I have recently completed my dissertation work at the University of Southern California, and successfully defended my thesis in May 2016. My dissertation research included testing of hypotheses pertaining to the evolution of body size, sexual size dimorphism, and sexually dimorphic traits in the North American Gopherus tortoises and other tortoise species. I compiled and analyzed existing datasets, developed models, and collected measurements of body size and other morphological traits to test my hypotheses.

Title – Evolution of Body Size and Sexually Dimorphic Traits in North American Gopherus Tortoises

Chapter 1 – Introduction and Theoretical Context. This chapter provides a context for questions pertaining to the evolution of intraspecific body size variation, sexual size dimorphism, and sexually dimorphic gular horn weaponry in tortoises, and an introduction to the North American Gopherus species.

Chapter 2 – Intraspecific Variation in Body Size and Sexual Size Dimorphism in Agassiz’s Desert Tortoise. This chapter examines body size and sexual size dimorphism variability in adult Agassiz’s desert tortoises across the range of the species, and tests hypotheses to determine the degree to which ecological and phylogenetic constraints have contributed to the evolution of the observed variation.

Chapter 3 – A Re-evaluation of Sexual Size Dimorphism in the Testudinidae. Through conducting a systematic review, I examines family-wide patterns of expression of sexual size dimorphism in tortoises (Testudinidae) in this chapter. This chapter offers findings that differ from seminal studies of the evolution of this trait in tortoises.

Chapter 4 – Fecund Females and Fighting Males: The Evolution of Sexual Size Dimorphism and Male Weaponry in the Testudinidae. This chapter is an examination of the relative influence of selection pressures (natural and sexual) that drove the evolution of sexual size dimorphism and gular horn weaponry in tortoises.  I test hypotheses to determine whether sexual selection (male-male combat) and/or natural selection favoring large-bodied females that produce large eggs and hatchlings, contributed to the evolution of the observed SSD patterns, and whether gular horn weaponry and male-biased SSD co-evolve.

Chapter 5 – Divergent Evolution of Sexual Size Dimorphism and Male Weaponry in the Gopherus Tortoises. This chapter examines the divergent pathways between the two sister clades in the genus Gopherus in which gular horn weaponry and both male-biased and female-biased SSD is expressed, and test hypotheses pertaining to the progression of trait evolution in the genus using fossil evidence, with comparisons to mating strategies, morphological traits, and burrow use behaviors that differ between the clades that provide context for the evolution of the traits.

Michael Tuma with Agassiz's desert tortoise

Michael Tuma with Agassiz’s desert tortoise