Why is the birth rate so low for giant pandas?

On September 6, 2006, 9-year-old giant panda Lun Lun gave birth to a cub. According to Zoo Atlanta, which hosts Lun Lun and her mate Yang Yang, this was only the fifth cub born in the United States since 1990. Since four United States zoos host breeding pairs of pandas, this number might seem a little low.

A lot of people view breeding programs as vital to the survival of the giant panda species. However, veterinarians and researchers have not always had much success with breeding giant pandas. In captivity, many male pandas appear uninterested in mating or do not seem to know how. For example, Ling-Ling and Hsing-Hsing, the first pair of pandas to live in the United States, tried unsuccessfully to mate for 10 years. After they eventually learned to mate, they had five cubs, but none survived to adulthood.

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Scientists once assumed that this failure to mate also existed in the wild. However, researchers have learned that several males compete for each female during mating season. The dominant male often mates with a female several times. This process might give younger pandas a chance to learn about mating. It also helps ensure that each fertile female becomes pregnant. Wild giant pandas usually give birth every two years for about 15 years.

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Life in captivity, however, is completely different. Outside of China, zoos generally have at most one or two breeding panda pairs, sodo not compete for females. Males and females usually live in different enclosures until the female is ready to mate. In addition, wild female pandas care for their young for about a year and a half and are not fertile during that time. Zoos used to separate captive cubs from their mothers at the age of six months in the hopes that the mothers would conceive more quickly. Some scientists believe this may cause behavior difficulties, including pandas' reluctance to breed.

Pandas' physiology also makes breeding in captivity a challenge. Female giant pandas are in estrus, or in heat, for between 12 and 25 days each spring. During that period, they are receptive to mating for two to seven days. They're fertile for only 24 to 36 hours. In other words, giant pandas have a very narrow window in which they can conceive, and that window only opens once a year.

In recent years, scientists have become more skilled helping captive pandas conceive. Zookeepers train pandas to submit to veterinary procedures that help them understand pandas' reproductive cycles. Other advances include:

­Even after successful mating or artificial insemination, it can be difficult or impossible to determine whether a panda is pregnant. Veterinarians often do not know if a panda is pregnant until she delivers. Many female pandas undergo pseudopregnancy, in which they show physiological and behavioral signs of pregnancy without actually being pregnant. It can be impossible to distinguish pseudopregnancy from real pregnancy. Often, veterinarians can't even locate a fetus via ultrasound. This is because a panda's fetus is so small and it does not implant itself in the uterus until about 45 days before delivery.