Female Sexual Behavior

Historical research on female sexual behavior

  • Unlike early studies on male sexual behavior, little progress was made studying female sexual behavior until the 20th century.

    • Female sexual behavior is more complex than male sexual behavior.

    • Ovariectomy refers to the surgical procedure for removing ovaries, which is a more difficult surgical procedure than orchiectomy (removal of testes).

    • The cyclical nature of female reproduction presents challenges for research due to the hormonal fluctuations impacting behavior.

    • There is historical bias among researchers (mainly males) who viewed females as passive participants in copulation.

Male versus Female Perspectives

  • Estrous cycles stop after ovariectomy, leading to the assumption that cycles in sexual behavior reflect ovarian function cycles.

  • At the beginning of the 20th century, cyclic ovarian changes could only be examined through surgical methods.

    • Female reproductive behavior was historically linked to ovarian function, with observations made post-surgery.

    • Vaginal cytological assay, developed by Stockard and Papanicolaou, shows that changes in types of vaginal cells are closely correlated with ovarian function, especially observed in guinea pigs.

Female animals as active participants

  • Prior to the 1970s, researchers believed females were passive recipients of male sexual attention.

  • Modern research acknowledges many species of females actively participate in initiating copulation.

Proximate bases of female sexual behavior

  • Female sexual behavior encompasses all behaviors necessary and sufficient to achieve the fertilization of ova (female gametes) by sperm.

  • Hormones related to maturing ova influence female nervous systems:

    • Neural changes serve to increase the likelihood of mating.

    • Reproductive hormones do not merely "turn on" sexual behavior, but rather change the likelihood that specific stimuli will elicit behaviors leading to successful copulation.

Components of female reproductive states in mammals

  • Female reproductive states are cyclical and typically consist of six components:

    • Courtship

    • Mating

    • Ovulation

    • Pregnancy

    • Parturition

    • Lactation

Vaginal cytological assay and ovarian correlates

  • In rodents:

    • Cells are swabbed from the vaginal lumen and examined microscopically.

    • In rats, the cellular content changes recur every 4-5 days:

    • Vaginal estrus lasts approximately 36 hours.

    • Vaginal diestrus is broken into diestrus I and II, lasting about 48 hours.

    • Proestrus lasts roughly 12 hours, during which the thin vaginal membrane disappears, leading to behavioral estrus.

  • Vaginal estrus correlates with the presence of ruptured follicles and the development of tertiary follicles from secondary ones, getting ready for ovulation.

Physiological changes during the reproductive cycle

  • In diestrus I:

    • Tertiary follicles grow larger and granulosa cells increase.

    • Antrum begins to fill with liquor folliculi.

    • Endocrine luteal cells form around former follicle sites leading to the formation of corpora lutea.

  • During diestrus II:

    • Connective tissue surrounding the theca interna develops into the theca externa.

    • Corpora lutea become fully formed, linking ovulation to behavioral estrus and mating.

  • In proestrus, follicles swell, and corpora lutea regress unless stimulated by specific vaginal stimuli.

The pap smear

  • The vaginal cytology assay has a modified version used today called the pap smear, which utilizes cells from the cervix and is often named after Dr. Papanicolaou.

Female sexual behavior in mammals

  • Male mammals can exhibit sexual behavior throughout the breeding season, whereas female mammals show fluctuating sexual activity.

    • Estrus is a period of copulatory receptiveness, translating from Latin as 2in a frenzy.2

    • Anestrus refers to the period when females are not in estrus; during this time, behaviors differ significantly compared to estrus.

Estrus versus Anestrus behavior

  • During estrus, females actively seek out males, initiate copulation, and prefer to be near them.

  • In contrast, females in anestrus often behave aggressively toward males; they are less attractive compared to estrous females.

Research on hormonal influence on sexual behavior cycles

  • Cycles of vaginal cytology and estrus behavior cease following ovariectomy, indicating that ovaries produce cyclical signals driving these changes.

  • Similarly, if ovaries are denervated, cycles persist, indicating hormonal mediation.

Source of estrus hormones

  • By the 20th century, ovarian structures, namely follicles and corpora lutea, were believed to produce hormones influencing estrus behavior.

    • Mice injected with extracts from follicles exhibited physiological responses common to estrus and exhibited mating behavior.

  • Substance extracted from ovarian follicles came to be known as estrogen, a term referring to substances generating estrus.

    • Two research teams independently crystallized estrone, a type of estrogen, from the urine of pregnant women.

Hormonal dynamics in rodents

  • Follicle fluid is rich in estrogens, released into circulation, leading to an increase in estrogen levels during proestrus. This spike induces behavioral estrus and changes in vaginal epithelial cells.

    • Early studies indicated that while estrogen injections typically induced estrus behaviors in many ovariectomized rodents, around 40% failed to mate post-injection, suggesting an unknown factor complementing estrogen's role in behavioral estrus.

  • When ovariectomized rabbits were injected with extracts from corpora lutea, physiological responses consistent with conception occurred, preparing the uterus for pregnancy. This luteal steroid hormone was identified as progesterone, named for its role in supporting gestation.

Effects of progesterone on sexual behavior

  • Progesterone exhibits biphasic effects on female sexual behaviors, varying across species.

    • For instance, in rats

    • High progesterone levels, post-copulation, suppress female sexual behavior.

Components and classifications of female sexual behavior

  • Beach proposed categorizing female sexual behavior into three components:

    • Attractivity: Stimulus value of a female towards males, typically inferred from male behavioral responses and measured in relational terms.

    • Proceptivity: Degree to which females initiate copulation, akin to sexual motivation.

    • Receptivity: Female responsiveness to male advances during mating.

Additional classifications of female sexual behavior

  • In recent research, female sexual behavior has also been divided into:

    • Precopulatory phase: Encompasses behaviors that attract males and initiate copulation (essentially aligning with proceptivity).

    • Copulatory phase: Consists of copulatory behaviors tied to receptiveness.

    • Ongoing classifications also include the specific facilitation of mating behaviors and underlying motivations.

Attractivity in detail

  • Evaluated through male preference, attractivity includes non-behavioral and behavioral components. Non-behavioral attributes can involve morphological changes coinciding with ovulation, again reflected in odor and other factors.

    • Behavioral attractivity encompasses behaviors soliciting males, which may include specific vocalizations and display postures.

Understanding proceptivity

  • Proceptivity consists of appetitive actions females engage in relating to male solicitation and attraction.

    • Rat studies showed estrous females exhibit high levels of initial attraction towards males.

    • Assessing proceptivity can involve:

    • Proximity behaviors

    • Solicitation counts

    • Alternating approach-retreat behaviors, common among many species.

Receptivity defined

  • Receptivity represents the consummatory phase of mating behavior, characterized by female reactions necessary for successful copulation. For non-primate mammals, specific mating postures are often involved.

  • Measurement of receptivity typically compares male mating attempts to successfully achieved copulations.

Neural mechanisms mediating female sexual behavior

  • Much of the understanding of endocrine effects on neural tissues involved in female sexual behavior comes from studies on lordosis in rats.

    • Lordosis posture is reflexively adopted when females are stimulated on their flanks by males. This arching position facilitates mating.

Neural mechanisms in detail

  • Activation of pressure receptors in the flanks leads to a response through sensory nerves projecting to the spinal cord, sending signals through multiple brain regions for lordosis behavior.

  • The ventromedial nucleus of the hypothalamus (VMN) plays a crucial role in lordosis, with lesions to this region decreasing or eliminating such behavior.

Hormones and neural structure interactions

  • Estradiol and progesterone receptors are distributed throughout the vertebrate brain, concentrated mainly in the medial preoptic area and hypothalamus.

    • After binding to nuclear receptors, the estrogen-receptor complex can act as a transcription factor affecting gene expression pertinent to estrus behavior and resulting responses to mating stimuli.

Copulatory behavior in rodents

  • Estrus behavior in rodents necessitates the lordosis posture to permit successful mating. Additionally, estrous females often engage in other behaviors to attract males, such as distinctive movements and vocalizations.

Female pacing of copulation

  • Wild mice and rats typically live in small groups, where females lead the mating behavior, often synchronizing estrus with group mates.

    • This coordinated breeding behavior can lead to competition for males and influence the sire's genetics for offspring due to which male achieves mating first and last.

Timing of mating and hormonal implications

  • Females tend to increase the mating interval when in natural settings compared to laboratory conditions, emphasizing the importance of the pacing of copulatory events to sustain corpus luteum functioning for pregnancy.

The vaginal code in rats

  • Evidence suggests a "vaginal code" exists in which specific patterns of copulatory stimulation are optimal for reproductive success. Studies indicate that clitoral stimulation also plays a vital role in female sexual behavior and mating success.

Interspecies variation in female reproductive cycles

  • Cycle lengths vary dramatically among species with optimal conditions for specific cycle types, as indicated in tables examining various species including rats, mice, guinea pigs, and canines. Such cycles range from distributed ovulations in certain species to spontaneous in others.

The ecology of reproductive cycles

  • The diversity of reproductive cycles can be viewed through an ecological lens; each cycle feels like an adaptation to enhance overall reproductive success in differing environments.

Female prairie voles and their reproductive cycle

  • Microtus ochrogaster exhibits irregular estrus cycles; its behavioral estrus can be induced by male proximity or pheromonal cues from male urine, triggering hormonal cascades that culminate in ovulation.

Social and environmental effects on reproductive cycles

  • The Lee-Boot effect determines that housing conditions without males leads to prolonged estrous cycles, and the Whitten effect induces estrus following male presence or pheromones.

    • The Bruce effect can cause pregnant females to terminate pregnancies based on exposure to males that are not the sire. Additionally, exposure to female odors can delay puberty or induce maturation variably.

Female mating behavior in canines

  • Canine estrous cycles recur approximately every 7-8 months, with periods of estrus lasting around 7-10 days. Diestrus persists for roughly two and half months, with anestrus lasting up to five months.

Sexual behavior in higher primates

  • Within various primate species, female sexual behavior shows a wide range of patterns, from highly stereotyped in certain species to more variable behaviors across species like humans, where there might be no strict estrous cycle.

Estrogens' influence on sexual behavior

  • In female rhesus monkeys, estrogen corresponding positively results in peak mating behavior.

    • Studies also showed performance alteration relating to estrus phases, providing insight into hormonal peaks affecting social and sexual behaviors.

Estrogens and sexual behavior in humans

  • Variability in study methods has revealed women gained increased social engagement and perks during ovulation, such as how female strippers averaged higher tips.

    • Research suggests varying preferences linked to male traits fluctuate across different menstrual cycle phases, better identified in non-contracepted women.

    • "Hidden ovulation" describes how ovulation is not overtly detected by females or males, complicating recorded reproductive behaviors.

Testosterone's role in female sexual behavior

  • Testosterone appears pivotal for sexual motivation in women, with levels varying by relationship status and dependent on factors tied to partner interactions.

    • Similar dynamics demonstrate that hormonal changes, particularly after premier relationships, can yield significant changes in motivation.

Menstrual and the reproductive cycle

  • The human menstrual cycle results from low estrogen levels, causing the shedding of the uterine lining each cycle. Hemorrhage may inform different reproductive processes in other species.

Social factors affecting reproductive cycles in humans

  • Evidence shows women cohabitating may synchronize their cycles, possibly modulated through chemosensory signals like androstadienone.

Environmental impacts on reproductive cycles

  • Nutritional deficiency, stress, and chemical messengers (cytokines) can negatively impact ovarian functionality and hormonal cycles, affecting reproductive receptivity and proceptivity.

Mechanisms of birth control pills

  • Combination pills stabilize estrogen and progesterone levels, inhibiting LH and FSH secretion, which prevents ovulation, while mini pills mainly affect cervical mucus and uterine lining but may occasionally suppress ovulation.

Phases of the menstrual cycle and hormonal changes

  1. Follicular Phase (Days 1-14)

    • FSH stimulates follicle development, releasing estrogen.

  2. Ovulation (Day 14)

    • Elevated estrogen triggers an LH surge, resulting in ovulation.

  3. Luteal Phase (Days 14-28)

    • Corpus luteum forms from the follicle and secretes progesterone and estrogen.

  4. Menstrual Phase (Days 1-5)

    • Shedding of the endometrial tissue occurs due to low steroid hormone levels.

  5. Proliferative Phase (Days 6-14)

    • The endometrium regrows in response to estrogen.

  6. Secretory Phase (Days 14-28)

    • The endometrium secretes nutrients in response to progesterone.

Hormone fluctuations across the ovarian cycle (HPO Axis)

  • Cycle dynamics reflect hormonal levels and include:

    • GnRH from the hypothalamus triggers FSH and LH release from the anterior pituitary, influencing ovarian estrogen (from follicles) and subsequent support for menstrual outcomes.

Kisspeptin and ovulation

  • The Kisspeptin neurons, identified within the AVPV, are instrumental in mediating positive feedback that triggers hormone surges for ovulation.

Comparative LH secretion patterns

  • Female and male LH concentrations exhibit distinct patterns, with females showing cyclical peaks following estrogen surges, while males show relatively stable levels across time.

Mechanisms of reproductive axis control

  • Sex differences in brain response to sexual steroid feedback affect the regulatory mechanisms governing ovulatory responses, with notable contrasts seen between ovariectomized and castrated rats receiving varied estrogen doses.

Menopause as a unique trait

  • Menopause typically begins around age 50, allowing for a significant post-reproductive lifespan, with theories like the grandmother hypothesis suggesting advantages in kin investment after reproductive years.