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Giant pan­das (Ail­uropoda melanoleuca) are unusual in being extremely spe­cial­ized her­bi­vores that feed almost exclu­sively on highly fibrous bam­boo, despite belong­ing to a clade (Car­nivora) of pri­mar­ily flesh-​eating car­ni­vores. But a recently pub­lished study sug­gests that the switch to a restricted veg­e­tar­ian diet wasn’t, in some respects, as big an evo­lu­tion­ary leap as it seems.

Giant panda in Madrid Zoo, 2012.
Image copy­right MoosMood

The study, pub­lished on 2 May in the jour­nal Cur­rent Biol­ogy, finds that the pro­tein and car­bo­hy­drate con­tent of the panda’s plant diet looks sur­pris­ingly like that of a hyper­car­ni­vore, ani­mals that obtain more than 70 per­cent of their diet from other ani­mals, they report. About 50 per­cent of the panda’s energy intake comes in the form of pro­tein, plac­ing them right along­side feral cats and wolves.

Fuwen Wei, co-​author, of Chi­nese Acad­emy of Sci­ences, Bei­jing. Key Lab­o­ra­tory of Ani­mal Ecol­ogy and Con­ser­va­tion Biol­ogy, Insti­tute of Zool­ogy, Bei­jing, and CAS Cen­ter for Excel­lence in Ani­mal Evo­lu­tion and Genet­ics, Kun­ming, Chi­nese Acad­emy of Sci­ences, China

The pan­das do have traits in com­mon with her­bi­vores, includ­ing a skull, jaw mus­cu­la­ture, and den­ti­tion that are adapted for fibrous diets, and a spe­cial­ized ‘pseudo-​thumb’ used for han­dling bam­boo. They’ve also lost the abil­ity to taste umami, which is often asso­ci­ated with meat eat­ing. On the other hand, giant pan­das have a diges­tive tract, diges­tive enzymes, and gut microbes that resem­ble that of car­ni­vores and not herbivores.

In the new study, Wei teamed up with nutri­tional ecol­o­gist David Rauben­heimer from the Uni­ver­sity of Syd­ney and col­leagues to explore the macro-​nutrient com­po­si­tion of their diet, includ­ing what the pan­das ingest and what they absorb. Using an approach called nutri­tional geom­e­try, the team showed that the macro-​nutrient mix that giant pan­das both eat and absorb is sim­i­lar to car­ni­vores, and unlike her­bi­vores. The macro-​nutrient com­po­si­tion of the panda’s milk also places it squarely among other carnivores.

The researchers say the find­ings can help resolve long-​standing ques­tions con­cern­ing the evo­lu­tion of the giant panda, includ­ing the unusual tran­si­tion to extreme spe­cial­ized her­bivory by a mem­ber of a car­niv­o­rous clade. “In fact,” they write, “the tran­si­tion was likely more super­fi­cial than assumed, com­bin­ing sub­stan­tial adap­ta­tion to new food types with rel­a­tively smaller changes in macro-​nutrient handling.”

The her­biv­o­rous diet led to evo­lu­tion­ary adap­ta­tions in their teeth, skull, and pseudo-​thumb needed to process bam­boo. But their gut and diges­tive sys­tem changed lit­tle, sug­gest­ing min­i­mal evo­lu­tion­ary mod­i­fi­ca­tion from their ances­tral state was needed to deal with the macro-​nutritional prop­er­ties of bam­boo. Their short gut, together with the abun­dance of bam­boo, allows the panda to con­sume and process large amounts of bam­boo, com­pen­sat­ing for the low diges­tive effi­ciency of such a fibrous diet.

David Rauben­heimer, co-​author, The Charles Perkins Cen­tre, The Uni­ver­sity of Syd­ney, Australia

Wei says they will con­tinue to study the evo­lu­tion and adap­ta­tion of the giant panda. They’ll also apply that work to the panda’s con­ser­va­tion man­age­ment as an endan­gered species.

(Source: Cell Press news release via EurekAlert!, 02.05.2019)