First direct measurement of food transit time through the gastrointestinal tracts of plant-eating primates using isotopic tagging

The time it takes for food to transit animal gastrointestinal tracts is essential information to fields from organismal evolution to ecosystem ecology. Nevertheless, current estimation methods do not measure food directly, are impractical for many species and often require unnatural conditions to administer. We overcome these limitations by adopting techniques from organic geochemistry. We trace isotopically tagged greens in lemurs, from ingestion to excretion, through isotopic analysis of higher plant biomarkers recovered in faeces. For frugivorous ruffed lemurs (Varecia rubra), omnivorous ring-tailed lemurs (Lemur catta) and frugo-folivorous sifakas (Propithecus coquereli), respectively, the biomarkers first appeared at 1.2, 4.3 and 23.4 h after ingestion and peaked at 2.9, 5.7 and 48.4 h after ingestion. These measurements generally match published estimates from pre-existing proxies. However, not all animals cleared the isotopic signal by the end of sample collection, yielding minimum gut retention times of 23.9–29.5 h for ruffed lemurs, 25.5–30.3 h for ring-tailed lemurs and 95.7 h for sifakas. We thus extend gut retention times approximately threefold from pre-existing proxies. Our interdisciplinary approach is applicable across diverse plant eaters, can help refine models of nutrient, seed and pathogen dispersal and be used to probe consequences of lifestyle and medical interventions on organismal nutrition and health.