Size Disparity in Putative Primate Adaptive Radiations and Other Mammalian Clades

Adaptive radiation is commonly viewed as the rapid production of phenotypic disparity and taxonomic diversity. Among primates, two clades have been identified as potential adaptive radiations: the lemurs of Madagascar and the platyrrhines of the Americas. This study examines these clades for one of the proposed signals of adaptive radiation: exceptional disparity in an ecologically important trait.

I sliced a time-scaled molecular phylogeny of extant and recently extinct mammals at the Eocene-Oligocene and mid-Oligocene boundaries, creating samples of clades that are comparable in age to or younger than Lemuriformes and Platyrrhini. For each clade, phenotypic disparity was quantified as the variance of species means for body mass. Differences in disparity attributable to clade age were controlled using phylogenetic regression.

Both primate clades have high age-adjusted disparity: lemuriforms are among the most size-disparate, ranking in the 95th percentile among clades originating after the Eocene-Oligocene boundary; platyrrhines are in the 84th percentile for clades originating after the mid-Oligocene boundary. None of the high-disparity clades in the samples are outliers, and the distributions of disparity are neither positively skewed nor bimodal.

If adaptive radiation has influenced size evolution in mammals, the effect is not apparent in the distributions of size disparity. Without outliers, positive skew, or bimodality, there are no criteria for identifying exceptional clades. The magnitudes of disparity found in lemuriforms and platyrrhines can be interpreted as consistent with adaptive radiation, but size disparity does not provide decisive support for recognizing them as such.