Is this mouse hurrying to countermark his turf?
Maricopa
County.
Mus domesticus, the common house mouse, may be marking turf in your kitchen at this instant.
Photo by Shana Dodd, EPA.
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.
Fluid signals
It turns out that mouse urine can carry more signals than the U.S. Army Signal Corps. It is used, for example, to mark turf. Jane Hurst, professor of animal science at the University of Liverpool, says, "Males cover their territory with urine, in lots of very small scent marks." If other males have the gall to mark the same territory, the territory's owner will vigorously "countermark" (using, you guessed it, more pee).
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
Now it appears that MUPS play a far larger role in signaling than simply serving as a tank-truck for other chemical signals. Hurst says the wild house mice she studies "there are lots of different MUPs and each male expresses a different pattern of about 10-12 MUPs. Each male produces a different pattern. Why would you have such variation, coded by different genes, if it's just a carrier?"
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?
And since the only difference is MUP, this supports the notion that these proteins play a central role in identification. Their exact job, however, remains uncertain. They could be serving as some sort of complicated carrier for pheromones and MHC fragments. In other words, the proportion of MUPs in the urine recipe might influence which specific pheromones or MHCs they carried. A mouse might carry a high proportion of "mouse-skunk" to "mouse-funk," based on the presence of MUPs that carry these perfumes.
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
BIBLIOGRAPHY
Individual Recognition in Mice Mediated by Major Urinary Protein, Jane L. Hurst, Nature, 6 Dec. 2001, pp. 631-4, see also pp. 590-1.
©2001, University of Wisconsin, Board of Regents.