Neurobiological bases of spatial learning in the natural environment : neurogenesis and growth in the avian and mammalian hippocampus

The amount of information amassed on the hippocampus is staggering. Certainly more than any other brain structure, the role of the hippocampus in learning has been explored at almost every level of analysis to include details about the gross anatomy, cytoarchitecture, physiology, biochemistry and possible mechanisms of plasticity primarily in humans, non-human primates and rodents. Much of the research addressing the relationship between spatial learning and the hippocampus has been performed in rodents, primarily rats. As noted by Nadel, the vast majority of these studies have been 'nature-blind'; that is, they ignore behavior that is significant to the success of animals in the wild. It is unclear, therefore, how and indeed if theories derived from these experiments generalize to species in more naturalistic settings. In their natural environment, both rodents and birds engage in behaviors that are believed to depend upon spatial learning ability such as home range navigation, migration, brood parasitism, foraging, and the storage and retrieval of food caches. We will review some of these biologically relevant models of the neurobiological bases of spatial learning in the natural environment. Towards this end we will provide an introduction to the evidence suggesting homology between the mammalian and avian hippocampal formations, a review of avian models, a review of mammalian models, a discussion of neural mechanisms of plasticity common to both and their possible involvement in learning and memory, and a summary and speculation that may provide insight into potentially fertile future directions.