Research

One of the most exciting challenges facing evolutionary biologists is to understand the selective pressures that drive morphological diversity. Competition for mates, in particular, is a potent driver in the evolution of male morphology and has given rise to many of nature’s most extreme body forms, including the flashy ornaments of female choice, and the bulky weapons of male-male combat. I am broadly interested in the evolution of sexually selected traits. Specifically, I am interested in the elaboration and diversification of sexually selected weapons, the interaction between different mechanisms of sexual selection, and the influence of ecological factors in driving the evolution of sexually selected traits. I describe some of my current research interests and projects below.

Male-male competition and the functional morphology of sexually selected weapons

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Finite element models of rhinoceros beetle horns

Animal weapons differ widely among species, but  the drivers of this diversity remain poorly understood. Because weapons are essentially “tools” to pinch, pry, and strike opponents in combat, the most intuitive and compelling explanation for weapon diversity is that it reflects structural adaptations to different fighting styles. That is, differences in the way males fight or in where they fight may favor corresponding changes in weapon shape.

My research on rhinoceros beetle horns supports the hypothesis that these weapons are structurally adapted to meet the functional demands of fighting. I have used mechanical testers and computer-based biomechanical modeling to study variation in horn size and shape. I find that variation in fighting styles has played a critical role in the diversification of weapon form. I expect that the integration between biomechanics, behavior, and evolution is critical to our understanding of weapon diversity, and a promising new direction for the field of sexual selection.

Population density affects the interaction between pre- and post-mating sexual selection

Horns and other secondary sexual traits are critical in determining a male’s mating success, but they are only part of the sexual selection story. Because females typically mate with more than one male, sexual selection can act both before and after mating. The interaction between pre- and post-mating episodes of selection is expected to be context dependent, but few studies have examined how total sexual selection changes under different ecological conditions.

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Dimorphic dung beetles: horned fighter male and hornless sneaker male (Photo credit: Doug Emlen)

Using the horned dung beetle as a model system, I am studying how population density affects the interaction between pre- and post-mating sexual selection. I find that the importance of post-mating processes increases with increasing population density, which reduces the benefit of pre-mating traits (horns) and increases the benefit of post-mating traits (testes). The results from experimental populations are largely consistent with the phenotypic variation observed between dung beetle populations in the field, and suggest that differences in density can contribute to variation in the nature and degree of male dimorphism within and among species.

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