The Panda House at the National Zoo will be off-limits to the public for the next three days, as zookeepers attempt to coax the giant pandas Tian Tian and Mei Xiang into sexual activity. Keepers hope “to create expectation between the two“—by separating the animals until hormone tests reveal that Mei Xiang has reached peak fertility. Why is it so hard to get animals in the mood?
We don’t really know what turns them on. Breeding some critters is easy, and zookeepers work to make sure they don’t reproduce too often. The endangered giant pandas happen to be quite finicky: Even a female in heat rarely elicits a response from a captive male panda. The reason for this remains unclear, but studies have shown that giant pandas breed most successfully when they’ve had direct physical contact with keepers, as well as access to climbable trees and private areas away from public scrutiny.
For other animals, missing social cues can cause problems. For many years, zookeepers had trouble breeding the white rhinoceros. Though they were often exhibited in male-female pairs, the animals rarely reproduced. In the wild, the white rhino lives in small herds; it turns out that a male needs to interact with a number of females in order to be properly aroused. Much of the difficulty breeding white rhinos disappeared as zoos began to keep them in larger groups.
A number of other factors can contribute to problems with captive breeding. Keepers might clean up waste too quickly and remove an important odor that signals fertility. The social tensions particular to zoo life can distract males from reproducing—a male guenon in a dysfunctional family group, for example, can become so preoccupied with aggressive behavior that he ignores the females. Aggression might even be directed out of the animal’s enclosure and toward animals of a different species in a nearby cage.
When individual animals seem unable to reproduce, keepers can call in physiologists to diagnose possible biological problems. The reproductive tract of an animal (especially among hoofstock) might begin to break down if she hasn’t bred regularly once reaching sexual maturity. Or cysts in the reproductive tract might make pregnancy impossible. The physiologist will also test semen samples for volume and concentration. (Cheetah semen is notorious for its poor quality.) Sometimes, zookeepers try to identify potential problems at the outset: When the zoo needs to bring in female elephants from the wild, for example, they will tranquilize more animals than they need and then let scientists perform ultrasound exams to determine which are most likely to reproduce.
But even an animal that seems perfectly healthy might not reproduce. Some individuals are just better at breeding than others, and zookeepers still haven’t figured out why. At the Chengdu Research Base of Giant Panda Breeding, a panda named Mei Mei (and nicknamed “the Heroine Mother”) gave birth to 10 cubs in her 21-year life.
Explainer thanks Barbara Durrant and Larry Killmar of the San Diego Zoo.