I aim to improve understanding of the role of environmental heterogeneity in the evolution of morphology, function, and behavior. I focus on sexual selection in insects, examining the importance of environmental variability both for the expression of sexually-selected traits and for the process of selection itself. Most of my research is conducted with leaf-footed bugs, Hemipterans in the family Coreidae.
Major Research Areas:
- Evolutionary ecology
- Behavioral ecology
- Sexual selection
- Chemical ecology
- Phenotypic plasticity
- Maternal effects
Sexually-selected traits are often very sensitive to the natural environment. Such sensitivity may have far-reaching implications, including affecting the reliability of sexual signals, the strength of selection pressures, the direction and speed of the response to selection, and the maintenance of genetic variation in sexually-selected traits. While there has been considerable work on sexual selection in constant environments, the implications of variation and uncertainty in the process of sexual selection are less well studied. Indeed, we are only just beginning to appreciate how dramatically environmental heterogeneity can influence the expression of sexual phenotypes as well as the agents of sexual selection themselves, female mate choice and male-male competitions.
Natural history of the leaf-footed bugs
Over 2000 species of leaf-footed bugs exist worldwide, and many have intriguing shapes associated with the process of sexual selection. Males of numerous species use their hind legs to squeeze each other in contests over host plant resources where females visit to feed, mate, and lay eggs. The hind femurs of these males are spiny and proportionately larger than the femurs of females. Males with larger legs can be more likely to win male-male contests and have greater mating success.
Environmental effects on the targets of sexual selection
My dissertation research examined maternal effects on the expression of sexually-selected traits in the heliconia bug, Leptoscelis tricolor (Hemiptera: Coreidae). Through field experiments conducted in Panama and Costa Rica, I found that sexually-selected trait expression varied over both space and time (Miller & Emlen 2010a). The host plant where mothers laid eggs affected secondary-sexual weapons (Miller & Emlen 2010b), the expression of color (Miller & Hollander, 2010), copulatory courtship behaviors, mating receptivity, and fecundity in offspring (Miller 2008). Moreover, seasonal changes in plant quality changed the strength and direction of these patterns. Such dynamic implications of maternal behavior for offspring may have potential effects on the intensity of sexual selection, evolutionary trajectories, and population dynamics.
Environmental effects on the agents of sexual selection
Currently, one of my major research interests is in examining the effect of environmental heterogeneity on the agents of sexual selection, female mate choice and male-male competitive behaviors.
Much of this work is conducted with the leaf-footed cactus bug, Narnia femorata (Hemiptera: Coreidae). Narnia lives on Opuntia sp. (cholla and prickly pear cactus) from the Southern U.S. to Central America, and is excellent for both field and laboratory study. Narnia routinely encounter two discrete natal environmental conditions. Some juveniles have access to cactus fruits for growth and development, while others only have access to cactus pads. Cactus with fruit is a higher quality resource for these insects; insects reared with fruit develop faster (Nageon de Lestang & Miller 2009), grow larger, are more competitively successful, and are generally more attractive as adults (Gillespie, Tudor, Moore, & Miller in prep).
Male Narnia compete with other males for territories on cactus. When a competition between males escalates, males turn around and squeeze each other with their exaggerated, spiny hind legs. Males with larger legs are more likely to win competitions and mate with females on the plant (Procter, Miller, & Moore 2010, M.Sc. thesis).
Narnia females visit male territories to mate, feed, and to lay eggs. Males defending a cactus territory with fruit provide direct benefits to females – a high-quality resource in which to feed and lay eggs. We recently discovered that females are much less choosy for male characteristics in this context. Females become significantly more discriminating when a male can only provide a female with sperm. Interestingly, male mate discrimination also varies with context. Males in possession of a high quality territory discriminate strongly based on cues of female fecundity, while males with a low quality territory do not discriminate based on these cues (Gillespie, Tudor, Moore, & Miller in prep for PNAS). Our results are among the first to experimentally demonstrate plastic mating tactics in a realistic context. Such environmentally-dependent mate discrimination has the potential to affect the strength and even the target of selection experienced by insects in natural populations.
Another agent of sexual selection, male-male competitions, may also vary in intensity and outcome according to environmental circumstances (Procter, Miller, & Moore 2010, M.Sc. thesis). However, this question is only beginning to be explored. We are currently manipulating male territory quality to examine adaptive plasticity in male-male competitive behaviors. Results are expected by the end of 2010.
