Does fat identity matter? The effect of saturated and unsaturated fatty acids on bumble bee consumption and fitness

Abstract Different animal species have specific nutrient needs, guiding their foraging preferences towards particular food properties, such as specific nutrient content or ratios, and thus their ecological interactions. Bumble bees, for instance, can assess pollen fatty acid content, impacting their foraging choices and reproductive success. While they avoid high fatty acid concentrations, certain fatty acids can enhance cognitive abilities and potentially benefit bee fitness. However, the effects of specific fatty acids on bees remain unclear, even though unsaturated fatty acids are generally considered more beneficial than saturated ones. To investigate this further, we conducted feeding experiments with pollen enriched with six fatty acids at varying concentrations fed to bumble bees. Our findings reveal that bees mostly preferred pure pollen over high fatty acid concentrations. Additionally, high fatty acid concentrations negatively affected reproduction, while lower concentrations, resembling pure pollen, had no such effect. Notably, pollen enriched with stearic acid was consistently consumed regardless of concentration without adverse effects on reproduction, even at high concentrations. Conversely, low palmitic acid concentrations boosted survival and reproduction, but this benefit vanished at higher concentrations. Overall, our results emphasise the importance of fatty acid concentration for bee nutrition and fitness, while also demonstrating that individual fatty acids can have differing effects on bee behaviour and health. Pollen fatty acid composition may therefore strongly affect the bees' foraging choices and hence plant–bee interactions.