Division of labour and worker size polymorphism in ant colonies: the impact of social and genetic factors

Division of labour among workers is central to the organisation and ecological success of insect societies. If there is a genetic component to worker size, morphology or task preference, an increase in colony genetic diversity arising from the presence of multiple breeders per colony might improve division of labour. We studied the genetic basis of worker size and task preference in Formica selysi, an ant species that shows natural variation in the number of mates per queen and the number of queens per colony. Worker size had a heritable component in colonies headed by a doubly mated queen (h(2)=0.26) and differed significantly among matrilines in multiple-queen colonies. However, higher levels of genetic diversity did not result in more polymorphic workers across single- or multiple-queen colonies. In addition, workers from multiple-queen colonies were consistently smaller and less polymorphic than workers from single-queen colonies. The relationship between task, body size and genetic lineage appeared to be complex. Foragers were significantly larger than brood-tenders, which may provide energetic or ergonomic advantages to the colony. Task specialisation was also often associated with genetic lineage. However, genetic lineage and body size were often correlated with task independently of each other, suggesting that the allocation of workers to tasks is modulated by multiple factors. Overall, these results indicate that an increase in colony genetic diversity does not increase worker size polymorphism but might improve colony homeostasis.

Effects of the social environment on the survival and fungal resistance of ant brood

The phenotype of social animals can be influenced by genetic, maternal and environmental effects, which include social interactions during development. In social insects, the social environment and genetic origin of brood can each influence a whole suite of traits, from individual size to caste differentiation. Here, we investigate to which degree the social environment during development affects the survival and fungal resistance of ant brood of known maternal origin. We manipulated one component of the social environment, the worker/brood ratio, of brood originating from single queens of Formica selysi. We monitored the survival of brood and measured the head size and ability to resist the entomopathogenic fungus Beauveria bassiana of the resulting callow workers. The worker/brood ratio and origin of eggs affected the survival and maturation time of the brood and the size of the resulting callow workers. The survival of the callow workers varied greatly according to their origin, both in controls and when challenged with B. bassiana. However, there was no interaction between the fungal challenge and either the worker/brood ratio or origin of eggs, suggesting that these factors did not affect parasite resistance in the conditions tested. Overall, the social conditions during brood rearing and the origin of eggs had a strong impact on brood traits that are important for fitness. We detected a surprisingly large amount of variation among queens in the survival of their brood reared in standard queenless conditions, which calls for further studies on genetic, maternal and social effects influencing brood development in the social insects.

Description of a new artificial diet for rearing ant colonies as Iridomyrmex humilis, Monomorium pharaonis and Wasmannia auropunctata (Hymenoptera, Formicidae)

A diet which does not desiccate was developed for rearing various ant species. this diet was found to be very attractive forIridomyrmex humilis. Monomorium pharaonis andWasmannia auropunctata in which species it permitted the rearing of colonies for long periods and the regular production of sexuals. Nous présentons la recette d'une nourriture artificielle élaborée pour élever des fourmis en laboratoire. Cette nourriture ne se dessèche pas et s'est avérée adéquate pour l'élevage de plusieur espèces telles queIridomyrmex humilis, Monomorium pharaonis etWasmannia auropunctata. Des sexués mâles et femelles ont été régulièrement produits dans ces sociétés.

Control of brood male production in the Argentine ant Iridomyrmex humilis (Mayr)

The influence of various social factors on the production of males was investigated in the Argentine ant, Iridomyrmex humilis. In this polygynous species, the workers which are monomorphic are unable to lay reproductive eggs, so all the males are the progeny of the queens. Although male eggs appear to be laid by mated queens throughout the year, in large stock colonies males are reared periodically (every 3 or 4 months); males develop from brood taken from these colonies at any point in the cycle and given queenless or queenright (1 to 5 queens) units. This is in striking contrast to many other species of ants where it is generally assumed that male eggs are laid seasonnally. Comparative experiments suggest that several related factors influence the rearing of males as far as the pupal stage. Worker/larva ratio: The proportion of male larvae developing in standardized units in which the worker/larva ratio was varied from 0.25 to 25 demonstrated that low ratios inhibit male production. Queen influence: In standardized units where the worker/larva ratio was high the presence of queens did not inhibit the rearing of males suggesting that there is no queen inhibitory pheromone controlling male experimental production. Data suggest evidence that queens prevent male production by means of appropriation of food. Diet: Male larvae failed to pupate in experimental societies deprived of protein. Thus, the production of males appears to be controlled by the amount of food available to larvae. This depends on foraging activity, the quantity of brood in relation to the number of workers and the number of queens in the society.

Male production by workers in the polygynous ant Prolasius advenus

The ability of workers to produce male individuals is reported here for the first time in a species of the formicine ant genus Prolasius. We show that Prolasius advenus workers possess ovaries and demonstrate that they are able to produce adult males in queenless colonies. We also experimentally tested the influence of queen volatiles on the level of worker reproduction. Workers produced fewer eggs in treatments where they could perceive odors from queens. Some volatile compounds emitted by queens may thus have a signaling or inhibitory effect on worker reproduction. This effect of queen presence did not entirely stop worker reproduction, however, as adult males still emerged under these conditions. Worker-produced males were absent only in treatments with the physical presence of queens. Dissections of workers collected from queenright nests in the field revealed signs of egg-laying activity in more than half of individuals. Together, these results suggest that in nature P. advenus workers produce males at least in orphaned colonies or in situations where the physical presence of queens is limited.

Influence of the number of queens on nestmate recognition and attractiveness of queens to workers in the Argentine ant, Iridomyrmex humilis (Mayr)

To investigate the influence of the number of queens per colony on nestmate recognition in Iridomyrmex humilis, comparative assays were performed to study the attraction of workers to queens. These assays demonstrated that a phenomenon of recognition is superimposed on the attraction of workers to queens. Workers could discriminate non-nestmate queens from their nestmate queen to which they were significantly more attracted. This discrimination is probably based on the learning by workers of queen and colony odour. The level of attraction of workers to non-nestmate queens was similar in monogynous and polygynous colonies, whereas the level of attraction of workers to nestmate queens was significantly lower in polygynous colonies. This difference in the level of attraction of workers to nestmate queens almost certainly resulted from a lower efficiency in nestmate recognition in polygynous colonies. It is hypothesized that the mixture of several pheromonal sources produced by less related individuals in polygynous colonies may result in a less distinct colony odour than in monogynous colonies. The results are discussed with regard to some implications of polygyny and particularly to the loss of intercolonial aggression in I. humilis as well as in other polygynous ant species

Introduced ant species and mechanisms of competition on Floreana Island (Galapagos, Ecuador) (Hymenoptera : Formicidae)

Simultaneous presence of several tramp ant species of relatively recent introduction on a remote island is an excellent opportunity to study competition mechanisms that lead to the establishment of invasive species. Using attractive food baits we collected 14 ant species among which 10 are well-known tramp species. The most important change between 1996-97 and 2003 is the spread of the tropical fire ant Solenopsis geminata at the detriment of Tetramorium simillimum, suggesting that the colonization process on Floreana is still very dynamic. The follow-up of 400 food baits for 21 hours permitted us to calculate indices of competition abilities for 11 species, revealing distinct strategies. The two small tramp species Monomorium floricola and Tapinoma melanocephalum are typically opportunists when large-sized Odontomachus bauri (possibly native species) and Camponotus macilentus (endemic species) are good interference competitors, out-competing other species at food baits. Dominant species S. geminata and Monomorium destructor reach high scores for all indices due to their high abundance.

Bifurcations des philosophes et corde raide des théologiens. Questions à Luc Ferry et à Pierre-André Stucki

L'A. compare l'approche éthique de deux philosophes francophones contemporains dont la situation culturelle et le rapport à la théologie sont fort contrastés. Il s'interroge notamment sur l'articulation du religieux et de l'éthique dans ces deux modèles. La différence des philosophies a au moins deux effets. Elle oblige le théologien à se situer dans l'espace du questionnement philosophique, mais elle le pousse également à une réflexion plus aiguë sur la contribution possible de la théologie dans le champ de l'éthique et de l'espace public laïque.

A stratified approach for modeling the distribution of a threatened ant species in the Swiss National Park

We present models predicting the potential distribution of a threatened ant species, Formica exsecta Nyl., in the Swiss National Park ( SNP). Data to fit the models have been collected according to a random-stratified design with an equal number of replicates per stratum. The basic aim of such a sampling strategy is to allow the formal testing of biological hypotheses about those factors most likely to account for the distribution of the modeled species. The stratifying factors used in this study were: vegetation, slope angle and slope aspect, the latter two being used as surrogates of solar radiation, considered one of the basic requirements of F. exsecta. Results show that, although the basic stratifying predictors account for more than 50% of the deviance, the incorporation of additional non-spatially explicit predictors into the model, as measured in the field, allows for an increased model performance (up to nearly 75%). However, this was not corroborated by permutation tests. Implementation on a national scale was made for one model only, due to the difficulty of obtaining similar predictors on this scale. The resulting map on the national scale suggests that the species might once have had a broader distribution in Switzerland. Reasons for its particular abundance within the SNP might possibly be related to habitat fragmentation and vegetation transformation outside the SNP boundaries.