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Thursday, March 28, 2024 | Back issues
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Giant pandas evolved a ‘thumb’ to help eat more bamboo, study finds

Unlike human thumbs, which can move independent from the other fingers, the study says the modern giant panda’s “thumb” rotates with the other digits.

(CN) — The giant panda has an enlarged wrist bone that acts as a sixth digit, according to a study published Thursday in Scientific Reports. Researchers reported that this is a trait passed down from the ancestral panda Ailurarctos from the late Miocene site of Shuitangba in Yunnan Province, China.

They thought it was strange that the “thumb,” which the study says barely protrudes from the panda’s palm-like surface, didn’t get bigger as time and many generations passed.

According to the study, the “thumb” serves a dual purpose — helping the panda eat bamboo and weight distribution.

Unlike human thumbs, which can move independent from the other fingers, the study says the modern giant panda’s “thumb” rotates with the other digits. The study adds that small muscles between the “thumb” and the first metacarpal act as a cushion for the bamboo stems when the panda grabs it. Also, the panda’s “thumb” has a hook that acts as a passive pincer. It’s less effective than a human grip but it gives the panda a tight grasp of their chosen bamboo stick.

As for the ancestral panda, the study says Ailurarctos’ thumb exceeded the size of the modern pandas, with the ancestral panda’s radial sesamoid index measuring 1.89 mm. The modern panda’s “thumb” measured 0.84-1.28 mm and has a slight hook at the distal end. The authors believe that this is the result of a “functional compromise between the need for grasping larger bundles of bamboo and the weight-bearing function of the false thumb.”

When it comes to weight distribution, a hooked thumb would create conflict for a creature that walks long distances with the palm touching the ground like the giant panda does. Per the authors, the “thumb” didn't increase in size to avoid interference with walking. The fattened surface of the “thumb” acts as a “as a means to spread the load within the external accessory pad to cushion stride impact,” says the study. This way, the pandas can comfortably get to where they need to go without sacrificing their ease in eating bamboo.

Interestingly, the red panda and distant relatives of the giant panda also developed a false thumb. However, the study says that the giant panda is a “highly specialized member of the bear family (Ursidae) that has become a dedicated herbivore, a rare case of a large carnivore with a short, carnivorous digestive tract that became a low-level consumer with a greatly altered gut microbiota." Meaning that the giant panda evolved their false thumb for the specific task of maximizing bamboo consumption.

Additionally, the study says, the giant panda’s evolution potentially influenced how their teeth grew.

The study claims that ursids have the most complex molars because their diet consists of more than 70% meat, either via active predation or by scavenging, with the other percentage consisting of non-meat. The modern giant panda has teeth with highly distinct crowns designed to crush tough bamboo. In the mouths of ancestral panda fossils, the authors found that their teeth possessed robust points, indicating that they developed their teeth that way for the same reason.

Lastly, the study notes that after at least six million years of a bamboo diet, once the false thumb became an adequate grip for the bamboo, the panda’s evolutionary transformation remained limited. However, the study cautions “that the fossil record is too incomplete to allow a full understanding of this process and future discoveries will likely reveal unforeseen details.”

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Categories / Environment, Science

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