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In desert areas, predation risk is one of the highest costs of foraging and is a major influence on animal behaviour. Several strategies are used by foragers for surviving and reproducing in desert areas. The foraging strategies of the small mammals of South American deserts are still poorly known. In this study, we investigated the foraging strategies of rodents of the Monte Desert in response to distance from seed sources to sheltered sites (i.e. shrubs) during two different seasons (wet and dry). We evaluated the relative rates of removal of two species of seeds (millet and sunflower) by rodents at two sites by establishing 80 seed sources, 40 in unsheltered microhabitats and 40 in sheltered microhabitats. We recorded both the number of caches and seed consumption for each source. We found that plant cover affected the foraging activity of rodents of the sand dunes in the Monte Desert because both consumption and numbers of caches constructed from sheltered seed sources were higher than those from unsheltered ones. Consumption of sunflower was higher in the wet season than it was in the dry season, when millet consumption increased. Sunflower was the preferred seed both from sheltered or unsheltered sources. We discuss the possible causes of the different foraging strategies used by rodents of the Monte Desert.

a b s t r a c tPatterns of evolution and systematics of sigmodontine rodents are matters of continuous revision and debate. The silky mouse, Eligmodontia, is a phyllotine rodent adapted to arid environments. Chromosomal and molecular data have identified six species in this genus. Among these E. puerulus and E. moreni are sister taxa from the high Andean and lowland deserts, respectively, with large chromosomal differences and intermediate levels of molecular divergence. The purpose of our study was to quantify the degree of variability (morphological, cytogenetic, and molecular) and to analyze its evolutionary implications within, and between, these sister species in the Monte and Puna biomes of Argentina. Our results show a high variability at the chromosomal and molecular level, but low morphological differentiation among populations of E. puerulus. Diploid numbers vary from 31 to 37 due to a complex Robertsonian system, whereas cytochrome-b distances range from 0.15% to 5.75%. On the other hand, E. moreni shows high morphological differentiation between populations, but low intraspecific differentiation at the molecular (from 0.73% to 1.4%) and chromosomal level (2n = 52). Comparison of E. puerulus with E. moreni reveals high morphological and chromosomal distinction between them, but absence of molecular differentiation. Our results suggest that: (1) the high genetic variability of E. puerulus could be associated to its geographic distribution in the complex topography of the high Andean Puna; (2) the high morphological differentiation between E. moreni and E. puerulus could be the result of natural selection; and (3) molecular polyphyly between E. puerulus and E. moreni could be due to introgression or a recent split of these taxa. Finally, our results emphasize the need to integrate different datasets in the analysis of species variability and diversification, as tools for understanding their evolutionary histories.

Foraging strategies have traditionally been modelled as a result of food selection in response to one factor, as for instance resource availability, deterrent compounds or nutrients. Thus, a trade-off is assumed between plasticity (generalist strategy) and efficiency (specialist strategy). Nevertheless, several studies have demonstrated that animals cope behaviourally with food supply variation. For instance, desertdwelling rodents partially compensate for nutritional bottlenecks through diet selection. The aim of our study was to test how foraging behaviour matches spatial and temporal variations in the trophic environment and how modelling hypotheses help us to understand the resultant foraging strategy. Our animal study model was the small cavy Microcavia australis, a widely distributed herbivorous rodent. Fieldwork was carried out in four places, in wet and dry seasons. We found significant differences in plant cover, plant diversity and niche breadth, and diet selection revealed a complex foraging strategy. M. australis shows a behavioural repertoire that exceeds single-criterion categories; therefore, we appeal to theoretical models that consider ecological and physiological perspectives. We classified the small cavy as a facultative specialist displaying a thoroughly opportunistic strategy based on the plasticity of the behavioural phenotype. We finally discuss the evolutionary relevance of our results and propose further investigation avenues.

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