by Matthew Cobb
Sex pheromones are widely used by mammals to communicate and detect the sexual status of a potential mate. This is particularly the case with female mammals, whose pheromones are primarily detected by a structure known as the vomeronasal organ (VNO), which is in the base of the nose/roof of the mouth. (And no, humans don’t have a functional VNO, although it does appear briefly during embryogenesis).
There are two kinds of smell receptor molecules in the VNO – V1Rs look pretty much like an ordinary smell receptor, and the neurons that house them send their axons into the part of the brain that deals with food and so on. But the other kind of receptor – V2Rs – look very different and project to a different part of the brain. The assumption is that key parts of mammalian pheromones are detected by the V2Rs, but that pheromones often contain a blend of compounds, some of which may be detected by V1Rs and by a specialised receptors called TAARs in the main part of the nose.
The really interesting thing is quite how far back these receptors go. The recent sequencing of the Platypus genome showed that there were V1R and V2R genes, strongly suggesting that this form of communication goes back at least 165 MY:
So what does a male mammal do when he detects a pheromone? Anyone with a horse – or a cat! – will know. He produces what is known as “Flehmen”, a characteristic curling back of the lip, with the mouth held open. Cats do this when they smell the urine sprayed by a male, and get a faraway, stoned look in the eyes while they’re about it. Here’s a picture of a tapir showing Flehmen:
So do marsurpials show Flehmen? You betcha! Here’s a video of a male kangaroo testing the reproductive status of a female, by tasting her urine. Note the “flehmen” response he makes with his mouth, just like a placental mammal. Note the way he shakes his head afterwards… Who can blame him? DON’T TRY THIS AT HOME!