Food for folivores: nutritional explanations linking diets to population density

Abstract

Ecologists want to explain why populations of animals are not evenly distributed across landscapes and often turn to nutritional explanations. In seeking to link population attributes with food quality, they often contrast nutritionally positive traits, such as the concentration of nitrogen, against negative ones, such as fibre concentration, by using a ratio of these traits. This specific ratio has attracted attention because it sometimes correlates with the biomass of colobine primates across sites in Asia and Africa. Although empirically successful, we have identified problems with the ratio that may explain why it fails under some conditions to predict colobine biomass. First, available nitrogen, rather than total nitrogen, is nutritionally important, while the presence of tannins is the major factor reducing the availability of nitrogen in browse plant species. Second, tannin complexes inflate measures of fibre. Finally, simple ratios may be unsound statistically because they implicitly assume isometric relationships between variables. We used data on the chemical composition of plants from three continents to examine the relationships between the concentrations of nitrogen, available nitrogen, fibre and tannins in foliage and to evaluate the nitrogen to fibre ratio. Our results suggest that the ratio of the concentration of nitrogen to fibre in leaves does sometimes closely correlate with the concentration of available nitrogen. However, the ratio may give misleading results when leaves contain high concentrations of tannins. The concentration of available nitrogen, which incorporates measures of total nitrogen, dry matter digestibility and tannins, should give a better indication of the nutritional value of leaves for herbivorous mammals that can readily be extrapolated to habitats.

Differential fruit choice in two simpatric monkey species and frugivorous bats in southern Mexico

“…This pattern is consistent with other studies in which it was found that monkeys of the genus Ateles are highly frugivorous to the degree that they are specialists (Di Fiore et al, 2008). Regarding the black howler monkeys, the results support the findings of other studies where A. pigra is considered mainly folivorous (which explains the high "Uf" proportion) (Wallis et al, 2012), but opposite to Garber et al (2015) who characterize this primate as one species with a balanced diet between leaves and fruits.…”

The effects of plant nutritional chemistry on food selection of Mexican black howler monkeys (Alouatta pigra): The role of lipids

“…In this regard Silver et al [2000] calculated the ratio as AP/ADF in leaves consumed by black howler monkeys in Belize, and also did not find differences between the protein-to-fiber ratio of young and mature leaves. This highlights the importance of measuring the amount of nutrients that can be digested and metabolized by the forager rather than the absolute amount of nutrients present in foods in understanding primate foraging decisions [DeGabriel et al, 2013;Wallis et al, 2012].…”

Linking feeding ecology and population abundance: a review of food resource limitation on primates

“…The mechanism on how this parameter affects primate abundance remains unclear as this ratio may correlate with other leaf constituents that really drive the relationship . For example, Wallis et al (2012) suggests that the available nitrogen concentration of leaves, which covers both fiber and tannin content, is a real influencing factor on folivore leaf choice. More research is needed to determine why the protein-to-fiber index predicts folivore biomass.…”