Lordosis behavior—also known as mammalian lordosis (Greek lordōsis, from lordos "bent backward") or presenting—is a body posture, hardwired in the brain, adopted mainly by non hominidea mammals including mice, cats, and others, usually associated with females and their receptivity to copulation. The primary characteristics are a lowering of the forelimbs whilst keeping 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 the apex points towards the abdomen. The term "lordosis" is sometimes used to describe, in humans, abnormal forward curvature of the spine in the lumbar region.
By simplifying, we can schematize the main phases of reproductive behavior. 1) Odors and especially pheromones make it possible to exchange sexual signals between potential partners. 2) Olfactory circuits (red arrows of the above diagram) make it possible in particular to recognize the partner of the opposite sex and to trigger sexual arousal, which induces vaginal lubrication, erection and copulation. 3) When the male mounts the female, the male's tactile stimuli on female's rump trigger the lordosis behavior reflex. 4) The lordosis circuits (orange arrows) cause the ventral arching of the spine, which elevates the hips and present properly the vagina to the male, thereby facilitating penetration by the penis. 5) 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' movements in the vagina trigger the reflex of ejaculation. 6) Tactile stimulation of the clitoris (and the penis for the male) during copulation are transmitted to the brain (blue arrows). 7) Activation of the reward system induces learning which optimize the copulation, particularly by the development of sexual motivation. Moreover, olfactive, auditive and visual signals perceived during the copulation may by conditioning become sexual signals, which optimizes the innate pheromonal signals. There is thus, in the innate neurobiological organization of the organism, a true heterosexual reproductive behavior in non primates mammals.
Lordosis reflex is a crucial body position in the reproductive behavior of the female of non primate mammals. Lordosis occurs both during pre-copulatory behavior and during copulation itself. 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"). 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.
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 properly the vagina 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.
From rodent to human, the corticalization of the brain induces several changes in the control of sexual behavior, including lordosis behavior. These changes induce a « difference between the stereotyped sexual behaviors in non-human mammals and the astounding variety of human sexual behaviors ».
Sexual reflexes, such as the motor reflex of lordosis, become secondary. In particular, lordosis behavior, which is a motor reflex complex and essential to carry out copulation in non-primate mammals (rodents, canines, bovids ...), is apparently no longer functional in women. Sexual stimuli on women do not trigger any more neither immobilization nor the reflex position of lordosis. On the level of olfactory systems, the vomeronasal organ is altered in hominids and 90% of the pheromone receptor genes become pseudogenes in humans. Concerning hormonal control, sexual activities are gradually dissociated from hormonal cycles. Human can have sex anytime during the year and hormonal cycles. On the contrary, the importance of rewards / reinforcements and cognition became major. Especially in humans, the extensive development of the neocortex allows the emergence of culture, which has a major influence on behavior. For all these reasons, the dynamics of sexual behavior was modified.
As a result of these evolutions, lordosis behavior becomes 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. Human sexual activities (kiss, mutual masturbation, fellatio, face to face coitus...) are not motor reflexes, but are mostly voluntary and learned activities, in order to obtain sexual rewards.
As lordosis behavior is a complex motor reflex of the female, designed to facilitate penile intromission, it cannot trigger innate auditive, olfactive or visual sexual responses from the male. Nevertheless, 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".