Serge Aron

Self-organized Shortcuts in the Argentine Ant

Les fourmis I. humilis choisissent le chemin le plus court pour aller de la colonie au lieu dapprovisionnement

How Trail Laying and Trail Following Can Solve Foraging Problems for Ant Colonies

One of the most striking features of an ant colony’s behaviour is its capacity for the spatial organisation of foraging activity. The use of trail pheromone to guide fellow workers in the nest to a large food source or rich foraging zone has been extensively studied (e. g. Wilson 1971) and obviously contributes to foraging efficiency. We have recently, however, been able to show that trail laying and trail following behaviour are more than just a means of communicating a food source’s location. When more than one trail is present at a time, the interactions between foragers and the trails can lead to the collective selection of the shortest path or the best food source, despite the fact that individual foragers have no means of making such choices.

Random behaviour, amplification processes and number of participants: how they contribute to the foraging properties of ants

Abstract Two major types of foraging organisation in ants are described and compared, being illustrated with experimental data and mathematical models. The first concerns large colonies of identical, unspecialised foragers. The communication and interaction between foragers and their randomness generates collective and efficient structures. The second concerns small societies of deterministic and specialised foragers, rarely communicating together. The first organisation is discussed in relation to the different recruitment mechanisms, trail-following error, quality and degree of aggregation of food-sources, and territorial marking, and is the key to many types of collective behaviour in social insects. The second is discussed in relation to spatial specialisation, foraging density, individual learning and genetic programming. The two organisations may be associated in the same colony. The choice of organisation is discussed in relation to colony size and size and predictability of food sources.

Memory and chemical communication in the orientation of two mass-recruiting ant species

SummaryThe relative contribution of visual and chemical components in the orientation ofLasius niger andIridomyrmex humilis (Argentine ant) workers during mass recruitment to newly discovered food sources is analyzed over short time intervals. While both species orient in response to the trail pheromone, a large number ofL. niger foragers rapidly switch to a more individual orientation, based on their memory of environmental cues.I. humilis workers, on the other hand, predominantly use collective chemical cues. The effect of the number of reinforcements on visual learning and its interference with chemical communication show that olfactory cues always prevail in the Argentine ant. InL. niger, the proportion of ants orienting to visual cues is independent of the trail concentration. Detailed observations of the trail-laying behavior of individually marked foragers show that nearly all theI. humilis workers initially lay a trail, whereas only half theL. niger foragers do so. This proportion decreases considerably with the number of trips performed byL. niger workers, while remaining constant for the Argentine ants. These results are interpreted with respect to the species behavioral ecology.

Visual cues and trail-following idiosyncrasy in leptothorax unifasciatus: An orientation process during foraging

SummaryLeptothorax unifasciatus workers use both chemical and visual cues when foraging. A visual orientation based on menotactical cues (60 watt light-bulb) and environmental cues (laboratory surroundings) dominate over a chemical orientation. The learning response to a 60 watt light-bulb cue occurs after a single trip to the food source.Workers lay down a trail when foraging which helps them to orient themselves but does not recruit other nestmates. Such trails are distinguished and preferred even when superposed by several other nestmates trails. Nevertheless, the foragers are able to follow their nestmates trails if their own is somehow missing. Newly recruited ants are unable to orient along chemical trails.ResumeLes ouvrières deLeptothorax unifasciatus utilisent des repères chimiques et des repères visuels au cours des activités de fourragement. Cependant, une orientation ménotactique (ampoule lumineuse de 60 watt) et reposant sur des repères visuels ambiants domient une orientation chimique. Un trajet effectué vers la nourriture suffit aux ouvrières pour sorienter ensuite sur une ampoule lumineuse.Les ouvrières tracent une piste au cours du fourragement nayant pas fonction de recrutement, mais agissant comme repère orientationnel individuel. Ces pistes sont distinguées et préférées même lorsquelles sont superposées à plusieurs autres. Néanmoins, les ouvrières acceptent de suivre les pistes de leurs congénères en labsence de leur propre piste. Les ouvrières nouvellement recrutées sont incapables de sorienter sur les pistes chimiques.

COLONY SEX RATIOS VARY WITH BREEDING SYSTEM BUT NOT RELATEDNESS ASYMMETRY IN THE FACULTATIVELY POLYGYNOUS ANT PHEIDOLE PALLIDULA

Abstract.— We investigated sex allocation in a Mediterranean population of the facultatively polygynous (multiple queen per colony) ant Pheidole pallidula. This species shows a strong split sex ratio, with most colonies producing almost exclusively a single‐sex brood. Our genetic (microsatellite) analyses reveal that P. pallidula has an unusual breeding system, with colonies being headed by a single or a few unrelated queens. As expected in such a breeding system, our results show no variation in relatedness asymmetry between monogynous (single queen per colony) and polygynous colonies. Nevertheless, sex allocation was tightly associated with the breeding structure, with monogynous colonies producing a male‐biased brood and polygynous colonies almost only females. In addition, sex allocation was closely correlated with colony total sexual productivity. Overall, our data show that when colonies become more productive (and presumably larger) they shift from monogyny to polygyny and from male production to female production, a pattern that has never been reported in social insects.

When Hymenopteran Males Reinvented Diploidy

In most plants and animals, a consistent relationship exists between the DNA content of a cell and its metabolic activity. The male-haploid sex determination of Hymenoptera and other arthropods may therefore impose a particular selective pressure upon males, which must evolve adaptations to cope with a genomic DNA reduced by half compared with that of females. Here, we show that a nuclear DNA content similar to that of females is restored in muscles of males in all hymenopteran lineages tested except the most basal one (Xyelidae). This doubling of DNA content in males does not occur in other haplodiploid insects, such as thrips (Thysanoptera) and whiteflies (Sternorrhyncha). These results indicate that this adaptation probably occurred early in hymenopteran history, possibly because males acquired strong flying and dispersal abilities.

Functional Self-Organisation Illustrated by Inter-Nest Traffic in Ants: The Case of the Argentine Ant

In many ant species the colony is not a single structure but rather a number of decentralised nests linked together by a network of trails. This network can be extended to include trails that form between the nests and long-lasting food sources or rich foraging areas. The formation of inter-nest networks was studied with laboratory colonies of the Argentine ant Iridomyrmex humilis. Bridges were placed to link isolated nests, with branches of equal length arranged in a triangle and a square linking three and four nests respectively, and two branches of different length linking two nests. The resulting ant traffic connected all the nests, neglecting redundant bridges, forming a minimum spanning tree. Cutting a frequented bridge caused the traffic to divert to a neglected bridge. Visual cues appear not to be essential as similar networks were generated both in light and darkness. Rotating the bridges caused the traffic to change correspondingly indicating a primary role of chemical cues with respect to memory or visual cues. Where the bridges between two nests were of unequal length, the ants neglected the longer one.

Error, communication and learning in ant societies

Abstract When large food sources are discovered, many species recruit using different mechanisms. Numerous recruits do not reach the food source and explore the foraging area. These ants make new discoveries. The interplay between communication and exploration generates collective and efficient structures. This organisation is discussed in relation to the mechanisms of recruitment, quality and degree of aggregation of food-sources and territorial marking. This first organization, characteristic of large colonies, is compared to another foraging organisation: small societies of deterministic and specialised foragers rarely communicating together.

Social parasitism in ants: effects of the inquiline parasite Plagiolepis xene St. on queen distribution and worker production of its host Plagiolepis pygmaea Latr.

Summary: The ant Plagiolepis pygmaea forms polygynous (several mated queens per colony) and polydomous (multiple nests per colony) colonies that may be parasitized by the workerless social parasite Plagiolepis xene. Earlier laboratory experiments showed that P. xene strongly restricts host worker production (Passera, 1972); however, this negative effect seems not to occur under natural conditions. Here, we explored the hypothesis that, in infested natural colonies of P. pygmaea, parasite queens affect the distribution of the host queens resulting in the latter inhabiting unparasitized nests of the colony at the period of egg-laying. Our field studies show that the presence of the inquiline ant P. xene influences host queen distribution, with a higher level of dispersion within parasitized colonies. In winter, there is a positive association between the number of host queens and the number of parasites in queenright nests. Such a relationship does not occur in summer, so that when egg-laying reaches its maximum activity most queens of the host are found in unparasitized nests. Moreover, the proportion of queenright nests parasitized was dramatically lower than in the winter. A detailed comparison of worker production between parasitized and unparasitized field colonies reveals no difference. Experiments show that reduced worker production in parasitized nests reared under laboratory conditions results from oophagy by the parasitic queens. Overall, our data suggest that the segregation of the host and parasite queens in different nests at the time of egg-laying results in part from host queens avoiding inhabiting parasitized nests to reduce oophagy by the parasite. We discuss the behavioral and ecological implications of our findings.