Describe the estrus cycle.

• A. The estrous cycle can be divided into four stages: proestrus, estrus, metestrus, and diestrus.
• B. It is a single-phase cycle
• C. It has no uterine changes
• D. It occurs only in humans

Answer: (A)

The estrous cycle in many mammals is organized into four distinct phases, each with specific hormonal changes and reproductive state. It begins with proestrus, when follicles mature and estrogen rises to prepare the reproductive tract. Next is estrus, the phase of sexual receptivity when ovulation typically occurs or is impending. After ovulation comes metestrus, where the corpus luteum forms and progesterone starts to rise, signaling the early luteal phase and continued preparation of the uterus. Finally, diestrus is the period of active luteal function with high progesterone; if pregnancy does not occur, the corpus luteum regresses and the cycle resets.

The other statements don’t fit because the cycle is not a single-phase process, it does involve clear uterine changes as the lining thickens and is prepared for possible implantation, and it is not limited to humans (humans have a menstrual cycle rather than an estrous cycle).