Giant Pandas Are Macronutritional Carnivores

Giant pandas are unusual in belonging to a primarily carnivorous clade and yet being extremely specialized herbivores that feed almost exclusively on highly fibrous bamboo [1]. Paradoxically, they appear inconsistently adapted to their plant diet, bearing a mix of herbivore and carnivore traits. Herbivore traits include a skull, jaw musculature, and dentition that are adapted for fibrous diets and a specialized "pseudo-thumb" used for handling bamboo [2, 3]. They have lost functional versions of the T1R1 gene codes for umami taste receptors, which are often associated with meat eating [3]. They also have an herbivore-like subcellular distribution of the metabolic enzyme alanine: glyoxylate aminotransferase [4]. But meanwhile, giant pandas have a digestive tract [5], digestive enzymes [6], and a gut microbiota composition that resemble those of carnivores and not of herbivores [6, 7]. We draw on recent developments in multi-dimensional niche theory [8] to examine this apparent paradox. We show that the pandas' diet clustered in a macronutrient space among carnivores and was distinct from that of herbivores. The similarity with carnivore diets applied not only to the ingested diet but also to the absorbed diet, with the absorbed macronutrient ratios similar to those of the ingested foods. Comparison of the macronutrient composition of pandas' milk with those of other species shows that the carnivore-like dietary macronutrient composition extends across the life cycle. These results cast new light on the seemingly incongruous constellation of dietary adaptations in pandas, suggesting that the transition from carnivorous and omnivorous ancestry to specialized herbivory might be less abrupt than it might otherwise appear.