Abstract:
Animals carefully regulate the amount of protein that they consume.
The quantity of individual essential amino acids (EAAs) obtained from
dietary protein depends on the protein source, but how the proportion
of EAAs in the diet affects nutrient balancing has rarely been studied.
Recent research using the Geometric Framework for Nutrition has
revealed that forager honeybees who receive much of their dietary
EAAs from floral nectar and not from solid protein have relatively low
requirements for dietary EAAs. Here, we examined the nutritional
requirements for protein and carbohydrates of foragers of the bufftailed
bumblebee Bombus terrestris. By using protein (sodium
caseinate) or an equimolar mixture of the 10 EAAs, we found that
the intake target (nutritional optimum) of adult workers depended on
the source and proportion of dietary EAAs. When bees consumed
caseinate-containing diets in a range of ratios between 1:250 and
1:25 (protein to carbohydrate), they achieved an intake target (IT) of
1:149 (w/w). In contrast to those fed protein, bees fed the EAA diets
had an IT more biased towards carbohydrates (1:560 w/w) but also
had a greater risk of death than those fed caseinate. We also tested
how the dietary source of EAAs affected free AAs in bee
haemolymph. Bees fed diets near their IT had similar haemolymph
AA profiles, whereas bees fed diets high in caseinate had elevated
levels of leucine, threonine, valine and alanine in the haemolymph.
We found that like honeybees, bumblebee workers prioritize
carbohydrate intake and have a relatively low requirement for
protein. The dietary source of EAAs influenced both the ratio of
protein/EAA to carbohydrate and the overall amount of carbohydrate
eaten. Our data support the idea that EAAs and carbohydrates in
haemolymph are important determinants of nutritional state in
insects.
