Journal of Animal Ecology 2009, 78, 848–856 doi: 10.1111/j.1365-2656.2009.01546.x © 2009 The Authors. Journal compilation © 2009 British Ecological Society Blackwell Publishing Ltd Individual-level diet variation in four species of Brazilian frogs M. S. Araújo 1 *†, D. I. Bolnick 2 , L. A. Martinelli 3 , A. A. Giaretta 4 and S. F. dos Reis 1 1 Departamento de Parasitologia, Instituto de Biologia, Universidade Estadual de Campinas, Caixa Postal 6109, 13083-970, Campinas, SP, Brazil; 2 Integrative Biology, University of Texas at Austin, 1 University Station C0930, Austin, TX 78712, USA; 3 Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, Av. Centenário, 303, 13416-000, Piracicaba, SP, Brazil; and 4 Laboratório de Ecologia e Sistemática de Anuros Neotropicais, Instituto de Biologia, Universidade Federal de Uberlândia, 38400-902, Uberlândia, MG, Brazil Summary 1. Many natural populations exploiting a wide range of resources are actually composed of relatively specialized individuals. 2. This interindividual variation is thought to be a consequence of the invasion of ‘empty’ niches in depauperate communities, generally in temperate regions. If individual niches are constrained by functional trade-offs, the expansion of the population niche is only achieved by an increase in interindividual variation, consistent with the ‘niche variation hypothesis’. 3. According to this hypothesis, we should not expect interindividual variation in species belonging to highly diverse, packed communities. 4. In the present study, we measured the degree of interindividual diet variation in four species of frogs of the highly diverse Brazilian Cerrado, using both gut contents and δ 13 C stable isotopes. 5. We found evidence of significant diet variation in the four species, indicating that this phenomenon is not restricted to depauperate communities in temperate regions. 6. The lack of correlations between the frogs’ morphology and diet indicate that trade-offs do not depend on the morphological characters measured here and are probably not biomechanical. The nature of the trade-offs remains unknown, but are likely to be cognitive or physiological. 7. Finally, we found a positive correlation between the population niche width and the degree of diet variation, but a null model showed that this correlation can be generated by individuals sampling randomly from a common set of resources. Therefore, albeit consistent with, our results cannot be taken as evidence in favour of the niche variation hypothesis. Key-words: carbon stable isotopes, individual specialization, niche variation, Niche Variation Hypothesis Introduction Many natural populations exploiting a wide variety of resources are actually composed of relatively specialist individuals (West 1986; Werner & Sherry 1987; Svanbäck & Bolnick 2007; Araújo et al. 2008). This ‘individual specialization’, which has been documented in more than 100 taxa (Bolnick et al. 2003), may have important ecological and evolutionary implications. For example, several models of demographic stochasticity and population dynamics predict that ecologically variable populations have more stable dynamics (Lomnicki 1988; Kendall & Fox 2002, 2003; Fox 2005), a prediction recently confirmed empirically in experimental populations of flour beetles (Agashe, in press). Additionally, if individuals use only a subset of the population niche, competitive interactions will be frequency dependent and will favour rare strategies (Wilson & Turelli 1986; Bolnick 2001). This in turn may drive disruptive selection (Bolnick 2004; Pfennig, Rice & Martin 2007; Bolnick & Lau 2008), which may increase the population genetic and phenotypic variance (Roughgarden, 1972) and cause evolutionary divergence (Dieckmann & Doebeli, 1999; Doebeli et al. 2007). While most studies published so far have only been able to document the presence of individual specialization in natural populations (Bolnick et al. 2003), available indices that actually *Correspondence author. E-mail: msaraujo@gmail.com †Present address: Integrative Biology, University of Texas at Austin, 1 University Station C0930, Austin, TX 78712, USA