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Giant pandas have the distinction of being extremely specialized herbivores that feed almost exclusively on highly fibrous bamboos, although they belong to a clade (Carnivora) composed mainly of carnivorous carnivores. But a study reported in Current biology May 2 suggests that switching to a restricted vegetarian diet was, in some ways, not a big evolutionary leap that it seemed there.
The study reveals that the protein and carbohydrate content of the panda's diet looks surprisingly like that of a hypercarnivore, animals whose food comes from more than 70% of other animals , they report. About 50% of the panda's energy intake comes in the form of protein, which places them alongside feral cats and wolves.
"As we know, the giant panda is a species of carnivore, but it is extremely specialized in a plant food, bamboo," said Fuwen Wei of the Beijing Chinese Academy of Sciences. "After what they eat, they absolutely belong to the herbivores, but given the macronutrient composition of diets ingested and absorbed, they belong to carnivores."
Pandas have common traits with herbivores, including the skull, jaw muscle and dentition, which are adapted to fibrous diets, and a specialized "pseudo-thumb" used to manipulate bamboo. They have also lost the ability to taste umami, which is often badociated with meat consumption. On the other hand, giant pandas have a digestive tract, digestive enzymes and intestinal microbes that resemble those of carnivores and not herbivores.
In this new study, Wei collaborated with nutrition ecologist David Raubenheimer of the University of Sydney and his colleagues to explore the macronutrient composition of their diet, including what pandas ingest and absorb . Using an approach called nutritional geometry, the team showed that the mixture of macronutrients that giant pandas eat and absorb is similar to that of carnivores and unlike herbivores. The macronutrient composition of panda milk also places it among other carnivores.
The researchers say the findings may help resolve long-standing issues regarding the evolution of the giant panda, including the unusual transition to an extremely specialized herbivore by a member of a carnivorous clade. "In fact," they write, "the transition was probably more superficial than supposed, combining a substantial adaptation to new types of food with relatively less significant changes in macronutrient management."
The herbivore diet has led to evolutionary adaptations of the teeth, skull and pseudo-thumb needed to treat bamboo. But their intestines and digestive systems have changed little, suggesting a minimal evolutionary change in their ancestral state to treat the macronutrient properties of bamboo. Their short bowel, badociated with the abundance of bamboo, allows the panda to consume and process large amounts of bamboo, thus offsetting the low digestive efficiency of such a fibrous dietary regimen.
"This study also contains a broader message," said Raubenheimer. "This demonstrates the importance of taking into account both nutrients and nutrients to understand the evolutionary ecology of animals – that's what nutritional geometry is designed to do."
Wei says that they will continue to study the evolution and adaptation of the giant panda. They will also apply this work to managing the conservation of the panda as an endangered species.
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Material provided by Cell press. Note: Content can be changed for style and length.
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