Sexually dimorphic structures in mammals: can we use them to draw inference about past environments?
Professor Christine Janis, University of Bristol, UK
The pattern of sexual dimorphism in ungulate (hoofed) mammals in relation to habitat preference was established by Peter Jarman in the early 1970s. He proposed five categories of ecomorphology within African antelopes that related to body size, size dimorphism, and dimorphism in horns. These ecomorphologies were correlated with differences in reproductive behaviour, which in turn were correlated with habitats ranging from closed forest to open grasslands. Thus the distribution of such ecomorphologies within fossil communities may provide information about the habitat independent of other palaeoenvironmental proxies. While sexual dimorphism in body size is difficult to determine in fossils, absolute body size and patterns of the possession of horns (or other types of cranial appendages) can be observed. The pattern of acquisition of cranial appendages and changes in lineages over time can provide information about habitat changes both within and between geographic areas. Cranial appendages first appeared in artiodactyls in the mid Cenozoic at a time when more open habitats started to spread in the higher latitudes: but patterns of acquisition and distribution of dimorphic ecomorphologies differ between ungulates in North America and the Old World, implying earlier and more significant aridity on the North American continent (an implication now confirmed by other proxies). Patterns of horn dimorphism in African bovids may serve to illuminate habitat change over time on that continent.
Sexual selection on colour signals: mechanisms and diversity
Associate Professor Devi Stuart-Fox, University of Melbourne, Australia
Sexual dichromatism, the difference between the sexes in coloration, is one of the most widely used indices of sexual selection in macroevolutionary studies. These studies have revealed evolutionary drivers of the strength of sexual selection, patterns of evolutionary losses and gains of sexual dichromatism, and associations between sexual dichromatism and rates of diversification (including speciation and/or extinction). But how robust is sexual dichromatism as a measure of sexual selection? What are the pitfalls and strengths of this measure? Measures of sexual dichromatism vary widely, limiting our ability to directly compare results between studies, and most do not account for receiver vision. This is important, both because animal colour vision varies substantially, and because the relationship between coloration and signal salience is likely non-linear. Colour is also produced by a variety of mechanisms, for example, pigmentary and structural. This has important implications for costs and constraints on colour variation, and therefore it is utility as a measure of sexual selection. Mechanisms of colour production also have implications for our ability to measure or reconstruct coloration of preserved and fossil specimens. Sexual dichromatism is a useful measure of the strength of sexual selection to identify patterns in the history of life, but coloration is multifaceted and must be understood in its biological context.
Variation rates as an indication of sociosexual display in horned dinosaurs, and other ornithischians
Dr Caleb Brown, Royal Tyrrell Museum, Canada
Phenotypic variation is the basic material upon which selection acts, and as such quantifying this variation is an important aspect of evolutionary biology. Specifically, research on a diverse array of living animals has documented higher rates of morphological variation in sociosexual display structures, than those under natural selection.
Many dinosaur species, particularly ornithischians, exhibit ‘exaggerated’ skeletal structures that lack obvious mechanical functions and have been hypothesized to have function is sexual and social display. These are often manifested as outgrowths/hypertrophy of the skull roof, including the solid and hollow crests of Hadrosauridae, the horns and frills of Ceratopsia, and the thickened domes of Pachycephalosauria. For dinosaur palaeobiology, however, intraspecific variation is often regarded as merely an obstacle to robust taxonomy.
Here morphological variation was quantified (using coefficient of variation) for the well-sampled species of horned dinosaur Centrosaurus apertus, as well as complementary dataset of the horned dinosaurs Anchiceratops, Chasmosaurus, Protoceratops, and the duck-billed dinosaurs Lambeosaurus, and Corythosaurus.
Levels of variation for the putative ornamentation structures (e.g. frills, horns, crests) are significantly higher (~2-3 times) than those for the remainder of the skull. These results are consistent with, and often statistically indistinguishable from, the known sexual displays of a comprehensive dataset of extant amniotes analogues (including mammals, birds, and squamates).
When combined with previous research suggesting these same structures in ornithischians are ontogenetically delayed, positively allometric, rapidly evolving, and highly species-specific, these data provide further support to the hypothesis that sociosexual selection was the evolutionary driver of these ‘exaggerated’ structures.
Sexual selection, adaptation and extinction: lessons from experimental evolution
Professor Jacek Radwan, Adam Mickiewicz University in Poznan, Poland
Failure of populations to adapt to sudden environmental changes may lead to extinction. Sexual selection can have multiple, and often opposing, influences on extinction probability. Whether the net outcome of these influences is increased, or decreased, extinction risk is an open empirical question. Professor Radwan will demonstrate how experimental evolution can be used to address this question, focusing on his own work utilising mite species with males differing in expression of a costly sexually selected trait.