Brain power: the nerve cells of dominant rats survive longer than those of subordinate animals. Credit: © Alamy

Assertiveness really is all in the mind. Dominant rats have more new nerve cells in a key brain region than their subordinates, a study reveals.

The finding hints that social hierarchies can influence brain structure, and raises questions over the use of standard animal behaviour tests in laboratory research.

Yevgenia Kozorovitskiy and Elizabeth Gould from Princeton University, New Jersey, studied the brains of around 40 rats that had been left to form social hierarchies in a semi-natural setting. Their results are published in The Journal of Neuroscience1.

In each experiment, four males and two females were placed inside a large box comprising an underground tangle of burrows and chambers and a feeding area above. Within three days, the males had established their preferred pecking order: an aggressive leader who attracted the females and three defensive subordinates.

Two weeks later, the high-status animals were found to have around 30% more neurons in their hippocampus, a brain region implicated in learning and memory, than they had before.

Neurons in this area are constantly recycled, says Kozorovitskiy. Around 9,000 new nerve cells are born every day, but most die within a week. In the dominant animals, however, the new cells survive for longer.

Although their exact function is unknown, it is thought that these extra neurons could help the animals adapt to their position of power. Whether the same phenomenon occurs in humans is not known.

Other factors, such as exercise and environmental enrichment, trigger the production of new nerve cells in the same area. "These cells are amazingly responsive to environmental conditions," says neuroscientist Tracey Shors from Rutgers, the State University of New Jersey.

Social climbing

The strain of rat that Kozorovitskiy and Gould used is normally passive, and does not form dominance hierarchies when housed in standard laboratory cages without tunnels.

Although the researchers had hoped that placing the animals in burrows would encourage them to form hierarchies, they were surprised to see the animals' social structure change so quickly, with a wealth of individual differences in behaviour appearing within days.

Kozorovitskiy suggests that studying rats in burrows is likely to be much more relevant to understanding human behaviour than tests carried out in standard cages.

"Standard laboratory housing may suppress individual variability," agrees neuroscientist Larry Young from Emory University in Atlanta, Georgia. "So the behaviour you see may be different from that you would observe in nature."

Most laboratory protocols suppress individual differences between test animals. But many researchers are now moving towards tests that enhance individual behaviour. These models should be more relevant to human disease, says Young.