Nutritional ecology of a prototypical generalist predator, the red fox (Vulpes vulpes)

Abstract

Generalist species, which exploit a wide range of food resources, are expected to be able to combine available resources as to attain their specific macronutrient ratio (percentage of caloric intake of protein, lipids and carbohydrates). Among mammalian predators, the red fox Vulpes vulpes is a widespread, opportunistic forager: its diet has been largely studied, outlining wide variation according to geographic and climatic factors. We aimed to check if, throughout the species' European range, diets vary widely in macronutrient composition or foxes can combine complementary foods to gain the same nutrient intake. First, we assessed fox's intake target in the framework of nutritional geometry. Secondly, we aimed to highlight the effects of unbalanced diets on fox density, which was assumed as a proxy for Darwinian fitness, as assessed in five areas of the western Italian Alps. Unexpectedly, the target macronutrient ratio of the fox (52.4% protein-, 38.7% lipid- and 8.9% carbohydrate energy) was consistent with that of hypercarnivores, such as wolves and felids, except for carbohydrate intakes in urban and rural habitats. The inverse relation between density and the deviation of observed macronutrient ratios from the intake target suggests that fox capability of surviving in a wide range of habitats may not be exempt from fitness costs and that nutrient availability should be regarded among the biotic factors affecting animal abundance and distribution.

Figure 1

Right-angled mixture triangle showing the macronutrient ratios of the 30 selected diet studies (squares: arable land, dots: urban habitats, triangles: mixed habitats, diamonds: forests). The yellow square marks the intake target (mean macronutrient ratio). The coordinate for the implicit variable is read as the difference between 100% and the value at which the diagonal with a slope of − 1 that passes through the point identified by the primary coordinates intersects the I‐axis.

Figure 2

Relationship between red fox (Vulpes vulpes) relative abundance (RA = N of scats/100 m) and density in the five study areas (valleys of the rivers CH-Chalamy, CV-Cervo, EL-Elvo, SB-San Barthelemy and NO-Nomenon).

Figure 3

Relationship between pre-reproductive density and mean deviation of the macronutrient ratio from the intake target in the five study areas (valleys of the rivers CH-Chalamy, CV-Cervo, EL-Elvo, SB-San Barthelemy and NO-Nomenon).

Figure 4

Location of the 30 selected studies (1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30). Map created by authors using QGIS 3.14 (https://qgis.org/).

Figure 5

The five study areas (red polygons) in the western Italian Alps where red fox (Vulpes vulpes) diets, macronutrient ratios and densities were assessed in 2021–2022 (S. Barthelemy: 25.9 km², Nomenon: 14.9 km², Chalamy: 31.4 km², Cervo: 12.3 km², Elvo: 12.1 km²). Map created by authors using QGIS 3.14 (https://qgis.org/).