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Sexual Selection in Flour Beetles

As an agricultural pest, Tribolium beetles have been living alongside humans for several thousand years, ever since we began harvesting and storing grains. In these and many other insects, post-mating sexual selection cannot be ignored because both sexes will mate repeatedly over their adult lifespans. As a result, a male’s reproductive success depends not only on acquiring mates, but also on maximizing his relative paternity share at each mating. Our research has developed Tribolium as a model system for studying integrative sexual selection, and explores several facets of Tribolium mating behavior, copulatory courtship, and post-copulatory sexual selection. 

Do males within a species differ repeatably in their paternity share?
When females mate with several different males, those males are expected to evolve traits that will increase their success at gaining fertilizations relative to the female’s other mates. Using Tribolium, our work provided the first experimental demonstration that within a species, males differ consistently and repeatably in the paternity share they gain during competitive mating.

Role of cryptic female choice in causing differences in paternity share
Our experiments have combined behavioral, anatomical, and physiological approaches to investigate whether post-mating female choice might contribute to differences among males in their reproductive success. These experiments provide several lines of evidence that support cryptic choice by Tribolium females. More attractive males (based on their pre-mating olfactory cues) gain higher post-mating paternity share. Females can also control spermatophore transfer during mating, and become more resistant to insemination during subsequent matings after they have mated with more attractive males. Also, within their reproductive tract females actively control the movement of sperm into long-term storage. When a female mates with a low-quality starved male, she stores less of his sperm.

How does nutrition alter Tribolium behavior?
Because Tribolium beetles can inhabit so many different stored foods, these beetles naturally experience a wide range of nutritional environments.  Our research, funded by the USDA, shows that nutritional changes affect male pheromone production as well as beetles’ response to pheromone-baited traps.

Why engage in same-sex matings?

The evolutionary maintenance of animal homosexuality has been a conundrum, since such behaviors are generally not expected to increase offspring production. We used the propensity of Tribolium males to engage in same-sex copulations to test several hypotheses concerning possible evolutionary benefits. We found some evidence that males benefit through indirect sperm translocation, a previously undocumented phenomenon. We also discovered that males often release spermatophores during homosexual copulations, suggesting that same-sex matings in this insect may be a behavioral mechanism that allows males to get rid of older and potentially lower quality sperm.

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