Leaf chemistry as a predictor of primate biomass and the mediating role of food selection: a case study in a folivorous lemur (Propithecus verreauxi).

Folivorous primate biomass has been shown to positively correlate with the average protein-to-fiber ratio in mature leaves of tropical forests. However, studies have failed to explain the mismatch between dietary selection and the role of the protein-to-fiber ratio on primate biomass; why do not folivores always favor mature leaves or leaves with the highest protein-to-fiber ratio? We examined the effect of leaf chemical characteristics and plant abundance (using transect censuses; 0.37 ha, 233 trees) on food choices and nutrient/toxin consumption in a folivorous lemur (Propithecus verreauxi) in a gallery forest in southern Madagascar. To assess the nutritional quality of the habitat, we calculated an abundance-weighted chemical index for each chemical variable. Food intake was quantified using a continuous count of mouthfuls during individual full-day follows across three seasons. We found a significant positive correlation between food ranking in the diet and plant abundance. The protein-to-fiber ratio and most other chemical variables tested had no statistical effect on dietary selection. Numerous chemical characteristics of the sifaka's diet were essentially by-products of generalist feeding and "low energy input/low energy crop" strategy. The examination of feeding behavior and plant chemistry in Old World colobines and folivorous prosimians in Madagascar suggests that relative lack of feeding selectivity and high primate biomass occur when the average protein-to-fiber ratio of mature leaves in the habitat exceeds a threshold at 0.4.

Relationship between taste thresholds and hunger under debate.

We determined taste recognition thresholds for six compounds (sucrose, fructose, sodium chloride, quinine sulphate, PROP and liquorice) in fasting students and, in the same subjects, after a meal. The testing procedure was the staircase-method in blind conditions. Although taste sensitivity may vary with hormonal status, our results did not show any significant difference in taste recognition thresholds between hunger and satiety. Our Bayesian analysis did not corroborate the hypothesis of increased sensitivity to nutrition-related tastants in the fasting state that was recently supported by data obtained with the two-alternative forced-choice method.