Which of the following is a true statement?
a. All mammals have menstrual cycles
b. The endometrial lining is shed in menstrual cycles but reabsorbed in estrous cycles
c. Estrous cycles are more frequent than menstrual cycles
d. Ovulation occurs before the endometrium thickens in estrous cycles

Verified step by step guidance

1. Understand the difference between menstrual and estrous cycles. Menstrual cycles involve the shedding of the endometrial lining, while estrous cycles involve reabsorption of the lining.
2. Consider the reproductive biology of mammals. Not all mammals have menstrual cycles; many have estrous cycles.
3. Analyze the frequency of cycles. Estrous cycles can vary widely among species, but they are not necessarily more frequent than menstrual cycles.
4. Examine the timing of ovulation and endometrial thickening. In estrous cycles, ovulation typically occurs after the endometrium has thickened.
5. Evaluate each statement based on the biological concepts discussed. Determine which statement accurately reflects the characteristics of menstrual and estrous cycles.

Key Concepts

Menstrual Cycle

The menstrual cycle is a reproductive cycle in female mammals, characterized by the shedding of the endometrial lining if fertilization does not occur. It typically involves phases such as menstruation, the follicular phase, ovulation, and the luteal phase. This cycle is common in humans and some primates.

Estrous Cycle

The estrous cycle is a reproductive cycle found in most non-primate mammals, where the endometrial lining is reabsorbed if fertilization does not occur, rather than being shed. This cycle includes phases like proestrus, estrus, metestrus, and diestrus, and is often associated with visible signs of fertility, such as behavioral changes.

Endometrial Lining

The endometrial lining is the inner layer of the uterus that thickens in preparation for potential implantation of an embryo. In menstrual cycles, this lining is shed during menstruation if fertilization does not occur, while in estrous cycles, it is typically reabsorbed. Its role is crucial for successful reproduction in mammals.