Geographical and taxonomic influences on cranial variation in red colobus monkeys (Primates, Colobinae): introducing a new approach to ‘morph’ monkeys

Abstract

ABSTRACTAim To provide accurate but parsimonious quantitative descriptions of clines in cranial form of red colobus, to partition morphological variance into geographical, taxonomic and structured taxonomic components, and to visually summarize clines in multivariate shape data using a method which produces results directly comparable to both univariate studies of geographical variation and standard geometric morphometric visualization of shape differences along vectors.Location Equatorial Africa.Methods Sixty‐four three‐dimensional cranial landmarks were measured on 276 adult red colobus monkeys sampled over their entire distribution. Geometric morphometric methods were applied, and size and shape variables regressed onto geographical coordinates using linear and curvilinear models. Model selection was done using the second‐order Akaike information criterion. Components of variation related to geography, taxon or their combined effect were partitioned using partial regresssion. Multivariate trends in clinal shape were summarized using principal components of predictions from regressions, plotting vector scores on maps as for univariate size, and visualizing differences along main axes of clinal shape variation using surface rendering.Results Significant clinal variation was found in size and shape. Clines were similar in females and males. Trend surface analysis tended to be more accurate and parsimonious than alternative models in predicting morphology based on geography. Cranial form was relatively paedomorphic in East Africa and peramorphic in central Africa. Most taxonomic variation was geographically structured. However, taxonomic differences alone accounted for a larger proportion of total explained variance in shape (up to 40%) than in size (≤ 20%).Main conclusions A strong cline explained most of the observed size variation and a significant part of the shape differences of red colobus crania. The pattern of geographical variation was largely similar to that previously reported in vervets, despite different habitat preferences (arboreal versus terrestrial) and a long period since divergence (c. 14–15 Myr). This suggests that some aspects of morphological divergence in both groups may have been influenced by similar environmental, geographical and historical factors. Cranial size is likely to be evolutionarily more labile and thus better reflects the influence of recent environmental changes. Cranial shape could be more resilient to change and thus better reflects phylogenetically informative differences.