There is perhaps no mammal that is less often in the mood for sex than the female giant panda. Each spring, female pandas enter estrous (aka "heat") for only 24 to 72 hours. If male pandas do not make their move during that brief window of time, they miss their chance to mate. Although researchers initially struggled to breed pandas in captivity, they achieved recent success by closely studying the females' reproductive behaviors. But what about the males? It now seems that male pandas have a reproductive cycle of their own, knowledge of which could strengthen the growing captive panda population by improving both breeding strategies and artificial insemination techniques. From the summer through the fall, male pandas are not interested in sex at all. But each winter and early spring, sex hormones flood the males' bodies, plump up their testes and fire up their sperm factories in preparation for a very brief mating season.

From a human perspective, the notion of a male who is only interested in sex for a short period of time each year is laughable. But not all male mammals are ready to go at it wherever, whenever. Many mammals mate exclusively during a breeding season. Some hibernating bears, for example, are sexually active only for a few weeks in the summer, before they hunker down for the winter. White-tailed deer enter a phase called a "rut" in late fall, during which antlers finish growing and males duke it out for the privilege of mating with females in heat. Pandas, which are solitary creatures, have one of the shortest breeding seasons of any mammal. They spend most of their time roaming bamboo forests, eating and marking their territories. Each spring, between February and May, male and female pandas gather and mate. The evolutionary reasons for this reproductive strategy remain unclear, but such brief rendezvous had been sufficient to maintain wild populations for millions of years—until people started destroying the forests where the bears live, reducing their habitat to a few mountain ranges in central China.

For a long time, scientists and conservationists struggled to help pandas reproduce in captivity. But in the past decade, researchers have made huge strides in breeding captive pandas, mostly because they have learned so much more about the female panda's hormonal cycles. Now, a three-year study by Copper Aitken-Palmer of the Smithsonian Conservation Biology Institute and her colleagues—as well as many collaborators in both the U.S. and China—has thoroughly chronicled the male panda's reproductive behavior for the first time. Not only do the findings expand the understanding of basic panda biology, they clarify when researchers should extract sperm from pandas for artificial insemination and when they should let the bears try to breed on their own. The new study appears in the April 4 issue of Biology of Reproduction–Papers in Press.

For three years, Aitken-Palmer and her colleagues studied eight male giant pandas at the Chengdu Research Base of Giant Panda Breeding and the affiliated Chengdu Zoo in China, recording the bears' behavior and how often they called to one another, collecting the bears' scat, measuring the size of their testes, and taking sperm samples.

Every two to three days, the researchers scooped up the pandas' feces, freeze-dried it, crushed it and diluted it in ethanol. Using enzymes and Y-shaped proteins called antibodies, the scientists analyzed the fecal extracts for levels of androgens—steroid hormones, such as testosterone, that promote male sexual behavior. Aitken-Palmer and her colleagues found more than twice the normal level of androgens in the pandas' poop between February and the end of March, just before the time of year when female pandas enter heat. But by the peak breeding season—March 22 to April 15—androgen levels had started to drop back down to baseline. Apparently, the hormones prepare the males for sex, but they are not needed after spring mating.

While the male giant pandas were under anesthesia, Aitken-Palmer and her teammates also measured the size of the animals' testes and induced erections and ejaculation with electrodes—a technique similar to the one doctors use on men who have trouble ejaculating on their own, but want to have children. Under the microscope, the scientists looked for sperm that were misshapen or had difficulty swimming. Between March 22 and April 15, the peak breeding season, the male giant pandas' testes were nearly three times larger than in the nonbreeding season, between June 1 and September 30. Similarly, male pandas ejaculated almost three times as much sperm between March 22 and April 15 than between October 1 and January 31. None of the pandas produced any sperm in the nonbreeding season. The pandas' sperm was also much more plentiful, motile and had fewer defects during the peak breeding season than at other times of the year.

"There is a lot we don't know about the male giant panda," says Aitken-Palmer. "The female panda is much better studied. That was really the impetus for the study. We were able to show that males have a very specific pattern of changes in androgens, reproductive behaviors and sperm quality throughout the year." Aitken-Palmer explains that the male's reproductive cycle largely matches the female's, but does not begin and end quite so abruptly. Because female pandas are only sexually receptive for one to three days a year, it would not make sense for male pandas to squander energy on year-round sperm production. It would also be disadvantageous, however, for the males to evolve too narrow a window of sexual preparedness and miss their opportunity to mate with more than one receptive female.

Aitken-Palmer says her new study suggests that the best time to collect sperm from male pandas is between December and February, when they are increasing their sperm quantity and quality in time for the spring mating season. In contrast, trying collecting sperm from male pandas in the fall or summer would be fruitless, because they are not producing much—if any. Likewise, sperm collection in the spring is not ideal because drowsy bears recovering from anesthesia might lose their opportunity to mate with ovulating females on their own. Because of continuing advances in the understanding of panda health, nutrition and reproductive physiology, Aitken-Palmer says that panda experts estimate that the current captive population is large and diverse enough to survive for at least the next 100 years.