Lordosis behavior, also known as mammalian lordosis (Greek lordōsis, from lordos "bent backward") or presenting, is a body posture adopted by some mammals including elephants, rodents, felines and others, usually associated with female receptivity to copulation. The primary characteristics of the behavior are a lowering of the forelimbs but with the rear limbs extended and hips raised, ventral arching of the spine and a raising, or sideward displacement, of the tail. During lordosis, the spine curves dorsoventrally so that its apex points towards the abdomen. In human physiology, the term "lordosis" is sometimes used to describe an abnormal forward curvature of the spine in the lumbar region.
Lordosis is a reflex action that causes many non-primate female mammals to adopt a body position that is often crucial to reproductive behavior. The posture moves the pelvic tilt in an anterior direction, with the posterior pelvis rising up, the bottom angling backward and the front angling downward. Lordosis aids in copulation as it elevates the hips, thereby facilitating penetration by the penis. It is commonly seen in female mammals during estrus (being "in heat"). Lordosis occurs both during pre-copulatory behavior and during copulation itself.
The main phases of reproductive behavior are:
- Odors, especially pheromones, make it possible to exchange sexual signals between potential partners.
- Olfactory circuits (red arrows in the above diagram) make it possible to recognize the partner of the opposite sex and to trigger sexual arousal, which induces vaginal lubrication, erection and copulation.
- When the male mounts the female, the male's tactile stimuli on the female's rump trigger the lordosis reflex.
- The lordosis circuits cause the ventral arching of the spine, which elevates the hips and presents the vagina to the male, thereby facilitating penetration by the penis.
- The tactile contact between the penis and the genital area triggers the reflex movements of the male's pelvis (pelvic thrusts), then intromission. After intromission, the penis's movements in the vagina trigger the reflex of ejaculation.
- Tactile stimulation of the clitoris (and the penis for the male) during copulation is transmitted to the brain (blue arrows).
- Activation of the reward system induces learning which optimizes copulation, particularly by the development of sexual motivation. Moreover, olfactive, auditory and visual signals perceived during the copulation may by conditioning become sexual signals, which optimizes the innate pheromonal signals.
During estrus, the estrogen hormone, estradiol, regulates sexual receptivity by the neurons in the ventromedial nucleus of the hypothalamus, the periaqueductal gray, and other areas of the brain. Sexual stimuli trigger activity in a number of brain areas, including the ventromedial hypothalamus, which sends impulses down axons synapsing with neurons in the periaqueductal gray. These convey an impulse to neurons in the medullary reticular formation which project down the reticulospinal tract and synapse with afferent nerve fibers in the spinal cord (L1-L6). These cause muscles along the spine to contract, thereby producing the lordosis posture. Because these afferent fibers are also part of a reflex arc, lordosis can be triggered reflexively.
More precisely, the lordosis sexual reflex is mainly hardwired in the spinal cord, at the level of the lumbar and sacral vertebrae (L1, L2, L5, L6 and S1). In the brain, several regions modulate the lordosis reflex. The vestibular nuclei and the cerebellum, via the vestibular tract, send information which makes it possible to coordinate the lordosis reflex with postural balance. More importantly, the ventromedial hypothalamus sends projections that inhibit the reflex at the spinal level. For this reason, in general, the lordosis reflex is not functional. Sex hormones control reproduction and coordinate sexual activity with the physiological state. Schematically, at the breeding season, and when an ovum is available, hormones (especially estrogen) simultaneously induce ovulation and estrus (heat). Under the action of estrogen in the hypothalamus, the lordosis reflex is uninhibited. The female is ready for copulation and fertilization.
During the copulation, when a male approaches the female, male pheromones (part 1 of the above diagram) are detected by the olfactory circuits (part 2). The pheromonal signals stimulate, among other things, the hypothalamus, which facilitates the lordosis reflex. Then, when the male mounts the female (part 3), tactile stimuli on the flanks, the perineum and the rump of the female are transmitted via the sensory nerves in the spinal cord. In the spinal cord, they are integrated with the information coming from the brain, and then, in general, a nerve impulse is transmitted to the muscles via the motor nerves. The contraction of the longissimus and transverso-spinalis muscles causes the ventral arching of the vertebral column (part 4). The lordosis position which results from it makes it possible to present the vagina properly to the male (part 5), facilitating penile intromission. Then, during intromission, tactile and deep sensations from the genital area and clitoris accentuate the lordosis reflex (part 6). It is thus observed that the physiological and neurobiological organization of the lordosis behavior reflex is specifically adapted to heterosexual copulation.
Evolution, from rodent to human
Increased corticalization of the brain induces several changes in the control of sexual behavior, including lordosis. Sexual reflexes, such as the motor reflex of lordosis, become secondary[clarification needed] and are apparently no longer functional in human women.
As a result of these evolved differences, lordosis behavior became secondary in hominidae and is apparently non-functional in humans. When a woman gets onto all fours, curves her back and remains still, it is no longer a reflex movement triggered by sexual stimuli, but a voluntary movement. The anthropologist Helen Fisher speculates that when a human female wears high-heeled footwear, the buttocks thrust out and the back arches into a pose that simulates lordosis behavior, which is why high heels are considered "sexy".
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