primates . A functional VNO is present in many other mammals , along with amphibians and all snakes and lizards . Despite claims to the contrary , it is widely accepted that adult humans do not possess a functioning VNO , and in fact there is evidence to suggest that our olfactory capabilities are on the wane .
In order to enhance the access of inhaled scent particles to the VNO some mammals , including species of cats , horses , sheep , cows , goats , deer and pigs , will raise their heads , curl their lips and wrinkle their noses ( thereby closing the nostrils ) in a behaviour known as the Flehmen response . Similarly , salamanders apparently rely on a bizarre form of ‘ nose tapping ’ to stimulate their VNO , while snakes and lizards employ the familiar ‘ tongue-flicking ’.
Studies have connected the VNO to pheromone recognition in relation to reproductive behaviour amongst rats , mice , hamsters , Guinea Pigs and pigs , and also territorial aggression . The link is strong enough for vomeronasal receptors to be sometimes referred to as ‘ pheromone receptors ’. Often urine or vaginal discharges will trigger a response from the VNO .
To return to the world of reptiles , garter snakes have also been demonstrated to use their VNO to detect pheromones , as well as locate prey items . Although snakes and lizards have a functional
Left : horses are amongst many mammals to exhibit the Flehmen response . Image by Sari O ’ Neal . Below left : Salamanders rely on a bizarre form of ‘ nose tapping ’ to stimulate their VNO . Image by Federico Crovetto . Above : The VNO ( Jacobson ’ s organ ) of squamates does not open to the nasal cavity . The tongue is therefore responsible for collecting scent molecules from the external environment , which are then transferred to the VNO via two small openings in the palate . Diagram by Rachael Hammond .
Smelly Facts !
Some turtles use their VNO to smell underwater .
Elephants transfer scent particles to their VNO via the prehensile ‘ finger ’ at the end of their trunks .
The area of conventional receptor cells in the nasal cavity is indicative of an animal ’ s sense of smell . Dogs may have around 170cm 2 of olfactory epithelium , compared to a human ’ s mere 5-10cm 2 .
In the same way that other mammals may utilise the Flehmen response to heighten the effectiveness of the VNO , dogs will sniff rapidly or run into the wind with their heads high while on the trail of a scent . Both strategies are designed to optimise olfactory performance by increasing airflow over the epithelium .