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Letter to the editor: Reply to Schillaci (2015): "Correlation between multiple variables among extant primates is insufficient for predicting unknown values in extinct hominins".

The thesis of our brief communication (Traynor et al., 2015) is that the comparative data about the orbits of other primates provide no basis for believing that Neandertals had smaller groups or more limited social cognition compared to the diversity of human hunter-gatherer societies. Pearce et al. (2013) had previously suggested that the cognitive abilities and group sizes of Neandertals were less than that of modern humans, based on a series of weak correlations among orbital aperture diameter, eyeball size, visual cortex volume, neocortex ratio, and group size among primates. We showed that when we consider only values observable in Neandertal specimens instead of those unknowns that must be estimated by regression (c.f. Smith et al., 1996), there is no basis for the conclusion that Neandertals were different from modern humans. Schillaci (2015) reminds readers that body mass often confounds phylogenetic comparisons in primates; we agree. Schillaci finds evidence for a positive correlation between body mass and group size among primates. If this correlation were taken at face value, the relatively large body mass estimates for Neandertals would be consistent with a larger group size than typical of modern human hunter-gatherers. Just as in the case of orbital aperture diameter, the skeletal and comparative data simply do not support a hypothesis of Neandertal cognitive inferiority or limits to group size imposed by cognitive limitations. Still, we caution that this comparative set of primates is limited: including gorillas and orangutans, for example, would substantially reduce the apparent strength of relationship of group size and body mass. A broader issue is that body mass for Neandertals is necessarily an estimate, and including this estimate in statistical models inevitably requires compounding regressions upon each other (Smith et al., 1996). This is problematic. Evidence from living primates, including the added complexity of body mass as suggested by Schillaci, does allow us to test the predictions of the hypothesis presented by Pearce et al. (2013), showing that it does not comport with comparative data. Yet this should not be mistaken for a prediction of Neandertal group size; indeed the weak correlations among these values in living primates do not show group size in Neandertals to have been significantly different from those in living people. For the Neandertals, the archaeological record and our ethnographic knowledge of human cultural variability are essential data that carry much more direct information about Neandertal social dynamics than comparisons across primates.