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Abstract
The apparent flexibility of behavior has long posed challenges for evolutionary biology. As such, there has been debate over whether behavior may be unconstrained in its evolution relative to other traits such as morphological, physiological, and life history characters. Here, I examine behavioral and mechanistic differences between species to ask whether behavior displays patterns more consistent with evolution driven by optimality or evolution driven by genetic constraint. Specifically, I studied differences in parental care behavior in two species of burying beetle, Nicrophorus orbicollis and Nicrophorus vespilloides, both of which breed on vertebrate carrion and directly provision their offspring through regurgitation. In order to address overall patterns of evolution between these species, I performed three studies at the phenotypic level designed to address parental variation between species as well as the fitness consequences of this variation. Next, I performed two studies designed to address the mechanistic basis of behavioral variation between species, using transcriptional assays to understand what factors might be influencing potential evolutionary trajectories of parental care. These studies together provide a deep and nuanced look at the way multiple traits vary in two related species. Furthermore, they suggest that while an optimality approach on its own clearly cannot account for species differences in behavior, there may be certain aspects of behavioral phenotypes evolving more freely than others.