The size of the hippocampus, a brain structure known to mediate spatial learning, is correlated with spatial behavior in wild rodents. In this career advancement award, a comparison will be made of the spatial learning ability of different species on abstract learning tasks. Using operant methods, pairs of species, known to differ in hippocampal size, will be compared on two operant tasks known to involve the hippocampus (spatial delayed- nonmatch-to-sample, simultaneous odor discrimination) and two tasks that do not require an intact hippocampus (auditory discrimination, successive olfactory discrimination). Two species of kangaroo rats (Dipodomys) and two species of voles (Microtus) will be compared. It is predicted that, within each genus, the species with a larger hippocampus will show superior learning ability on hippocampal tasks, particularly the spatial task, but that there should be no species differences on nonhippocampal tasks. The proposed study will be significant for several reasons. It will be the first comparative study of different types of memory in wild rodents. Because our understanding of the physiology of memory in mammals is largely based on the laboratory rat, it is important to know if laboratory rats show typical rodent behavior. More importantly, comparative studies of closely related rodent species can identify what features of a species's habitat can lead to increases in learning and memory capacity. Not only can this study help us to understand the evolution of animal memory, but it can also help us to understand the hippocampus, a structure critical in both animal and human memory. For example, no other animal study has attempted to compare performance of different tasks involving the hippocampus (maze learning, odor discrimination). Thus, this study will shed light not only on the evolution of memory, but on the function of the hippocampus, a structure of clinical importance.
