Alternative life-histories in a socially polymorphic ant

Social organisms vary greatly in the number of breeders per group; yet, the causes and consequences of this variation remain poorly known. Here, we show that variation in social structure is tightly linked with changes in several fundamental life-history traits within one population of ants. Multiple-queen colonies of Formica selysi were much more populous than single-queen ones. They also occurred in areas of higher nest density, had longer colony lifespan, produced smaller queens that presumably disperse less, and invested less in reproductive individuals relative to workers. These multiple changes in life histories are consistent with a shift in the mode of colony foundation and the degree of philopatry of queens. They may also provide various fitness benefits to members of multiple-queen colonies and are likely to play a central role in the evolution and maintenance of polymorphic social structures.     

The influence of social structure on brood survival and development in a socially polymorphic ant: insights from a cross‐fostering experiment

Animal societies vary in the number of breeders per group, which affects many socially and ecologically relevant traits. In several social insect species, including our study species Formica selysi, the presence of either one or multiple reproducing females per colony is generally associated with differences in a suite of traits such as the body size of individuals. However, the proximate mechanisms and ontogenetic processes generating such differences between social structures are poorly known. Here, we cross‐fostered eggs originating from single‐queen (= monogynous) or multiple‐queen (= polygynous) colonies into experimental groups of workers from each social structure to investigate whether differences in offspring survival, development time and body size are shaped by the genotype and/or prefoster maternal effects present in the eggs, or by the social origin of the rearing workers. Eggs produced by polygynous queens were more likely to survive to adulthood than eggs from monogynous queens, regardless of the social origin of the rearing workers. However, brood from monogynous queens grew faster than brood from polygynous queens. The social origin of the rearing workers influenced the probability of brood survival, with workers from monogynous colonies rearing more brood to adulthood than workers from polygynous colonies. The social origin of eggs or rearing workers had no significant effect on the head size of the resulting workers in our standardized laboratory conditions. Overall, the social backgrounds of the parents and of the rearing workers appear to shape distinct survival and developmental traits of ant brood.     

Experimentally increased group diversity improves disease resistance in an ant species

A leading hypothesis linking parasites to social evolution is that more genetically diverse social groups better resist parasites . Moreover, group diversity can encompass factors other than genetic variation that may also influence disease resistance. Here, we tested whether group diversity improved disease resistance in an ant species with natural variation in colony queen number. We formed experimental groups of workers and challenged them with the fungal parasite Metarhizium anisopliae . Workers originating from monogynous colonies (headed by a single queen and with low genetic diversity) had higher survival than workers originating from polygynous ones, both in uninfected groups and in groups challenged with M. anisopliae . However, an experimental increase of group diversity by mixing workers originating from monogynous colonies strongly increased the survival of workers challenged with M. anisopliae , whereas it tended to decrease their survival in absence of infection. This experiment suggests that group diversity, be it genetic or environmental, improves the mean resistance of group members to the fungal infection, probably through the sharing of physiological or behavioural defences.     

Variation in queen size across a behavioral transition zone in the ant Messor pergandei

Summary. Ant colonies should be selected to optimally allocate resources to individual reproductive offspring so as to balance production costs with offspring fitness gains. Different modes of colony founding have different size-dependent fitness functions, and should thus lead to different optimal queen sizes. We tested whether a behavioral transition from solitary colony founding (haplometrosis) to group colony founding (pleometrosis) across the range of the ant Messor pergandei was associated with a difference in queen size or condition. Both winged gynes and founding queens were significantly smaller and lighter at pleometrotic than at haplometrotic sites, with an abrupt shift in these characters across the 8.5 km-wide behavioral transition zone. Both the mutualistic advantages of grouping and among-queen competition within associations are likely to be important in selecting for smaller queen size in pleometrotic populations.Received 16 January 2004; revised 13 August 2004; accepted 16 August 2004.     

Variable queen number in ant colonies: no impact on queen turnover, inbreeding, and population genetic differentiation in the ant Formica selysi

Variation in queen number alters the genetic structure of social insect colonies, which in turn affects patterns of kin-selected conflict and cooperation. Theory suggests that shifts from single- to multiple-queen colonies are often associated with other changes in the breeding system, such as higher queen turnover, more local mating, and restricted dispersal. These changes may restrict gene flow between the two types of colonies and it has been suggested that this might ultimately lead to sympatric speciation. We performed a detailed microsatellite analysis of a large population of the ant Formica selysi, which revealed extensive variation in social structure, with 71 colonies headed by a single queen and 41 by multiple queens. This polymorphism in social structure appeared stable over time, since little change in the number of queens per colony was detected over a five-year period. Apart from queen number, single- and multiple-queen colonies had very similar breeding systems. Queen turnover was absent or very low in both types of colonies. Single- and multiple-queen colonies exhibited very small but significant levels of inbreeding, which indicates a slight deviation from random mating at a local scale and suggests that a small proportion of queens mate with related males. For both types of colonies, there was very little genetic structuring above the level of the nest, with no sign of isolation by distance. These similarities in the breeding systems were associated with a complete lack of genetic differentiation between single- and multiple-queen colonies, which provides no support for the hypothesis that change in queen number leads to restricted gene flow between social forms. Overall, this study suggests that the higher rates of queen turnover, local mating, and population structuring that are often associated with multiple-queen colonies do not appear when single- and multiple-queen colonies still coexist within the same population, but build up over time in populations consisting mostly of multiple-queen colonies.     

Restricted effective queen dispersal at a microgeographic scale in polygynous populations of the ant Formica exsecta

Ecological constraints on effective dispersal have been suggested to be a key factor influencing social evolution in animal societies as well as the shift from single queen colonies (monogyny) to multiple queen colonies (polygyny) in ants. However, little is known about the effective dispersal patterns of ant queens. Here we investigate the microgeographic genetic structure of mitochondrial haplotypes in polygynous populations of the ant Formica exsecta, both between pastures and among nests within pastures. An analysis of molecular variance revealed a very high genetic differentiation (phiST = 0.72) between pastures, indicating that queens rarely disperse successfully between pastures, despite the fact that pastures were sometimes as close as 1 km. Most of the pastures contained only a single haplotype, and haplotypes were frequently distinct between nearby pastures and even between groups of nests within the same pasture. In the three pastures that contained several haplotypes, haplotypes were not randomly distributed, the genetic differentiation between nests being phiST = 0.17, 0.52, and 0.69. This indicates that most queens are recruited within their parental colonies. However, a large proportion of nests contained more than one haplotype, demonstrating that colonies will sometimes accept foreign queens. The relatedness of mitochondrial genes among nestmates varied between 0.62 and 0.75 when relatedness was measured within each pasture and ranged between 0.72 and 1.0 when relatedness was assessed with all pastures as a reference population. Neighboring nests were more genetically similar than distant ones, and there was significant isolation by distance. This pattern may be due to new nests being formed by budding or by limited effective queen dispersal, probably on foot between neighboring nests. These results show that effective queen dispersal is extremely restricted even at a small geographical scale, a pattern consistent with the idea that ecological constraints are an important selective force leading to the evolution and maintenance of polygyny.     

Offspring size effects in the marine environment: A field test for a colonial invertebrate

Abstract A central tenet of life‐history theory is the presence of a trade‐off between the size and number of offspring that a female can produce for a given clutch. A crucial assumption of this trade‐off is that larger offspring perform better than smaller offspring. Despite the importance of this assumption empirical, field‐based tests are rare, especially for marine organisms. We tested this assumption for the marine invertebrate, Diplosoma listerianum, a colonial ascidian that commonly occurs in temperate marine communities. Colonies that came from larger larvae had larger feeding structures than colonies that came from smaller larvae. Colonies that came from larger larvae also had higher survival and growth after 2 weeks in the field than colonies that came from smaller larvae. However, after 3 weeks in the field the colonies began to fragment and we could not detect an effect of larval size. We suggest that offspring size can have strong effects on the initial recruitment of D. listerianum but because of the tendency of this species to fragment, offspring size effects are less persistent in this species than in others.     

Effects of nest size and dispersion on brood production in a North American population of wood ant Formica fusca (Hymenoptera: Formicidae)

We examined several key parameters of the population ecology of a North American population of Formica fusca (L.), including nest dispersion, colony size and brood production. Physical nest size was significantly correlated with colony size, and colony size, in turn, was significantly correlated with brood production. Sex allocation was male biased, although larger nests were more likely to produce reproductive female brood (gynes). Neither nest temperature nor moisture level was significantly correlated with brood production. Formica fusca nests in this population had a comparatively low average nearest-neighbor distance with a significantly even pattern of dispersion, which suggests relatively high intraspecific competition. However, nearest-neighbor distance was not significantly associated with either colony size or relative brood production.     

Inbreeding and sex-biased gene flow in the ant Formica exsecta

Abstract.— The objective of this study was to assess breeding and dispersal patterns of both males and females in a monogyne (a single queen per colony) population of ants. Monogyny is commonly associated with extensive nuptial flights, presumably leading to considerable gene flow over large areas. Opposite to these expectations we found evidence of both inbreeding and sex-biased gene flow in a monogyne population of Formica exsecta . We found a significant degree of population subdivision at a local scale (within islands) for queens (females heading established colonies) and workers, but not for colony fathers (the males mated to the colony queens). However, we found little evidence of population subdivision at a larger scale (among islands). More conclusive support for sex-biased gene flow comes from the analysis of isolation by distance on the largest island, and from assignment tests revealing differences in female and male philopatry. The genetic similarity between pairs of queens decreased significantly when geographical distance increased, demonstrating limited dispersal and isolation by distance in queens. By contrast, we found no such pattern for colony fathers. Furthermore, a significantly greater fraction of colony queens were assigned as having originated from the population of residence, as compared to colony fathers. Inbreeding coefficients were significantly positive for workers, but not for mother queens. The queen-male relatedness coefficient of 0.23 (regression relatedness) indicates that mating occurs between fairly close relatives. These results suggest that some monogyne species of ants have complex dispersal and mating systems that can result in genetic isolation by distance over small geographical scales. More generally, this study also highlights the importance of identifying the relevant scale in analyses of population structure and dispersal.     

Sex ratio conflicts, mating frequency, and queen fitness in the ant Formica truncorum

We examined the effect of facultative sex allocation by workerson queen fitness in a Furnish population of the ant Formicatruncorum. Workers rear female-biased broods in colonies headedby a singly mated queen and male-biased broods in colonies headedby a multiply mated queen. As a result, multiply mated queenshave a 37% fitness advantage over singly mated queens. Neitherreproductive output nor worker population of colonies variedwith queen mating frequency. We suggest that singly mated queenspersist in the population because fitness benefits to multiplymated queens via sex allocation are balanced by costs of additionalmatings. Alternatively, singly mated queens may persist simplybecause some queens lack opportunities to mate multiply or becausemale control sometimes prevents additional matings by queens.     

The ant Lasius niger shows no relationship between task efficiency and body size variation among workers

In ants, workers of different sizes may perform various tasks, even in so-called monomorphic species with relatively low body size variation. However, it is unclear if the body size diversity of monomorphic workers correlates with task efficiency, especially in stressful contingencies. Here we tested if the body size variation of workers corresponds with its efficiency in transferring pupae. Transferring brood is a pre-set behavioral response to stress, e.g. suboptimal temperature. Here we applied a laboratory experiment simulating nest damage. The study was performed on the common garden ant (Lasius niger (Linnaeus, 1758)) – a species with no distinct worker subcastes. The efficiency of workers was measured as the latency of transferring pupae from a lit part of the experimental colony to a darkened part, while the body size diversity was expressed as the within-colony coefficient of variation in head width. We did not find any significant correlation between efficiency and body size variation. Summarizing the existing studies and the present results, we propose the hypothesis that the body size diversity of L. niger may have implications for workers’ division of labor but not for their task efficiency in a stressful contingency.     

The interspecific range size–body size relationship in Australian frogs

Aim There is substantial residual scatter about the positive range size–body size relationship in Australian frogs. We test whether species’ life history and abundance can account for this residual scatter. Location Australia. Methods Multiple regressions were performed using both cross‐species and independent contrasts analyses to determine whether clutch size, egg size and species abundance account for variation in range size over and above the effects of body size. Results In both cross‐species and independents contrasts models with body size, clutch size and egg size as predictors, partial r² values revealed that only egg size was significantly and uniquely related to range size. Contrary to expectation, neither body size nor clutch size could account for significant variation in range size. Incorporating species abundance as a predictor in further multiple regression analysis demonstrated that while abundance accounted for a significant proportion of range size variation, the contribution of egg size was reduced but still significant. Notably, non‐significant relationships persisted between range size and both body size and clutch size. Conclusions The weak positive correlation between body size and range size in Australian frogs disappears after accounting for species abundance and egg size. Our findings demonstrate that species with both high local abundance and small eggs occupy comparatively wider geographical ranges than species with low abundance and large eggs.     

Population genetics of the socially polymorphic ant <Emphasis Type="Italic">Formica podzolica</Emphasis>

Summary. We used microsatellite markers to analyze the hierarchical genetic structure of the North American mound building ant, Formica podzolica. About one-third of all colonies were headed by a single queen (monogynous) whose effective mating frequency was close to one (nestmate worker relatedness r = 0.70), while the remaining colonies were polygynous, with low average nestmate relatedness (r = 0.16). The low worker relatedness found in most polygynous colonies furthermore suggested that the numbers of queens in polygynous colonies of this ant are usually high. Contrary to what has been described from other ants with a queen number dichotomy, we did not find an effect of social form variation on the partitioning of genetic variation above the level of the colony. We found no significant differentiation between the sympatric social forms of F. podzolica, nor did differentiation among populations appear to be affected by colony social organization. These unexpected patterns of genetic structure may have resulted from differences either in the spatial distribution of the social forms or in their social flexibility.Received 12 January 2004; revised 23 February 2004; accepted 10 March 2004.     

Estimating quantitative genetic parameters in wild populations: a comparison of pedigree and genomic approaches

The estimation of quantitative genetic parameters in wild populations is generally limited by the accuracy and completeness of the available pedigree information. Using relatedness at genomewide markers can potentially remove this limitation and lead to less biased and more precise estimates. We estimated heritability, maternal genetic effects and genetic correlations for body size traits in an unmanaged long‐term study population of Soay sheep on St Kilda using three increasingly complete and accurate estimates of relatedness: (i) Pedigree 1, using observation‐derived maternal links and microsatellite‐derived paternal links; (ii) Pedigree 2, using SNP‐derived assignment of both maternity and paternity; and (iii) whole‐genome relatedness at 37 037 autosomal SNPs. In initial analyses, heritability estimates were strikingly similar for all three methods, while standard errors were systematically lower in analyses based on Pedigree 2 and genomic relatedness. Genetic correlations were generally strong, differed little between the three estimates of relatedness and the standard errors declined only very slightly with improved relatedness information. When partitioning maternal effects into separate genetic and environmental components, maternal genetic effects found in juvenile traits increased substantially across the three relatedness estimates. Heritability declined compared to parallel models where only a maternal environment effect was fitted, suggesting that maternal genetic effects are confounded with direct genetic effects and that more accurate estimates of relatedness were better able to separate maternal genetic effects from direct genetic effects. We found that the heritability captured by SNP markers asymptoted at about half the SNPs available, suggesting that denser marker panels are not necessarily required for precise and unbiased heritability estimates. Finally, we present guidelines for the use of genomic relatedness in future quantitative genetics studies in natural populations.     

Genetic and social structure of the queen size dimorphic ant Leptothorax cf. andrei

1. The study of queen polymorphisms can provide insight into the evolution of alternative life histories in ants. In this paper, results of morphological, social and genetic investigations of the newly discovered queen size dimorphism in Leptothorax cf. andrei are presented. 2. Queens had a bimodal size distribution, and were classified as large (macrogynes) or small (microgynes) queens. Despite their small size, microgynes had a fully developed external flight apparatus and a functional reproductive tract. 3. Queen morphology was not correlated with colony social structure, and the relatedness among nestmate queens was high, indicating secondary polygyny by re‐adoption of related queens (daughters) into existing colonies. 4. The distribution of microsatellite alleles indicated that there is a genetic separation between macro‐ and micro‐gynes but the two morphs belong to the same species. 5. The results support the hypothesis that microgyny in Leptothorax has not evolved as a specialisation to inter‐ or intra‐specific social parasitism but rather is an adaptation to alternative dispersal behaviour.     

Resource characteristics and competition affect colony and individual foraging strategies of the wood ant Formica integroides

Abstract.  1. Resource characteristics and competitive pressure can affect an ant colony's foraging strategy. This study examined the ability of the wood ant Formica integroides to respond, at both the colony and individual levels, to changes in competitive pressure for access to terrestrial and arboreal resources.
2. Because foraging behaviours depend on resource characteristics, foraging for different resource types (e.g. terrestrial and arboreal habitats) produces different spatial or territorial arrangements. In this study, terrestrial contests for resources followed an interference-exploitation tradeoff, while arboreal foragers defended entire trees as absolute territories.
3. Competitive pressure for access to arboreal resources was shown to increase with distance from F. integroides nests.
4. In this study, the ability of F. integroides to defend a resource varied with body size. Large foragers were better defenders than small foragers. For groups of foragers, the ability to defend a resource increased with the ratio of large to small foragers.
5. In response to competitive pressure, F. integroides colonies altered the size distribution of arboreal, but not terrestrial, foragers. An increase in competitive pressure was matched by an increase in the number of large foragers allocated to trees. This response to competition affected the relationship between body size and distance from the nest for arboreal foragers.
6. Foraging behaviours for individual arboreal foragers also varied with competitive pressure. As competition increased, large arboreal foragers spent more time in direct contact with the resource rather than standing between resource patches.     

Disentangling determinants of egg size in the Geometridae (Lepidoptera) using an advanced phylogenetic comparative method

We present a phylogenetic comparative study assessing the evolutionary determinants of egg size in the moth family Geometridae. These moths were found to show a strong negative allometric relationship between egg size and maternal body size. Using recently developed comparative methods based on an Ornstein-Uhlenbeck process, we show that maternal body size explains over half the variation in egg size. However, other determinants are less clear: ecological factors, previously hypothesized to affect egg size, were not found to have a considerable influence in the Geometridae. The limited role of such third factors suggests a direct causal link between egg size and body size rather than an indirect correlation mediated by some ecological factors. Notably, no large geometrid species lay small eggs. This pattern suggests that maternal body size poses a physical constraint on egg size, but within these limits, there appears to be a rather invariable selection for larger eggs.