1Université Pierre-et-Marie Curie, Laboratoire d'Ecologie, CNRS-UMR 7625, 7 quai Saint Bernard, 75005 Paris, France.
2University of Pretoria, Department of Zoology, Pretoria 0001, South Africa.
Streblognathus aethiopicus lacks queens, and the morphologically similar workers all have the potential to mate and lay fertilized eggs. Reproduction is regulated by intense agonistic interactions which lead to the formation of dominance relationships. Only the top-ranking worker can mate ('gamergate'). When the gamergate is established, overt aggression stops, and subordinates have undeveloped ovaries. There is clear olfactory recognition of the gamergate by nestmate workers, who crouch when she comes within 1 cm. As in other queenless ants, experimental removal of the gamergate quickly leads to increased aggression among a proportion of workers, one of which gains the top rank. The nature of the dominance behaviours suggests that a chemical signal of low volatility originates on the abdomen; direct antennal contact is not needed for its perception. We studied the cuticular hydrocarbons from the abdomens of different functional categories of workers (gamergates, new dominants, callows less than 10 days, older foragers). Fertile and infertile workers can be distinguished by the relative proportions of 17 hydrocarbons heavier than C23. Unlike its close relative Dinoponera quadriceps (Peeters et al. 1999 Proc. R. Soc. Lond. B 266: 1323-1327), the dissimilarities between the reproductive and infertile workers of S. aethiopicus are not accounted for by a single compound. Workers that had been allowed experimentally to gain the top rank for a few days were recognized and aggressed by the gamergate, even though 2 weeks are needed for their ovaries to become fully active. Importantly, workers having the top rank for less than 24 hours were also recognized immediately, suggesting a very rapid modification of the putative chemical signal.