The question is then: how do these variations occur, which we believe to coincide with those of the secretion of testosterone by the testicles and those of the repertoire produced? We now know that the mechanisms governing the variations in volume of HVC and RA – the most extensively studied nuclei – are not the same. In RA, we observe modifications in the size of the neurons, their distribution and their dendritic connections. In spring, for instance, they are bigger, make more connections and are more spread out. Hence, a more voluminous nucleus. In HVC, the mechanism in force for RA is partly present, but it is not the only one. We also observe the incorporation of new neurons in the nucleus. “In other words, neurogenesis is involved here”, says Jacques Balthazart.
When Nottebohm made this discovery, this phenomenon was something of a revelation, because it went against an old dogma of neurobiology: the brain of warm-blooded adult vertebrates (homeotherms) does not produce new neurons. So what was then discovered? In short, the existence of an extremely active neurogenesis in the entire telencephalon of songbirds. This discovery led researchers to reconsider the problem in a more general manner. As a consequence, the presence of significant neurogenesis in at least two regions of mammals’ brains has been demonstrated.
How can we define the type of link uniting a morphological element, the change in size of some of the brain’s nuclei, and a learnt behavioural expression, the production of songs? Such is the key question that numerous laboratories concerned with research on songbirds are studying.