Final answer:
Thirteen-year-old Eric has developed tufts of hair under his arms. This change is what is known as one of the secondary sex characteristics.
Step-by-step explanation:
Secondary sex characteristics are physical features that emerge during puberty and are not directly related to reproductive organs but distinguish individuals based on their sex.
The development of tufts of hair under the arms is a classic example of a secondary sex characteristic associated with puberty. These characteristics are driven by hormonal changes, primarily the increase in sex hormones like testosterone and estrogen.
During puberty, the hypothalamus-pituitary-gonadal axis becomes activated, leading to the release of sex hormones. In boys like Eric, the increased production of testosterone stimulates the growth of body hair, including the appearance of hair in the axillary (underarm) region. This hair growth is a visible sign of sexual maturation and plays a role in the overall development of sexual dimorphism.
In summary, the emergence of tufts of hair under Eric's arms is a secondary sex characteristic influenced by the hormonal changes that occur during puberty. These characteristics serve as external markers of sexual maturity and contribute to the development of distinct male and female physical traits.