Flexible, but not enough: How an omnivorous ant copes with macronutrient imbalances

Abstract

Omnivores may have high flexibility in adjusting their ingestion rates and degree of food selectivity to regulate the nutrient uptake and maintain homeostasis. However, if resources are limited, their ability to regulate the nutrient balance may be constrained, affecting their performance and ecological functions. Here, we studied how an omnivorous ant species, Dorymyrmex tener, deals with a nutritionally unbalanced diet, which in turn may affect population dynamics. We first, determined the elemental composition of D. tener under natural scenarios of resource variability. Then, to test how different fixed diets (in terms of C:N ratio) influence pre- and post-ingestive regulating mechanisms, we exposed colonies to two diets with different C:N ratios (18:1 and 15:1). We measured food consumption as a proxy of pre-ingestive mechanism, and phenotypic and behavioral plasticity as a proxy of post-ingestive mechanisms (i.e., body weight and C:N ratio, and activity level, predator behavior, and interspecific aggression, respectively). Finally, to determine the colony performance we assessed worker and juvenile’s mortality. We found that field colonies of D. tener maintained their homeostasis (measured as C:N ratio) despite seasonal variations in resource availability. Furthermore, experimental colonies fed a 15:1 C:N ratio diet showed a higher per capita food consumption and had marginally heavier workers compared to those fed the 18:1 diet. However, these dietary differences did not lead to detectable changes in behavioral traits such as activity levels, predation, or aggression, nor did they impact overall colony performance. Our findings suggest that this omnivorous ant possesses a well-developed regulatory mechanism for maintaining macronutrient balance, primarily through pre-ingestive strategies. However, its ability to compensate for nutritional imbalances may be constrained in environments with limited food diversity, potentially affecting its ecological performance and resilience.