Production of sexuals in a fission-performing ant: dual effects of queen pheromones and colony size

Abstract

Models based on the kin selection theory predict that in social hymenopterans, queens may favor a lower investment in the production of sexuals than workers. However, in perennial colonies, this conflict may be tuned down by colony-level selection because of the trade off between colony survival and reproductive allocation. In this study, we present a survey of sexual production in colonies of Aphaenogaster senilis, a common species of ant in the Iberian Peninsula. Similar to most species that reproduce by fission, males were found in large excess compared to gynes (172:1). Sexuals were more likely to be found in queenless than in queenright (QR) field colonies. However, we also found a few gynes and numerous males in very large QR colonies. We compared these data with those available in the literature for A. rudis, a congeneric species from North America that has independent colony founding. The sex ratio in this species was only five males for each female, and sexuals were mostly found in QR nests, irrespective of colony size. We confirmed queen inhibition of sexual production in A. senilis in laboratory experiments and provide evidence that this inhibition is mediated by a nonvolatile pheromone. To seek the potential source of such a queen pheromone, we analyzed the secretions of two conspicuous exocrine glands, the Dufour's and postpharyngeal glands (DG and PPG, respectively) in both queens and workers. Both secretions were composed of hydrocarbons, but that of DG also contained small quantities of tetradecanal and hexadecanal. The hydrocarbon profile of the DG and PPG showed notable caste specificity suggesting a role in caste-related behavior. The PPG secretions also differed between colonies suggesting its role in colonylevel recognition. We suggest that in A. senilis, there are two modes of colony fission: First, in very large colonies, gynes are produced, probably because of the dilution of the queen pheromone, and consequently one or more gynes leave the mother colony with workers and brood to found a new nest. This is beneficial at the colony level because it avoids the production of costly sexuals in small colonies. However, because the queen and workers have different optima for sexual production, we hypothesize that queens tend to overproduce the pheromone to delay their production. This in turn may drive workers to leave the mother colony during nest relocation and to produce sexuals once they are away from the queen's influence, creating a second mode of colony fission.

Intraspecific competition affects population size and resource allocation in an ant dispersing by colony fission

“…Proximately, such a shift in the allocation of resources may have resulted from a qualitative change in colonies' resource intake, such as an increase in the proportion of proteins or lipids, which is known to affect the production of sexuals, as well as the sex ratio in other species (Deslippe and Savolainen 1995, Bono and Herbers 2003, Brown and Keller 2006. In A. senilis, queens are produced in very small numbers and only in large or queenless colonies (Boulay et al 2007b). It has been suggested that queens are actually produced only after fission (Ledoux 1971), which explains why we did not find unmated queens in the colonies excavated just before the reproductive season.…”

A proactive–reactive syndrome affects group success in an ant species

“…In Doñana National Park (southern Spain), A. senilis is particularly abundant in xeric sandy areas dominated by open scrublands. Colonies are strictly monogynous (1 queen per colony) and contain between 200 and 3000 workers (Boulay et al 2007). This species occupies a relatively low rank in the community behavioral dominance hierarchy: it is a subordinate, risk-prone species that forages very close to its critical thermal maximum-the temperature at which locomotor ability is so reduced that individuals can no longer escape lethal temperatures (Cerdá et al 1998a).…”

Chemical Integration of Myrmecophilous Guests inAphaenogasterAnt Nests

“…An internal standard (eicosane) was added to the extract or deposited on the fiber to measure the hydrocarbon quantities. Hydrocarbons of A. senilis were previously identified [24,25] and we added some new compounds present in very small quantities.…”