Sexual dimorphism in northern map turtles (Graptemys geographica): Ecological causes and consequences

Sexual dimorphism in traits such as colour, size, and shape is very ubiquitous in animals. The direction and intensity of sexual dimorphism, however, varies between species and understanding the causes for the evolution and maintenance of sexual dimorphism has been a central quest in evolutionary biology. The purpose of my thesis was to explore the ecological causes and consequences of sexual dimorphism in northern map turtles. I integrated a variety of approaches to test hypotheses associated with sexual size dimorphism (SSD) as well as sexual dimorphism in trophic morphology. In my first chapter provided evidence through dietary and functional analysis that dimorphism in feeding structures has evolved to increase the energy intake of females to fuel egg production. In my second chapter, I investigated factors contributing to sex differences in diet and habitat use. I found no sex differences in habitat despite marked differences in prey distribution. Using stable isotopes analysis and fecal analysis, I found a large dietary overlap between males and females, indicating no intersexual competition for food. Patterns of prey selection in females, however, were again concordant with the reproductive role hypothesis. In my third chapter, I studied SSD from an ontogenetic perspective. I investigated sexual bimaturation (sex differences in age at maturity) and its relation to the operational sex ratio. Females take twice as long as males to reach sexual maturitd but the estimated operational sex ratio was even in my study population contrary to a male bias sex ratio as predicted by the pattern of maturation in this species. I also tested if fast growing juvenile females incur the metabolic cost of growth compared to similar size non-growing males. Based on respirometry, I found no evidence of such metabolic cost. In my fourth chapter, I investigated thermoregulatory implications of sexual size dimorphism. I showed that large females have a more limited range of daily body temperature than small turtles. This difference appears to lead to a lower accuracy of thermoregulation in large females. Maturation in males, however, does not appear to involve a thermoregulatory cost that could lead to a decrease in growth rate