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istr itia l 20A new colony of the slave-making ant Polyergus breviceps is initiated when a newly mated gyne invades a host nest and kills the resident queen. This process seems to result in chemical camouflage of the invading gyne and allows her to usurp the position of colony reproductive. Young, recently mated Formica gynes, however, are not attacked. To determine whether worker and/or immature presence is the basis for aggression, we placed eggs, larvae, pupae and workers from mature F. gnava queens with newly mated F. gnava queens and observed the responses of introduced P. breviceps queens. Because no newly mated gyne was attacked, we tested newly mated F. gnava queens (1) once they had produced eggs, (2) when the offspring reached the larval, pupal and callow stages of development, and (3) every 2 weeks until aggression ensued. Eventually all F. gnava queens were attacked but only 29 weeks after having mated. Thus, although offspring are the ultimate benefit from attacking an established F. gnava queen, P. breviceps queens detect mature queens using another time-dependent feature that is reliably indicative of reproductive status. The similarity of host queen hydrocarbon profiles, often correlated with reproductive status in other ant species, suggests that other compounds reflect queen fecundity and produce a kairomonal effect, or that another cue signals host queen and colony suitability. Our findings indicate P. breviceps gynes have evolved to respond aggressively to a host gyne cue that appears long after mating, preventing attacks on gynes without the workers necessary for colony founding. 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved. Socially parasitic ant species rely on workers of other ant species to fulfil certain tasks that enable a colony to survive, such as foraging, feeding and brood care (Hölldobler & Wilson 1990). New gynes of socially para-sitic ant species often lack the fat stores that most non-parasitic ants use to survive the isolation period before the first brood matures (Keller & Passera 1989) and are, therefore, typically incapable of independently founding new colonies. Occasionally, these queens initiate new colonies by departing from their natal nests with a suppl