Prolactin Hormone

Prolactin is a peptide hormone produced in the anterior pituitary, mammary glands, and deciduas (maternal placenta) as part of the endocrine system. It is composed of 199 amino acids with the mass of 24 kDa. Its production is stimulated by estrogen, which induces the growth of prolactin-producing cells and increases prolactin production directly within cells. The release of prolactin from the pituitary is regulated by the hypothalamus. Dopamine, a neurohormone secreted by the hypothalamus into the bloods vessels leading to the pituitary, inhibits the production of prolactin directly. Without dopamine, prolactin is produced at a high, continuous rate. 

Its predominant role is to induce the production of milk in the mammaries, but also produces the sensations of satisfaction after sexual activity by suppressing the effects of dopamine that causes sexual arousal. Recent studies have shown how prevalent prolactin is in the animal kingdom as a player in parental behavior.

Prolactin in Emperor Penguins

Prolactin is highly important in the expression of parental behavior in males and females of many bird species, especially penguins. High prolactin concentrations are necessary in emperor penguins to ensure that the individual penguins will return to their egg/chick and mate after long stints out on the ocean.

The remaining level of prolactin in a penguin’s bloodstream upon their return is still quite high despite the absence of interaction between it and the egg/chick. If the prolactin levels are above a certain threshold in a penguin that has lost its egg/chick, and the penguin experiences the stimuli of other chicks asking for food from its own parents, kidnapping behavior is very likely to occur. The interaction between the external stimuli and the high prolactin levels leads to kidnapping behavior.