ARTICLES Caste-biased acceptance of non-nestmates in a polygynous ponerine ant TOMONORI KIKUCHI, KAZUKI TSUJI, HITOSHI OHNISHI & JULIEN LE BRETON Faculty of Agriculture, University of the Ryukyus (Received 24 May 2005; initial acceptance 13 July 2005; final acceptance 6 July 2006; published online 14 January 2007; MS. number: 8553R) We investigated the influence of caste on nestmate discrimination in the ponerine ant Pachycondyla luteipes, where workers lack functional ovaries and are totally sterile. Both a mark-and-recapture field experiment and an introduction experiment in the laboratory revealed intermixing of both nestmate and non-nestmate workers between nests. In the laboratory experiment, conspecific workers, both nest- mate and non-nestmates, were almost always accepted. Workers’ internest hostility was weak and did not correlate with the distance between nests over the geographical scale studied (<130 m). However, workers responded differentially to nestmate and non-nestmate workers, grooming non-nestmates more frequently than nestmates. In contrast, non-nestmate queens were usually violently attacked by resident workers, and as a result only 30% were accepted. Nestmate queens were always accepted with no aggres- sion. Our results indicate that P. luteipes workers have the ability to recognize nestmates but are not aggres- sive when the non-nestmates are sterile workers. Such caste-biased acceptance has been predicted by kin selection in relation to the avoidance of intraspecific social parasitism and regulation of queen numbers. Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Keywords: caste; nestmate recognition; Pachycondyla luteipes; polygyny; ponerine ant In most social Hymenoptera, kin discrimination is well developed and there is strict nestmate discrimination (Crozier & Pamilo 1996). The ability to discriminate kin is important for the evolution and maintenance of altru- ism by workers through kin selection (Hamilton 1987). Nestmate and/or kin discrimination consists of three parts: the production of recognition cues, the perception of these cues and subsequent behavioural discrimination (Reeve 1989; Sherman et al. 1997). The perception of rec- ognition cues, however, does not always lead to discrimi- nation. For example, in a unicolonial population of the wood ant, Formica paralugubris, workers showed no aggres- sion towards non-nestmate workers, but they did engage in trophallaxis more frequently with nestmate workers than with non-nestmate workers (Chapuisat et al. 2005). This implies that, although F. paralugubris workers are able to discriminate nestmates from non-nestmates, they do not show aggression against the latter. Reeve (1989) indicated that the expression of kin discriminatory behav- iour will depend on the costs and benefits of discrimination, and predicted that when the cost of accepting is high, the probability of rejecting both nonkin and kin should in- crease. Conversely, if the fitness cost of accepting is low, the probability of rejecting may decrease. Empirical support for Reeve’s (1989) model has been observed from studies of honeybees, Apis mellifera (Downs & Ratnieks 2000). Intrusion by non-nestmates leads to various costs for recipient colonies, ranging from parasites and disease transmission to selfish reproduction by the intruders (Schmid-Hempel 1998; Neumann & Moritz 2002; Lopez- Vaamonde et al. 2004). Given that cost, the acceptance probability might be influenced by the reproductive abil- ity of the potential invaders. In ants, the difference in re- productive potential between queens and workers is usually pronounced and in some species workers are totally sterile (Bourke & Franks 1995). A colony accepting non-nestmate workers will suffer relatively low costs if the non-nestmate workers are sterile, since the cost of self- ish reproduction by nonkin is absent, unless the workers resort to another selfish behaviour (e.g. food robbing in Correspondence: T. Kikuchi, Faculty of Agriculture, University of the Ryukyus, Okinawa 903-0213, Japan (email: g036001@agr.u-ryukyu. ac.jp). 559 0003e 3472/07/$30.00/0 Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2007, 73, 559e565 doi:10.1016/j.anbehav.2006.04.015