Who is who matters-The effects of pseudoreplication in stable isotope analyses.

Stable isotope analysis in free-ranging primates is a promising new avenue in reconstructing feeding niches and temporal dietary variation. Particularly, the large sample sizes obtained from non-invasively collected hair and fecal samples from nests of great apes offer great potential. However, analyzing repeated observations of the same individuals without controlling for potential differences among them means to "pseudoreplicate" and can lead to a greatly inflated probability of erroneous significance. We here test the effects of pseudoreplication in stable isotope data of great ape hair by means of simulations. We show that pseudoreplication can severely affect the probability of erroneous significance as well as non-significance. We suggest several strategies to avoid pseudoreplication in primate isotope ecology. First, if applicable, information on individual identity should be included in statistical analyses. Second, if samples derive from unhabituated animals, sampling at far apart locations or territories should avoid resampling of the same animal. In great apes, sampling of independent nests within nest groups can ensure that each sample derives from a different individual. Third, we encourage the combination of genetic surveys with sampling for isotope analyses to ensure the (genetic) identification of individuals. Am. J. Primatol. 78:1017-1030, 2016.