RONALD G. OLDFIELD, PHD
Lecturer
Research Interests
My primary area of interest is evolutionary ecology. I investigate behavior, development, ecology, morphology, and physiology in fishes in order to better understand evolutionary patterns and processes.
At the individual level, I use resource defense theory to better understand how ecological factors influence social behavior and the distribution of individuals in space and time. If a resource (e.g., food, mates, shelter) is distributed unpredictably then individuals will not be able to defend it. Competition will be exploitative and individuals may roam or form large groups. As predictability of a resource increases, individuals will be more capable of defending it. Contest competition will emerge and individuals may form territories. In addition to affecting social structure, social behavior may also affect growth and development. In fishes, these effects can be especially pronounced, with individuals of some species going so far as to change sex in response to changes in social conditions. To elucidate the relationships between ecological conditions, social behavior, and growth and development, I perform experiments in which I manipulate factors such as resource availability, group size, and available space using a model organism, the Midas cichlid, Amphilophus citrinellus, a perch-like fish from Nicaragua. Environmentally-induced aggressive behavior has currently unrecognized implications for captive fish welfare and growth responses have important applications for aquaculture. Some responses of social behavior to ecological factors may be genetic instead of plastic.
At the species level, I am interested in how social responses to ecological factors are related to the evolution of traits. This approach requires comparative analysis of closely related species, in a phylogenetic context. The most likely ancestral state can be determined through phylogenetic optimization. The evolutionary pattern of extant social phenotypes can then be interpreted. One recent project has been a study of the evolution of socially controlled sex change in fishes. Previous reports had claimed that sex in A. citrinellus was determined by social interactions experienced at the juvenile stage. This was thought to represent an intermediate stage in the evolution of functional sex change, frequently seen in adult marine fishes, and to support a hypothesis that a change in developmental timing was the process responsible. This work had been extensively cited, including a detailed description in the leading ichthyology textbook, although it had not been subsequently tested. Through a series of field observations, laboratory experiments, and histological preparations I determined that this species is not sexually labile, and that the original authors came to their conclusion in error.
An upcoming research project will be an analysis of the evolution of mating systems in Méxican cichlids of the genus Herichthys. Most Herichthys species form monogamous pairs to spawn and raise offspring, but some species are polygynous (one male mates simultaneously with several females). Moreover, in most Herichthys species both males and females change color during spawning from a drab gray to having a bright white background overlaid with a large black patch. In one polygynous species, the Cuatro Ciénegas cichlid, Herichthys minckleyi, from the desert valley of Cuatro Ciénegas, México, females turn completely white and males completely black. The isolated lakes of Cuatro Ciénegas provide a habitat very different than the coastal rivers where the other Herichthys species live, and the change in mating system and coloration may be a response to local ecological conditions. First, I will perform laboratory experiments to determine whether mating behavior and coloration are the results of phenotypic plasticity or of evolution. Using an existing molecular phylogeny I will reconstruct the evolution of mating systems in Herichthys. Second, I will quantify ecological differences between Cuatro Ciénegas and the habitats of other Herichthys species. I will analyze these differences in the context of resource defense theory in an attempt to understand how polygyny could be favored in the lakes. Finally, I will analyze hormone profiles in an attempt to determine how environment might have selected specific molecular mechanisms that would result in a change in mating behavior.
Diversity of social behavior has important conservation implications because it demonstrates a level of phenotypic biodiversity beyond that observed using more typical methods involving characterizations of morphology or genetics in preserved specimens. More importantly, it highlights the importance of the natural environment in shaping and preserving that biodiversity.
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