Male songbirds perform songs to attract mates, meaning the brain nuclei that control song are over 5x bigger in males. Giving newborn females testosterone or estradiol allows these nuclei to develop larger (in line with the organizational hypothesis). If given more testosterone in adulthood, female songbirds can sing just like males. However, canaries only require hormone treatment in adulthood to start singing. The explanation is that birds that reproduce year-round need both rounds of treatment, but if they are seasonal breeders, the nuclei grow in the spring alongside testosterone and singing and all decline in the fall.

Song Control Regions in Male Songbirds

The sexually dimorphic nucleus of the POA (SND-POA) is a region that is over 5x as big in male rats than female. The organizational hypothesis is in line as males castrated at birth have a smaller SND-POA whereas androgenized females have a larger region. There is no difference if castration is done in adulthood. This also follows the aromatization hypothesis.

Preoptic Area (POA) of Rats

Rats contain the spinal nucleus of the bulbocavernosus (SNB) is a group of motor neurons that innervate the muscles that control the penis, meaning that while males have about 200 of these cells, females have far less and the muscles die at birth. This follows the organizational hypothesis as androgens given at birth can spare SNB cells and their muscles and castration causes them to die. To the androgen-insensitive the muscles also die, meaning aromatization is not a factor. 

Spinal Cord in Mammals

There a number of possible environmental influences that can impact sexual differentiation from chemical composition of the environment to behaviors of other individuals, including: 

 • Neonatal exposure to bisphenol A

 • Development of the spinal nucleus of the bulbocavernosus

 • Traditional gender roles

Environmental Influences

Coined by Charles Darwin, this term describes how males and females show pronounced sex differences in appearance. 

Rutgers University

Gonadal hormones drive sexual differentiation; the presence of testosterone promotes masculine physical development, whereas the absence of testicular hormones leads to feminine development. Furthermore, evidence suggests that these hormones influence brain development in a similar manner.

Gonadal Hormones

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

A permanent change in an individual's nervous system and behavior because of steroid exposure during development.

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