What is black and white and a puzzle all over?

They say that every family has a black sheep, but the order Carnivora has its black and white giant panda Ailuropoda melanoleuca (Fig. 1)—a strict and fussy vegetarian that feeds almost exclusively on bamboo leaves and shoots. These finicky eaters even have seasonal preferences for different bamboo species and parts. While they feed on the hard culm (stalk) for a good part of the year, they prefer the sugar- and starch-rich shoots in late spring and early summer. Due to this highly restricted diet, the panda's lifestyle revolves around its energy requirements. Up to 14 hours each day is spent consuming about 30 pounds of bamboo; a lot of this food is undigested, and consequently the pandas defecate about 50 times per day. Movements are kept to a minimum, climbing minimized, territorial defense and social interactions avoided, and the females usually give birth to a single tiny newborn (weighing about 140 grams) after an unusually short gestation period.

Figure 1.

Ailuropoda melanoleuca, the carnivore that went vegetarian.

The panda's diet does not make sense anatomically or physiologically. Like their carnivorous cousins, they have a short GI tract, a simple stomach and a straight colon. The panda genome encodes all the enzymes required to break down a juicy steak, but does not show evidence for genes that could contribute to cellulose breakdown.¹ In many herbivores the gut microbiome typically processes the cellulose-rich diet. But the absence of a sacculated stomach and the rapid passage of food through the panda's gut means that there is little space or time for fermentation. Also, the panda gut is mostly populated by organisms such as Escherichia coli and Streptococcus, and not the cellulolytic bacteria such as Ruminoccocci or Fibrobacter typically found in ruminants.^2,3

Genome sequence analyses suggest that taste may have something to do with the giant panda's diet.¹ Meat and cheese are rich in l-glutamine, the compound that binds taste receptors (the T1R1/T1R3 heterodimer) to produce the sensation known as umami (the other taste receptors sense sweet, salty, sour and bitter tastes). A frameshift mutation in the panda's T1R1 gene suggests the absence of a functional umami receptor, and this may reduce the desire to eat meat. However, taste may not be the main issue with the panda's selective diet.⁴ Giant pandas occasionally eat rodents and birds, and captive pandas are sometimes fed high energy foods such as sugar cane, apples and rice gruel. More intriguingly, goat meat is the standard bait for capturing pandas.

High energy (and low fiber) foods alter the giant panda's digestion pattern, reflected in lower fecal output and the abnormal presence of mucus in the stool. The more troubling issue is that an insufficient bamboo diet also results in reduced reproductive success. With fewer than 3,000 giant pandas in the world, this iconic endangered species is the logo for the World Wide Fund for Nature (WWF). Extensive habitat conservation is essential to provide an adequate bamboo supply for the giant panda population to survive and thrive. If we understand the giant panda's gut better, we may be in a position to help it thrive even if there is a shortage of bamboo forests. This is one more puzzle in the inside passage that does not appear to have a black and white answer.