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1. Changes in climatic factors could have major effects On the foraging performance of animals. To date, however, no study has attempted to examine the concurrent effect of different climatic factors On foraging performance of individual organisms.

2. In the present study, this issue was addressed by studying changes in foraging performance of seed-eating ant colonies of the genus Messor in response to variation in precipitation and ambient temperature along a macroecological gradient. In addition, we examined the way three colony-level attributes, foraging distance, forager number, and variance in worker-size, could affect foraging performance in those ants. Foraging performance was measured as size matching, i.e. the correlation between forager size and load size. The study was carried out for 2 years in six sites along a south-north productivity gradient in a semi-arid region of the Eastern-Mediterranean.

3. Size matching increased with increased precipitation as well as with an increase worker-size variability, but slightly decreased with increasing temperatures, as predicted by foraging-decision models. In contrast, foraging distance had no effect on size matching. Interestingly, size matching showed a unimodal relationship with forager number,

4. These results indicate that interplay between climate and body size affects foraging performance either directly via physiological constraints, or indirectly through their effect on food availability. Moreover, this is one of the first evidences to support the assumption that ant colonies can differ in their ability to optimally allocate their workforce in natural environments. This emphasises the importance of studying the way foraging strategies vary across environmental gradients at macroecological scales.