Review Article Children's competition in a natural setting: evidence for the ideal free distribution ☆ Gérald Disma a , Michel B.C. Sokolowski a, ⁎ , François Tonneau b a Unité “Dynamiques des Systèmes Anthropisés” (JE 2532), Dpt. de Psychologie, Université de Picardie Jules Verne, 80025 Amiens Cedex 1, France b Escola de Psicologia, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal Initial receipt 31 January 2010; final revision received 30 November 2010 Abstract Little is known of the foraging abilities of children in modern cultures, especially when children forage in groups. Here we present a test of optimal foraging theory in groups of street children working for money. The children we observed were selling bottles of water to drivers distributed in two lanes at a crossroad of Istanbul, Turkey. As predicted by the ideal free distribution (a model of optimal group foraging), the ratio of children working in the two lanes was sensitive to the ratio of cars (and therefore the ratio of potential buyers) present in each lane. Deviations from the ideal free model arose largely from numerical restrictions on the set of possible ratios compatible with a small group size. When these constraints were taken into account, optimal behavior emerged as a robust aspect of the children's group distribution. Our results extend to human children aspects of group foraging that were previously tested in human adults or other animal species. © 2011 Elsevier Inc. All rights reserved. Keywords: Ideal free distribution; Children; Foraging; Competition 1. Introduction Some of the salient characteristics of human life histories are an extended period of juvenile dependency and a delayed age of first reproduction (Gurven & Kaplan, 2006; Kramer, 2002; Sear & Mace, 2008; Walker et al., 2006). From an evolutionary perspective, an individual's most important developmental goal is to reach reproductive age in optimal conditions (Bjorklund & Pellegrini, 2000; Bjorklund & Pellegrini, 2002). Accordingly, human life histories may have evolved so that children display morphological and behavioral features that are advantageous to reproduction (Ellis, 2004; Kaplan & Gangestad, 2005; Vigil, Geary & Byrd-Craven, 2005). Human children's extended period of dependency on adults, for example, may allow juveniles to acquire food procurement abilities that are complex and time consuming to master (Bock & Johnson, 2004; Bock & Sellen, 2002; Gurven & Kaplan, 2006; Walker, Hill, Kaplan & McMillan, 2002). The advantages derived from accessing diets that are technologically difficult to acquire but rich in energy could compensate for the costs of a delayed reproductive onset (Kramer, 2002; Lancaster, Kaplan, Hill & Hurtado, 2000). Even very young children can acquire simple foraging or food procurement techniques (Horner, Whiten, Flynn & de Waal, 2006; Rakoczy, Tomasello & Striano, 2005), however, and differences between children and adults in food gathering efficiency may depend more on children's smaller body size or slower gait than on the mastery of specific skills (Bird & Bliege Bird, 2000; Bird & Bliege Bird, 2002; Blurton Jones & Marlowe, 2002). An extended period of juvenile dependency also has fitness costs. During this period, children may become orphans though natural disasters, wars or conflicts among groups (Hill, Hurtado & Walker, 2007; Wrangham, Wilson & Muller, 2006). The parents may also abandon their children in poverty. Another source of potential fitness costs is the presence, within the same family, of children of different ages and energetic needs (Lawson & Mace, 2009). Competition among siblings may have major effects on development through a reduced quantity and quality of parental care. Some data suggest that younger children tend to be less fed (Horton, Evolution and Human Behavior 32 (2011) 373 – 379 ☆ This study is part of the first author's doctoral dissertation. François Tonneau was supported by the Portuguese FCT. ⁎ Corresponding author. E-mail address: michel.sokolowski@u-picardie.fr (M.B.C. Sokolowski). 1090-5138/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.evolhumbehav.2010.11.007