Sperm may follow rising temperature to egg. (Heat-seeking missiles).

As a blast of Arctic air chills much of the United States this winter, many people travel long distances seeking warmth. Much like sperm, apparently.

A new study suggests that rabbit sperm find their way toward an unfertilized egg by heading toward higher temperatures within the animal's fallopian tubes. The egg rests at a spot slightly warmer than the site where sperm begin the final leg of their journey, and sperm can sense that temperature difference, report Michael Eisenbach of the Weizmann Institute of Science in Rehovot, Israel, and his colleagues in the February Nature Medicine.

About a decade ago, Eisenbach's team found that mammalian sperm cells can move toward chemicals secreted by unfertilized eggs, a phenomenon called chemotaxis. That led the investigators to argue that mammalian eggs lure sperm by using attractants. The finding challenged the dogma that the egg is a passive partner in the fertilization process.

Yet Eisenbach wasn't convinced that sperm chemotaxis could occur over the full 2 to 3 centimeters of the mammalian fallopian tube. The tube's natural pulsing prevents a gradient of the attractant from being stable except near the egg, he says.

Recently, Eisenbach read about experiments performed more than a decade ago that showed temperatures varying within female mammals' genital tracts. In one example, English reproductive biologist Ronald Hunter had found that during ovulation, the isthmus--a site in the fallopian tube where sperm rest and mature before moving on--is slightly cooler than the site where fertilization occurs. Hunter had even raised the notion that sperm perceive this temperature difference.

Intrigued, Eisenbach and his colleagues designed a chamber mimicking a fallopian tube. It contains two liquid-filled wells, one held at 39[degrees]C--about the body temperature of rabbits--and one at 37[degrees]C. After videotaping the swimming of rabbit sperm placed in this chamber, Eisenbach's team concluded that a small share of rabbit sperm, 7 to 17 percent, exhibit a clear preference for moving toward the warmer well, a process called thermotaxis.

The researchers obtained similar results when the wells had only a half-degree temperature difference between them. They also found evidence of thermotaxis with human sperm. Eisenbach and his team determined that the isthmus of live rabbits is nearly 2[degrees]C cooler than the fertilization site--a result similar to Hunter's.

"The evidence for thermotaxis is, in my opinion, as good as the evidence for chemotaxis," says Eisenbach. His group's experiments indicate that only the small percentage of sperm that are fully mature can sense the temperature gradient and chemical cues needed to reach an egg.

While finding the new work "intriguing," sperm researcher Susan Suarez of Cornell University calls for more studies. "I would stop short of concluding that the data provide strong proof for the existence of thermotaxis in rabbit sperm," she says. "The problem with [the] studies is that so few sperm appear to respond."

Assuming that human sperm navigate by temperature, Eisenbach envisions physicians' using a thermotaxis assay to evaluate the fitness of a man's sperm. An unusually large number of sperm unresponsive to temperature differences might explain some cases of male infertility.