Giant pandas metabolise like hyper-carnivores

Giant pandas adoption of bamboo as a sole food source has puzzled scientists for decades. Now a macronutrient analysis of the bears’ diet suggests that this was not as big an evolutionary leap as previously thought.

Giant pandas (Ailuropoda melanoleuca) present somewhat of an evolutionary paradox, with a mix of herbivore and carnivore traits. Specialised jaw muscles, teeth and a “pseudo-thumb” for gripping and handling the bamboo are herbivorous traits. Yet their digestive systems have the same digestive tract, enzymes and gut microbiota as carnivores.

Researchers from the Chinese Academy of Sciences in Beijing teamed with nutritional ecologist David Raubenheimer from the University of Sydney, Australia, to analyse the macronutrients within the panda diet.

The study utilised recent developments in niche theory – an approach first introduced in 1972 to probe that regards environment as the ultimate distribution measure of any species.

Work conducted by ecologist Gabriel Machovsky-Capuska from the University of Sydney, and others, in 2016, extended the idea of diet in determining environmental niche, focussing on nutrient mix rather than actual foodstuffs.  

In the new research China’s Yonggang Nie and colleagues used this framework to test whether, they state, “the combination of herbivore and carnivore associated traits in giant pandas is not incongruous at all, but suggests hitherto hidden dimensions of their diet”.

The findings, published in the journal Current Biology, reveal that, despite an exclusive bamboo menu, panda diet composition has protein and carbohydrate percentages on par with hyper-carnivores, such as feral cats or wolves, that obtain over 70% of their diet from animal foods.

“As we know, the giant panda is a Carnivora species, yet extremely specialized on a plant food, the bamboo,” says co-author Fuwen Wei. 

“Based on what they eat, they absolutely belong to the herbivores, but considering the macronutrient composition of the ingested and absorbed diets, they belong to the carnivores.”

As with other specialised herbivores, such as koalas (Phascolarctos cinereus), not all plants are regarded equal. Pandas range over their habitat seasonally in order to forage on bamboo plants with the highest nutritional profile, selecting shoots, for instance, in preference to young leave.

The researchers calculate that the macronutrient composition of the wild foods in a typical panda diet comprise 61% protein energy, 23% carbohydrates and 16% fat. 

The study provides an example of an animal that occupies different trophic niches at the food and macronutrient levels, and the researchers suggest that in giant pandas, the transition from carnivore to specialised herbivore required minimal evolutionary modification from their ancestral state.

The species has a short gut, characteristic of carnivores, which facilitates rapid throughput, as well as carnivore-like gut microbiota, well suited to the high protein content of bamboo.

The abundance and accessibility of the plant enables large amounts to be eaten, compensating for the large amounts of indigestible cellulose.

Panda herbivorous traits, such as jaw muscles, are adaptations for the processing of bamboo, while carnivorous traits are ready-made for the absorption of a high protein diet, albeit a plant-based one.

“There is also a broader message from this study,” says co-author David Raubenheimer. “It demonstrates the importance of considering both foods and nutrients in understanding the evolutionary ecology of animals.”