The hypothesis presented here should be considered as one of several constraining factors of evolutionary relevance. It is based on food quality and does neither consider quantitative aspects, nor does it take into account the need to match the animals�� energy requirements, specific mineral needs, or avoidance of plant secondary components. However, these possibly confounding variables can not be separated as long as we do not have the means to measure the qualitative and quantitative availability of food, its individual and seasonal variation as well as seasonal or ontogenetic variation in ingestion and nutrient assimilation by the animals. We used nitrogen concentrations as measured by the Kjeldahl procedure rather than crude Ixabepilone protein in this comparison, because different conversion factors from nitrogen to crude protein have been suggested. As other measures of protein concentrations, such as ninhydrin, Biorad, or amino acids can not be transformed to nitrogen concentrations using a simple transformation factor, only studies reporting total nitrogen measured with the Kjeldahl method were used in the present analysis. Few ripe fruits contain digestion inhibitors such as frequently found in leaves. Thus, there was no need to control for these digestion-inhibiting components. Except for the samples collected by Polowinsky and Schwitzer, all other samples collected by the 20(S)-Protopanaxatriol authors for the present paper were analyzed with the same equipment and procedure in the labs of JUG. Community-wide data on the chemical composition of fruits are scant. However, of 10 published primatological studies addressing protein selection in fruits in Madagascar and the Neotropics, none found a positive significant difference in nitrogen concentrations between those fruits eaten and not eaten by the primates under study. One study that did report a significant difference, reported a negative correlation between consumption and protein concentrations. Therefore, we consider the fruits consumed by primates as a conservative representative sample of the nitrogen concentrations for all fruits available at each site.