Caste-biased acceptance of non-nestmates in a polygynous ponerine ant

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 nestmate 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 aggression. Our results indicate that P. luteipes workers have the ability to recognize nestmates but are not aggressive 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.