HOW MICE CHOOSE PARTNERS BY THEIR SMELL
When mice choose a mate, they like to smell them first. That's because the smell tells them a lot about the genes that their potential partner might pass onto their children. And mice want a good match. For a rough example, if one mouse is immune to diseases A and B, it would want to mate with a mouse which is immune to diseases C and D. Then their baby mouse might come out immune to A, B, C, and D, and wouldn't get sick as much as mum and dad.
The mouse's immune system has a spy. He's called the Major Histocompatibility Complex, aka MHC, and like the best of informants, he figures out the safety threat based on the very littlest of information. When we say littlest, we mean, literally, littlest – MHC tracks down tiny, tiny molecules called peptides. Peptides are produced when proteins in the body's cells break down. MHC catches those peptides and carries them up to the surface. And when those peptides are brought to light, the immune system can recognise if the proteins they come from are good for the mouse's body – or if they are intruders, viruses, which will make the mouse sick.
But just like no two mice have the same nose or eyes or make the same squeak, and every spy lurks around a different part of the world, every mouse's MHC informs about slightly different peptides. Mice want to find other mice whose MHC finds peptides that their MHC doesn't. When they find that mouse, well, they're happy – it's time to mate.
Mouse Nose, Nase der Maus Quelle:Max-Planck Gesellschaft
That's where the nose comes in. Inside the nose is an organ like you see in the picture on the right. It's called the vomeronasal organ, and scientists at the Max Planck Institute for Immunobiology in Freiburg, Germany have discovered that inside that part of the mouse's nose, the nerve cells are arranged to go on alert when a particular kind of MHC comes wafting through the air. The green part of the picture is the peptide sensitive part. And click on the link below so Dr. Thomas Boehm can tell you that it's not just mice that have this organ – so do other animals, and so, even, do humans – even though, in humans, unlike mice, nobody is exactly sure why it's there.