Faeces as food: a framework for adaptive nutritional coprophagy in vertebrates

Nutritional coprophagy has been under-recognized in many vertebrates despite its potential impact on social behaviours, health outcomes and ecological relationships. We reviewed published instances of apparent nutritional coprophagy across all vertebrate taxa, casting a wide net to capture incidental observations as well as focused studies. We found that coprophagy is a recognized, natural behaviour in five of seven vertebrate classes (all except Agnatha and Chondrichthyes), with reports of nutritional coprophagy in over 150 species. Nutritional coprophagy is common among some families and appears to be more common among herbivores and omnivores compared to faunivores. Across all taxa in which they occur, coprophagic behaviours cluster into seven distinct groups, distinguished by source of faeces, life stage, frequency and dietary ecology, as follows. (A) Routine reingestion to fulfil nutrient requirements (20 families of mammals and amphibians); (B) Juvenile nutritional support (11 families of mammals, reptiles and birds); (C) Nutritional supplementation of adults during parental care (18 families of mammals and birds); (D) Adaptation to aphotic, nutrient-poor habitats (2 families of amphibians and bony fish); (E) Frequent supplemental feeding strategy in biodiverse environments (38 families of mammals, reptiles, birds and bony fish); (F) Rare supplemental nutrition (12 families of mammals, reptiles and birds); (G) Targeted ingestion of seeds and fruits found in faeces (7 families of mammals, reptiles and birds). We propose that these groups represent different manifestations of adaptive nutritional coprophagy. Despite the numerous reports of coprophagy that have been documented, the behaviour is still widely considered abnormal or pathological in many animals. Applying this framework may help field researchers, wildlife managers and animal caretakers recognize nutritional coprophagic behaviours and characterize them as part of a dietary strategy. The framework also suggests possible evolutionary pathways for coprophagy as an adaptive behaviour.