sex, using the same ten tusk variables, illustrate that similar 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, using the same tusks and measurements as in DFA, shows that male tusks are larger than female tusks across all measurements, especially in maximum tusk circumference and pulp cavity depth, for both genera. CVA also emphasizes differences in tusk morphology between genera that imply mastodon tusks are, in general, more robust than elephant tusks, although this difference does not affect the nature of tusk dimorphism. Overall, this study illustrates that there is a characteristic male and a characteristic female tusk form shared by elephants and mastodons. This suggests selection pressures favoring tusk dimorphism remained relatively constant for these lineages since at least the time of their divergence. Thus, mastodons, like modern elephants, likely exhibited behaviors associate with strongly dimorphic species, and aspects of modern elephant behavior may have emerged prior to the divergence of elephants and mastodons.