Hormone releasing mechanisms develop as penguins reach sexual maturity, which gives them the ability to produce prolactin as soon as the can reproduce. However, since mating and subsequent reproductive behavior induce prolactin production, prolactin is virtually non-existent prior to the first pregnancy.


Experiments have revealed that prolactin is expressed in penguins during their second and third pregnancies, but no evidence has been gathered to examine how far prolactin levels drop in between pregnancies. Yet, based on the metabolic costs of producing prolactin, which are not offset by the benefits of raising progeny, it is expected that prolactin is not produced during those periods. Further investigations are needed to illuminate the development and release of prolactin over the course of a penguin's lifetime.