For years the story on rat sex has been this: the male seeks above all else to ejaculate quickly, and once he has done it with one female, he is eager to move on to new partners. The female, meanwhile, seeks to extend the sex encounter through "pacing." A new study finds that if pacing is slow enough, the male will prefer that familiar partner to someone new. The wait, it seems, makes the female more attractive.

"It's an awful lot like what we were taught in high school," says Concordia University psychologist James Pfaus, who co-authored the study with Nafissa Ismail, the graduate student who conceived it.

The experiment made innovative use of standard research devices called pacing chambers, which are cages with dividers having either one or four holes big enough to let a female rat through but too small for the larger male. Thus, the female can join or leave the male, allowing her to significantly lengthen her arousal and, studies have shown, her chance of pregnancy. But the mating rituals last longer in the one-hole chambers, because the male, eager to get at the female, often sticks his big head in the hole, blocking her only passage back to his side and delaying her return.

The researchers let 20 couples mate in one-hole chambers and 20 in four-hole chambers. Then they placed each couple, along with a novel female, in a larger, open area. Among males from four-hole chambers, about half preferred their familiar mates. Among males who mated more slowly in the one-hole chambers, 80 percent preferred the familiar partner.

Driving this behavioral dynamic is, as always with rat sex, some neurochemical reward. Boston University biologist Mary Erskine notes that "sexual preferences come from chemical rewards, and we can be sure there are some here." Sexual climax, in fact, unleashes a flood of pleasure-producing hormones and neurotransmitters, such as testosterone and dopamine. Pfaus speculates that the higher level of arousal created by the longer wait generates a stronger release, and a more substantial reward, thereby enforcing the preference.

"Whether it's simply a stronger dose of the usual chemical rewards or some in addition, we don't know," Pfaus says. "But something is making this sort of mating more rewarding to the male or rewarding in a different way."

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