Of Pandas and Bamboo

Today, there are eight living species of bear. Six of these are "ursine bears", members of a subfamily distributed across the northern hemisphere, and including the familiar brown, black, and polar bears, among others. The other two are distinct enough that modern taxonomists place each of them in their own subfamily, representing lineages that diverged from the common ancestor of the ursine bears a long time ago. One of these is the spectacled bear of South America, the last survivor of a group of American "short-faced" bears, some of which were really quite impressive.

The other, of course, is the giant panda (Ailuropoda melanoleuca).

This is a most peculiar bear. So much so, in fact, that for much of the 20th century it was unclear whether it really was a bear at all - a misunderstanding due to its similarity, and presumed close relationship to, the decidedly un-bearlike red panda. However, we now know that the two animals are not particularly close relatives and the resemblances between them are largely due to the fact that they share a similar diet.

Such is the iconic status of the panda and such are the efforts that the Chinese government have undertaken to preserve it, that its population has been steadily increasing for about the last three decades. So much so, in fact, that it was officially delisted as an endangered species in 2016. Nonetheless, it remains "threatened" and there is little doubt that it would be in dire trouble were it not so effectively protected.

Even if the current rate of population increase means that it can't truly be considered an endangered species any more, there are probably no more than around 1,000 adult pandas alive today. These are found scattered across a few mountain ranges in central China, notably the Minshin, Qionglai, and parts of the larger Qinling ranges, all of which run along the eastern edge of the Tibetan Plateau. Many of the surviving populations are separated from one another in different parts of the mountains, not least because giant pandas don't really venture below about 1,200 metres (4,000 feet) elevation.

Or, at least, they don't today.

China, of course, is an ancient civilisation that has had a large human population for a very long period of time. But, especially from the middle of the 20th century onwards, there has been a huge growth in industrialisation, and there were significant expansions of large-scale agriculture for at least a hundred years prior to this. This has been bad news for pandas, resulting in the massive decline in their population - and area of habitation - that put them on the endangered species list in the first place.

Pandas used to live in lowland forests across much of China but, by some estimates, a third of all of China's forests were felled between 1950 and 2004, although the decline has slowed, and perhaps even reversed since then. Still, for all of the successful (if far from completed) efforts to preserve the panda, the places they live now are just a remnant of their former range - essentially the remote mountains where humans don't really want to live and farm. 

In short, pandas may be a mountain-dwelling species now, but only because they have nowhere else to go. Clearly, forested mountain slopes are good enough for them, but if pandas had their druthers, they'd also live much further afield. Which means that, if we want to know about where they lived in the past and how the current populations came to be, we can't simply rely on looking at the details of their current habitat. And understanding the history of populations can be useful if we want to understand the impacts of current genetic diversity in a threatened species.

It can be difficult to fully estimate the range that a species lived in the past, at least in the absence of clear physical evidence. There are at least three different kinds of factor that influence where a wild animal lives. Firstly, there is the physical environment - climate, altitude, ruggedness of the terrain, and so forth. Secondly, there are biotic factors, such as what sort of predators are around, and whether there is anything suitable to eat. And thirdly, there's what has been termed the species' M-region, which is a fancy way of saying where the animal can physically get to without crossing an impassable barrier such as the sea.

The hardest part of this to evaluate are the biotic factors, due to the complex range of possible interactions there might be with other species. Therefore, a new study trying to evaluate the former range of the giant panda narrowed this down to a single factor: where could pandas find enough food to eat before the advance of agriculture?

This is rather easier to evaluate for pandas than it might be for many other animals, including other bears. That, of course, is because pandas have a very restrictive diet. While it isn't absolutely true that they never eat meat at all, if the choice is between that and starvation, they don't seem to like it, and in fact, the gene that would normally allow a carnivore (or human) to detect the taste of meat is switched off. 

Under normal conditions, therefore, giant pandas essentially eat nothing but bamboo. 

Bamboos are a specific subfamily within the wider grass family. They may be much, much larger, but their closest relatives are the "cool-season grasses", which include wheat, barley, and most of what's probably on your lawn. There are a great many species of bamboo, even once we ignore those found outside of China but, even so, it's not what you'd call a varied diet - especially for an animal that lacks any of the adaptations to the digestive system that enable hoofed animals to subsist on leafy diets.

Using a combination of known climactic factors and the distribution of edible bamboo, the study was able to predict where pandas should have been found during the Last Ice Age, and this turns out to correlate well with where we have, indeed, found panda fossils of the relevant age. If it's accurate, it suggests that pandas once lived, not only at lower altitudes, but in warmer and moister climates than they do today, supporting earlier claims that pandas once lived into the subtropics - and that they were probably even more widespread across China and southeastern Asia than the known fossils indicate.

This implies that pandas have been being pushed into colder, drier habitats since well before modern times, and that their current mountainous home is close to the limits of where they can actually survive. Indeed, we know from DNA analysis of ancient panda remains that just a few thousand years ago, pandas were more genetically diverse than they are now, perhaps with those more suited to a colder climate surviving while others did not. 

This fits into and expands our understanding of longer-term panda evolution. Pandas represent the oldest branch within the living bear family, with fossils related to pandas dating back over ten million years, well into the Miocene. But many of these were likely omnivores, perhaps with a diet not that different from brown bears today. Although herbivory may have evolved more than once in the lineage, giant pandas as we know them today only switched towards vegetarianism with the evolution of the modern genus (Ailuropoda) around 2.7 to 2 million years ago. 

The time since then has been marked by pandas becoming increasingly specialised in their diet. They likely already ate a significant amount of bamboo when the Ice Ages began, but as they went on they began to eat less of anything else. By the time we get to the living species, the switch-over was complete, leaving pandas reliant on a food source that has become ever harder to obtain over the last three thousand years.

[Photo by "Wayne77" from Wikimedia Commons.]