The flehmen [fley-muhn] response is a common animal behavior when investigating sites of particular interest (e.g. a male smelling female urine) characterized by curling back the top lips exposing the front teeth and gums, then inhaling and holding the posture for several seconds. The behavior may be performed over particular locations, in which case the animal may also lick the site of interest, or it may be performed with the neck stretched and head held high in the air.
Flehmen (German: ‘to bare the upper teeth’) is performed by a wide range of mammals including ungulates (hoofed animals) and felids (cats). The behavior facilitates the transfer of pheromones and other scents into the vomeronasal organ (pheromone detector) located above the roof of the mouth via a duct which exits just behind the front teeth of the animal.
The main function of flehmen is intraspecific communication. 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 is exhibited in the reproductive behavior of sheep. The ram often exhibits flehmen after sniffing the ewes’ external genital region, but this occurs most frequently on the day before estrus (i.e. ‘in heat’) when the ewes are sexually receptive.
Flehmen behavior also plays a role in reproductive synchrony between females. Female sable antelopes use it to manipulate the timing of both conception and birth of offspring. The frequency of flehmen changes seasonally, with the highest levels just prior to conception. Female antelopes associate 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. In the American bison, flehmen behavior in females has also been shown to stimulate the onset of estrus and copulation synchronization.
Mares commonly show a peak in flehmen response during the first few hours after giving birth. Smelling the newborn foal and the amniotic fluids associated with birth often produce the reaction. 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. However, it has been reported that young colts flehmen up to five times more frequently than fillies, and fillies flehmen more frequently than mature mares. 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 pheromones to a functional chemosensory system.
The flehmen response is not limited to intraspecific 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.
The Daily Omnivore 
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