Social Brain Energetics: Ergonomic Efficiency, Neurometabolic Scaling, and Metabolic Polyphenism in Ants

Abstract

Metabolism, a metric of the energy cost of behavior, plays a significant role in social evolution. Body size and metabolic scaling are coupled, and a socioecological pattern of increased body size is associated with dietary change and the formation of larger and more complex groups. These consequences of the adaptive radiation of animal societies beg questions concerning energy expenses, a substantial portion of which may involve the metabolic rates of brains that process social information. Brain size scales with body size, but little is understood about brain metabolic scaling. Social insects such as ants show wide variation in worker body size and morphology that correlates with brain size, structure, and worker task performance, which is dependent on sensory inputs and information-processing ability to generate behavior. Elevated production and maintenance costs in workers may impose energetic constraints on body size and brain size that are reflected in patterns of metabolic scaling. Models of brain evolution do not clearly predict patterns of brain metabolic scaling, nor do they specify its relationship to task performance and worker ergonomic efficiency, two key elements of social evolution in ants. Brain metabolic rate is rarely recorded and therefore the conditions under which brain metabolism influences the evolution of brain size are unclear. We propose that studies of morphological evolution, colony social organization, and worker ergonomic efficiency should be integrated with analyses of species-specific patterns of brain metabolic scaling to advance our understanding of brain evolution in ants.

Transcriptomic analysis of mosaic brain differentiation underlying complex division of labor in a social insect

“…Enriched GO terms in this module related to metabolic processes (Figure 2b), consistent with our GO enrichment results, and contained several genes related to neurotransmission, steroid hormone activity, as well as nucleic acid binding and regulation. Therefore, it appears likely that metabolic processes scale in a size‐determined manner, consistent with other findings (Coto & Traniello, 2022).…”

White Paper: An Integrated Perspective on the Causes of Hypometric Metabolic Scaling in Animals

“…Brain size scales hypometrically with respect to body size across diverse animal clades ( Eberhard and Wcislo 2011 ; Polilov and Makarova 2017 ; Burger et al 2019 ; Godfrey et al 2021 ; McCurry et al 2021 ), with few exceptions (e.g., Groothuis and Smid 2017 ). If increased total brain or brain compartment size allows increased information processing ability to meet complex behavior/cognitive demands ( Chittka and Niven 2009 ; Dunbar and Shultz 2017 ; DeCasien and Higham 2019 ; Kilmer and Rodríguez 2019 ; ), then selection for neural performance may drive the evolution of increased relative brain investment in smaller animals to maintain similar behavioral/cognitive capacities during predator avoidance or competition with larger animals ( Seid et al 2011 ; Coto and Traniello 2021 ; Muratore et al 2022 ; Coto and Traniello 2022 ). One potential mechanism linking brain and body size scaling may be genes that simultaneously influence brain and whole-body growth, with greater expression early in life or in smaller species ( Riska and Atchley 1985 ).…”