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panda reproduction

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Our Panda Conservation Program

Bai Yun has been a wonderful ambassador for pandas.

Bai Yun has been a wonderful ambassador for pandas.

When Bai Yun arrived at the San Diego Zoo back in November 1996, we all had great expectations for the San Diego Zoo’s panda conservation program. And we knew that these expectations rested squarely on Bai Yun’s beautiful black-and-white shoulders. In the years since, our panda conservation program has grown and has achieved a number of notable successes.

At the center of it all is Bai Yun. Of course, Gao Gao, too, has been extremely important to the success of our breeding program at the San Diego Zoo. Not all male pandas show appropriate breeding behavior, so Gao Gao’s arrival in San Diego in 2003 enabled us to fulfill our goal of studying giant panda reproduction, from breeding to maternal care. However, Bai Yun’s importance to our conservation program goes beyond her successes as a mother, as she has truly exemplified the role of conservation ambassador. Engaging and fascinating the public for the last 18 years, she is the quintessential giant panda, emblematic of the inherent beauty and value of wildlife.

Bai Yun will be 23 years old in September. For those of us who have watched her over the years, we are amazed at her consistent good health, youthful behavior, and appearance. However, this year, her estrous behavior has not been what it has been in the past. Can Bai Yun be heading toward reproductive senescence? Heading into her 23rd year, the answer, most likely, is yes; however, we won’t know for sure until next spring. As of this writing, Bai Yun has not shown more than a minimal level of the behavioral changes that are typically associated with estrus. Back in March, we saw a bit of scent marking and some water walking, behaviors that normally indicate that estrus is coming. However, the expression of these behaviors did not escalate, and soon after they began, they ceased. Since then, Bai Yun has been “quiet.” While estrus can occur into June, the vast majority of breeding, including for our bears here, occurs in March and April,

When Bai Yun gave birth to Xiao Liwu in 2012, it was widely noted that she was the second-oldest giant panda to give birth. While an impressive statistic, that notable milestone provided us with valuable information regarding the finite nature of a female’s biological capacity to produce offspring. Male giant pandas, like other male mammals, can theoretically sire offspring later in life, though for wild pandas, other factors may get in the way of this, including competition with other males for breeding access to females and choosy females that may not be interested.

Bai Yun has given birth to 6 cubs over the past 15 years. While some other females have given birth to 10 or more cubs, the number of litters a female has is typically no more than 6 or 7. For example, between 2004 and 2013, Bai Yun’s first daughter, Hua Mei, has had 10 cubs from 7 litters. While Hua Mei is 8 years younger than Bai Yun, it will be interesting to see whether or not she has more cubs in the coming years. These contrasting mother-daughter patterns are at the heart of one of our research questions: What are the limits of reproductive output in the species?

In some panda breeding facilities, cubs are weaned earlier in order to promote successive annual breeding opportunities. In other facilities, cubs are weaned at about 18 months, mimicking what we believe is the more natural timing of weaning. In these cases, females will only be able to breed every two years. Given this, we might expect to see females that breed every year producing 15 litters over their reproductive lives. However, this does not appear to be the case.

Understanding what governs female reproductive output in giant pandas has implications for both captive breeding and conservation of wild giant pandas, and we are currently analyzing a fairly large volume of data to address this question. Is reproductive output governed exclusively by chronological age? Or is it governed in part by health and vigor? And how does variation in inter-birth-interval (the time between successive pregnancies) influence a female’s lifetime reproductive output? We hope to have some answers to these questions in the coming months.

I have to admit that I never get tired of watching our giant pandas here at the San Diego Zoo. While the excitement of a new cub is undeniable, I know that I will enjoy watching Bai Yun and Gao Gao relax this summer, while young Xiao Liwu explores and plays, enjoying his first summer as a solo panda. Our panda family exemplifying their roles as ambassadors for conservation!

Panda Yun Zi in China.

Update on panda Gao Gao, May 11, 2014: Thank you for all the Gao Gao well wishes! He is doing well post surgery and is enjoying spending time in his back bedrooms. There he is catered to by his keepers 3 to 4 times a day, and he lets them know when he wants back scratches. Gao does have access daily to an off-view exhibit that has a panda camera in it, although he seems to prefer to enjoy the air-conditioned bedrooms, his black sleeping tub, and his keepers’ attention.

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Estrus Peaks and Valleys

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:

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Bai Yun and Gao Gao

Bai Yun April 15, 2009A second breeding occurred this morning, April 15, between the beautiful Bai Yun and Gao Gao the Great at the Giant Panda Research Station. [The first happened last night at 5:16.] The main viewing area was, of course, closed to the public to allow the pandas and researchers the quiet and time they needed, as it has been since yesterday’ s mating bout. Zhen Zhen has been in the alternate “classroom” exhibit for visitors, but the excitement has been all about the adults.

Gao Gao April 15, 2009Initially this morning there was no real interest, but enclosure swapping was implemented and the interest level quickly increased. It took about 38 minutes for Gao and Bai to coordinate their positions, but the actual copulation lasted nearly 2 minutes. It only requires one mating, so this morning’s interaction just increases the likelihood of a future pregnancy. Whether there will be another mating today will depend on the pandas, but the amount of energy already expended by Gao Gao may be the determining factor.

Now, to anticipate some of your questions:

Panda gestation is about 45 days, but they can delay implantation for up to 6 months or longer. We don’t understand how this works, but it has the advantage of having cubs born later in the year when food is more plentiful for the mother. This delay is common to other bears, as well as rabbits and other species.

Determining panda pregnancy is challenging, since females can go into pseudo pregnancies that behaviorally and hormonally mimic a real pregnancy. Ultrasound and thermal imaging are employed, but it can still be difficult to locate a tiny fetus in the body of a 200+ pound bear with a gut full of bamboo.

Once mating season is concluded, the pandas may once again be rotated on and off exhibit, but the timing of this is determined by the animal care staff and research needs – and, of course, whether we are on birth watch.

Keep your fingers crossed for our wonderful mom as we begin the careful watching- and-waiting phase of the panda breeding process.

Ellie Rosenbaum is a panda narrator at the San Diego Zoo.

View video

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Moving Forward

Note from Public Relations: Guest access to the San Diego Zoo’s Giant Panda Research Station is expected to be interrupted over the next few days as female Bai Yun and male Gao Gao are introduced for breeding. Giant panda adults are solitary, normally coming together for a few days each year for breeding. Animal care staff have been closely monitoring Bai Yun and Gao Gao, observing behaviors, vocalizations and physical signs that indicate that the female will be receptive to the male. On the afternoon of April 14, the viewing area was closed as breeding behavior escalated and possible introduction of the two adults was considered. In order to ensure that there are no distractions that might interrupt the interaction of the rare pair, the panda viewing area has been closed to guests.

Zoo officials expect that guest viewing at the Station will be interrupted over the next few days. Viewing of Panda Cam may also be interrupted as staff focus their efforts on monitoring the interaction directly and are not available to manage Panda Cam views for online guests.

Things are progressing here at the Giant Panda Research Station at the San Diego Zoo in many ways. As of April 12, Bai Yun and Gao Gao swapped exhibits. This involves releasing them into one anothers’ enclosures for olfactory exploration and scent marking. As Bai Yun is showing increasing signs of readiness for mating, the door to the introduction area, or “howdy door,” was opened between the exhibits April 13, and Gao lost no time in taking up his usual spot in the tunnel-way inself, not visible to the guests, usually, but straight line-of-sight on Bai, the most important thing. A knowing guest described him as “keeping vigil,” an excellent description of his tendency to plant himself in there for naps, scent-marking, vocalizing, even eating, so as not to give up his place of priority near Bai Yun. We’ve seen this in previous years; what was pretty amazing to me was the speed with which he entered the area and his memory of this from past mating seasons.

As always, actual mating will take place when Bai Yun is ready, a very short, perhaps one-day, window now that she’s getting older and her hormones are more erratic (“pandapause”?). It could be as soon as today, given her past history, or any time thereafter. The area will be closed, of course, and the classroom area will be open for panda viewing, but whether guests will be seeing Su Lin or ZZ will depend on the keepers and animal care managers. Efforts will be made to have our Panda Cam at the ready should mating occur, so don’t be surprised at anything or anyone you see over the next few days on our Web site. Should breeding occur, pandas will be rotated on and off exhibit again as we play the next phase of the “waiting game” that accompanies panda breeding season: is she or isn’t she?

Bamboo coolerThe bamboo cooler has taken shape, and is nearly completed. It should be finished over the next few days; racks to hold the bamboo are being installed along with the drainage, the adjacent concrete pads for prepping, and even a litttle shaded area for short-term storage. It will make the job of keeping the bears well fed a lot easier for our dedicated keepers.

“Bear” with us over these next exciting days and weeks. We’ll give you as much info as we can, as often as we can.

Ellie Rosenbaum is a panda narrator at the San Diego Zoo.

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The 1-2-3s of Panda Breeding

Bai Yun’s estrus is progressing, with a mixture of the dampened behavior we saw in 2007 and the heightened behavior we have seen in previous years. Her rate of scent marking was high over the weekend, but today, April 13, on exhibit we haven’t seen anything of the sort. She is probably past the time when scent marking begins to drop off, and we would expect her vocalizations to kick in soon. As of this morning, however, very little bleating and no chirping has been heard.

Behavioral indicators aren’t the only measures of her estrus we are tracking. Some of her physical traits also show us that her hormones are very actively preparing her for breeding.

Today our howdy gate was opened for the first time, and Gao Gao checked it out several times in the first few hours. At first, his visits were very brief and included a short “hello” bleat. Bai Yun noticed him within a few minutes, stared at him a moment, then went about her business for the rest of the observation period. Gao’s visits became more regular, and before long he was scent marking at the gate and standing there, admiring Bai Yun from a distance. Always the gentleman, he did not make a nuisance of himself and gave Bai Yun plenty of space. When the time comes, he will demonstrate the persistence he needs to be a successful breeder.

Part of Gao’s success may be in his relaxed attitude prior to her peak of estrus: he is not bothering Bai Yun now, when she isn’t yet ready to mate. We manage our pandas in such a way that adults are not housed together except during breeding episodes. We believe that this mimics the natural situation as closely as possible. Gao Gao’s caution and reluctance to impose himself on Bai Yun may be heightened because, most of the year, he has no active relationship with her.

Our data suggests Bai Yun will peak later this week. When this occurs, we will be closing down the queue where she and Gao Gao reside. Guests to the San Diego Zoo can expect to see one of our younger pandas in the alternate “classroom” viewing area. Once her peak has passed and Gao Gao indicates that no further efforts on his part are necessary, we will reopen the main viewing areas to our guests. Bai Yun’s reproductive behavior may continue to be evident for several days after the fact. Stay tuned: it’s going to be a fun week!

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

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All Eyes On…Gao Gao

Gao GaoWell, well. We may be closing in on an exciting time at the Giant Panda Research Center. Over the weekend, keepers noted some changes with Bai Yun that signal to us that her hormones are kicking in.

The female panda responds to the seasonal flush of hormones that lead to mating in many ways. Early behavioral changes may include increased scent marking, decreased appetite, and increased restlessness and motoring about. Since males and females do not typically hang out in close proximity, the effect of these changes is to draw the male in to her at the time of her estrus.

Her scent, laid down with lots of marks left behind during her wandering about, catches the attention of males who may stumble across her path and then know to look for a female in estrus. Later behavioral changes, like increases in bleating and chirping vocalizations, call to any males who have begun to zero in on her, so that she can be found among the dense bamboo cover.

Physiological changes usually include an enlargement in her vulva and a change in color of these tissues, becoming redder and fuller as she approaches her peak. At mating time, these changes even include a dilation, or opening, of this part of her reproductive organ. What is usually a gray, bumpy patch of skin under her tail becomes a bright red, enlarged visual signal for the male, exuding scent. When touched on the back or hips, a female in peak estrus has a seemingly involuntary tail-up response, which serves to expose the reddened area. This clearly facilitates the mating process and helps the male to get the job done.

Gao Gao, for his part, appears to have noticed some changes in Bai Yun. Staff has reported he is showing an increased interest in Bai Yun’s scent in the tunnels. This is very significant to us. For all of our experience and education and understanding of the breeding process with pandas in general and Bai Yun in particular, no one knows better than Gao Gao how to ensure a successful mating with our panda matriarch.

You will be seeing some new things on Panda Cam in the coming days as we respond to our bears’ changes. The Howdy Gate will open soon, allowing the two bears to meet across a barrier and reacquaint themselves. It also allows us to assess their interactions and read Gao Gao’s responses. We will also begin pen swapping, moving Bai Yun into Gao’s pen and vice versa. This offers our male better exposure to our female’s scent. Again, we can watch his response to guide our understanding of where “White Cloud” is in the process.

In 2007, Bai Yun rocketed from mild signs of estrus to mating activity overnight. Literally. We are ready to be on our toes this time. Who knows how this will happen, and there is still the possibility it won’t happen, if Bai Yun never reaches a peak that our boy is sufficiently motivated by. Keep your eyes on Gao Gao: he’ll let you know.

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

Addendum (posted April 8): To clarify, I indicated the staff will be making some changes to the panda protocol over the next few days, but I think it would be premature to say we think breeding is going to occur in the next few days. We anticipate we are probably a few weeks away from an actual breeding at this time.