Pandas are descended from predators. So why do they eat almost exclusively
vegetarian food? Along with their black and white fur, lazy behavior, and cute antics, pandas have another exceptional feature. And that is eating bamboo, which is currently an endangered species for the great panda or the so-called bamboo bear (Ailuropoda melanoleuca).
One panda can eat anywhere from 12 to 38 kilograms of bamboo per day and spend up to 16 hours each day eating this fibrous, chewy plant time after time.
This consumption is so intense that Feng Li, a researcher from China's Western Normal University, calls the panda a "mobile bamboo eater." But do panda bears expand their diet and eat anything other than this fast-growing member of the flora? As a rule, the answer is no - although there are rare exceptions.
The panda's menu
"There have been cases where wild large pandas have sometimes eaten small animals," Li reported in an email to Live Science. Li cited rare cases recorded in China's Qinling mountainous region, where pandas were found gnawing on the bones of takin (Budorcas taxicolor), a wild pair-hoofed mammal in the polorhynchus family. In addition, they have sometimes been found hunting Chinese bamboo rats (Rhizomys sinensis).
John Speakman, head of the Department of Zoology at the University of Aberdeen (UK), confirms that these cases are opportunistic and unusual: "On the Internet there is a picture of a panda in a zoo that caught and ate a peacock. But as far as I know, these are really random events".
Barring such rare instances of carnivory, pandas stick exclusively to bamboo in their diet. However, there is an interesting twist to this story, as there is plenty of evidence to suggest that pandas should actually have a much more varied diet and way of eating. In fact, they are not particularly adapted to eating bamboo.
Studies have shown that pandas' gut structure is similar to carnivores like their "cousins" bears, and that their stomachs lack the multi-chambered structure that other herbivores use to process large quantities of plants.
Moreover, the microbes that inhabit their gut are more similar to those of meat-eaters and are able to process and extract protein. These microbes are not suitable for breaking down plants. This is also evidenced by panda feces: researchers found pieces of undigested bamboo in them.
Evolutionary takeoff
These maladaptive traits appear to be evolutionary atavisms left over from the past ancestors of the great panda. The fossil record suggests that the ancestors of modern giant pandas fed not only on meat but also on vegetation.
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Then, about 7 million years ago, the fossil record suggests that one ancestor in particular, Ailurarctos (Ailurarctos), an extinct panda genus from late Miocene China, began to try bamboo.
According to Lee, this was "the final point of the transition from omnivory to herbivory," which may have been accelerated by changes in food availability. "The popular current view is that ecological changes in ancient times led to a dramatic reduction in food resources. The ancestors of giant pandas competed fiercely with other predators," says Li.
According to the researchers, having already started supplementing its diet with bamboo, Ailurarctos had an adaptive advantage and was able to survive in conditions of scarcity by occupying this ecological niche. "The evolutionary process of transforming giant pandas from omnivores to bamboo-exclusive is a classic case of ''survival of the fittest but not necessarily the best' in evolutionary biology," Lee said.
However, while it may seem that their evolutionary structure is ill-suited to their exclusively bamboo diet, giant pandas have also evolved several adaptations to suit the situation.
For example, a gene in their DNA that is supposed to allow them to experience the unique umami-like flavor of meat has been inactivated, which means they likely don't have a natural appetite for fresh meat, Lee says. Pandas also have a very slow metabolism, which helps them survive on a low-calorie diet of bamboo. In fact, pandas expend only 38% of their daily energy compared to other land mammals of similar size.
Specific mutations
Speakman was involved in a study that revealed a possible reason for this low metabolic rate: a gene mutation unique to pandas that controls thyroid hormones, which play a key role in regulating metabolism.
"We all expected pandas' metabolic rate to be low because otherwise they couldn't eat enough food to match their energy expenditure. The big discovery was the discovery of the genes that underlie this," Speakman tells Live Science. "We have since created mice with the same mutation and confirmed that these mice also have a reduced metabolic rate."
Photo: livescience.com
Meanwhile, Lee led a study that found that bamboo itself may even play a key role in helping pandas adapt to their unusual diet. When Lee examined blood samples taken from panda bears, he found molecules called microRNAs, or miRNAs, in them.
MicroRNAs are molecules found in animals and plants that are used by cells to control gene expression and are also found in bamboo, from where they are ingested by pandas and can accumulate in tissues over time.
When Lee and his colleagues studied some of the microRNAs found in panda blood, they found that those targeted certain genes that are involved in increasing appetite, regulating the sense of smell and suppressing bitter taste, among other things. So it's possible that these molecules derived from bamboo helped develop and curb the pandas' appetite for the plant.
This could be very important for their survival, "ensuring the daily consumption of enough bamboo to meet the body's energy needs," Lee explained.
Studying the body of pandas
Other studies have found that pandas' gut bacteria are not entirely inconsistent with their vegetarian diet. In fact, their gut flora changes during the season when bamboo shoots - a more nutritious part of the plant than the leaves - appear, allowing pandas to store more energy during this short period when they noticeably gain weight.
Pandas have also developed a "pseudofinger" that allows them to grip bamboo stalks firmly. Their molars are wider than those of other bears, which is essential for crushing tough bamboo stalks, as are pandas' uniquely powerful jaws. "The reason their large, round faces are the way they are is because they have massive jaw muscles that provide greater bite force than polar bears," says Speakman.
Pandas are adored but equally derided as ill-adapted, evolutionarily "dead-end animals." But while it may seem like they just lie around and eat bamboo all day long, according to Lee, "'laziness' of pandas is the wisdom of survival."