Workers agonistic interactions in queenright and queenless nests of a polydomous ant society

Abstract

In eusocial Hymenoptera, the ability of workers to reproduce is a cause of conflict, both between the queen and workers and among workers. Reproductive decisions by workers depend on parameters such as colony size (which reduces the cost of selfish reproduction for the colony) or queen fertility. Indeed, queen signals inhibit reproduction from workers either by coercion or because of self-restraint. Multinest societies called polydomous facilitate the collection of scattered food and colony movement in an unstable environment. In these societies, queen signal dispersion could be hindered by the physical absence of queen in some nests. Here we investigate how polydomy influences worker behaviour in a monogynous polydomous ant, the ponerinae Pachycondyla goeldii. A recent field study has shown that P. goeldii workers display a significantly higher ovarian development in secondary (queenless) than in primary (queenright) nests. In the present study we show that P. goeldii workers change they behaviour when they colonize new nests, displaying significantly more agonistic behaviours in secondary than in primary nests. Behavioural changes and ovary development of workers from secondary nests are probably because of a decrease of queen signal intensity in their nests. The rise of agonistic behaviour in these nests stems from mechanisms regulating the reproduction of workers such as policing or hierarchy set-up. Our results lead us to favour the hypothesis of hierarchy establishment between workers in the secondary nests, which characterize large colonies. Pachycondyla goeldii colonies therefore maximize their reproductive output by triggering worker reproduction when colony size increases.