Why Are Giant Pandas Black And White?

The giant panda's black and white markings are unique in the animal kingdom so the reason for this particular color pattern has remained mysterious -- until now.

A new study published by a collaboration between researchers from the University of California, Davis and the California State University, Long Beach, scored and compared fur coloration in thousands of photographs of bears and other terrestrial carnivores. Based upon this analysis, the research team reports that the giant panda's unique black-and-white markings have two functions: camouflage and communication. The panda's white body markings help it hide in snow, whilst its black body markings help it hide in shade. The panda's distinctive facial markings are used to communicate with other pandas.

Pandas are not your average bear

As coloration and color pattern goes, most mammals are drab brownish or greyish to match their background, making them really quite boring to look at -- with the exception of a few remarkable creatures, such as zebras and giant pandas, which possess sharply contrasting black-and-white color patterns. The rarity and stunning noticeability of such coloration has long fascinated scientists: why did such distinctive color patterns evolve and what is their functional significance?

Last year, a team of researchers from the University of California, Davis, discovered that zebras stripes' polarize light and thereby reduce biting flies' ability to home in on these animals for a meal (ref). This year, the zebra research team collaborated with colleagues at the California State University, Long Beach, to investigate the evolutionary reason for giant pandas' unique color pattern.

Ecologically speaking, giant pandas, Ailuropoda melanoleuca (which translates to "black and white cat-foot"), are a mere shadow of what they once were. They are now are restricted to a highly fragmented geographic range between 2000–3000 meters (6600-9800 feet) in elevation in south-central China, but historically, they occupied a much larger and more continuous range throughout most of southern and eastern China. Further, panda fossils indicate they once lived as far south as northern Myanmar and northern Viet Nam and northward nearly to Beijing. Although their closest relatives are carnivores, pandas are specialised on a diet of low-quality bamboo. With the exception of mothers with cubs, adult pandas are solitary animals and are known to roam widely through low- and mid-elevation forests at various times of the year.

Despite their large body size -- adult pandas weigh between 86 and 107 kilograms (190-235 pounds) -- they are a menu item for variety of animals, ranging from the comparatively small dhole, Cuon alpinus, a native Asian wild canid, to large predators such as leopards, Panthera pardus, Asiatic black bears, Ursus thibetanus, and brown bears Ursus arctos, and formerly, wolves, Canis lupus, and tigers, Panthera tigris.

But pandas' strange color pattern has been the feature that has most fascinated people. Because pandas' markings are unique, it has not been possible to directly compare them to another, similarly marked species to understand the evolutionary forces that drive and maintain this pattern. Throughout the years, scientists proposed a number of hypotheses for why giant pandas have evolved their unique coloration:

1. panda coloration advertises it is an aggressive animal that is not to be trifled with
2. the panda's white fur acts as camouflage against a snowy background
3. the panda's dark fur retains heat in cold environments
4. the dark patches around the panda's eyes reduce daytime glare
5. the contrasting color pattern on the panda’s face is used to communicate with other pandas

Hiding in snow or shade, or intimidating competitors

"Understanding why the giant panda has such striking coloration has been a long-standing problem in biology that has been difficult to tackle because virtually no other mammal has this appearance, making analogies difficult," said lead author Tim Caro, a professor in the department of Wildlife, Fish and Conservation Biology at UC Davis.

Based upon the assumption that selective pressures drive color pattern and coloration, the researchers compared and scored the color of specific body areas in thousands of photographs of 195 land-dwelling carnivore species and 39 other bear species.

"This really was a Herculean effort by our team, finding and scoring thousands of images and scoring more than 10 areas per picture from over 20 possible colors," said study co-author Ted Stankowich, a professor at CSU Long Beach, in a press release.

By comparing color for each body (Method 1; Figure 1) and facial (Method 2; Figure 1) region and correlating that to that species's particular ecology and social behaviors, the researchers were able to assess all proposed hypotheses as potential evolutionary explanations for panda coat color pattern.

"The breakthrough in the study was treating each part of the body as an independent area," Professor Caro pointed out.

The team's analysis revealed that different regions of the giant panda fur coloration serve different purposes. The white fur covering the majority of the animal's body -- its face, nape, back, flank, belly and rump -- is adapted for camouflage against a snowy background, whilst its dark shoulders and legs provide crypsis in shade.

The team concluded that panda coloration is ultimately driven by panda ecology: because they eat such a poor diet that they never manage to store enough body fat to allow them to hibernate, pandas must constantly for prowl for food. Since they are always seeking their next meal, pandas are never able to molt their fur quickly enough so it matches their background, as most other carnivores can do. Thus, they evolved a "compromise" color pattern and coloration consisting of black-and-white fur.

"This is an alternative evolutionary strategy to smaller carnivores like the ermine and arctic fox that have winter and summer coats," write the authors in their study. "Interestingly, some wolverines (Gulo gulo), another relatively large species that does not go into winter torpor, and that travels long distances across several habitat types, sometimes has a black and white body that resembles the pattern of the giant panda."

Interestingly, the researchers did not find any evidence to suggest that panda's fur color might play a role in either temperature regulation or disruptive coloration.

According to the study, the markings on the panda's face have a different function: communication. The dark ears convey a warning to would-be predators that this animal can defend itself whilst the dark patches around the eyes may help pandas recognize individuals of their own species, and they may also play a role in threat displays. It is interesting to note that the dark areas around the pandas' eyes are not associated with reduced glare.

Based on this study, the research team proposed another working hypothesis for the panda's remarkable pelage: the giant panda's body coloration provides it some camouflage in either snowy or shadowy habitats, whereas its facial markings are used for communication (Figure 2).

Source:

Tim Caro, Hannah Walker, Zoe Rossman, Megan Hendrix, Theodore Stankowich (2017). Why is the giant panda black and white? Behavioral Ecology. Published online on 28 February 2017 before print. doi:10.1093/beheco/arx008

Read more:

Kenneth H. Britten, Timothy D. Thatcher, and Tim Caro (2016). Zebras and Biting Flies: Quantitative Analysis of Reflected Light from Zebra Coats in Their Natural Habitat, PLoS ONE, 11(5): e0154504 | doi:10.1371/ journal.pone.0154504

Why Are Giant Pandas Black And White? | @GrrlScientist