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. Step 1: Let's go through each statement one by one. The first statement says 'All mammals have menstrual cycles.' This is not true. Not all mammals have menstrual cycles. Some mammals, like humans and some primates, have menstrual cycles, but many others, like dogs and cats, have estrous cycles instead.
2. Step 2: The second statement is 'The endometrial lining is shed in menstrual cycles but reabsorbed in estrous cycles.' This is true. In a menstrual cycle, the endometrial lining is shed if no pregnancy occurs. In an estrous cycle, the lining is reabsorbed into the body.
3. Step 3: The third statement is 'Estrous cycles are more frequent than menstrual cycles.' This is not necessarily true. The frequency of these cycles can vary greatly among different species. For example, a dog's estrous cycle typically occurs twice a year, while a rat's estrous cycle occurs every 4 to 5 days.
4. Step 4: The last statement is 'Ovulation occurs before the endometrium thickens in estrous cycles.' This is not true. In both menstrual and estrous cycles, the endometrium thickens before ovulation to prepare for potential pregnancy.
5. Step 5: So, the correct answer is b. 'The endometrial lining is shed in menstrual cycles but reabsorbed in estrous cycles.'

Key Concepts

Menstrual and Estrous Cycles

Menstrual cycles are reproductive cycles in which the endometrial lining is built up and then shed if fertilization does not occur, typical in humans and some primates. In contrast, estrous cycles involve the reabsorption of the endometrial lining if fertilization does not happen, common in many mammals. Understanding these cycles is crucial for distinguishing between the reproductive strategies of different species.

Endometrial Lining

The endometrial lining is the inner layer of the uterus that thickens in preparation for potential implantation of a fertilized egg. In menstrual cycles, this lining is shed during menstruation, while in estrous cycles, it is typically reabsorbed if no pregnancy occurs. This difference is key to understanding the reproductive physiology of mammals.

Ovulation Timing

Ovulation is the process where a mature egg is released from the ovary, and its timing relative to the thickening of the endometrium varies between menstrual and estrous cycles. In menstrual cycles, ovulation occurs after the endometrium has thickened, while in estrous cycles, ovulation can occur before significant thickening. This timing is essential for understanding reproductive success and strategies in different species.