What is an example of a secondary sex characteristic in males?

Secondary sex characteristics are physical traits that emerge during puberty and are not directly involved in reproduction but signify sexual maturity. In males, one prominent example of a secondary sex characteristic is the change in voice quality, which typically becomes deeper as the larynx grows and vocal cords lengthen during puberty.

The production of sperm, which is mentioned in another choice, is a primary sex characteristic as it is directly related to the reproductive system and its functions. Menstruation is also not applicable in males, as it is a biological function specific to females. Testosterone levels, while crucial in the development of male characteristics and reproductive functions, do not represent a physical change; instead, they indicate hormonal changes that facilitate the development of secondary sex characteristics.