Birds Got Talent: How young males find their voice by making new neurons
Image Source: https://www.newscientist.com/article/2160842-birds-dream-sing-by-moving-their-vocal-muscles-in-their-sleep/ via Arco Images GmbH / Alamy Stock Photo
For humans, learning and singing songs might be for a music recital, for performing in the shower, or on car rides alone. To learn these songs, new neurons are generated in a process known as neurogenesis. However, for male songbirds, learning to sing may influence their ability to find a mate, and can lead to an overall decline in songbird populations. A 2021 study by Dr. Adriana Diez from Western University revealed that male songbirds show signs of increased neurogenesis in the vocal control centers of their brains as they begin learning to sing, around 25 days after hatching.
Songbirds have evolved special “song learning” regions of the brain that are bigger in male birds compared to their female counterparts. One such area is known as the High Vocal Center (HVC), which is important for learning and producing songs. Male birds with a decreased HVC volume also exhibited less singing during the mating season, which was associated with less reproductive success.
The phases of song learning that songbirds go through are well documented. Song learning in songbirds can be broken into 4 phases: sensory, sensorimotor, motor, and finally, crystallization. Young songbirds do not sing much during the sensory and sensorimotor phases, which occur at 25 days and 40 days after hatching, respectively. Male songbirds typically sing at 70- and 110-days post-hatch, corresponding to the motor and crystallization phase, respectively. It is unclear what differences in development occur during song learning that cause the special “song learning” regions of the male songbird brain to be so much bigger than female birds. The researchers thought that neurogenesis, the process of forming new neurons, might play a role.
There is a popular idea that you cannot form new neurons during adulthood. However, different studies have shown neurogenesis in humans from youth to adulthood. New neurons are formed by the division of neural stem cells, specialized cells that can replenish most brain cells, which eventually develop into immature neurons. Immature neurons migrate in the brain and establish new connections, eventually becoming mature neurons. These immature neurons express a protein called doublecortin (DCX), which is a marker of neurogenesis. The researchers looked at DCX to assess neurogenesis at different phases of song learning.
To study these questions, the researchers used a colony of zebra finches, a species of songbirds that is native to Australia and Indonesia. The researchers looked at male and female zebra finch brains using immunohistochemistry, a technique used to visualize the expression and location of neurogenesis in different brain regions when seen under a microscope.
At 25 days post-hatch, or the sensory phase, male zebra finches had more immature neurons in the HVC. In contrast, females showed consistently lower levels of DCX expression in the HVC, aligning with their lack of song production in adulthood. This sex difference remained consistent all the way until the crystallization phase. This suggests that early neurogenesis in the HVC may lay the groundwork for the male-specific development of the song system and set up structural differences in the brain long before behavioural differences become apparent.
Songbird populations have recently been on the decline, which is a warning sign that our environment is suffering. Efforts like this study allow more insight into what happens in the brains of songbirds during distinct stages of their lives, and identify environmental concerns that may impact their function. The newfound knowledge that neurogenesis is important for the development of regions of the brain, like the HVC, can help inform future policies for limiting environmental contamination of chemicals that can interfere with neurogenesis. Several factors, such as environmental contaminants, can decrease overall neurogenesis in both humans and birds.
Featured article: Diez, A., An, H. Y., Carfagnini, N., Bottini, C. & MacDougall‐Shackleton, S. A. Neurogenesis and the development of neural sex differences in vocal control regions of songbirds. Journal of Comparative Neurology 529, 2970–2986 (2021).