The effect of dietary protein quality on nitrogen isotope discrimination in mammals and birds

Abstract

We tested the competing hypotheses that (1) nitrogen discrimination in mammals and birds increases with dietary nitrogen concentration or decreasing C:N ratios and, therefore, discrimination will increase with trophic level as carnivores ingest more protein than herbivores and omnivores or (2) nitrogen discrimination increases as dietary protein quality decreases and, therefore, discrimination will decrease with trophic level as carnivores ingest higher quality protein than do herbivores. Discrimination factors were summarized for five major diet groupings and 21 different species of birds and mammals. Discrimination did not differ between mammals and birds and decreased as protein quality (expressed as biological value) increased with trophic level (i.e., herbivores to carnivores). Relationships between discrimination factors and dietary nitrogen concentration or C:N ratios were either the opposite of what was hypothesized or non-significant. Dietary protein quality accounted for 72% of the variation in discrimination factors across diet groupings. We concluded that protein quality established the baseline for discrimination between dietary groupings, while other variables, such as dietary protein intake relative to animal requirements, created within-group variation. We caution about the care needed in developing studies to understand variation in discrimination and subsequently applying those discrimination factors to estimate assimilated diets of wild animals.

Stable carbon and nitrogen incorporation in blood and fin tissue of the catfish Pterygoplichthys disjunctivus (Siluriformes, Loricariidae)

“…S1) were higher than the widely used average of 3.4‰ for this isotope for muscle tissue (Fry and Sherr 1984;Focken 2004;Fry 2007), but were not inconsistent with Δ 15 N tissue-diet (> 4‰) in herbivorous mammals (Robbins et al 2005) or fishes (Mill et al 2007). The major contributing factors to Δ 15 N tissue-diet are growth rate, dietary protein content, protein quality (i.e., the biological value, or how well the amino acid profile of the dietary protein meets the needs of the animal; Robbins et al 2005;Robbins et al 2010), and the dietary δ 15 N (Caut et al 2009;Martínez del Rio et al 2009). Herbivorous animals generally consume protein that is low-quality, and/or has low δ 15 N (Robbins et al 2005), and hence, herbivorous animals have larger Δ 15 N tissue-diet than carnivores.…”

Use of carbon and nitrogen stable isotopes to study the feeding ecology of small coastal cetacean populations in southern Brazil

“…Based on these revisions, it has been observed that nitrogen trophic fractionation depends on several aspects of both diet and the consumer. With regards to the former, the quantity and the quality of the protein is a key factor (Robbins et al 2005, Martínez del Rio et al 2009, Caut et al 2011). In the latter, the magnitude of nitrogen trophic fractionation depends on the type of tissue in question, as there are variations in turnover rates for different tissues (Newsome et al 2010).…”

Naturally-occurring stable isotopes as direct measures of larval feeding efficiency, nutrient incorporation and turnover

“…Epstein 1978, 1981;Fry and Sherr, 1984;Fry, 1991;Van der Zanden et al, 1999). However, the relationships between nitrogen isotopic discrimination and other factors such as dietary protein supply and quality remain poorly understood (Robbins et al, 2005;Martinez del Rio et al, 2009). Moreover, there is increasing evidence that isotopic discrimination factors are species-and tissuespecific (DeNiro and Epstein, 1981;Tiezen et al, 1983;Yokoyama et al, 2005;Stenroth et al, 2006) and the considerable variance in the reported values demonstrate that careful validation of assumptions about discrimination factors are critical for interpreting stable isotope data from experimental feeding studies (Cabana and Rasmussen, 1996;McCutchan et al, 2003;Crawley et al, 2007;Martinez del Rio et al, 2009).…”