Science Watch

The phrase "bird brained" may take on a more positive meaning, thanks to new evidence that chickadees in a harsher climate can find their food cache more efficiently than chickadees from a milder climate, and have a corresponding superiority in brain size.

The findings, which appear in the August 2002 issue of Behavioral Neuroscience (Vol. 16, No. 4), reflect a successful test of the "adaptive specialization hypothesis." Based on the theory of natural selection, this hypothesis outlines a chain of events in which specific environmental conditions, coupled with the drive to survive, result in animals adapting to those conditions in specialized ways--in this case, through changes to the hippocampus and a corresponding rise in spatial memory.

The findings add to the growing body of research indicating that, "the number of neurons in the brain is not fixed," says John Disterhoft, PhD, of the Institute for Neuroscience at Northwestern University Medical School in Chicago, "but rather can be increased with use, exercise and hormonal manipulations."

"Understanding the association between different parts of the brain and their specific functions," says co-author Vladimir Pravosudov, PhD, of the University of California, Davis, "may help researchers to pinpoint the origin of some behavioral health problems. The knowledge of brain structures and their functions is always helpful when we need to understand any unusual, or clinical, pattern."

An evolutionary path?

A bird hippocampus sits on top of the brain, rather like a mushroom cap. Despite its different appearance and location in mammalian brains, the hippocampus is associated with spatial memory in both birds and mammals. Spatial memory is not as simple as sense memory, says Howard Eichenbaum, PhD, of Boston University's Laboratory of Cognitive Neurobiology. Rather, it captures "complex relationships among cues in the environment. It involves learning and remembering one's previous experiences in that environment--in other words, higher-order memory."

Pravosudov and co-author Nicola Clayton of the University of Cambridge in the United Kingdom, compared how well black-capped chickadees (Poecile atricapilla, a fairly common, easy-to-train research bird) from Alaska and Colorado found their self-cached food--shelled sunflower seeds, crushed peanuts and mealworms--in a lab setting with simulated trees and other naturalistic objects. The researchers then examined the birds' brains to compare hippocampus volume and neuron density.

"Food-caching birds," explains Pravosudov, "are excellent subjects to study memory because they have evolved exceptionally good memory. For these birds, survival depends on how well they can retrieve their thousands of food caches. Thus, we can gain insights into both the evolution of memory and the hippocampus, and the relationship between them. And because spatial memory in humans is also hippocampus-dependent, we can draw parallels between bird studies and human memory."

Pravosudov and Clayton compared same-species birds from two different regions to test the hypothesis that accurate cache recovery is more critical for birds that live in harsh conditions where food is limited and unpredictable--as it would be in Alaska compared with Colorado.

"Showing that differences in memory exist within different sub-populations of the same species could indicate the evolutionary path for memory and the hippocampus in food-caching animals," write the authors.

In their experiments, Pravosudov and Clayton studied eight birds each captured near Anchorage, Alaska, and Windsor, Colo., and compared their performance and hippocampus size with that of seven Alaskan birds and eight Coloradans who didn't do any caching and recovery.

As the experimenters watched through a one-way window, the birds were individually released into a large test room with 70 caching sites, some holes in "trees" and some in wooden caching blocks hung from wire-mesh screens. Birds couldn't see inside the holes, due to knots at the end of strings attached just above each hole. Each hole had a wooden perch just below it. The birds took food freely and cached it in all sites, and also inspected the sites for food.

For a cache recovery task, hungry birds were allowed to eat and cache sunflower seeds for 20 minutes. After a five-hour break, they were released back into the experimental room, where they could eat only the food they'd stashed. To retrieve a cache or inspect a hole, a bird had to pull away the string. Pravosudov and Clayton recorded the number of caches each bird recovered in terms of the number of looks (pulling at the strings) to find each seed. The duo tested each bird twice.

To locate their caches, both Alaskan and Colorado chickadees inspected significantly fewer sites than they might have at random. However, there were regional contrasts. Alaskan chickadees inspected significantly fewer sites than did the birds from Colorado, and were more efficient at finding hidden caches--suggesting an edge in spatial memory.

Pravosudov and Clayton also set up an associative learning task that allowed them to directly compare how accurately the birds found a single peanut cache hidden by the experimenters. In this way, all the birds had exactly the same experience--whereas in the cache recovery task, each bird hid food in different numbers of places. Again, birds from both areas did better than they would in a random search, but Alaskan chickadees once again pulled ahead, inspecting fewer sites to find the peanuts. Alaskan chickadees also performed better when the number of available sites was reduced to 15. However, when the feeder with the peanuts had a unique color pattern--erasing the need to rely on spatial memory--the differences disappeared.

When the experimenters analyzed the birds' brains, Pravosudov and Clayton found that the hippocampal formation was significantly larger in Alaskan chickadees. They also had significantly more neurons in their hippocampal formation. Caching and retrieval experience in the lab had no significant effect on the relative volume of this part of the brain, meaning the birds came from the wild with these neurological differences.

Future implications

Thus, on both a behavioral and an anatomical level, chickadees from two very different climates showed just how well they and their brains have adapted to the environment--through superior spatial memory that serves the Alaskan variety particularly well, supported by a larger, denser hippocampus. Because there were no brain differences between birds that underwent the experiments and those that did not, the superior caching, efficient cache recovery and larger hippocampal formation of Alaskan chickadees were not caused by temporary differences in the local environment.

Could the differences between Alaskan and Colorado chickadees perhaps have been shaped by something other than food supply?

"Biologists familiar with food-caching birds are quite sure that food supply is the key to their survival in winter," says Pravosudov. "At the same time, day length, temperature, snow cover and more, all of which differ between Alaska and Colorado, significantly affect the availability and predictability of the food supply and energy balance of wintering birds." Thus, by connecting the scientific dots, the researchers find the adaptive specialization hypothesis the "most plausible biological explanation" of their findings.

"My guess is that the hippocampus is larger in species where there is a greater demand for an organized representation of memories, not simple stimulus-reward associations or acquired habits," Eichenbaum says.

The results suggest that specific brain properties can evolve when there is pressure on memory and supporting brain structures. "The same can be implied for humans," adds Pravosudov, "except we don't need such exceptionally good memories as food-caching birds, because we have evolved other means to increase our survival."

Disterhoft says, "This research adds an interesting piece to understanding the puzzle of how important learned information is stored in the brain. We can now frame a series of molecular, biochemical or biophysical experiments to look at the mechanism of this change.

"The better we understand how specific parts of the brain work, and what they contribute to normal cognition," he adds, "the better we are able to understand what the problem is when the brain does not function properly, as well as where that dysfunction is. Because the human brain works the way that bird brains do, at the cellular and sub-cellular levels, studies like this help us appreciate how our own brains change during learning and memory."

Rachel Adelson is a science and technology writer based in Raleigh, N.C.