Seasonal effects of food macronutrient content on ants: Colony and individual responses

Authors: Cook, Steven; EUBANKS, MICKY - Texas A&M University; GOLD, ROGER - Texas A&M University; BEHMER, SPENCER - Texas A&M University

Submitted to: Journal of Insect Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/5/2016
Publication Date: 2/6/2016
Citation: Cook, S.C., Eubanks, M.D., Gold, R.E., Behmer, S.T. 2016. Seasonal effects of food macronutrient content on ants: Colony and individual responses. Journal of Insect Physiology. doi: 10.1016/j.jinsphys.2016.02.001.

Interpretive Summary: Long-lived animals show seasonal changes in their foraging behavior and food selection that follow shifts in food quality and abundance. Animal physiology also changes seasonally. These changes may be adaptive, allowing such animals to best match seasonal nutritional demands while minimizing costs. Previously we demonstrated that food collection behaviors and nutrient regulation strategies differed between summer and fall fire ant colonies. In this paper we explore the adaptive nature of these contrasting responses by quantifying colony- and individual-level traits in both summer and fall-collected fire ant colonies presented with synthetic foods that differed in their protein-carbohydrate ratio (ranging from carbohydrate-biased, to equal ratio, to protein-biased). Two experiments were conducted: In the first experiment, colonies received a single food item. In the second experiment, colonies received two nutritionally complementary food items (these colonies could self-select their protein-carbohydrate intake). We found that nutrient availability and consumption had only transient effects on colony-level characteristics, suggesting that colony level responses to seasonality per se are fixed, and shift seasonally following a circannual rhythm. At the end of five weeks, total worker mortality was nearly three times greater in summer colonies, but total worker biomass was similar between summer and fall colonies because summer colonies produced more brood. Lipid content of individual workers was positively correlated with carbohydrate consumption in summer colonies, but over time, summer ants maintained lipid content at initial levels while fall colonies significantly compounded lipid reserves, even when feeding on protein-biased foods. Finally, total brood mass at the end of experiments was not significantly different seasonally, or by nutrient availability or consumption. Our results suggest that that colony-level seasonal differences are adaptive, and the information presented should help other researchers interpret results from studies conducted during different seasons.

Technical Abstract: Long-lived animals experience different conditions as seasons change. Associated with seasonality are shifts in food quality and abundance. Animals can respond to seasonality by modifying their foraging behavior and adjusting the manner in which they utilize and process ingested nutrients. This allows them to best match nutritional demands while minimizing costs. Previously we demonstrated that food collection behaviors and nutrient regulation strategies differed between summer and fall fire ant colonies. In this paper we explore the adaptive nature of these contrasting responses by quantifying colony- and individual-level traits. Summer and fall fire ant colonies were collected and maintained under identical temperature, humidity, and photoperiod regimes. Two experiments were performed in which colonies were presented with foods differing in their protein-carbohydrate ratio. In the first experiment, colonies received a single food item. In the second experiment, colonies received two nutritionally complementary food items. After 5 weeks, total worker mortality was nearly three times greater in summer colonies, but total worker biomass was similar between summer and fall colonies because summer colonies produced more brood. Lipid content of individual workers was positively correlated with carbohydrate consumption in summer colonies, while fall colonies significantly compounded lipid reserves, regardless of diet. Nutrient availability and consumption had only transient effects on colony-level characteristics, suggesting that colony level responses to seasonality per se are fixed, and shift seasonally following a circannual rhythm.