Gonads and singing play separate, additive roles in new neuron recruitment in adult canary brain.

New neurons are constantly added to the high vocal center (HVC) of adult male canaries, Serinus canaria. Singing and testosterone (T) are known to promote this addition, but it is not known whether either variable can act on its own and what is their effect when acting together. We studied this question by castrating adult male canaries in late summer and quantifying their song in early fall. Intact birds served as controls. A 5 d systemic treatment of two daily injections of the cell birth marker 3H-thymidine started 10 d after surgery. Twenty days after the first 3H-thymidine injection and for a period of 1 month, we quantified the singing of all birds, which were then killed. Amount of singing, syllable diversity, and song stability were similar in intacts and castrates. When castrates and intacts that sang comparable amounts were compared, the number of 3H-labeled HVC neurons was 2.6 times higher in intacts than in castrates. In castrates with plasma T levels that were undetectable, the mean amount of singing was positively related to the number of new neurons. We suggest that singing and gonadal factors promote, separately, the recruitment of new neurons and that when they exert this effect together they do so in an additive manner.