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Humans may not be the only species that struggles to eat the right amounts of the ideal foods. A new study led by researchers at Penn State suggests that what bumble bees choose to eat may not line up with their ideal nutritional needs.

The study— in the Journal of Insect ÌÇÐÄÊÓƵiology—examined whether giving bumble bees diets of pollen and nectar enriched with different amounts of proteins, fats and carbohydrates had an effect on how much they ate, as well as a variety of physical characteristics.

The researchers found that bumble bees consumed different amounts depending on which diet they were assigned, but these amounts didn't translate to better physical fitness. For example, bees over-consumed the protein-enriched pollen and under-consumed the fatty, or lipid-enriched, pollen, even though neither of these choices enhanced fitness.

Etya Amsalem, associate professor of entomology in the College of Agricultural Sciences and lead author on the study, said the findings challenge previous beliefs that scientists can make assumptions about bees' nutritional needs based on what they choose to consume.

"Bee preferences are unlikely to reflect true needs and more likely result from evolutionary constraints," she said. "For instance, bees may under-consume lipids not because high-lipid diets are harmful, but because their physiology is not well-suited for digesting or storing large amounts of lipids."

Similarly, Amsalem added, bees may over-consume proteins because they have evolved to seek them out whenever they're available, even though excess protein can be detrimental because it affects how efficiently bees can eliminate nitrogen from their bodies.

As bee populations decline around the globe, the researchers said, more studies are pointing toward nutritional stress as a contributing factor, making diet an important factor to understand. Habitat loss and agricultural development have reduced the numbers of flowers, forcing bees to rely on suboptimal food sources.

While bumble bees have been shown to regulate their food intake, Amsalem said, scientists don't know if bee preferences necessarily align with what benefits them the most.

"It's an odd assumption when you think about it—my toddler prefers chocolate over broccoli, but I wouldn't conclude from that that chocolate is healthier for her," she said. "So, why do we assume bees are different? Understanding potential mismatches between consumption and fitness outcomes is important, as most conservation and management strategies assume bees can self-regulate their diets for optimal health."

For the study, the researchers split the bees into groups and provided them with different diets manipulated by enriching the pollen or modifying the sugar solution—which mimicked nectar—with different macronutrients in different concentrations.

They then measured consumption and several indicators of fitness—mass gain, ovary activation and number of eggs laid by workers—for 10 days. All worker bees were the same age, and all tests were conducted in microcolonies containing three workers, which Amsalem said enabled the researchers to directly test if consumption aligned with improved fitness.

These groups were compared with a separate control group of bumble bees, which were allowed to feed freely from wild pollen and a 60% sugar solution.

The researchers found that increasing protein levels in pollen resulted in bees consuming more pollen, while increasing lipid levels led to bees eating less. Additionally, worker bees over-consumed sucrose when it was offered at lower concentrations.

However, despite bees choosing these eating patterns, these choices ended up negatively affecting their fitness compared to the control. In all cases, egg-laying decreased and body mass either declined or remained unchanged.

Amsalem said the findings suggest that conservation and agricultural management strategies may want to consider diet composition impacts on fitness, not just preference or intake.

"Future research could investigate why bees fail to optimize diet—for example, metabolic limitations, gustatory biases or ecological tradeoffs—and may explore nonmacronutrient components like vitamins and minerals that may explain fitness discrepancies," Amsalem said.

Anna Cressman, who earned a master of science in entomology from Penn State, and Seyed Ali Modarres Hasani, postdoctoral scholar at Penn State, also co-authored this paper.