Every fall, the songbird roams a territory covering tens of square miles, gathering seeds and storing them in hundreds of hiding places in trees and on the ground. Over the harsh winter that follows, the tireless chickadee, which weighs about 12 grams and fits inside the average human hand, faithfully re-visits its caches to feed.
The chickadee’s unerring spatial memory is remarkable enough, says Colin Saldanha, assistant professor of biological sciences and an anatomist who has studied songbirds for six years.
But it is what happens inside the tiny songbird’s brain that Saldanha finds amazing.
In the fall, as the chickadee is gathering and storing seeds, Saldanha says, its hippocampus--the part of the brain responsible for spatial organization and memory in many vertebrates--expands in volume by approximately 30 percent due to the addition of new nerve cells. In songbirds, the hippocampus is located on the dorsal surface of the forebrain right beneath the skull. In mammals, the hippocampus is located beneath the cortex.
In the spring, when its feats of memory are needed less, the chickadee’s hippocampus shrinks back to its normal size, Saldanha says.
"To see this happen under natural conditions is truly awe-inspiring,".says Saldanha, who began studying the black-capped chickadee upon his appointment to the faculty in the fall of 2001. "Our hypothesis is that this exaggerated growth occurs when the birds need it the most--and we’re interested in finding out what exactly triggers it."
The estrogen connection
Songbirds are the first species of vertebrate in which brain growth during adulthood has been found to occur, Saldanha says. By studying neurogenesis in the black-capped chickadee, Saldanha hopes to learn how hormones help guide the brain’s development and reorganization. He is particularly interested in the role played by the hormone estrogen in the growth of the hippocampus. Songbirds (like most vertebrates) make estrogen in their ovaries; scientists have determined that their brains also express aromatase, the enzyme that makes estrogen. Perhaps not surprisingly, the area of the songbird brain with the highest estrogen-making capability is the hippocampus.
"We know hormones affect the reorganization of the brain in ovo, in utero and during the early physical development of most vertebrates," Saldanha says. "We are trying to figure out whether the ability to make estrogen in the hippocampus is helping the dramatic reorganization of the [adult] brain."
Saldanha uses transmission electron microscopy (TEM) to examine neurons (nerve cells) and synapses (connections between nerve cells, where scientists think learning occurs) from the brain of the black-capped chickadee. His goal is to determine whether estrogen is being made in the cellular body or in the synapse, and whether the location of this estrogen-making ability changes seasonally.
"We’re looking at the ability of nerve cells and connections to make estrogen in the brain and asking if this ability is involved in brain reorganization," he says. "We are the first lab, I think, to look at estrogen-synthesizing neurons in the songbird hippocampus at the electron-microscope level. We may, in fact, be the only lab using this technology to investigate songbird spatial memory."
Potential hope for humans
Saldanha, who was appointed as part of the biological sciences department’s Bioscience and Biotechnology Initiative, is licensed to catch and house birds by the U.S. Department of the Interior Bird Banding Laboratory and the Pennsylvania Game Commission. He has placed feeders around the Mountaintop Campus, and he keeps captive chickadees in an aviary outside his office in Iacocca Hall.
The behavior of the black-capped chickadee has been studied for a long time, he says, and much is known about its lifecycle and habits. Only recently have scientists begun to study its brain.
"It’s nice to have an ecological umbrella under which to ask biological questions. Because we know the birds’ natural behavior, I feel I can be more confident in the validity of my studies. It can be difficult sometimes to extend findings from the Petri dish to the real world."
Many neuro-degenerative diseases involve the hippocampus, Saldanha says. In humans, strokes can affect the hippocampus and cause a "profound deficit" in memory, especially in the ability to make new memories. In the brains of Alzheimer’s patients, the hippocampus shrinks.
But Saldanha stresses that his studies are new and that any applications lie far in the future.
"Maybe in the very long term, we can understand how to prevent and restore memory loss in patients with Alzheimer’s," he says. "Often times, the best way to fix something that’s broken is to figure how it works when it’s not broken."