Patterns of tooth size variability in the dentition of primates.

Published data on tooth size in 48 species of non-human primates have been analyzed to determine patterns of variability in the primate dentition. Average coefficients of variation calculated for all species, with males and females combined, are greatest for teeth in the canine region. Incisors tend to be somewhat less variable, and cheek teeth are the least variable. Removing the effect of sexual dimorphism, by pooling coefficients of variation calculated for males and females separately, reduces canine variability but does not alter the basic pattern. Ontogenetic development and position in functional fields have been advanced to explain patterns of variability in the dentition, but neither of these appears to correlate well with patterns documented here. We tentatively suggest another explanation. Variability is inversely proportional to occlusal complexity of the teeth. This suggests that occlusal complexity places an important constraint on relative variability within the dentition. Even when the intensity of natural selection is equal a t all tooth positions, teeth with complex occlusal patterns must still be less variable than those with simple occlusion in order to function equally well. Hence variability itself cannot be used to estimate the relative intensity of selection. Low variability of the central cheek teeth (Mi and M:) makes them uniquely important for estimating body size in small samples, and for distinguishing closely related species in the fossil record. The existence of variation is a prerequisite for evolution by natural selection. No description of an organ, organism, or population is complete without characterizing both its typical characteristics and the variations or deviations from typical. Books have been written about variation and variability in mammals (e.g., Berry and Southern, "70; Yablakov, '74; see also Long, '69), but relatively little is yet known about the variability of many anatomical systems. The study of variation and variability is time consuming because measurements of large samples are required for adequate characterization. In recent years tooth size has been studied in many primates. Hooijer ('48) described the variability of the orangutan dentition in one of the earliest quantitative studies based on a large sample size. Additional studies have since appeared that document dental variability in other speAM. J. PHYS. ANTHROP. (1979) 51: 457-466 cies of living primates (Schuman and Brace, '55; Freedman, '57; Olson and Miller, '58; Hooijer. '60; Swindler et al., '63; Biggerstaff, '66; Zingeser, '67; Pilbeam, '69; Leutenegger, '71: Wolpoff, '71; Gingerich, '74; Johanson, '74; Swindler, '76; Gingerich and Ryan, '791. Most of these papers describe dental variability in anthropoid primates, and relatively little has been reported of variability in prosimians. In this study we have attempted to achieve something of a synthesis by analyzing variability across a broad range of primates. The analysis shows that there is a definite pattern to dental variability in primates. This pattern is important for understanding functional fields in the dentition. Knowledge of the pattern is also very useful for interpreting the systematic relationships and the adaptations of fossil primates.