2. Th e presence or absence of testosterone determines whether a mammal will diff erentiate as a male or a female. In birds, the story is the opposite: Th e presence or absence of estrogen is critical (Adkins &

Adler, 1972). What problems would sex determination by estrogen create if that were the mechanism for mammals? Why do those problems not arise in birds?

(Hint: Th ink about the diff erence between live birth and hatching from an egg.)