Ant Queen Egg-Marking Signals: Matching Deceptive Laboratory Simplicity with Natural Complexity

Abstract

BackgroundExperiments under controlled laboratory conditions can produce decisive evidence for testing biological hypotheses, provided they are representative of the more complex natural conditions. However, whether this requirement is fulfilled is seldom tested explicitly. Here we provide a lab/field comparison to investigate the identity of an egg-marking signal of ant queens. Our study was based on ant workers resolving conflict over male production by destroying each other's eggs, but leaving queen eggs unharmed. For this, the workers need a proximate cue to discriminate between the two egg types. Earlier correlative evidence indicated that, in the ant Pachycondyla inversa, the hydrocarbon 3,11-dimethylheptacosane (3,11-diMeC27) is more abundant on the surface of queen-laid eggs.MethodologyWe first tested the hypothesis that 3,11-diMeC27 functions as a queen egg-marking pheromone using laboratory-maintained colonies. We treated worker-laid eggs with synthetic 3,11-diMeC27 and found that they were significantly more accepted than sham-treated worker-laid eggs. However, we repeated the experiment with freshly collected field colonies and observed no effect of treating worker-laid eggs with 3,11-diMeC27, showing that this compound by itself is not the natural queen egg-marking pheromone. We subsequently investigated the overall differences of entire chemical profiles of eggs, and found that queen-laid eggs in field colonies are more distinct from worker-laid eggs than in lab colonies, have more variation in profiles, and have an excess of longer-chain hydrocarbons.ConclusionsOur results suggest that queen egg-marking signals are significantly affected by transfer to the laboratory, and that this change is possibly connected to reduced queen fertility as predicted by honest signaling theory. This change is reflected in the worker egg policing response under field and laboratory conditions.

Identification of an ant queen pheromone regulating worker sterility

“…when the queen becomes old or ill), worker behaviour associated with the absence of the queen may be initiated before the queen dies. Where the same pheromones are present on brood (Endler et al 2004;van Zweden et al 2009), declining brood number may similarly contribute to these worker responses (Edwards 1987;Endler et al 2004), as well as allowing queens to tune their reproductive rate to the current number of brood as mentioned above. If the queen pheromone also affects worker aggression (as implicated here and in several other ants; Vander Meer & Alonso 2002; Peeters & Liebig 2009;Smith et al 2009;Moore & Liebig 2010;Wurm et al 2010), the pheromone could also be used by the colony to decide who should reproduce.…”

“…Lastly, 3-MeC 31 might serve as an egg-marking signal used by workers to decide which eggs to rear (e.g. Endler et al 2006;van Zweden et al 2009). …”

Kinship, greenbeards, and runaway social selection in the evolution of social insect cooperation

“…Although both workers and queens can produce male eggs, an increasing body of evidence shows that queen eggs are recognizably different from worker eggs in morphology and chemical signatures (22,46,47). As a result of nutritional differences, queens may always be able to imbue their eggs with specific signals that cannot be faked by workers.…”

Honest and dishonest communication in social Hymenoptera

“…At present, no obvious trends have been identified concerning the chemical and physical properties of such compounds. In some species, the bouquets of reproductives and non-reproductives differ in longchained hydrocarbons, while in other species, shorter-chained and branched hydrocarbons are more dominant (Endler et al, 2004;de Biseau et al, 2004;Monnin, 2006;van Zweden et al, 2009).…”