Colony disassociation following diet partitioning in a unicolonial ant

Discriminating nestmates from alien conspecifics via chemical cues is recognized as a critical element in maintaining the integrity of insect societies. We determined, in laboratory experiments, that nestmate recognition in an introduced population of the Argentine ant, Linepithema humile, is modified by hydrocarbons acquired from insect prey, and that workers from spatially isolated colony fragments, each provided with prey that possessed distinct cuticular hydrocarbons, displayed aggressive behavior towards their former nestmates. Isolation for 28 days or more between colony fragments fed different prey was sufficient to prevent re-establishment of inter-nest communication for at least an additional 28 days through the introduction of a bridge between the nests. Ants possessed intrinsic cuticular hydrocarbons plus only those hydrocarbons from the prey they received during the isolation period. Colony fragments which were isolated for less than 28 days reunited with workers possessing both prey hydrocarbons. Therefore, L. humile nestmate recognition may be dynamic, being in part dependent on the spatio-temporal distribution of prey, along with physical factors permitting or restricting access of subcolony units to those prey. Eusocial insects utilize chemical odors to maintain colony integrity and to discriminate against non-nestmate conspecifics. These odors are heritable but may also be derived from the environment and, when exchanged continuously, promote nestmate homogeneity (Crozier and Dix 1979; Howard and Blomquist 1982; Breed 1983; Gamboa et al. 1986; Hölldobler and Wilson 1990). Here we provide the first evidence for an environmentally derived nestmate recognition mechanism causing colony disassociation in a unicolonial population of the Argentine ant, Linepithema humile. The loss of colony uniformity occurs following the physical separation of colony subunits and the exploitation of distinct prey by each subunit. These sequential events produce aggression and lack of integration between former nestmates, despite a brief period of separation, with prey-acquired hydrocarbons reinforcing isolation by overlaying individual intrinsic cuticular hydrocarbons, thereby creating distinct nestmate recognition profiles.