GENEVA - The female mice are chasing the males around the cage; they raise their behinds in order to sniff their genitals, some even biting the males' private parts. It is a strange spectacle, monitored by researchers from the University of Geneva (Unige).

The phenomenon is described in an article in the latest issue of the scientific review Current Biology. This behavior, a type of rodent sexual harassment, has never been seen before in mice. A plausible cause would, of course, be genetic engineering.

It all started with an ugly surprise one morning, when scientists from the Department of Genetics and Evolution at Unige discovered some male mice in the laboratory with mutilated genital organs.

"It was shocking, as we have never seen these types of injuries in our mice before, and we couldn't figure out what had caused them," a geneticist, Jozsef Zakany, says. The injuries continued and would often happen overnight, when mice are most active. Researchers therefore decided to install video cameras in the cages in order to film the rodents' nocturnal behavior.

The video footage revealed various surprises. Firstly, they discovered that the female mice were actively participating in mating rituals. The films also revealed some explicit scenes. The females, when they were in a sexually receptive phase, would seek out the males and sniff their intimate parts to initiate mating. Even though this can appear in other species of animals, scientists had never seen female mice initiating mating.

The videos also unmasked the males' attackers: a handful of female mice that could be described as nymphomaniacs. Like normal mice, the females approached the males, but in such an aggressive manner that they injured them. "In some of the videos, it really looked as if they had lost control of their impulses," says Zakany.

Of mice and men (and women)

Zakany says that there was no doubt about the sexual character of these attacks, especially as the females never attack members of their own sex. The males show a surprising amount of tolerance. They are not at all aggressive toward the females, even when the females bite them.

This unusual sexual behavior among female mice could be caused by genetic modification. Researchers from Unige, specialists in the study of "genetic architecture," had suppressed one strand in the genetic structuring of the mice. This strand is situated in an area named "HoxD." Their goal was to study the consequences of this deletion in animal development. However, they never expected to unleash a breed of sexually hyperactive female mice.

"One of the most surprising finds in the study is that only the females seem to be impacted by the genetic modification," says Pierre Roubertoux, a specialist in behavioral genetics at the University of Marseille.

Researchers have not, in fact, observed any change in the mating ritual of the males among this mutated breed. "The effect of this deletion is, therefore, most probably influenced by the mouse's hormonal condition," the French geneticist suggests.

Nevertheless, there is a high level of variability among the individual mice that have been genetically modified, and those conducting the study have concluded that the behavior of these mutant mice is significantly different from that of average mice.

How genetic modification influenced the mice's behavior is yet to be explained. Geneticists have observed that a strand of genes named "HoxD," adjoining the area that they suppressed, reacted abnormally in the brain of mutant mice. This gene should normally act as a protein provider, which could be the cause of these abnormalities.

"However, the team in Geneva were not able to identify this protein, which meant they had to stop their research," says Pierre Roubertoux.

In human beings, similar genetic modifications can appear as a result of natural causes. No sexual abnormality was, however, observed in patients who experience this deletion. "However, maybe these people were simply not asked about their sexual lives," Zakany suggests.

Roubertoux acknowledges that "the same genetic modification can also have different effects on mice and humans." Some human illnesses can also produce wild and compulsive sexual behavior. However, it is normally linked to complex physical problems, such as brain damage or some forms of epilepsy, and not to genetic engineering.