Mammals share the same reproductive system, including the regulatory hypothalamic system that produces gonadotropin-releasing hormone in pulses, the pituitary gland that secretes follicle-stimulating hormone and luteinizing hormone, and the ovary itself that releases sex hormones, including estrogens and progesterone.

However, species vary significantly in the detailed functioning. One difference is that animals that have estrous cycles resorb the endometrium if conception does not occur during that cycle. Animals that have menstrual cycles shed the endometrium through menstruation instead. Another difference is sexual activity. In species with estrous cycles, females are generally only sexually active during the estrus phase of their cycle. This is also referred to as being "in heat." In contrast, females of species with menstrual cycles can be sexually active at any time in their cycle, even when they are not about to ovulate.

Humans have menstrual cycles rather than estrous cycles. They, unlike most other species, have concealed ovulation, a lack of obvious external signs to signal estral receptivity at ovulation (i.e., the ability to become pregnant). There are, however, subtle signs to which heterosexual human males may favorably respond, including changes in a woman's scent[1] and facial appearance. Some research also suggests that women tend to have more sexual thoughts and are more prone to sexual activity right before ovulation. Animals with estrous cycles often have unmistakable outward displays of receptivity, ranging from engorged and colorful genitals to behavioral changes like mating calls.