E-Mail karger@karger.com www.karger.com/bbe Original Paper Brain Behav Evol 2013;81:86–92 DOI: 10.1159/000345945 Breaking Haller’s Rule: Brain-Body Size Isometry in a Minute Parasitic Wasp Emma van der Woude a Hans M. Smid a Lars Chittka b Martinus E. Huigens a a Laboratory of Entomology, Wageningen University, Wageningen, The Netherlands; b Queen Mary University of London, Research Centre for Psychology, School of Biological and Chemical Sciences, London, UK Introduction Across and within all animal species investigated so far, Haller’s rule holds that smaller animals have propor- tionally larger brains than larger-bodied forms [Rensch, 1948]. In a double logarithmic plot, such an allometric brain-body size relationship is described by a straight line with a slope (the brain scaling coefficient b ) lower than 1. The lower b is, the larger is the discrepancy in relative brain size between differently sized animals. Brain-body size allometries that have been reported to date range from a within-species b = 0.20 for a tiny ant species to a between-species b = 0.77 for mammals, with a tendency of within-species coefficients to be lower (fig. 1a) [Martin, 1981; Wehner et al., 2007; Riveros and Gronenberg, 2010; Eberhard and Wcislo, 2011; Seid et al., 2011]. In various species of ants, for example, intraspecific coefficients have been found to range between 0.20 and 0.40, whereas the interspecific coefficient based on the mean brain and body mass of the same species is 0.57 [Wehner et al., 2007]. In very small animals, brain-body size allometry implies that brain size becomes a limiting factor of body miniaturization because costs for development and main- tenance of energetically expensive brain tissue [Aiello and Wheeler, 1995] will become an excessively high bur- Key Words Brain size  Allometry  Isometry  Miniaturization  Trichogramma evanescens Abstract Throughout the animal kingdom, Haller’s rule holds that smaller individuals have larger brains relative to their body than larger-bodied individuals. Such brain-body size allom- etry is documented for all animals studied to date, ranging from small ants to the largest mammals. However, through experimental induction of natural variation in body size, and 3-D reconstruction of brain and body volume, we here show an isometric brain-body size relationship in adults of one of the smallest insect species on Earth, the parasitic wasp Trichogramma evanescens. The relative brain volume consti- tutes on average 8.2% of the total body volume. Brain-body size isometry may be typical for the smallest species with a rich behavioural and cognitive repertoire: a further increase in expensive brain tissue relative to body size would be too costly in terms of energy expenditure. This novel brain scal- ing strategy suggests a hitherto unknown flexibility in neu- ronal architecture and brain modularity. Copyright © 2013 S. Karger AG, Basel Received: May 16, 2012 Returned for revision: June 18, 2012 Accepted after second revision: November 12, 2012 Published online: January 26, 2013 Emma van der Woude Laboratory of Entomology, Wageningen University PO Box 8031 NL–6700 EH Wageningen (The Netherlands) E-Mail emma.vanderwoude  @  wur.nl © 2013 S. Karger AG, Basel 0006–8977/13/0812–0086$38.00/0 Downloaded by: Wageningen UR Library 137.224.18.34 - 3/24/2014 9:25:59 AM