ORIGINAL PAPER The role of per-capita productivity in the evolution of small colony sizes in ants Boris H. Kramer & Inon Scharf & Susanne Foitzik Received: 18 March 2013 /Revised: 3 July 2013 /Accepted: 13 August 2013 /Published online: 13 September 2013 # Springer-Verlag Berlin Heidelberg 2013 Abstract The evolution of colony size in social insects is influenced by both extrinsic and colony-intrinsic factors. An important intrinsic trait, per-capita productivity, often declines in larger colonies. This pattern, known as Michener’ s para- dox , can limit the growth of insect societies. In this study, we first describe this problem, survey its occurrence across dif- ferent ant species, and present a case study of eight cavity- dwelling ants with very small colony sizes. In these species, colonies might never reach sizes at which per-capita produc- tivity decreases. However, in six out of the eight focal species, per-capita productivity did decline with increasing size, in accordance with other studies on per-capita productivity in ants. Several mechanisms, such as resource availability or nest-site limitation, may explain the decrease in per-capita productivity with increases in colony size in our focal species. In these central-place foragers, the individual foraging mode is expected to lead to an increase in travel time as colonies grow. We suggest that polydomy, the concomitant occupation of several nest sites, could serve as a potential strategy to over- come this limitation. Indeed, for one species, we show that polydomy can help to circumvent the reduction in productiv- ity with increasing colony size, suggesting that limited re- source availability causes the observed decrease in per-capita productivity. Finally, we discuss the influence of other factors, such as the nesting ecology and colony homeostasis, on the evolution of colony size in these cavity-dwelling ants. Keywords Colony size . Leptothorax . Social insects . Productivity . Per-capita productivity . Michener effect . Temnothorax Introduction Evolution shapes growth, reproduction, and aging in animals (Stearns 1992). In social insects, colony size is a characteristic trait that forms the basis for various life-history trade-offs (Bourke 1999; Dornhaus et al. 2009; Kramer and Schaible 2013). Colony size varies among populations and species: Some species reach enormous colony sizes of several million individuals (Beckers et al. 1989), whereas the mature colonies of other species (e.g., our focal species) contain only a few dozen individuals. Species-specific colony sizes are deter- mined by factors both extrinsic (e.g., predation) and intrinsic (e.g., foraging mode) to the colony (Jeanne and Nordheim 1996). The advantages of large colony size include better fighting abilities and defense, higher annual survival, and increased production of sexuals (Wilson 1974; Cole 1984; Kaspari and Vargo 1995; Bourke 1999; Palmer 2004; Sorvari and Hakkarainen 2007). On the other hand, colony growth can be restricted by the limited availability of con- struction materials or nesting sites, by a limited egg laying rate of the queen or due to resource limitation caused for example by the specific foraging mode of the species (Beckers et al. 1989; Wenzel and Pickering 1991; Foitzik and Heinze 1998). An important intrinsic measure of colony fitness is the biomass production, which has been used to quantify the benefits of group living (Wcislo and Tierney 2009). Two common fitness measures in social insects, colony productiv- ity and per-capita productivity, are defined as the total number Communicated by D. Naug B. H. Kramer (*) Max Planck Institute for Demographic Research, Konrad-Zuse-Str. 1, 18057 Rostock, Germany e-mail: kramer@demogr.mpg.de I. Scharf Department of Zoology, Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel S. Foitzik Institute of Zoology, Johannes Gutenberg University of Mainz, Mainz, Germany Behav Ecol Sociobiol (2014) 68:41–53 DOI 10.1007/s00265-013-1620-8