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.
In the 1980s, Fernando Nottebohm suggested the existence of a correlation between the number of different songs produced by the canary and the volume of the HVC. Hence, the idea already evoked that this nucleus was a sort of library and its size, the reflection of the complexity of the animal’s repertoire. Subsequent studies showed that this conclusion was not always valid. According to Jacques Balthazart, we now tend to believe that the size of HVC does not necessarily correlate with the diversity of the bird’s repertoire, but rather with the quantity of vocalisations it makes.
“Perhaps there is an inversion in the causal link”, he insists. Indeed, it was initially thought that the volume of HVC influenced the variety in the repertoire, while we now think that the size of the nucleus is the consequence of a more abundant vocal activity, as if the bird was “exercising” its brain by singing a lot.”
In these conditions, it is justifiable to question the role of the new neurons that establish themselves in the HVC. It has been revealed that they all project onto the RA, and never onto Area X. And yet, the HVC and RA constitute the motor pathway, while X belongs to the frontal pathway involved in learning the songs. The decrease in the size of HVC in the summer and part of the autumn is the result of an accelerated neuronal death that affects the exchange of information between the two components of the motor pathway. If such a phenomenon affected the frontal pathway, the song memory would perhaps deteriorate and diminish. “Is that the reason why it is preserved?” Jacques Balthazart wonders. “Undoubtedly. But this is a finalist interpretation that is not currently based on any scientific certainty.”
On the other hand, we know that seasonal variations in the nuclei and neurogenesis coincide with those in the size of the testicles and, subsequently, the production of testosterone. The volume of the male gonads is itself controlled by the photoperiod which, incidentally, also has a direct effect on the size of the brain, regardless of the action of the testosterone.
In fact, this is unbelievably complex and the web is very difficult to untangle. Why? Because there are several types of closely interlinked correlations involving the photoperiod, the level of testosterone, the number of songs emitted, the size of the HVC nuclei and, up to a certain point, the vocal repertoire.