Many generalist populations may actually be composed of relatively specialist individuals. This ‘individual specialization’ may have important ecological and evolutionary implications. Although this phenomenon has been documented in more than one hundred taxa, it is still unclear how individuals within a population actually partition resources. Here we applied several methods based on network theory to investigate the intrapopulation patterns of resource use in the gracile mouse opossum Gracilinanus microtarsus. We found evidence of significant individual specialization in this species and that the diets of specialists are nested within the diets of generalists. This novel pattern is consistent with a recently proposed model of optimal foraging and implies strong asymmetry in the interactions among individuals of a population.

The diet of the gracile mouse opossum Gracilinanus microtarsus was studied in a cerrado remnant in south-eastern Brazil through the analysis of faeces sampled from adult individuals. Patterns of food resource consumption were assessed using the statistics of per cent occurrence. Intrapopulation variation in the number of food items detected in faeces as a function of relevant factors was inferred using generalized linear models. The latter statistical formalism also allowed variation in the number of food items detected in faeces to be interpreted in terms of rate ratios among levels of significant factors. Insects, spiders, snails and fruits were detected in the faeces of G. microtarsus, with insects, particularly termites, beetles and ants, being the most frequently detected food resource. These food resources were consumed in proportion to their relative abundance in the cerrado, suggesting that G. microtarsus is an opportunistic forager feeding primarily on insects. The generalized linear model identified sex, season and food resource as significant factors affecting the number of food items detected in faeces, as well as interactions between sex and season and season and food resource. Rate ratios calculated between sexes within seasons showed that the number of food items detected in the faeces of males was larger than that detected in the faeces of females in the warmwet and cool-dry seasons. Rate ratios calculated between seasons within sexes showed different trends in the number of food items detected in faeces for each significant food resource. It is suggested that differences in the number of food items detected in faeces between sexes within season and between seasons within sexes are related to energetic requirements associated with reproduction.

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Bourguyia hamata females oviposit almost exclusively inside the rosette formed by the curled leaves of the epiphytic bromeliad Aechmea nudicaulis. We investigated whether the architecture of the individual bromeliads influences oviposition site selection by this harvestman species. We collected data on the presence of clutches inside bromeliads, rosette length, rosette slope in relation to tree trunks, and the amount of debris inside the rosette. Additionally, we measured the water volume inside the rosettes as well as the variation in the humidity inside and outside bromeliads with long and short rosettes. Longer rosettes were preferred as oviposition site possibly because they accumulate more water and maintain lower internal humidity variation than the external environment. Although the slope of the rosettes did not influence the occurrence of oviposition, the probability of debris accumulation inside the rosettes increased with their slope, and the frequency of clutches was greater in bromeliads with small amounts of debris. A field experiment showed that bromeliads with water inside the rosette were more frequently used as oviposition sites than bromeliads without water. In conclusion, females oviposit predominantly in bromeliads that accumulate more water and have small amounts of debris inside the rosettes, probably because these characteristics promote a more adequate microhabitat for egg development.

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