Memories are made of this

Injury to a small curlicue of brain tissue called the hippocampus produces such profound amnesia that people so affected greet as a stranger someone they met and talked with minutes earlier. Reasoning and intelligence remain intact in these people, but recent experiences vanish, destroying all hope of a normal life. Humans are not alone in their dependence on an intact hippocampus. The ability of many other animals to learn from experience has also been shown to involve this part of the brain.

The hippocampus -- more properly called the hippocampal formation because it consists of a number of structures -- was given its name by the sixteenth-century Italian anatomist Giulio Cesare Aranzi, who saw in the overall shape of the human hippocampus a resemblance to Hippocampus, the seahorse. The human hippocampus is distinguished by curving, interleaved fields of neurons. It is a bilateral structure about the size and shape of a small green bean, occupying 2.5 to 3.0 cubic centimeters in the right and left temporal lobes. In most other mammals, the hippocampus is a C-shaped curve deep in the brain with the same distinctive arrangement of neuron fields. In birds, the hippocampus is wedge shaped, descending from the top of the brain into the crevasse dividing the left and right hemispheres. Despite these differences in appearance, embryological and neuroanatomical evidence shows that the hippocampuses of birds and mammals are true homologues, that is, they evolved from a brain structure possessed by an ancestor common to both groups.

Recently, laboratory and field studies have shed light on the different functions of the hippocampus in a variety of animals. jays, chickadees, and nuthatches, for example, are champion food storers, hiding thousands of nuts, seeds, and other comestibles over the course of a year; they have a remarkable ability to remember, weeks or even months later, where they stashed their food. Part of the secret to their success is a big hippocampus.

The black-capped chickadee stores seeds and invertebrate prey in scattered hiding places, mostly in fall and winter. Its hippocampus is about 10 cubic millimeters in size and takes up about 4 percent of the forebrain, compared with about 4 cubic millimeters and only 1.5 percent of the forebrain in the house wren, a species that feeds on insects and other invertebrates and does not store its food. The forebrain in both species is about 300 cubic millimeters, about the size of a raisin. Hippocampus size varies even among species of birds that store food. In western North America, the life of a Clark's nutcracker largely revolves around collecting, transporting, and storing pine seeds in the fall and retrieving them during the winter and spring. The pinyon jay, gray-breasted jay, and scrub jay -- other western members of the corvid (crow) family -- store nuts, seeds, and acorns, too, but not to the same degree; they also are less successful at relocating caches and solving other spatial problems in laboratory experiments. Significantly, the hippocampus of these species is considerably smaller than the Clark's nutcracker's. Similar relationships between food storing and hippocampal size have been found in European corvids and in the parids, the group that includes chickadees and titmice.

Researchers working with vole -- small rodents in the genus Microtus -- have also uncovered a connection between hippocampal size and an animal's ability to remember the places it has been and where it needs to go. Working in the laboratory at the University of Pittsburgh, Steve Gaulin and his colleagues tested the skills of meadow voles at finding their way around a maze and discovered that males are better at it than females. The male meadow vole's hippocampus, as shown in further studies by Lucia Jacobs, now at the University of California, Berkeley, turns out to be larger than the female's. There is a good ecological reason for these sex differences. Like most species in this genus, the meadow vole is polygamous. During the breeding season, the male expands its home range to encompass the smaller ranges of several females and then faces the challenge of navigating within this home range, finding the females, and mating with them. Prairie and pine voles are closely related to meadow voles but are monogamous; males thus have no need to search out multiple females. In these species, there are no sex-based differences in home range size, spatial ability, or hippocampal size.

Another example of the interplay of ecology, behavior, and hippocampal size comes from the brown-headed cowbird, a member of the blackbird family. Rather than build nests or care for their young, these brood parasites lay their eggs -- up to forty a year -- in the nests of other birds, including warblers, vireos, fly-catchers, finches, and thrushes. Female cowbirds usually lay an egg shortly before dawn, spending the remainder of the morning searching for nests to parasitize on subsequent days. They may sit quietly in the trees, spying on unsuspecting potential "hosts," or walk silently along the forest floor while looking up into the canopy; sometimes they fly noisily through the brush, perhaps to locate a nest by flushing out incubating birds. Female cowbirds must find and remember -- perhaps for several days -- a variety of nest sites dispersed through the woods.