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As Chapter 17 in the text describes, the hippocampus is important for memory formation, and in some species especially for spatial memory, which is needed to recover stored food. The families of birds that store food are no more closely related to each other than they are to other, non-food-storing families and subfamilies of birds. Thus food storers are not all descended from an ancestral species that stored food and possessed a large hippocampus. Rather the evidence suggests that a large hippocampus is necessary for successful storage and recovery of food. Food-storing species of birds have twice as large a hippocampus in relation to their forebrain (the telencephalon) as do species that do not store food. Several families of birds—in an example of convergent evolution—independently evolved a large hippocampus for these attributes (Sherry, 1992).
Investigators have expanded this analysis by asking the following question: among closely related species of birds that all store food, but in which some species depend more on stored food than others, is there a relationship between the amount of storing and the relative size of the hippocampus? The answer is yes for species of corvids (including jays and nutcrackers) (Basil et al., 1996) and for species of parids (including chickadees and titmice) (Hampton et al., 1995). So even within closely related species, reliance on storing and recovering food appears to have been a selection pressure that has led to larger hippocampal size.
References
Sherry, D. F. (1992). Memory, the hippocampus, and natural selection: Studies of food-storing birds. In L. R. Squire and N. Butters (Eds.), Neuropsychology of memory (2nd ed., pp. 521-532). New York: Guilford.
Basil, J. A., Kamil, A. C., Balda, R. P., and Fite, K. V. (1996). Differences in hippocampal volume among food storing corvids. Brain, Behavior, and Evolution, 47, 156-164.
Hampton, R. R., Sherry, D. F., Shettleworth, S. J., Khurgel, M., et al. (1995). Hippocampal volume and food-storing behavior are related in parids. Brain, Behavior, and Evolution, 45, 54-61