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The flehmen response (//; German: [ˈfleːmən]), also called the flehmen position, flehmen reaction, flehming, or flehmening (from the German verb flehmen, to bare the upper teeth), 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 response, animals draw back their lips in a manner that makes them appear to be "grimacing" or "smirking". The action, which is adopted when examining scents left by other animals either of the same species or of prey, helps expose the vomeronasal organ and draws scent molecules back toward it. This behavior allows animals to detect scents, for example the urine of other members of their species, or clues to the presence of prey. Flehming allows animals to determine the presence or absence of estrus, the physiological state of the animal in question, and how long ago it passed by.
The vomeronasal organ, also called Jacobson's organ, is a chemoreceptor organ that plays a role in 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.
Animals exhibiting this behavior 
This behavior is not limited to predators. Horses are well known to exhibit flehmen response. It is recognizable, for example, in stallions smelling the urine of a mare in heat. To detect estrus the male giraffe's flehmen response includes actual taste-testing of the female's urine.
The flehmen response has been observed in almost all ungulate species and some felid species of the Felidae family. It is an animal behavior that utilizes the vomeronasal organ, a part of the accessory olfactory system, for chemical communication.
Purpose of flehmen behavior 
Male individuals commonly use the flehmen response as an olfactory mechanism for identifying the reproductive state of females of the same species based on pheromones in the female's urine or genitals. This has been exhibited in sheep, where flehmen by rams, after sniffing the ewes’ external genital region, occurred most frequently on the day before estrus, when the ewes were sexually receptive.  Females and young also carry out this behavior. In young horses, both colts (males) and fillies (females) exhibit flehmen behavior towards other conspecifics with neither sex performing the behavior more than the other. Young elephants also have a flehmen response to stimulants. The vomeronasal organ of newborn elephants displays a structural maturity similar to adults, which supports the conclusion that flehmen at only six weeks of age is used to deliver chemical pheromones to a functional vomeronasal organ.
This response is not limited to conspecific communication. Goats have been tested for their flehmen response to urine from 20 different species, including several non-mammalian species. This study suggests there is a common element in the urine of all animals, a pheromone, which elicits flehmen behavior. Specifically, chemical pheromone levels of a modified form of androgen, a sex hormone, were associated with the response in goats.
Flehmen behavior also plays a role in reproductive synchrony between females, as studied in the sable antelope. The frequency of flehmen changed seasonally, with the highest levels just prior to conception. Female antelopes associated closely with other females in the same reproductive state. Flehmen rates between females anticipated birth synchrony. Additionally, the level of synchrony was predicted by the frequency of female urine sampling during the previous year. Flehmen is a mechanism used by female sable antelopes to manipulate the timing of both conception and birth of offspring. In the American bison, flehmen behavior in females has also been shown to stimulate the onset of estrus and copulation synchronization.
Chemical cues 
The chemical cue obtained by an animal exhibiting the flehmen response is the presence of a non-volatile organic compound. In contrast to volatile organic compounds (VOCs), non-volatile organic compounds are those carbon compounds that do not participate in atmospheric photochemical reactions or evaporate under normal atmospheric conditions. The vomeronasal organ detects non-VOCs, which must have direct contact with the odor source. Sources of non-VOCs relevant to the flehmen response include pheromones and hormones excreted from the genital regions or urine of animals.
Similar responses 
A grimace similar to the flehmen response may also be seen in association with pain. In horses, it is often associated with low-grade abdominal pain.
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Further reading 
- 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.