Available online at www.sciencedirect.com Subordinate wasps are more aggressive in colonies with low reproductive skew D. FANELLI * †, J. J. BOOMSMA † & S. TURILLAZZI * *Dipartimento di Biologia Animale e Genetica, University of Florence yDepartment of Population Biology, Institute of Biology, University of Copenhagen (Received 15 January 2007; initial acceptance 19 March 2007; final acceptance 10 July 2007; published online 18 October 2007; MS. number: 9237) The small societies of primitively eusocial wasps have provided interesting testing grounds for reproductive skew theory because all individuals have similar reproductive potential, which is unusual in social insects but common in vertebrate societies. Aggression is a key parameter in testing the theory, but empirical stud- ies have seldom quantified aggression together with the entire array of other relevant variables. The few studies that have done so were recently criticized for failing to control for the overall level of social activity. We analysed behaviour and reproductive partitioning patterns in the stenogastrine wasp Parischnogaster mellyi. We used aggression of the subordinate (b) breeder as key variable and analysed how relatedness, body size, number of breeders and productivity affect the interaction between the reproductive skew and the aggression while controlling for nest activity and actual interaction time between a and b. We showed that (1) more even reproductive partitioning (lower reproductive skew) is associated with higher levels of aggression initiated by the b subordinate independent of colony activity and (2) none of the cur- rently available reproductive skew models is convincingly supported. Comparison of our results with ear- lier studies suggests that a common aggression-based mechanism for reproductive partitioning may apply across all primitively eusocial wasps despite complications arising from variable activity levels; however, currently available models may not apply. Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Keywords: microsatellites; Parischnogaster mellyi; primitively eusocial; reproductive transactions; social behaviour; Stenogastrinae; tug of war; wasps Elucidating why natural selection often favours individ- uals that give up reproduction to rear the brood of others is a fundamental problem in evolutionary biology (Leigh 1999). A series of models have produced predictions about how reproduction could be partitioned without becoming evolutionarily unstable, but their empirical support has remained controversial. Optimal skew models (OSMs) assume that the level of reproductive partitioning is ‘optimal’ for all members of a society and cooperation is made possible by reproductive ‘transactions’. Provided that both a dominant and a sub- ordinate are able to monitor their share of reproduction and the dominant is in control of group composition, OSMs predict a minimum ‘staying incentive’ that the dominant must concede to keep the subordinate in the group and an upper limit of reproductive sharing that the subordinate can claim, above which the dominant should prefer to evict the subordinate (Johnstone 2000; Reeve 2000; Reeve & Keller 2001). Between these lower and up- per limits there should be a ‘window of selfishness’ (WOS) where breeders are expected to compete directly over reproduction leading to a compromise situation (Reeve et al. 1998; Johnstone 2000; Reeve 2000). The reproduc- tive transactions paradigm is usually contrasted with ‘pure’ compromise models, which assume that neither dominants nor subordinates have complete control over reproductive partitioning and never peacefully concede shares of reproduction. The most widely discussed com- promise model is the ‘tug of war’ (TOWM), which assumes that the degree of reproductive skew in a society is the re- sult of a compromise between the conflicting optima of its Correspondence: Daniele Fanelli, Dipartimento di Biologia Animale e Genetica, Universita ` di Firenze, Via Romana 17, 50125 Florence, Italy (email: daniele_fanelli@yahoo.it). J. J. Boomsma is at the Department of Population Biology, Institute of Biology, University of Copenhagen, 2100 Copenhagen, Denmark. 879 0003e 3472/08/$32.00/0 Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2008, 75, 879e886 doi:10.1016/j.anbehav.2007.07.009