Knowing whether an animal is pregnant sounds simple. Truth is, it's anything but.
By Jackson Breyer
Late this past January, it was the giant panda Mei Xiang’s time of year again. She was in estrus. Unusual calls echoed in the panda house, and keepers scrambled for urine samples to monitor her hormones. Tian Tian, our male giant panda, noticed too, and the pair made vigorous attempts at mating. But despite the bears’ efforts, Zoo staff soon realized they would once again need to artificially inseminate Mei Xiang.
Timing was crucial. Pandas ovulate just once a year, and their reproductive window opens for merely a day or two. Fortunately, scientists at the National Zoo are quite accustomed to this reality, and hormonal analyses allowed them to inseminate Mei Xiang when she was most fertile—on January 29 and 30. With luck, their efforts would yield the Zoo’s second panda cub. (The first, Tai Shan, was born in 2005.)
That was the easy part of panda reproduction. Then came the challenge—the waiting. For the next several months, biologists eagerly tracked Mei Xiang’s behavior and hormonal data, administering regular ultrasounds in the hopes of confirming her pregnancy. The signs weren’t clear, but things looked promising. Mei Xiang built a nest. She slept in it. She became grouchy and picky with her food. The more hopeful among us could almost hear the pitter-patter of tiny panda paws.
Week gave way to week; month followed month. No cub appeared. Mei Xiang’s hormones and behaviors returned to normal. On July 21, the Zoo issued a press release. “Researchers have determined,” it said, “that Mei Xiang experienced a pseudopregnancy.”
A Discouraging Word
Pseudopregnancy. That hope-draining word is all too familiar to fans of the Zoo’s giant pandas. It’s a general condition where an animal shows all the signs of pregnancy—except for the baby. The result is a female who seems pregnant but does not give birth. She can develop a large abdomen, begin nesting, and even lactate; there are just no offspring on the way. For that reason it’s also known as false pregnancy.
Pseudopregnancy raises a lot of questions. For starters, why does it happen at all? Being fruitlessly pregnant seems like an inefficient way to spend time. Why is it so hard for biologists to tell a pseudopregnancy from a real one? Even with modern technology, it takes waiting out the gestation to tell if Mei Xiang is actually pregnant. And does this condition occur in other animals, or are giant pandas the only pregnancy pranksters?
To answer the last question first, pseudopregnancy is not unique to giant pandas. It occurs in cats and dogs, among other mammals. Both mice and men (yes, men) have endured false pregnancies. At the Zoo, this baffling phenomenon arose repeatedly in a cheetah named Wandu.
Feline False Alarms
Wandu bred regularly with a male named Norok, yet their couplings yielded no offspring. Since cheetah pseudopregnancies cause the same symptoms as real ones during the first two months, the staff at the Zoo never quite knew if cubs were on the way. They would prepare each time for the impending birth, only to find their efforts
wasted. (The only sure way to figure out if the pregnancy was real would have been to give Wandu an ultrasound or similar procedure. But that requires the patient’s cooperation or anesthesia.)
The news that Wandu might not be pregnant would usually come at the beginning of her third trimester, around the 55th day. The clue lay in her progesterone—“the hormone of pregnancy,” as Janine Brown, head of the Smithsonian Conservation Biology Institute’s endocrinology lab, calls it. If Wandu were pregnant, her progesterone levels would have been elevated at the beginning of her third trimester. They just the opposite. The huge drop in progesterone showed clearly that she was not actually pregnant, despite having shown many signs of being so.
The keepers would then mark her down as having gone through a false pregnancy. It’s interesting to note that cheetah false pregnancies don’t last for the entire gestation, as the giant panda’s does. The times are different because the specific workings of cheetah and giant panda pseudopregnancies are themselves slightly different.
Cheetahs undergo the most common sort of pseudopregnancy, which also afflicts many other species. It’s caused by an odd phenomenon called the non-pregnant luteal phase, and it’s different from what happens in giant pandas.
When many species ovulate, a follicle in the ovary releases an egg. The egg travels away to be fertilized, and the follicle develops into what’s called a corpus luteum (Latin for “yellow body”). This corpus luteum soon begins producing progesterone. That in turn makes the female start to show signs of pregnancy. The longer the progesterone is released, the more pregnant she’ll look.
This can have interesting effects in species which have long luteal phases. Even if the egg is not fertilized, progesterone is released for a long duration, and the animal will develop many signs of an advanced pregnancy.
Things work slightly differently for giant pandas. They go through a phenomenon called delayed implantation. The egg pauses for a while in its development before resuming normal cell division. Because of this phenomenon, the corpus luteum shuts down soon after ovulation. Later on, though, the corpus luteum reactivates and produces progesterone—whether or not the panda has conceived. This can cause a pseudopregnancy.
The changes in Mei Xiang’s progesterone levels follow a fairly predictable pattern as she progresses through a pregnancy, either true or false. This allows the Zoo to roughly estimate when Mei Xiang will give birth if she is actually pregnant. This obligatory pregnancy or pseudopregnancy happens after every annual ovulation.
That Mei Xiang’s hormones aren’t noticeably different between regular and pseudopregnancies presents the Zoo’s biologists with a significant challenge. If they can’t use hormones to determine if she’s pregnant, then they must find an alternative way to run a pregnancy test. Unfortunately there aren’t many options. That raises the question of ultrasound.
Unlike cheetahs and many other animals at the Zoo, Mei Xiang will usually participate in ultrasounds. Yet they haven’t proven especially helpful. Even in 2005, when she gave birth to Tai Shan, Mei Xiang’s ultrasounds never revealed any hint of a cub.
There could be many explanations for this. Panda cubs, at birth, are very small—about the size of a stick of butter. Spotting something that size on an ultrasound would be a challenge to begin with, but doing so in a giant panda is nearly impossible. So while it’s worth conducting the ultrasounds for the off chance of seeing something, they’re hardly the final word. Unfortunately, the only sure pregnancy test for a panda is to wait and see if she gives birth.
This takes a toll on humans’ patience. Almost inevitably, one starts musing: Why would pandas evolve this way? That’s a good question, for which unfortunately we have no answer. Some biologists speculate that pseudopregnancy could be nature’s insurance, guaranteeing that a female’s body is ready if there is actually a cub.
Meantime, Zoo scientists and their colleagues continue to delve into this enigmatic phenomenon, trying to tease out subtle differences between true and false pregnancies. In time, they hope to be able to answer that deceptively simple question: Is she or isn’t she?
—JACKSON BREYER, the 2011 summer intern for Smithsonian Zoogoer, is a student at James Madison University.
If you have a comment about Smithsonian Zoogoer magazine, please email it to us.Smithsonian Zoogoer 40(6) 2011. Copyright 2011 Friends of the National Zoo. All rights reserved.