Ecology dictates the value of memory for foraging bees

"Ecological intelligence" hypotheses posit that the benefits of cognitive investment vary with foraging ecology, and provide a key framework for understanding the evolution of animal learning and memory1-4. However, although certain ecological selection pressures have been found to correlate with brain or neural region size5-8, empirical evidence to show that any specific cognitive trait is useful in certain environments but not others is currently lacking. Here, we assay the short-term memory of bumblebee (Bombus terrestris audax) workers from 25 identically reared colonies, before allowing each colony to forage in a landscape where forage availability varies seasonally. Through analysis of the bees lifetime foraging careers, comprising >1700 foraging trips over two years, we show that performance on a task designed to test short-term memory predicts individual foraging efficiency - a fitness proxy that is key to colony reproductive output - in plentiful spring foraging conditions. However, this relationship is reversed during the summer floral dearth, when the costs of cognitive investment may outweigh the benefits. Our results provide evidence that the value of a cognitive trait depends upon the prevailing ecological conditions and suggest that temporal changes in that environment could place contrasting selection pressures on memory within a single species.