Flehmen response

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The flehmen response, also called the flehmen position, flehmen reaction, flehming, or flehmening (from German flehmen, meaning to curl the upper lip), is a particular type of curling of the upper lip in ungulates, felids, and many other mammals, which facilitates the transfer of pheromones and other scents into the vomeronasal organ, also called the Jacobson's Organ. In the flehmen reaction, animals draw back their lips in a manner that makes them appear to be "grimacing". The action, which is adopted when examining scents left by other animals of the same species or prey, helps expose the vomeronasal organ and draws scent molecules back toward it. This behavior allows animals to detect scents, for example from urine, of other members of their species or clues to the presence of prey. Flehming allows the animals to determine several factors, including the presence or absence of estrus, the physiological state of the animal, and how long ago the animal passed by. This particular response is recognizable in stallions when smelling the urine of a mare in heat.

The vomeronasal organ, also called Jacobson's organ, is a chemoreceptor organ thought to have to do with the perception of certain scents and pheromones. It is named for its closeness to the vomer and nasal bones, and is particularly developed in animals such as cats and horses. The organ is located on the roof of the mouth. In addition to house cats and horses, animals who exhibit the flehmen reaction include buffalo, tigers, tapirs, lions, giraffes, goats, and llamas.[1][2][3]

A grimace similar to the flehmen response may also be seen in association with pain.[4] In horses it is often associated with low grade abdominal pain.

Although most adult humans lack a functional Jacobson's Organ (at least generally), a similar human behavior is the instinctive pulling back of the upper lip in a grimace when faced with something considered "disgusting" -- which may have a quite different function.

Source

• Pion PD and Spadafori G. The feline behavior series. VeterinaryPartner.com

Swaisgood, R. R., D. G. Lindburg, X. Zhou, and M. A. Owen. 2000. The effects of sex, reproductive condition and context on discrimination of conspecific odours by giant pandas. Anim. Behav. 60; 227-237.de:Flehmen