Total recall

The little chickadee sheds light on how certain humans have bigger, more retentive brains

By Gayathri Vaidyanathan
Published: Wednesday 31 March 2010

imageIn the  Alaskan cold, a tiny non-migratory bird called the black-capped chickadee lives and forages for food. It does not eat all it finds but stores a little for winter and spring.

The sharpness of this bird’s recall ability has now been harnessed as support for a fundamental theory of evolution. The theory, called adaptive specialization, is that the environment can modify neural mechanisms of the brain and, ultimately, its behaviour in an effort to make the species more fit for survival in that habitat.

The chickadee hides almost hundred thousand food items every year in unique caches; its brain directs it to successfully retrieve several.
This is a result of selection by environmental pressures, argue Timothy Roth and Vladimir Pravosudov, biologists at the University of Nevada in usa. “The birds need the food caches to survive so there is high selection pressure on cache retrieval abilities such as memory. Better memory is life,” Pravosudov said.

The range of the chickadee extends from the farthest northern reaches to the warmer plains of North America, and the theory predicts that birds living in regions of high environmental severity will need better recall. Rather than using smell or other cues, the birds memorize the hiding locations using landmarks.

The region of the brain essential for memorization is the hippocampus. The study of memory was revolutionized by Henry Gustav Molaison, an infamous subject in psychology who had two-thirds of his hippocampus removed to cure severe epilepsy. Molaison lost all forms of long-term and spatial memory. “In humans the hippocampus is assumed to be a kind of relay station where memories are initially stored before they are transferred to the cortex,” said Anders Brodin, ecologist at Lund University in Sweden.

In their study, published in the February 2009 issue of the journal Proceedings of The Royal Society B, the scientists theorized that chickadees with a greater necessity for recall might have a larger hippocampus. They collected birds from five locations and examined the size of their hippocampus by volume and by the number of neurons. Birds from the north had a larger hippocampus and more neurons than relatives in southern latitudes. “We had a stark, linear relationship,” said Roth. “Usually, we don’t see anything so pretty.”

Earlier studies had compared the food caching ability in different species, but given the number of variables between species that can affect hippocampus size, the evidence was rather weak. The present study compares brain size within a single species—the black-capped chickadee. All other behaviours held equal, the main differences between birds from Alaska and Kansas should be their environment, said Pravosudov. “There appears to be a local evolution of traits within different populations,” he added.

This attempt to bridge a gap between assigning a neural basis for behaviour, and correlating that to the environment is called neuroecology—an emerging field. In 2001, two researchers from the University of York in the UK challenged the idea that a single species can use different mental processes to solve problems.

“Darwin argued that between-species differences in intelligence were differences of degree, not of kind,” wrote Euan MacPhail and Johan Bolhuis in the August 2001 issue of Biological Reviews of the Cambridge Philosophical Society. They called neuroecology anti-Darwin. Pravosudov disagreed. “Birds just rely more on one of the available functions and such heavy reliance on a specific function leads to enhancement of the specific brain area,” he said.

This means answers to questions such as why certain human “are so smart and have big brains” lie within the field of neuroecology, Pravosudov said. “If induced growth in the brain exists, this would be a major discovery,” Brodin added. “Training the brain would be analogous to training muscles. It would be important for research on diseases such as Parkinson’s and Alzheimer’s,” he said.

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