Estrus Peaks and Valleys

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Bai Yun watches Yun Zi's antics.

In recent years, Bai Yun has displayed behavioral signs of estrus in an unpredictable pattern. The pattern of expression has been very truncated, encompassing a few short days of intense behavior alerting us to her mating readiness. This has been a change from the pattern of her younger years, in which the behavioral trends in her estrus were more lengthy and signaled well in advance that her body was preparing for mating.

When she first arrived at the San Diego Zoo in 1996, Bai Yun was a young adult bear. She had an annual estrus the first three years, until daughter Hua Mei was born. During that time, we would see an increase in scent marking about two weeks prior to ovulation, and an increased restlessness would set in. Why might a female undergo such changes so far out from her breeding window?

Recall that in the wild, pandas maintain overlapping home ranges. In those home areas, the bears are generally solitary as adults. However, they are exposed to the scents of other neighboring bears that have crossed over their path days or weeks before. If a female is a few weeks out from peak receptivity, it makes sense that she would need to begin to advertise her status to any males that might be in the area. She scent marks, and a male who comes across her scent a few days later can recognize the change in her status via that scent mark. Our research in Wolong has confirmed that males are more interested in scent from a female who was known to be in estrus at the time she left the scent. Once he has identified this change in a female’s status, a male will then likely remain closer to this female, assessing her status more frequently and keeping closer tabs on her in order to be present at the time for mating.

A younger Bai Yun has demonstrated that about a week prior to her peak, her rate of bleating begins to climb. This friendly, goat-like vocalization picks up at a time when males in the area are likely to be closer than usual, thanks to her increased scent marking. Within a few days of her peak, she increases her rate of chirping, a sharp, louder vocalization that we can often hear through the doors and windows of our building! Recent research from our collaboration with Zoo Atlanta has revealed some interesting information about these vocalizations and how they relate to male-female interactions: it appears males can use elements in the chirp to identify the precise time when a female is most fertile. Thus, when a male that has been hanging around waiting for his opportunity to mate hears his female chirping, he can assess whether or not she is ready to breed yet.

I have often wondered about Gao Gao’s ability to assess Bai Yun’s readiness for mating. Even with a truncated estrus in the last few years, and limited exposure to her scent, he seems able to pinpoint the time to breed with her. The next day, despite her willingness to breed again, he may often show no interest. Perhaps it is a change in the sound of her chirp that he is assessing and determining that it is not worth his effort to endure another breeding encounter. For a wild male, following a female closely can be an energetically costly endeavor: he risks coming into close contact with other males, and fighting may result. The process of mating itself is laborious and may take up most of a day or two, leaving him physically drained. If a female turns on the male during courtship, he could be injured. And all the effort in assessing and breeding with her detracts from his feeding schedule. Thus, it would seem that making judicious choices about when to push his luck could be advantageous to a male panda.

Why am I discussing this now, when Bai Yun is sure to experience a lull in her estrus cycle due to the fact that she is still nursing Yun Zi? Because, despite Yun Zi, we have an estrus in progress in San Diego: this time, with Su Lin. Unlike her little estrus last year, our young bear (four years old) appears to be experiencing a more adult-like, full-on estrus. She is deep into the scent-marking stage, complete with restlessness. Bleating has begun, and chirping should be on the horizon soon. This is likely her first fully fertile estrus, as females in Wolong have been bred at her age, resulting in a birth a few months later. However, Su Lin will not be breeding here, since she clearly cannot breed with her father Gao Gao. Nonetheless, this is an opportunity for the public to observe a full, lengthy behavioral estrus in a panda female, something we haven’t seen around this facility in many years!

Suzanne Hall is a senior research technician for the San Diego Zoo’s Institute for Conservation Research.

Here’s the most recent chart comparing our panda cubs’ growth during their first 200 days: