The plastic brains of birds

There has been a lot of talk about brain plasticity, the idea that our brains can be shaped and moulded by experience, in popular books and articles over the past several years. The notion that new neurons can be born in our brains, even in adulthood, is gripping and at times very encouraging.

However, our brains are not nearly as plastic as the more primitive brains of fish, amphibians and birds. Some of these organisms experience fluctuations in brain volume so drastic that if they existed in humans, they would probably lead to startling changes in intelligence and behaviour throughout adult life.

In birds, changes in singing behaviour that occur as the seasons change are linked to radical changes in the size of brain regions that control singing (explained here). During mating season, when birds sing more, new neurons are born (i.e. neurogenesis) in the song control system, whereas neurons die during the offseason.

Neurogenesis occurs in widespread regions of adult bird brains, making them a good model for studying the mechanism of neuron birth. In mammals, neurogenesis has only been identified in the hippocampus and the olfactory bulb. In humans, most research attention is given to the hippocampus because of its prominent roles in memory and cognition.

David Sherry and Jennifer Hoshooley at the University of Western Ontario recently published a review on the study of plasticity/neurogenesis in the hippocampus of birds who store food during specific seasons. The authors discuss studies that show changes in hippocampal size and neurogenesis during periods of food-storing behaviour. They propose that hippocampal neurogenesis may be a consequence of the behavioural and cognitive involvement of the hippocampus in storing and retrieving food.

It is important to note, however, that it is difficult to directly link hippocampal neurogenesis to food-storing behaviour when this type of behaviour always comes during a specific season that is associated with changes in social system, size and appearance of home range, and diet. Important evidence that supports the notion that hippocampal neurogenesis is due to food-storing behaviour alone comes from studies of food-storing birds (e.g. chickadees) versus non-storing birds (e.g. house sparrows). The non-storing birds experience the same environmental changes as food-storing birds, but the food-storing birds show much more hippocampal neurogenesis than the non-storing birds (a study that showed this was conducted by David Sherry’s group).

It thus appears that hippocampal plasticity is linked to food-storing behaviour, an activity that involves memorizing multiple locations at once – something that birds are very good at. What remains to be seen is the aspect(s) of food-storing behaviour that the hippocampus is important for. We know that a reliable way to stimulate hippocampal neurogenesis in mammals is getting them to exercise. Could it be that birds are much more physically active during food-storing season, and could this account for the birth of new neurons? Neurogenesis is extensively studied in birds, but the question of exercise and neurogenesis has not been directly investigated.

We know that we share genes with birds that control language and singing; perhaps further study of plasticity and neurogenesis will illuminate more similarities between us and our flying friends.

References:

Sherry DF, & Hoshooley JS (2010). Seasonal hippocampal plasticity in food-storing birds. Philosophical transactions of the Royal Society of London. Series B, Biological sciences, 365 (1542), 933-43 PMID: 20156817

Hoshooley JS, & Sherry DF (2007). Greater hippocampal neuronal recruitment in food-storing than in non-food-storing birds. Developmental neurobiology, 67 (4), 406-14 PMID: 17443797