Name the hormone responsible for milk letdown in lactating animals.

Oxytocin is the hormone primarily responsible for facilitating milk letdown in lactating animals. This process occurs during nursing or milking when oxytocin is released from the posterior pituitary gland. The presence of an infant suckling or the sound of a milking machine stimulates sensory nerves, which trigger oxytocin release. As oxytocin levels rise, it promotes the contraction of myoepithelial cells surrounding the mammary glands, leading to the expulsion of milk from the alveoli and into the ducts, making it available for the nursing animal.

In contrast, the other hormones listed play different roles in the reproductive and endocrine processes. Estrogen, while important in many reproductive functions, does not directly influence milk letdown. Prolactin is crucial for milk production itself rather than the release, and testosterone is primarily associated with male reproductive functions and does not have a role in lactation. Therefore, the specific action of oxytocin in milk letdown highlights its essential role in the lactation process in animals.