Evolution and Development of Sexual Dimorphism in African Antelope
To understand sexual dimorphism as a morphological question, we must understand how it manifests, how it develops, and how it has evolved. To do this, I use comparative osteology, morphometrics, and comparative phylogenetic analyses to characterize sexual dimorphism in the crania and post-crania of African antelopes. This group was selected for their morphological and phylogenetic diversity while imposing some geographic control. Sexual differences in bovids have never been systematically studied or reported in a way that permits direct, quantitative comparisons of features and taxa.
To remedy this situation, in Chapter 2 I report the sexual differences in nine linear cranial traits for 54 taxa. These results are derived from 15935 measurements obtained from 1969 adult specimens. I statistically describe the magnitude of sexual difference for each trait in teach taxon and compare groups of taxa. I discuss which traits are the best candidates for distinguishing the sexes at different sample sizes and levels of classification. Using relative sexual differences (i.e., percentage difference between the sexes) did not substantially change the patterns of sexual differences at the level of subfamilies. For most taxa, this is the first study to report and statistically contextualize the utility of specific craniometric traits for sex identification, and it is the first to enable direct comparison of sex differences across so many antelope species.
In Chapter 3, I use springbok as a test-case for the quantitative study of sexual dimorphism in antelopes. Using an age-estimated ontogenetic series of skulls from one population of Namibian springbok (Antidorcas marsupialis), the development of three horn traits, skull size, and skull shape was measured and described for both sexes. Males and females were then compared across five age classes to better understand how sexual dimorphism develops.
Dental aging criteria and age classes were expanded, statistically evaluated, and validated for both maxillary and mandibular dentitions. Regions of the skull most useful for distinguishing the sexes were confirmed. Sexual dimorphism in skull traits changes significantly over post-natal ontogeny, with a significant overall increase in sexual dimorphism occurring between 12 and 32 months of age. Some aspects of sexual dimorphism continue to change well into adulthood, although the vast majority of development in sexual dimorphism occurs before approximately 22 months of age. The development of sexual dimorphism happens at different times in different traits. Sexual differences in horn length and skull shape develop almost entirely before one year of age. Differences in horn circumference are well established by one year of age, and continue to increase through subadulthood. Most of the skull size differences develop between 22 and 32 months of age, and sexual differences in horn rings first increase in subadulthood and then reverse in mid-late adulthood. An inverse relationship between the development of sexual size dimorphism and the development of sexual shape dimorphism was observed in the lateral aspect of the skull. The largest fluctuations in sexual differences in size and shape were driven by higher growth rates in subadult males, but the fluctuations in shape differences in subadulthood were likely influenced by both male and female changes in skull shape. This points to more than one developmental trajectory over the lifetime of individuals and a possible physiological trade-off between changes in size and changes in shape at ontogenetic time scales.
Expanding on the results from Chapter 2, I describe the anatomical and phylogenetic distribution of sex differences in Chapter 4. Using the only complete, dated phylogeny available for bovids and nearly two dozen traits, I show where the sexes differ, and by how much, and how sexual differences compare across taxa and among well-established clades of antelopes. The amount (i.e. rate) of evolution in the difference between the sexes for each trait is compared to the magnitude of sexual difference. Two hypotheses (not mutually exclusive) are proposed to explain the pattern of horn evolution in female bovids.