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Lordosis behavior, also known as mammalian lordosis (Greek lordōsis, from lordos "bent backward") or presenting, is the naturally occurring body posture for sexual receptivity to copulation adopted by some mammals including elephants, rodents, felines. 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.
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The lordosis position is facilitated by the appropriate sensory input, such as touch or smell. For example, in female hamsters, when they are in the correct hormonal state there are areas on the flank that, when touched by the male, facilitate lordosis.
Description
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.
Neurobiology
Lordosis brain circuits are connected with other neural circuits, especially the olfactory and reward systems.
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.
There is thus, in the innate neurobiological organization of the organism, a true heterosexual reproductive behavior in non-primate mammals.
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.
Lordosis can also be elicited by manual cutaneous stimulation of the flanks followed by the rump-tail base-perineum region, or induced by injections of estradiol benzoate and progesterone.
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.
Brain function
Several brain areas are known to be important for the regulation of lordosis, including the ventromedial nucleus of the hypothalamus (VMN). The activity of the VMN in the regulation of lordosis is estrogen-dependent and when it is lesioned lordosis is abolished. Displays of lordosis can be affected by exposure to stress during puberty. Specifically, stress can suppressed the hypothalamic-pituitary-gonadal (HPG) axis and therefore decrease concentrations of gonadal hormones. Consequently, these reductions in exposure to gonadal hormones around puberty can result in decreases in sexual behavior in adulthood, including displays of lordosis.
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 and are apparently no longer functional in human women.
In humans
As a result of these evolved differences, lordosis behavior became secondary in hominidae and is apparently non-functional in humans. If 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".
The Red dress effect is the sexual response to the color red in apes. Sexual swelling is an involuntary display that can occur in some female primates when they are fertile. Desmond Morris in his book "The Naked Ape" speculates wearing red lipstick is a sexual signal.