The distribution of grooming among female primates: Testing hypotheses with the Shannon-Wiener diversity index

The Shannon-Wiener diversity index (H) has been used to characterize grooming relationships among adult female primates. To make comparisons among different groups, the value of H has been divided by the maximum value it can reach for a particular group size. This ratio, the grooming diversity ratio (GDR), has been used to test predictions from hypotheses that may explain the distribution of social relationships among adult female primates. Using grooming data from different primate populations and random computer simulations I show that the mean value of H and GDR are positively affected by the In of the mean number of grooming bouts recorded per dyad (LnMNBD) and negatively by the coefficient of variation in the frequency of grooming bouts recorded for the females within a group (CV). These two variables reflect the combined effect of sampling effort and the rate of interactions among individuals (and their variance) and should be statistically controlled to make sensible use of H or GDR. After controlling for LnMNBD and CV, I found no significant effect of female group size, the degree of female involvement in inter-group encounters and the patterns of female dispersal on the mean value of GDR. The socioecological model of female social relationships (Sterck et al., 1997) predicts that groups categorized as resident-egalitarian should have higher GDRs than resident-nepotistic ones. I find some support for this prediction when using a data set where some species contributed more than one data point but not when using mean values per species as in the previous analyses. This result may be confounded by a phylogenetic effect: langurs seem to have more diverse relationships than other primates. Data from more species are necessary to corroborate these results and to disentangle the effect of phylogeny from that of the social categories since most monkey groups characterized as resident-egalitarian were langurs.