Colony-level macronutrient regulation in ants: mechanisms, hoarding and associated costs

Social and nonsocial organisms both require a suite of nutrients in correct amounts and ratios to promote growth and fitness, but the nutrient profiles of available foods are rarely optimal. Nutrient acquisition in insect societies is more complex compared to that of nonsocial organisms however, because foraging is restricted to only a proportion of the colony, and these members must satisfy their own nutritional requirements and those of other members having distinct nutritional needs. In this study we used laboratory colonies of the. re ant Solenopsis invicta to quantify how ants regulate their protein-carbohydrate intake when restricted to diets with different fixed protein-carbohydrate (p:c) ratios, and to quantify, at both the individual and colony level, behaviours and costs associated with nutrient regulation when feeding on these foods. We found that ants were most attracted to foods with equal or moderately protein-biased p:c ratios. However, colonies on these two treatments created large hoards, and the p:c ratios of these foods differed from that of collected food. In general, carbohydrates were extracted and protein was retained. As a result, carbohydrate intake on all diets except the extremely protein-biased diet was similar. However, carbohydrate regulation on diets with equal and moderately protein-biased p:c ratios may be costly through elevated worker activity and mortality, and through reduction of worker lipid reserves. For colonies feeding on heavily protein-biased food, energy production may have been achieved via gluconeogenesis. We discuss our results in relation to how dietary p:c imbalances in naturally encountered foods may be driving ant foraging behaviour in the field. (C) 2009 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.