Geographical and Taxonomic Influences on Cranial
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Global Ecology and Biogeography, (Global Ecol. Biogeogr.) (2009) 18, 248–263 Blackwell Publishing Ltd RESEARCH Geographical and taxonomic influences PAPER on cranial variation in red colobus monkeys (Primates, Colobinae): introducing a new approach to ‘morph’ monkeys Andrea Cardini1,2* and Sarah Elton2 1Museo di Paleobiologia e dell’Orto Botanico, ABSTRACT Universitá di Modena e Reggio Emilia, via Aim To provide accurate but parsimonious quantitative descriptions of clines in Università 4, 41100, Modena, Italy, 2Hull York cranial form of red colobus, to partition morphological variance into geographical, Medical School, The University of Hull, Cottingham Road, Hull HU6 7RX, UK 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 mor- phometric 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 parsi- monious 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. Keywords Akaike information criterion, Central Africa, clinal variation, cranial shape, cranial *Correspondence: Andrea Cardini. size, curvilinear models, geometric morphometrics, Piliocolobus, partial regression, E-mail: [email protected] surface rendering. DOI: 10.1111/j.1466-8238.2008.00432.x © 2009 The Authors 248 Journal compilation © 2009 Blackwell Publishing Ltd www.blackwellpublishing.com/geb ‘Morphing’ red colobus cranial variation (1988) argued that aridity during glacial maxima caused tropical INTRODUCTION forest contractions and confined forest fauna to refugia. These Understanding and appreciating clinal variation in size and potentially included highland areas (Mayr & O’Hara, 1986) like shape is a key aspect of biogeographical research. Recent studies the Fouta Djallon highlands (Guinea, West Africa), the Adamawa have demonstrated the utility of advanced morphometric Plateau (Cameroon, western equatorial Africa) and the moun- techniques in the study of clinal variation (Fadda & Corti, 2001; tains between the Rift Valley and the Lualaba River (central and dos Reis et al., 2002; Frost et al., 2003; Monteiro et al., 2003; eastern equatorial Africa). From there, according to this Santos et al., 2004; Cardini et al., 2007). However, although size hypothetical scenario, lowlands were recolonized during inter- differences are relatively easy to illustrate, it is much more glacials, when forests expanded. Indeed, ranges of assemblages of challenging to accurately display spatial variation in shape. red colobus (Grubb et al., 2003) are more or less centred around Visualizing differences is considered to be one of the strengths of those mountain refugia. Forest expansion interrupted the geometric morphometrics, so developing efficient methods for process of divergence and created contact areas like those found illustrating multivariate predictions of clinal shape is crucial to today in some regions of Central Africa (see map in Gautier-Hion the development of morphometric research within biogeography. et al., 1999, p. 81). A fundamental aim of this paper is therefore to report a new Patches of lowland forests along major rivers could also have method that uses a principal components analysis of clinal allowed the survival of small populations of forest animals shapes to produce a visual summary that is directly comparable (Colyn, 1991; Colyn et al., 1991). River barriers were and still are with results from classical biogeographical studies of single crucial to the evolution of red colobus and other Central African variables (e.g. size). By building on previous work (Cardini et al., primates (Colyn, 1991). The existence of several differentiated 2007), this method takes full advantage of powerful visualization taxa within interfluvial blocks of central equatorial Africa suggest tools like surface rendering to illustrate three-dimensional shape that during the last dry climatic period, populations may have variation along vectors. We apply this new method to a geometric survived in isolated patches of lowland forest within the Congo– morphometric study of clinal variation in a group of African Lualaba river basin (Colyn, 1991). Thus, in this region, primate primates, the red colobus monkeys, and examine the effects of diversity might not only be due simply to emigration during geography alongside taxonomy to investigate the factors that times of forest expansion from a major mountain refugium in might influence their cranial form. the Rift Valley, but also to the survival of islands of lowland forests Red colobus [Procolobus (Piliocolobus) de Rochebrune 1887 – during the arid interglacials. Phylogenetic reconstructions based Primates, Cercopithecidae] are medium-sized colobine monkeys on vocalizations (Struhsaker, 1981) give tentative support to the that are patchily distributed within the moist lowland forests of idea that refugia, particularly mountain regions, acted as major west, central and east equatorial and tropical Africa (Davies & originating centres for modern red colobus populations. Oates, 1994). They are currently considered to show more Interestingly, the sole published molecular phylogenetic analysis biological variability than is found in a single species and the (Ting, 2008) is fairly congruent with this earlier phylogeny, sup- prevailing trend is to treat them as a superspecies divided into porting an initial Late Pliocene split of the red colobus clade into assemblages (Grubb et al., 2003; Grubb, 2006). Grubb et al. (2003) three main lineages, which approximately correspond to the western, recognized four main Piliocolobus groups, which largely reflect western equatorial and central equatorial/eastern assemblages, the allopatric or parapatric distribution of their members followed by Pleistocene radiations within the three clades. (Fig. 1a): (1) Piliocolobus badius, consisting of three populations Quantifying clinal variation in cranial form is an important living in West Africa from Senegal to Ghana; (2) Piliocolobus element in understanding Piliocolobus population history and pennantii, including four populations from western equatorial biogeography. Although studies of cranial variability within red Africa; (3) several populations found to the east of the range colobus were conducted in the past (Verheyen, 1957; Colyn, 1991), of P. pennatii, which together form the ‘central equatorial African their main purpose was to assess the validity of taxa proposed on assemblage’ (of these, Piliocolobus sp. ellioti, Piliocolobus sp. foai, the basis of pelage colour rather than examining broader spatial Piliocolobus sp. oustaleti, Piliocolobus sp. tephrosceles and patterns. As briefly reviewed in Cardini et al. (2007), several Piliocolobus sp. tholloni were included in the study reported primate species, including Brazilian tufted-eared marmosets, here); (4) an eastern assemblage comprising three small and Malagasy sifakas, Kenyan vervets, pig-tailed and crab-eating isolated populations in Kenya and Tanzania (Piliocolobus macaques in Southeast Asia, Japanese macaques, African baboons gordonorum, Piliocolobus kirkii and Piliocolobus rufomitratus). and vervets are featured in the vast literature on clinal and Geographical factors probably played a major role in producing ecogeographical size variation. However, only a few of these the morphological variation and taxonomic complexity evident studies investigated African monkeys and none of them con- in red colobus today. Within West and Central