The effect of oxygen tension on colony formation and cell proliferation of amniotic fluid cellsin vitro

The effects of reduced oxygen concentrations in the gas phase over the culture medium on colony formation and cell proliferation were investigated in high and low cell density primary and secondary cultures of amniotic fluid cells. Using two standard culture methods (25 cm² plastic flasks and Leighton type tubes) a significantly reduced culture time was observed at high cell density for mass cultures by incubation within a low oxygen tension gas phase (2.5 per cent to 7.5 per cent O₂) instead of conventional air (18 per cent O₂). At low cell density colony formation was significantly enhanced in cultures grown at reduced oxygen tension. Using gas permeable membranes as support, lowering the oxygen tension from 7.5 per cent to 2.5 per cent yielded an increase in plating efficiency of cells from approximately 5 per cent to 25 per cent, whereas plating efficiency was less than 2 per cent for cells grown at ambient 18 per cent O₂. It is suggested that low oxygen tension in the gas phase is an effective means of enhancing clonal growth in amniotic fluid cell cultures, thereby reducing both culture time and risk of culture failure.     

Constraints on foraging success due to resource ecology limit colony productivity in social insects

The benefit of group living is a fundamental question in social evolution. For sociality to evolve, each individual must gain in terms of some fitness component by living in larger groups. However, in social insects, a decrease in per capita success in brood production has been observed in larger groups. While it has been proposed that this decrease could be outweighed by an increase in the predictability of success, a functional basis to this hypothesis has so far never been demonstrated. In this paper, using foraging economics as a functional proxy to colony productivity, we construct a model to explore how number of foragers in the colony interacts with the ecology of resources to influence per capita foraging success and its predictability. The results of the model show that there is no increase in per capita foraging success in larger colonies under most circumstances, though there is an increase in its predictability. We then test the model with empirical data on the foraging behavior of the primitively eusocial wasp, Ropalidia marginata. The consistency between the data and the model suggests that foraging economics could provide a robust functional basis in explaining the relationship between colony size and productivity.     

The control of water collection in honey bee colonies

A honey bee (Apis mellifera) colony adaptively controls the collection of water by its foragers, increasing it when high temperatures necesssitate evaporative cooling inside the hive and decreasing it when the danger of overheating passes. This study analyzes the mechanisms controlling water collection once it has begun, that is, how a colony's water collectors know whether to continue or stop their activity. M. Lindauer suggested that water collectors acquire information about their colony's need for more water by noting how easily they can unload their water to bees inside the hive. In support of this hypothesis, we found that a water collector's ease of unloading does indeed change when her colony's need for water changes. How does a water collector sense the ease of unloading? Multiple variables of the unloading experience change in relation to a colony's water need. Three time-based variables – initial search time, total search time, and delivery time – all change quite strongly. But what changes most strongly is the number of unloading rejections (refusals by receiver bees to take the water), suggesting that this is the primary index of ease of unloading. Why does a water collector's ease of unloading change when her colony's need for water changes? Evidently, what links these two variables is change in the number of water receivers. These are middle-aged bees that receive water just inside the hive entrance, then transport it deeper inside the hive, and finally smear it on the walls of cells or give it to other bees, or both. A colony increases the number of water receivers when its water need increases by having bees engaged in nectar reception and other tasks (and possibly also bees that are not working) switch to the task of water reception. Evidently the activation of additional water receivers does not strongly reduce the number of nectar receivers in a colony, since a colony can increase greatly its water collection without simultaneously decreasing its collection of rich nectar. This study provides a clear example of the way that the members of a social insect colony can use indirect indicators of their colony's labor needs to adaptively control the work that they perform.     

Emergence of increased division of labor as a function of group size

Empirical evidence suggests that division of labor in insect societies is positively related to group size both within and across taxa. Response threshold models (RTM) have been commonly used to analyze patterns of division of labor. However, these models have been explored empirically and theoretically for only a limited number of tasks, and few studies have examined predictions of the model as colony size and work availability change. We theoretically examine how group size influences division of labor using a fixed response-threshold model. We simultaneously explore how expected by-products of increased colony size, including demand (total work need relative to total work force available) and task number, affect this relationship. Our results indicate that both low demand and high task number positively influence division of labor. We suggest that these changes parallel what is observed within social groups as their size increases, and that, in part, the commonly observed increased division of labor with increasing group size is emergent.     

Helpers in colonial cooperatively breeding sociable weavers <Emphasis Type="Italic">Philetairus socius</Emphasis> contribute to buffer the effects of adverse breeding conditions

Some studies on the effects of helpers in cooperatively breeding vertebrates show a positive effect of helper presence on reproductive output whereas others find no effect. One possibility for this discrepancy is that helpers may have a positive effect when breeding conditions are adverse, while their effect might go unnoticed under good conditions. We investigate this hypothesis on sociable weavers Philetairus socius, a colonial cooperatively breeding passerine that inhabits a semi-arid region where breeding conditions vary markedly. We used multivariate mixed models to analyse the effect of helpers on reproduction under contrasting environmental and social conditions while controlling for parental and colony identity. We found that reproductive success in sociable weavers was primarily influenced by nest predation and rainfall. In addition, colony size was negatively associated with hatching and fledging success and number of young fledged per season. Helpers had a less prominent but significant influence on feeding rates and reproductive outcome. In agreement with expectations, the presence of helpers counteracted some of the negative effects of breeding in periods of low rainfall or in large colonies and was also associated with an increased number of young fledged per season. Our results illustrate that the effect of helpers might be detectable mostly under unfavourable conditions, but can contribute to improve reproductive performance in those situations.     

Stable relatedness structure of the large-colony swarm-founding wasp Polybia paulista

In social-insect colonies, cooperation among nestmates is generally stabilized by their high genetic similarity. Thus, fitness gained through cooperation drops quickly as the number of reproductive females (queens) increases. In this respect, wasps of the tribe Epiponini have attracted special attention, because the colonies have tens, or even hundreds of queens. It has been empirically or genetically confirmed that relatedness within nestmates can be elevated by a mechanism known as cyclical monogyny, under which new queens are produced only after the number of old queens is reduced to one. Another likely factor that can increase relatedness within nestmates under polygyny is comb partitioning by queens. If queens concentrate their egg laying on one or a subset of the available combs, then workers may be able to rear closer relatives by focusing their work on the comb where they emerged. Using microsatellite markers, we tested the hypotheses of cyclical monogyny and comb partitioning by queens increasing relatedness within nestmates under polygyny in the large-colony epiponine wasp, Polybia paulista. There were no significant differences between relatedness within combs and between combs, and thus we ruled out the possibility that each queen partitions reproduction between combs. However, as cyclical monogyny predicts, a lower effective number of queens contributed to queen production than to worker production. Cyclical monogyny explained well the observed smaller effective number of queens for new queens than that for workers, but failed to explain the stable relatedness values throughout colony cycles.Communicated by L. Keller     

Colony foundation and polygyny in the ant Formica podzolica

We document the variation in number of queens occurring naturally in founding, immature and mature nests of the ant Formica podzolica, and compare development of colonies and survivorship of queens in experimental nests started with 1–16 foundresses. Number of queens per nest was associated with stage of colony development. Most nests were monogynous, but 20% of immature nests (n = 66) and 25% of mature nests (n = 92) were oligogynous or polygynous. Colonies were usually established by single queens (i.e., haplometrosis), but colony establishment by multiple queens (i.e., pleometrotis) was also common, occurring in 27% of founding nests (n = 492). Foundress groups in the field were small ( = 1.47 ± 0.04 queens/nest), and large groups experienced high mortality and low productivity in artificial nests. Therefore, the many queens (up to 140) in some immature and mature colonies were probably secondarily pleometrotic. Experimental nests started with 1–4 queens were more successful than those initiated by 8 or 16 queens. Small groups (2–4 queens) produced more pupae before the first nests reared workers than single foundresses or larger groups (8 or 16 queens). Although single foundresses were less productive than queens in small groups, they experienced greater survivorship and less weight loss than queens in pleometrotic associations. Besides low productivity, queen mortality and weight loss were greatest in large groups.     

Proclivity and effectiveness in gall defence by soldiers in five species of gall-inducing thrips: benefits of morphological caste dimorphism in two species (<Emphasis Type="Italic">Kladothrips intermedius</Emphasis> and<Emphasis Type="Italic"> K. habrus</Emphasis>)

The essence of eusociality is a trade-off between producing ones own offspring and helping collateral kin via such activities as defence, foraging and brood rearing. This trade-off often involves morphological differences between helper and reproductive castes, but the advantages, correlates and phylogenetic context of morphological caste differentiation have seldom been analysed. Six species of Australian gall-inducing thrips on Acacia show morphological polymorphism. One morph, referred to as a soldier, has reduced wings and antennae but greatly enlarged fore-femora, which are thought to be adaptations for gall defence. The other, dispersing morph, has fully developed wings and relatively slight fore-femora. Here, we quantify the defensive behaviour of soldier morphs, and compare soldier and foundresses, using behavioural assays designed to measure proclivity to attack kleptoparasites (specialised invaders in the genus Koptothrips) and effectiveness in killing them. In all five species investigated, soldiers were able to kill Koptothrips. Moreover, the effectiveness of soldiers was relatively high in the more-derived species in the phylogeny of the clade, Kladothrips intermedius, K. habrus and K. waterhousei. Soldiers of K. intermedius and K. habrus also killed kleptoparasites more effectively than did foundresses, and K. habrus soldiers exhibited higher proclivity to attack than did foundresses. Data from naturally invaded galls demonstrate that soldiers in the field do kill Koptothrips, and vice versa. These results show that soldiers of Australian gall thrips are motivated and effective for gall defence.Communicated by D. Gwynne     

Relatedness,recognition errors,and colony fusion in the termite <Emphasis Type="Italic">Nasutitermes corniger</Emphasis>

Loss of aggression between social groups can have far-reaching effects on the structure of societies and populations. We tested whether variation in the genetic structure of colonies of the termite Nasutitermes corniger affects the probability of aggression toward non-nestmates and the ability of unrelated colonies to fuse. We determined the genotypes of workers and soldiers from 120 colonies at seven polymorphic microsatellite loci. Twenty-seven colonies contained offspring of multiple founding queens or kings, yielding an average within-colony relatedness of 0.33. Genotypes in the remaining 93 colonies were consistent with reproduction by a single queen and king or their progeny, with an average within-colony relatedness of 0.51. In standardized assays, the probability of aggression between workers and soldiers from different colonies was an increasing function of within-colony relatedness. The probability of aggression was not affected significantly by the degree of relatedness between colonies, which was near zero in all cases, or by whether the colonies were neighbors. To test whether these assays of aggression predict the potential for colony fusion in the field, we transplanted selected nests to new locations. Workers and soldiers from colonies that were mutually tolerant in laboratory assays joined their nests without fighting, but workers and soldiers that were mutually aggressive in the assays initiated massive battles. These results suggest that the presence of multiple unrelated queens or kings promotes recognition errors, which can lead to the formation of more complex colony structures.     

Self-organized digging activity in ant colonies

Many ant species adjust the volume of their underground nest to the colony size. We studied whether the regulation of the volume of excavated sand could result from an interplay between recruitment processes and ant density. Experiments were performed with different group sizes of workers in the ant Messor sancta. When presented with a thin homogeneous sand disk, these groups excavated networks of galleries in less than 3 days. The excavation dynamics were logistic shaped, which suggests the existence of a double feedback system: a positive one resulting in an initial exponential growth phase, and a negative one leading the dynamics to a saturation phase. The total volume of excavated sand was almost proportional to the number of workers. We then developed a model in which we incorporated the quantitative behavioral rules of the workers digging activity. A positive feedback was introduced in the form of a recruitment process mediated by pheromones. The model predicts that the excavation dynamics should be logistic shaped and the excavation should almost stop despite the absence of any explicit negative feedback. Moreover, the model was able to reproduce the positive linear relationship between nest volume and colony size.Communicated by K. Ross