A preliminary analysis of the dental and cranial morphology of presbytis and trachypithecus in relation to diet

Abstract

The object of this exercise is to examine the variation in two widespread groups of Asian colobine, or leaf eating monkey. I will deal primarily with dental variation, and, to a lesser extent, the relation of these dental features, to. the cranium as a whole. To the extent that each suite of dental characteristics may be related to the. ecological setting of the animals in question, I will attempt to extrapolate from dentition to foraging habit. The systematics of Asian colobines is a massive, and regrettably underworked subject. I cannot do it justice in the present context. However, a number of terminological questions have arisen out of my own work (and, more particularly, out of the work of my colleagues). As a consequence, I propose to raise a few taxonomic issues, but only those pertinent to the morphological information being described. I will limit the scope of this work to more substantial issues, leaving a detailed analysis for a later thesis; I present several appendices of tables (these are merely a sample of what I could provide)partly to demonstrate my interest in computer analysis and partly provide my associates with some basic metric data. Time does not permit me to make more than a brief review of this cranial data. I will deal with primates of what has been, until recently, the genus Presbytis. Others have proposed, and I tardily concur, that the systematics of Asian leaf-monkeys might more reflect phylogeny if Presbytis were separated into its major (formerly subgeneric) components. The dental information I am about to present is not contrary to this interpretation, but a proper assignment of taxa to the generic level is a topic largely beyond the present work and will not be pursued at any length. What is important in the present functional context is that one of these two groups is significantly different in the shape of its dental components from all other colobines. Of itself, this may not merit generic status, for the new, restricted Presbytis. It does, however, give us something more important, an insight into the machinery of foraging in a specialized branch of an already specialized primate subfamily. I will compare Presbytis with one other common colobine, Trachypithecus. These genera are largely sympatric. In terms of body size, they are roughly equivalent, though Trachypithecus shows a degree of clinal variation in size (which is certainly worth ex­ ploring). Trachypithecus in addition shares dental traits in com­ mon, firstly with Asian colobines (exclusive of Presbytis) and, secondly, with African colobines. The cranium of Trachypithecus should tell us as much about the distinctiveness of Presbytis as it tells us about itself. With generic determination as the major aspect of this thesis, I believe we can also make reasonable steps in the direction of differentiating species. This is of secondary importance at this stage of my work, since the suite of characters in this study is not exhaustive. Within Trachypithecus, I will organize the dental information supporting a generic-level determination for T. francoisi, the Francois' langur of Vietnam, the allocation of which which has been in some dispute. Also, I would like to make a case for the resurection of the species-level term auratus for the indiginous Trachypithecus of Java. Not only is this suggested from material uncovered by Colin Groves, the erection of T. auratus as a species also makes sense morphologically. One exception to the limited degree of variation seen among Presbytis is potenziani, of the peculiar environment of the Men­ tawai Archipelago. For some time P. potenziani was thought to be either a member of Trachypithecus or an intermediate between Presbytis and Trachypithecus. Though unusual cranially, P. potenziani is, dentally, well within the Presbytis fold. This, also, I will show. I have limited myself to the more common statistics: for the most part, univariate and bivariate analyses. I have used discriminant analysis more as a method of defining the bivariate relationships, than as a systematizing technique on its own. This is, to an extent, because of the complicating effect of allometry, and also because relationships based on covariance matrices seem less suited to the analysis of biological structures than to structures in which ex­ traneous influences may be easily removed from the analysis. Some of these problems will be mitigated by the use of logarithmic or standardized variables. They do not, however, overcome these deficiencies. I am, at this point, still unsatisfied with the methods available for us to study metric variation among organisms. For that reason, I have chosen to begin with the simplest, perhaps most cumbersome methods. I rely rather heavily on bivariate plots, and statistics based around the Student's-'t' statistic. The 't' statistic also has formidable limitations, particularly with some of the small samples I am forced to use. These small samples also limit the effectiveness of analysis of variance techniques which would otherwise be very useful in identifying functional complexes. Graphic methods, however, provide a visually intuitive aspect to the study of variation. I have employed discriminant analysis to both raw and standar­ dized data (in Trachypithecus), though I will not directly discuss the latter. Given the warnings above, Discriminant function is an additional useful graphic technique but I would emphasize that this must be backed up by a more detailed and thoughtful analysis of the conponents of that analysis. True, none of us like to peruse the pages of 't' statistics such as Appendices F and G; the analysis of regression slope is even less intuitively meaningful than ratios. We like all-inclusive, simple answers. In viewing the plot of the first two Cannonical Functions derived from SPSS, it is easy to forget how much the data has been reduced. Returning to simpler methods (par­ ticularly for a preliminary analysis)gives one a much better feel for the data. Finally, I would like to say something about the Figures presented in Chapter 5. Some readers may object to figures composed from al- phabetic characters; I will not deny that these are less attractive, and, if I have done a poor job, they are also less instructive than an actual drawing. On the other hand, using this kind of illustration, I can keep an entire thesis on tape in a computer. Copies of the. thesis may be cheaply line-printed and distributed with a minimum of collation and without expensive photographic reproductions. As with many things in this life, each way of doing things has its plusses and its minuses.