A faecal index of diet quality that predicts reproductive success in a marsupial folivore
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Estimating the nutritional value of a herbivore’s diet is difficult because it requires knowing what the animal eats, the relative quality of each component and how these components interact in relation to animal physiology. Current methods are cumbersome and rely on many assumptions that are hard to evaluate. We describe a new method for estimating relative diet quality directly from faeces that avoids the problems inherent in other methods. We combine this method with near infrared reflectance spectroscopy (NIRS) to analyse many samples and thus provide a technique with immense value in ecological studies. The method stems from the correlation between the concentrations of dietary and faecal nitrogen in herbivores eating a tannin-free diet, but a weaker relationship in browsers that ingest substantial amounts of tannins, which form complexes with proteins. These complexes reduce the availability of nitrogen and may increase faecal nitrogen concentrations. Using the tannin-binding compound, polyethylene glycol, we showed that tannin-bound nitrogen is a significant and variable part of faecal nitrogen in wild common brushtail possums (Trichosurus vulpecula). We developed a technique to measure faecal available nitrogen and found that it predicted the reproductive success of female brushtail possums in northern Australia. Faecal available nitrogen combined with NIRS provides a powerful tool for estimating the relative nutritional value of the diets of browsing herbivores in many ecological systems. It is a better indicator of diet quality than other commonly used single-nutrient measures such as faecal nitrogen and foliage analysis paired with observed feeding behaviour.
KeywordsNutrition Tannins Available nitrogen PEG Browser
We thank R. and E. Fryer, Australian Wildlife Conservancy, and other land owners for permission to use their land. We thank Elesha Curran for assistance with field and laboratory work. Funding was provided by grants from the Australian Research Council to C.N. Johnson and W.J. Foley and the Ecological Society of Australia and Royal Zoological Society of N.S.W. to J.L. DeGabriel.
Conflict of interest
The authors declare that they have no conflict of interest.
The procedures were approved by the Animal Experimentation Ethics Committee of the Australian National University and conform to the Guiding Principles in the Care and Use of Animals. Experiments comply with the current laws of Australia.
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