The development of a second thumb enabled pandas to switch from a meat-based diet to a plant-based diet containing bamboo at least six million years ago. Using the pseudo-thumb, which is actually an elongated carpal bone, the bears can grip the bamboo poles tightly and then chop them with their teeth, an international team of scientists reports in the specialist magazine Scientific Reports. The thumb is designed in such a way that the animals have no difficulty walking on four paws and they can easily move from one bamboo plant to the next while eating.
It has long been known that pandas have, in addition to the normal five fingers, a kind of sixth finger that is juxtaposed with the rest of the fingers in a manner similar to a thumb. It improves the animals’ gripping ability, especially gripping bamboo, the pandas’ main food. However, the pseudo-thumb looks stunted and protrudes only slightly from the palm. When did the additional finger arise in the course of evolution? And why, experts have been asking for a long time, hasn’t it developed into a complete finger?
Looking for an answer, Xiaoming Wang’s team at the Natural History Museum of Los Angeles County analyzed fossil remains of an early panda (Ailurarctos) dating back about six to seven million years. The researchers compared the structure of the hand bones with that of today’s giant panda (Ailuropoda melanoleuca), among others.
With the switch to plants, the pandas’ diet became less nutrient-poor
In fact, the pseudo-thumb was already present in the early panda, at least six million years ago. Compared to body size, it was even larger than that of modern-day pandas. Another difference: In the end, early pandas’ thumbs weren’t as crooked as modern-day giant pandas are. The researchers hypothesize that the shape of the pseudo-thumb is a compromise between the benefits of eating bamboo and the need to be able to walk unhindered on all fours – a thumb that is too long would interfere.
“Five to six million years should be enough time for the panda to evolve longer false thumbs, but it appears that evolutionary pressures to travel and support body weight have kept the thumb short – strong enough to be useful without being so big that it gets in the way,” explains co-author Denise Su of Arizona State University.
Giant pandas live in dense bamboo forests and feed almost exclusively on these grasses. However, the animals are descended from carnivores. “In the process of evolving from a carnivorous ancestor to a pure bamboo eater, pandas faced many obstacles,” says Wang. “An opposable thumb made from a wrist bone is perhaps the most amazing development given these hurdles.”
With the switch from meat to plants, the pandas exchanged a very high-protein diet for a low-nutrient diet. To make up for the disadvantage, pandas have to eat most of the day. Today’s giant pandas devour up to 45 kilograms of bamboo a day. They also make a heap with impressive frequency: around 100 times a day. That’s because their guts – a legacy of their carnivorous ancestors – are short and poorly designed for extracting nutrients from plants. The food therefore crosses the digestive tract in less than twelve hours.
Switching to a plant-based diet had one major advantage: Bamboo is available in large quantities in the animals’ habitat all year round, so searching for food does not use up a lot of energy. After the pandas adapted to eating huge amounts of bamboo instead of meat, the development of the false thumb was a big help when eating, the researchers write.