Functional Ecology 2006 20, 1037–1044 1037 © 2006 The Authors. Journal compilation © 2006 British Ecological Society Blackwell Publishing Ltd Ant stoichiometry: elemental homeostasis in stage-structured colonies A. D. KAY,*†‡ S. ROSTAMPOUR* and R. W. STERNER* *Department of Ecology, Evolution and Behavior, University of Minnesota, St Paul, MN, USA Summary 1. Organisms facing variation in food quality maintain elemental composition within limited bounds. Such stoichiometric homeostasis has often been considered a species- specific parameter, but stoichiometry can also vary intraspecifically across life stages, sexes and sizes. In colonial organisms with overlapping generations, stoichiometric vari- ation among stages could lead to flexibility in colony-level elemental composition due to changes in internal demography. 2. We examine how the balance of energy (sucrose) and nutrients (prey) affects growth rate and carbon : nitrogen : phosphorus (C : N : P) homeostasis in a eusocial insect, the pavement ant Tetramorium caespitum. 3. Colony growth depended heavily on prey availability. However, sucrose scarcity led to higher worker mortality and production of smaller workers, suggesting sucrose availability will affect colony-level performance in a competitive environment. 4. In contrast, C : N : P stoichiometry of larvae, pupae, and workers varied mostly with sucrose availability. Biomass P content within life stages was lower in colonies receiving less access to sucrose. We suggest this difference arose primarily from shifts in individual ant mass coupled with negative P-body mass relationships. 5. Life stages differed considerably in elemental composition, and resource conditions affected colony stage structure. Nevertheless, variation in colony-level stoichiometry primarily reflected compositional differences within stages rather than shifts in internal demography. Key-words: carbohydrates, growth rate hypothesis, nutrient balance, phosphorus, RNA Functional Ecology (2006) 20, 1037–1044 doi: 10.1111/j.1365-2435.2006.01187.x Introduction Homeostasis has served as a conceptual framework for animal physiologists for over a century (Bernard 1872; Randall, Burggren & French 2001). More recently, it has emerged as a central idea in ecological stoichiom- etry, the study of the balance of energy and materials in living systems (Sterner & Elser 2002). The degree to which organisms regulate elemental composition may influence the magnitude and nature of their ecological impact. For example, strict elemental homeostasis may result in consumers disproportionately depleting resources containing scarce materials and increasing recycling rates of materials ingested in excess of demand (Vanni 2002). Alternatively, elemental composition may be flexible, and stoichiometric shifts in individuals could reflect changes in functional capabilities (Kay et al . 2005). For example, the growth rate hypothesis (GRH) predicts a causal link between variation in whole-body P concentration and growth rate, a key life-history attribute, due to the role that P-rich ribo- somal RNA plays in regulating protein synthesis (Elser et al . 1996). Elucidating the degree of homeostasis in different taxa and the factors that explain regulatory variation should help clarify the mechanisms that link food quality to consumer traits that affect ecological interactions (Sterner & Elser 2002). In this paper, we examine elemental homeostasis at the individual- and colony-level in the pavement ant, Tetramorium caespitum . No previous study has exam- ined the C : N : P stoichiometry of a social insect, but this group may prove to be an important system for exploring the maintenance and ecological relevance of elemental homeostasis. Relative to solitary animals, ants and other social insects may be more likely to vary in chemical composition in response to changing food quality due to within-colony specializations. Social insect colonies contain multiple life stages with †Author to whom correspondence should be addressed. E-mail: adkay@stthomas.edu ‡Present address: Department of Biology, University of St Thomas, St Paul, MN, USA, 55105