DOI: 10.5061/dryad.pvmcvdnjt
title: Seasonal isotopic niche of a rodent: High between-individual variation but no changes in individual niche width during the rich-resource period

description: 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. 
description: <strong>1. Capture procedures </strong>
 We captured <i>Rhipidomys macrurus</i> 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).
 <strong>2. Sample collection and processing of fecal
 samples</strong> 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 <i>R. macrurus</i>. 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 <i>R. macrurus</i>. 
 <strong>3. Food resources collection and
 processing </strong>  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 <i>R. macrurus</i>. 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).
 <strong>4. Isotopic analysis </strong>
 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 (<i>δ</i>) 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 <i>δ</i><sup> 13</sup>C = -13.1‰
 and <i>δ</i><sup> 15</sup>N = 5.1‰; tropical soil
 with <i>δ</i><sup> 13</sup>C = -26.5‰ and
 <i>δ</i><sup> 15</sup>N = 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.