Careful comparisons of natural behavior and brain anatomy have revealed that in many species, the way they make a living has left an imprint on the hippocampus. For example, species of birds that hide caches of food in spatially scattered locations are reliably found to have larger hippocampi than noncaching species have, even when the comparison species are very close relatives that have otherwise similar lifestyles (see Figure 1) (Krebs et al., 1989; Sherry, 1992; Sherry et al., 1989). Lesions of the hippocampus impair the ability of these birds to retrieve caches of food (Sherry and Vaccarino, 1989). Homing pigeons also have enlarged hippocampi relative to other varieties of pigeons, presumably serving the spatial demands of their prodigious navigational abilities (Rehkamper et al., 1988). A relationship between spatial cognition and hippocampal size is evident in mammals too. Just as with the food-caching birds, Merriam’s kangaroo rat, which stashes food in scattered locations, has a significantly larger hippocampus than its noncaching cousin, the bannertail kangaroo rat (Jacobs and Spencer, 1994).

Figure 1  Food Storing in Birds as Related to Hippocampal Size
Food-storing species of birds have twice as large a hippocampus in relation to their forebrain (telencephalon) as do species that do not store food. Note that both axes on this graph are logarithmic. (After Sherry et al., 1989.)

In voles, mating strategies and sex differences in spatial behavior, rather than food caching, seem to have shaped the hippocampus. In nature, pine voles and prairie voles are monogamous, and males and females have comparably sized home ranges. But their close relatives, the meadow voles, are polygynous (mating with more than one female at a time), so meadow vole males’ home ranges are much larger and encompass the home ranges of several females. In meadow voles, but not in pine voles, males have larger hippocampi than females have (Jacobs et al., 1990), reflecting this sex-related species difference in spatial processing (see Figure 2). Accordingly, when studied in the laboratory, a significant male-favoring sex difference in spatial ability is found only for the meadow voles (Gaulin and Fitzgerald, 1989). Even within individual life spans, spatial learning can change the anatomy of the hippocampus.

Figure 2  Sex, Memory, and Hippocampal Size
Males and females of two species of voles were compared on three variables: (a) size of home range, (b) score on a spatial learning task, and (c) hippocampal size divided by brain size. (After Jacobs et al., 1990.)


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