Lordosis behavior—also known as mammalian lordosis (Greek lordōsis, from lordos "bent backward") or presenting—is a body posture adopted by mammals including humans, 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.
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.
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".