Scent marking is when an animal uses scented secretions, urine, feces or saliva to convey information to other animals. Many mammals do it – think of dogs; rhinoceroses depositing dung in heaps in designated spots to mark their territories; antelopes such as dik-diks rubbing tarry secretions from glands under their eyes onto bushes; ring-tailed lemurs rubbing their wrist or perianal glands on nearby objects, or rubbing their tails between glands on their wrists and then wafting their scents at each other by flicking their tails; etc. But other animals including insects also scent mark; some territorial bumblebees mark their territory, for instance.
Why do they do it? The reasons may be many and are not exclusive. One would be to mark territory; this may be as a warning to other conspecifics to stay out - a kind of property sign -or it may be friendlier – “I was here.” Animals such as cats leave scent marks that others assess; depending on how old the scent is they make decisions in order to avoid encounters. The ethologist Konrad Lorenz though of this as a timetable. Males especially may mark to attract mates or ward off competitors – “I was here and I’m big and strong.” Some mark others of their own species as a sign of dominance – “you are mine and I am the boss.” Scent marking may just be a means of communication, as in communal dung piles. Dogs, for instance, like to mark many different objects on their travels and to sniff the marks left by others – the canine equivalent of newspapers, perhaps, or like leaving a calling card.
Cats also scent mark; this link amusingly discusses some of the reasons why.
Photo: Ringtail lemur scent marking in a zoo; photo by Maky (Alex Dunkel)
Jacobson's Organ, or the vomeronasal organ (VNO), is an auxiliary olfactory sense organ that mainly detects chemical signals such as pheromones. It is found at the base of the nasal cavity and is found in many animals including reptiles and mammals. (Humans do have a VNO but not all the genes are functional.)
Cats, horses and other hoofed mammals use a distinctive movement to direct compounds to the VNO. They lift their heads, wrinkle their noses and curl back their lips (see picture); this is called the Flehmen response.
"Panthera leo Flehmen Zoo Leipzig 2013" by Appaloosa - Own work. Licensed under CC BY-SA 3.0
Snakes flick their tongues constantly to gather odorant particles from the air and then direct them to the VNO.