Nutritional ecology and digestive response to dietary shift in the large South American fox, Pseudalopex culpaeus

TL;DR

Captive culpeo foxes fed rat, mixed rat–peppertree fruit, or fruit-only diets showed strong diet-quality effects on intake, digestibility, retention time, and body mass. Foxes maintained or gained mass on rat and mixed diets but lost mass on fruit alone, leading the authors to conclude that *Pseudalopex culpaeus* cannot survive exclusively on fruits beyond about seven days.

Key findings

• Body mass at 10 days differed significantly among diets (F₂,₁₂ = 5.18, P = 0.02): rat and mixed diets produced positive mass balance, whereas fruit-fed foxes lost mass; the authors report a 37% decrease in body mass on fruit diets versus a 46% decrease in foxes without food.
• Dry-matter intake was highest on fruit (310.5 ± 41.7 g d⁻¹), intermediate on mixed (218.9 ± 60.45 g d⁻¹), and lowest on rat (115.3 ± 15.2 g d⁻¹; F₂,₁₀ = 4.22, P = 0.04), while energy intake was similar across diets and nitrogen intake trended downward from rats to fruits.
• Apparent dry-matter digestibility was 87.3 ± 0.9% on rat, 53.4 ± 1.3% on mixed, and 32.7 ± 7.7% on fruit (F₂,₁₀ = 775.84, P < 0.0001); energy digestibility was 92.9 ± 0.5%, 52.7 ± 1.7%, and 29.6 ± 8%, respectively (F₂,₁₀ = 792.67, P < 0.0001).
• Mean retention time was shortest on fruit (12.0 ± 0.7 h), intermediate on mixed (18.8 ± 1.0 h), and longest on rat (28.2 ± 1.6 h; F₂,₁₀ = 737.14, P < 0.0001), and was negatively correlated with dry-matter intake; the authors hypothesize that mixed diets may yield higher assimilation efficiencies and positive energy/mass balance during low prey availability.

Methods

• Fifteen nonreproductive adult culpeo foxes (nine males, six females; 4,480–6,140 g) were captured in central and northern Chile between May 1998 and June 2000 and assigned to three diets with five foxes each: live laboratory rats (400 g d⁻¹), peppertree (*Schinus molle*) fruits ad libitum, or a mixed diet (1:4 minced laboratory rats to peppertree fruits) ad libitum for 10 days.
• Nutritional trials used metabolic cages with feces collection; food intake was measured gravimetrically, diets and feces were analyzed by proximal chemical analysis and bomb calorimetry, digestibility was calculated for dry matter, nitrogen, and energy, and mean retention time was estimated using marker particles (100 2-mm² flagging tape) with feces collected every 2 h for two days.
• Data were analyzed with one-way ANOVA and Tukey tests (α = 0.05); results are reported as mean ± SE in the text, though Table 2 indicates mean ± SD.

Limitations

• The paper states that because food intake and mean retention time covary, “it is difficult to assign causality”; the field benefit of mixed diets is presented as a hypothesis rather than a tested outcome.
• Evident from the text, not stated as a formal limitation: each diet had only five captive foxes, trials lasted only 10 days, and only one fruit species (*S. molle*) was tested.
• Evident from the text, not stated as a formal limitation: survival conclusions for wild foxes are extrapolated from short-term captive feeding trials, and the authors note that temporal and spatial variation in prey nutrient, energy, and water contents may affect fox nutrition and mass balance.