The dynamics of trophic niche width in animals at both population- and individual-level is potentially influenced by temporal variation of food resources, by between-individual differences in food-resource rank preferences, and also by competition. Using stable isotope of carbon and nitrogen (δ¹³C and δ¹⁵N) of fecal samples, we investigated the trophic niche dynamics and individual variation in food-resource use by the arboreal rat Rhipidomys macrurus, in the highly seasonal Brazilian savanna (Cerrado). We tested the hypothesis that dietary niche expansion during the rich-resource period (wet season) occurs via individual specialization and consequently lower individual niche overlap in contrast with niche retraction during the low-resource period (dry season) via increase in niche overlap and expansion of individual niches. The results indicated that R. macrurus is primarily frugivorous and presents a wider isotopic niche in the rich-resource period in comparison to the low-resource period. The increase in niche width was achieved by individual specialization (decrease in niche overlap), as expected. During the low-resource period, however, individual niche widths were not wider than during the rich-resource period. Additionally, individual body condition was lower in the wet season than in the dry season, suggesting higher competition in this period. We conclude that an increase in the population niche may involve only between-individual variation and not necessarily requiring changes in individual niche width. We propose that the combination of ecological opportunity (high resource diversity) in addition to a greater competition in the warm-wet season leads to expansion of the population trophic niche width via individual specialization. 

1. Capture procedures 

We captured Rhipidomys macrurus individuals in the Brazilian savanna (Cerrado) using Sherman live traps, and registered the body mass (to the nearest g) and body length (to the nearest mm) of each captured individual, which was also ear-tagged for further identification (National Band and Tag Co.; Monel tag, size 1).

2. Sample collection and processing of fecal samples

We collected the fecal samples from traps or during handling of the trapped animals. The fecal samples were taken to the laboratory and first washed in two superimposed sieve meshes (0.1 mm and 0.7 mm) to obtain the remains of the food items consumed by R. macrurus. Then, the remains were oven-dried at 60°C for 72 h, mill-grounded into a homogeneous powder, and weighed (minimum sample weight=1.5 mg) in tin capsules on an analytical scale. The homogenization of the fecal samples minimized the chance of analyzing only a sub-set (considering the total fecal sample) of food items consumed by R. macrurus. 

3. Food resources collection and processing 

 For arthropods, we used pitfall traps (200-ml cups) and window traps (2-l plastic bottles with a 12 x 17 cm front-opening) filled with water and a few drops of soap, which were deployed randomly in the study site during the cool-dry and warm-wet seasons. Each arthropod morphotype consisted of one sample submitted to isotopic analysis. Concomitantly with arthropod sampling, we randomly deployed collectors (1.5 m by 1.5 m fabric squares attached by four aluminum rods fixed in the ground) to obtain fruits potentially consumed by R. macrurus. Complementarily, along trails we manually collected fruits of the plant families potentially consumed by small mammals. As with arthropods, each fruit morphotype consisted of one sample submitted for isotopic analysis. Each sample of arthropods or fruits collected was washed in distilled water, oven-dried at 60°C (arthropods for 72 h and fruits for 240 h), mill-grounded into a homogeneous powder, and weighed (minimum sample weight=1.5 mg) in tin capsules on an analytical scale (0.001 g precision).

4. Isotopic analysis 

Fecal samples and food resources placed into tin capsules were analyzed for C and N isotopic analysis using an elemental analyzer (Carlo Erba, model 1110, Milan, Italy), interfaced directly to a mass spectrometer for isotopic analysis (ThermoQuest-Finningan Delta Plus, Finnigan-MAT, California, USA) at the Isotope Ecology laboratory at CENA/USP (Piracicaba, Brazil). The results were reported using delta notation (δ) in parts per thousand (‰), relative to standard international references (Vienna Pee Dee Belemnite for carbon and atmospheric air for nitrogen). During analysis, the target samples are interspersed with several replicates of two laboratory reference materials (sugarcane with δ ¹³C = -13.1‰ and δ ¹⁵N = 5.1‰; tropical soil with δ ¹³C = -26.5‰ and δ ¹⁵N = 12.1‰). Long-term standard deviations of both internal standards used at CENA/USP are of 0.2‰ for carbon and 0.3‰ for nitrogen. These reference laboratory materials are calibrated against at least four different IAEA and USGS reference materials.