Aggression regulates monogyny in non-mutilating Diacamma ants
In queenless species of ants, colonies consist of workers with equivalent reproductive potentials. Aggressive interactions regulate fertility and sexual activity. The genus Diacamma is unusual, because monogyny is regulated by mutilation (i.e., removal of a pair of tiny innervated thoracic appendages) of all young workers. One exception is the ‘nilgiri’ population from south India, where only 6 % of workers were mutilated in ten field colonies (range 2.8–12.5 %). Nonetheless, all colonies were monogynous. To investigate the behavioural mechanisms underlying the replacement of the ‘gamergate’ (mated reproductive worker) in ‘nilgiri’, we experimentally divided colonies in two. In the groups lacking the gamergate, aggression soon started among the younger workers. One of these workers exhibited a dominant posture after 1–2 days, and this new alpha started ovipositing and sexual calling within 2–3 weeks. When she was confronted with the original gamergate, olfactory recognition occurred immediately, and this sometimes led to a characteristic dominance behaviour (‘sting smearing’). The fate of 85 young workers of known age was studied: they were usually the target of aggression from either gamergates or new alphas. Their gemmae elicited attention, although these were seldom removed. A small change in the gemma pheromone apparently caused an evolutionary switch from mutilation (as occurs in the very closely related D. ceylonense) to a reversible regulation of reproduction in ‘nilgiri’.