Below are some current and ongoing projects from the McCullough lab:
Impact of sexual selection on the invasion dynamics of exotic dung beetles
Biological invasions are a pervasive feature of the Anthropocene that provide unique opportunities to study the process of rapid evolutionary change. Current invasion studies are aimed at understanding the eco-evolutionary factors that influence invasion dynamics, but one potent evolutionary factor remains curiously unexplored: sexual selection. Variation in sexual selection can affect the success, spread, and speed of biological invasions by promoting adaptation and species persistence in stressful or altered environments, or affecting life-history tradeoffs between reproduction and dispersal. However, the details of how sexual selection varies across the range of exotic species remain unknown. We are studying the impact of sexual selection on invasion dynamics, using the recently introduced, agriculturally important dung beetle Onthophagus taurus as a model system. 
Behavioral ecology of Onthophagus orpheus
The tunneling species Onthophagus orpheus is a beautiful, iridescent beetle that is native to Massachusetts. Conveniently, it can be collected in large numbers from pitfall traps baited with dog poop at Clark’s Hadwen Arboretum – just half a mile from campus. Large males have a large thoracic horn, small males have a tiny thoracic horn, and females are hornless. But we know essentially nothing else about its natural history! We are conducting behavioral observations to learn about their fighting, sneaking, courtship, and parental care behaviors. 
Form and function of animal weapons
Animal weapons differ widely among species, but the drivers of this diversity remain poorly understood. Horns, antlers, and claws are essentially “tools” that males use to pinch, pry, and strike opponents in combat, so 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 form. Our research on rhinoceros beetle horns supports the hypothesis that weapons are structurally adapted to meet the functional demands of fighting. We have used mechanical testers and finite element modeling to study variation in horn size and shape. We find that horns are both stronger and stiffer in response to species-typical fighting loads, demonstrating the critical link between the form and function of beetle horns. 
Because weapons are used as tools, they can experience substantial stresses and strains that ultimately cause them to break during fights. In collaboration with Sarah Lane at the University of Plymouth, we are studying the costs and consequences of weapon damage across the animal kingdom. We find that rates of damage range from 0% (stag beetles) to 82% (tule elk), and the severity of damage varies from punctured claws (fiddler crabs) to the entire loss of horns (rhinoceros beetles). We are exploring the factors that affect the likelihood of weapon damage and the associated fitness costs, and how these factors contribute to variation in fighting styles and escalation decisions within and among species.
Molecular interactions between male and female reproductive proteins
Fertility depends, in part, on interactions between male and female reproductive proteins that trigger postmating responses in females. For example, when a male fruit fly mates, he transfers not only sperm, but also a cocktail of seminal fluid proteins, which decrease the female’s willingness to re-mate, increase her rate of ovulation and oviposition, and modify the molecular composition of the female reproductive tract. Seminal fluid proteins are among the most rapidly evolving proteins in the animal kingdom, so coevolution between interacting male and female reproductive proteins is expected to drive specific, coordinated molecular interactions within species and reproductive incompatibilities between species. However, the nature of these interactions are understudied. Our research uses quantitative proteomics to compare the postmating responses in sibling Drosophila species. We find substantial divergence in the extracellular environments of the female reproductive tract in each species, and dramatic differences among species in the female postmating response. Our research suggests that postmating responses are rapidly evolving, which highlights the importance of ejaculate-female interactions in maintaining species boundaries.