title: Predation risk drives social complexity in cooperative breeders.
abstract: Predation risk is a major ecological factor selecting for group living. It is largely ignored, however, as an evolutionary driver of social complexity and cooperative breeding, which is attributed mainly to a combination of habitat saturation and enhanced relatedness levels. Social cichlids neither suffer from habitat saturation, nor are their groups composed primarily of relatives. This demands alternative ecological explanations for the evolution of advanced social organization. To address this question, we compared the ecology of eight populations of Neolamprologus pulcher, a cichlid fish arguably representing the pinnacle of social evolution in poikilothermic vertebrates. Results show that variation in social organization and behavior of these fish is primarily explained by predation risk and related ecological factors. Remarkably, ecology affects group structure more strongly than group size, with predation inversely affecting small and large group members. High predation and shelter limitation leads to groups containing few small but many large members, which is an effect enhanced at low population densities. Apparently, enhanced safety from predators by cooperative defense and shelter construction are the primary benefits of sociality. This finding suggests that predation risk can be fundamental for the transition toward complex social organization, which is generally undervalued.
authors.affiliation: Department of Behavioural Ecology, Institute of Ecology and Evolution, University of Bern, 3032 Hinterkappelen, Switzerland; Behavioural and Physiological Ecology Group, Groningen Institute for Evolutionary Life Sciences, Faculty of Mathematics and Natural Sciences, University of Groningen, 9747 AG, Groningen, The Netherlands; frank.groenewoud@iee.unibe.ch.
authors.forename: Frank
authors.identifier: http://orcid.org/0000-0002-5490-086X
authors.initials: F
authors.lastname: Groenewoud
authors.affiliation: Department of Behavioural Ecology, Institute of Ecology and Evolution, University of Bern, 3032 Hinterkappelen, Switzerland;
authors.forename: Joachim Gerhard
authors.initials: JG
authors.lastname: Frommen
authors.affiliation: Department of Behavioural Ecology, Institute of Ecology and Evolution, University of Bern, 3032 Hinterkappelen, Switzerland;
authors.forename: Dario
authors.initials: D
authors.lastname: Josi
authors.affiliation: Laboratory of Animal Sociology, Department of Biology and Geosciences, Graduate School of Science, Osaka City University, Sumiyoshi, Osaka 558-8585, Japan.
authors.forename: Hirokazu
authors.initials: H
authors.lastname: Tanaka
authors.affiliation: Department of Behavioural Ecology, Institute of Ecology and Evolution, University of Bern, 3032 Hinterkappelen, Switzerland;
authors.forename: Arne
authors.initials: A
authors.lastname: Jungwirth
authors.affiliation: Department of Behavioural Ecology, Institute of Ecology and Evolution, University of Bern, 3032 Hinterkappelen, Switzerland;
authors.forename: Michael
authors.initials: M
authors.lastname: Taborsky
mesh_terms: D000818:Animals; D005075:Biological Evolution; D023681:Cichlids / Q000502:physiology*; D017753:Ecosystem; D011235:Predatory Behavior; D012726:Sexual Behavior, Animal
keywords: cooperative breeding; ecological constraints; predation risk; social complexity; social evolution