Pandas have two pseudo thumbs for eating bamboo. But since when?

In its evolutionary history, the panda hand has never developed a truly opposable thumb, but something close to it. This unique adaptation allows these bears to feed almost exclusively on bamboo. But when did this trait appear? In a new article, a team of researchers report the discovery of the first ancestral bamboo-eating panda equipped with such a “thumb”.

Pandas and their pseudo thumbs

Giant panda (Ailuropod melanoleuca) may be a member of the order Carnivora, but its diet is 99% plant-based, including lots of bamboo. And since they don’t have a multi-chambered stomach to extract nutrients from hard plant matter, these plump bears must consume plenty of it to sustain themselves (at least thirteen pounds a day).

To grasp stems, pandas use a thumb-shaped sixth “finger”: radial sesamoid. This is not a finger per se, but a protruding extension of the carpal bone. However, these pseudo-thumbs are enough to allow pandas to perform several complex movements, including grabbing stems to better eat them.

Scientists have long puzzled over this rudimentary thumb. Since when has it existed? Until now, the absence of a petrified leg did not allow answering this question. There are only 150,000 records. In fact, this trait is much older.

In a study published in the journal Scientific Reports, a team of Chinese researchers led by Dr. Wang presented evidence that panda parents have indeed been using pseudo-thumbs for millions of years.

Fossil dated at least six to seven million years ago.

The fossil was discovered in 2015 at the Shuitanba site in southern China. Dr. Wang, lead author of the new work, was digging in a quarry with his team when he came across a spoon-shaped piece of bone. ” Intuitively, I thought it was a fossilized panda finger.“, he stated.

A comparison of the fossil with the skeletons of a modern panda confirmed his hunch. After analyzing several teeth found nearby, the researchers determined that this fake thumb belonged to Ailurarctos, an extinct genus of pandas that lived during the Miocene era, six to seven million years ago.

As the first example of a panda’s pseudo-thumb, the researchers expected that this extra “finger” would still be very primitive. In fact, it was significantly larger than that of modern pandas. However, unlike the straight thumbs of the petrified bear, the pseudo-thumbs of modern pandas are curved inward like a hook, making them easier to grip.

Artistic reconstruction of Ailurarktos. Credits: Mauricio Anton

Will this be an evolutionary compromise?

Pushing back the origin of panda pseudo-thumbs millions of years ago, the uncomfortable question arises: why did these pseudo-thumbs never develop into real universal thumbs? In their study, Dr. Wang and his team hypothesize that the size of the pseudo-thumb is limited by how pandas move. When they’re not lounging around, they really do walk on all fours. However, the presence of a true sixth toe may have interfered with this mode of locomotion. So the fact that it remains only a simple standout extension would be sort of evolutionary compromise. ” You need to grab him, but you also keep stepping on him.“, the researcher concludes.

However, not all researchers are convinced of this reasoning. Juan Abella, a paleontologist at the Catalan Institute of Paleontology in Spain, points out that having a toe extended towards the back of the foot is only little effect on movement pandas. Thus, the advantages outweighed the potential disadvantages. Remember that we are talking about a very slow animal that sometimes passes over sixteen hours a day to eat.

At this point, the question “why” is still open to debate. However, the discovery of additional fossils may one day allow a decision to be made.

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