POSTED 6 DEC 2001
| Identity. It's as central to a rodent as it is to Lt. Col. Frank Slade, the blind, bitter ex-soldier played by Al Pacino in Scent of a Woman. Pacino's identity -- a swaggering, gritty fighter with a heart of lead and a first-name relationship with Jack Daniels -- had enough charisma to light up the screen -- not to mention the hearts of the young and restless characters he scraped up against.
Identity is also central to the lowly guy who happens to be a house mouse, Mus domesticus. While Slade marks his turf with a swagger and a pathetic, howling "Hoohah!", these territorial rodents use urine.
The point, naturally, is reproduction. "Females judge males for how effectively they can defend their territory," says Hurst. "The more you countermark, the more fit you are." But if Mouse A and Mouse B are genetically identical, and B marks A's territory, A will not fret about the intrusion because the intruder's marks smell just like his own.
One group of chemicals used in this individual recognition system is the MHC (major histocompatibility complex, part of the immune system).
If this all sounds, well, lousy-mousy, remember that people also seem to use MHC for mating.
Mouse urine also carries pheromones, trace chemicals that can cause females to become sexually receptive. Sensing a mature male's pheromones can also cause abortion in a pregnant female.
As if that's not complicated enough, Hurst has fingered a new group of proteins for a major role in the yellow chat-room of urinary communication. These so-called major urinary proteins, or MUPs, are much more common than MHC fragments or pheromones. The research appears in tomorrow's (Dec. 6, 2001) issue of Nature.
It's already known that MUPs carry pheromones and release them slowly, guaranteeing that urine can mark turf for long periods. MUPs are also, intriguingly, the cause of most human allergies to rodents.
Not a simple vessel
To test the notion, Hurst and colleagues trapped a bunch of wild house mice, then compared the behavior of brothers with the identical or different MUPs.
The test depended on territorial behavior. Mice that are genetically different easily distinguish each other, she says. "Normally, a male mouse defends it territory. If you introduce the urine of another male, the territory owner gets very excited and really increases the marking rate."
Otherwise, she adds, a female would "think it's wimpy and will not mate."
Hurst knew that urine from an unrelated mouse would cause an uproar. But she found that even a brother's urine would cause a fight -- as long as it carried a different stew of MUPs.
So far, so good. But here's the corker. Urine from a brother with the same MUP type, she says, "causes no response at all. They hardly investigate it and don't bother to respond."
In fact, the mice are so blase that they might as well be responding to their own urine, she adds. "The only interpretation we can put on it is that they don't recognize it as different urine. We've shown that MUPs are deeply involved in individual recognition."
How do they do the things they do?
It's also possible that mice recognize MUPs directly; that MUPs are not just perfume bottles, but also carry information about the contents. At this point, the question remains open, Hurst says. "Do mice detect the proteins themselves, or are they detecting the volatiles held by the MUPs?"
Why should we care? For one thing, animals use scents in skin and sexual secretions, urine and saliva to indicate gender, sexual maturity and receptivity, social ranking and identity.
For another, scent is not just a rodent thing; it appears among elephants, gazelles and antelopes, most predators, foxes, wolves, dogs and lower primates.
And that's the scent of a story that might even interest Lt. Col. Frank Slade.
-- David Tenenbaum
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