McDonald (1977) has identified two different forms of jaws and caniniforms in Megalonyx–individuals with small caniniform teeth and a large diastema, or gap, between the caniniform and first molar, and individuals with larger caniniforms, essentially growing into the diastema, leaving a smaller gap. Greg’s theory is this is sexual dimorphism and the sloths with the larger caniniforms are male–possible displaying their choppers to impress the females and intimidate competitors.

By Greg’s theory, #23034 and hence “Rusty” are female; the Tarkio adult is a male, so we can’t simply say the Tarkio caniniform is 14% longer so the animal was 14% larger. We’re going to have to use a different bone to compare–femurs probably, like Greg says. Sounds like another calculation for Meghann. . . .

Greg has recently been doing some thinking about sexual dimorphism in sloths and published a paper about the possible ecological meaning in Harlan’s ground sloth (McDonald, 2006). He admits males are generally the more robust forms in mammals, but there are solid reasons for evolution to take a species in the opposite direction–large newborns, for example, so he could have the theory backwards. It’s a terrific paper.

References

McDonald, HG. 1977. Description of the osteology of the extinct gravigrade edentate Megalonyx with observations on its ontology, phylogeny and functional anatomy. Masters Thesis. University of Florida.

McDonald HG. 2006. Sexual dimorphism in the skull of Harlan’s ground sloth, Contributions ion Science, Natural history Museum of Los Angeles County 510: 1-9.