by Xinhua science writer Yu Zheng     

    BEIJING, June 19 (Xinhua) -- If you have ever wondered why Jerry the animated mouse was also cozying up to tormented feline Tom, Chinese scientists say they have the answer. They hope their finding will someday help in understanding human depression.

    According to the recent finding by a rodent zoologist and his research group, a male mouse is more attractive to potential mates if his scent features even the faintest whiff of his arch enemy --the common house cat.

    Zhang Jianxu, author of two cat and mouse tales that appeared in the respected scientific publication "Chemical Senses" and "The Journal of Ethology", respectively, said a mouse would appear more macho and attractive to the opposite sex if its odor included that of cat's urine.

    In an interview with Xinhua on Thursday, Zhang said he worked with Sun Lixing from Central Washington University and Kevin Bruceand Milos Novotny, both from Indiana University, to research predator-prey interaction between cats and mice.

    Zhang, who lives in Beijing, and the three U.S.-based scientists studied changes in the male mouse urine that had been affected by cat urine. This in turn received a favorable reaction from female mice.

    A scientific judge from "The Journal of Ethology" of Japan called the findings of the research paper "new and interesting."

    "The authors substantiate their results with a chemical analysis showing an increase of the pheromones (chemicals that enable same-species communication) in the urine of experimental mice," he wrote.

    Predator odor has been widely documented as something that scares prey, induces anxiety and stress, and, understandably, lowers the female preference for males.

    Such an academic understanding has been employed by industries to develop repellents that contain certain smells of predators to fend off prey. Due to an extensive knowledge on insect odors, mosquito or bug repellents are manufactured and have largely proved effective. Almost anyone spending lengthy periods of time outdoors is equipped with such repellents.

    Previous studies have shown that chronic exposure to predator odor reduced aggression and disrupted the love lives of rodents.

    In the past, an overwhelming amount of research has been done on the relationship between predator pressure and prey behavior. In contrast, very little information was available on the effect of predator odor-induced stress on pheromonal change and its consequence in social behavior in prey species, said Zhang of the Chinese Academy of Sciences (CAS) Institute of Zoology.

    Mammalian pheromone production is controlled by hormones. This helps stimulate cells' functions, which have a profound and diffusive effect on animal behavior.

    In their studies, Zhang's team kept 48 sex-naive male mice, which were eight weeks old when purchased, for 58 days.

    The scientists exposed the mice to a tiny dose of cat urine in hair-thin tubes. After repeatedly inhaling the cat fumes, the urine of the affected mice was tested and revealed coincident elevations of two male pheromones -- the alpha and beta farnesenes, the exact chemicals that drive female mice crazy.

    With innovative spectrum analysis, the researchers, for the first time, pinpointed the two chemicals that had previously been missed.

    For comparison, they also designed two other groups that were exposed to rabbit urine and water respectively. They discovered that the cat odor-treated males were more likely to initiate an attack than those treated with rabbit urine or water.

    "The levels of the male pheromone compounds which indicate dominance became significantly higher in mice exposed to cat urine," Zhang said.

    "The key to this provocative result was that predator odor might stimulate preputial glands secretion to produce sexually-attractive farnesenes in urine; estrous females spent significantly more time in investigating the urine of those males."

    Although the effects of predators and their odors were reported to suppress rodent behavior, physiology and reproduction, the latest finding suggested potent evidence of sexual attractiveness and dominance.

    A plausible explanation on why the suppressed poor mice were more attractive to females was that they were more motivated to keep fit and be ready to attack, or, more precisely, counter-attack. Stronger males were able to survive high predation pressure.

    An animal olfactory mechanism study has fanned the enthusiastic curiosity among biologists. U.S. biologist Linda Buck won the 2004Nobel Prize in physiology for unraveling the sense of smell.

    Zhang has initiated a collaborative research with psychologists, hoping his newly-found cat-mouse model can be used to extend the understanding of human depression and bring insight to depression treatment.