Harvester ant nests, soil biota and soil chemistry

Many ant species accumulate organic debris in the vicinity of their nests. These organic materials should provide a rich resource base for the soil biota. We examined the effect of harvester ant nests (Pogonomyrmex barbatus) on the soil community and soil chemistry. Ant nest soils supported 30-fold higher densities of microarthropods and 5-fold higher densities of protozoa than surrounding, control soils. The relative abundances of the major groups of protozoa differed as well: amoebae and ciliates were relatively overrepresented, and flagellates underrepresented, in ant nest versus control soils. Densities of bacteria and fungi were similar in the two soil types. Concentrations of nitrate, ammonium, phosphorus, and potassium were significantly higher in ant nest soils, while concentrations of magnesium, calcium, and water were similar in nest and control soils. Ant nest soils were marginally more acidic than controls. The results demonstrate that P. barbatus nests constitute a significant source of spatial heterogeneity in soil biota and soil chemistry in arid grasslands. Received: 17 March 1997 / Accepted: 10 June 1997     

Queen adoption in the polygynous and polydomous ant, Leptothorax curvispinosus

Alate female reproductives of the facultatively polygynous andpolydomous ant, Lepiothorax curuispinosus, were reared fromfield-collected nests, mated, and introduced into either theirparental nests or alien conspecific nests. The 41 queens introducedinto alien nests were usually attacked and rejected (97. 6%),but one queen was accepted after initial aggression. The 27queens introduced into their parental nests received a variableresponse. Some were accepted without any apparent aggression(59. 3%), but others were strongly attacked and rejected (40.7%). Sequential introductions of up to four queens into particularparental nests indicated that nests consistently either acceptor reject their mated offspring. The presence or absence ofresident queens in parental nests had no apparent influenceon the acceptance of offspring queens. Nests that accepted queenshad significantly fewer workers than those that did not, butthis slight difference is unlikely to explain these dichotomousresults and could be spurious. Dissections of the introducedqueens revealed that 79. 0% were inseminated and 98. 3% haddeveloping, yolked eggs in their ovarioles, but these variableshad no apparent effect on acceptability. Similarly, the sizeof the introduced queens and the time that elapsed between matingand introduction had no apparent effect. The consistent responseof parental nests in either accepting or rejecting their matedoffspring indicates a mechanism of queen number regulation inthis species that involves characteristics of the colony ornest rather than variability among offspring queens. This mechanismcould be responsible for maintaining relatively low numbersof queens and high genetic relatedness in colonies (or individualnests) while promoting flexibility in colony reproduction bycolony fission ("budding") and the dispersal of young queens.This mechanism could also involve an important conflict of interestbetween parental colonies and their mated offspring and mightcontribute to the evolution of socially parasitic colony foundationstrategies. Acceptance of mated offspring by their parentalcolonies might only occur during certain periods in colony development,depend on the level of genetic diversity within the colony (ornest), reflect the condition of the colony, nest, queen(s),brood, or local habitat, or result from a genetic polymorphism.     

Sex-ratio conflict between queens and workers in eusocial Hymenoptera: mechanisms, costs, and the evolution of split colony sex ratios

Because workers in the eusocial Hymenoptera are more closely related to sisters than to brothers, theory predicts that natural selection should act on them to bias (change) sex allocation to favor reproductive females over males. However, selection should also act on queens to prevent worker bias. We use a simulation approach to analyze the coevolution of this conflict in colonies with single, once-mated queens. We assume that queens bias the primary (egg) sex ratio and workers bias the secondary (adult) sex ratio, both at some cost to colony productivity. Workers can bias either by eliminating males or by directly increasing female caste determination. Although variation among colonies in kin structure is absent, simulations often result in bimodal (split) colony sex ratios. This occurs because of the evolution of two alternative queen or two alternative worker biasing strategies, one that biases strongly and another that does not bias at all. Alternative strategies evolve because the mechanisms of biasing result in accelerating benefits per unit cost with increasing bias, resulting in greater fitness for strategies that bias more and bias less than the population equilibrium. Strategies biasing more gain from increased biasing efficiency whereas strategies biasing less gain from decreased biasing cost. Our study predicts that whether queens or workers evolve alternative strategies depends upon the mechanisms that workers use to bias the sex ratio, the relative cost of queen and worker biasing, and the rates at which queen and worker strategies evolve. Our study also predicts that population and colony level sex allocation, as well as colony productivity, will differ diagnostically according to whether queens or workers evolve alternative biasing strategies and according to what mechanism workers use to bias sex allocation.     

Colony structure and reproduction in the ant, Leptothorax acervorum

We analyzed the sociogenetic organization of the ant (Leptothoraxacervorum) from Nrnberger Reichswald in Southern Germany. Accordingto relatedness estimates from allozyme analyses, virgin femalesexuals produced in polygynous colonies were on average fullsisters, whereas workers in a pooled sample of polygynous colonieswere significantly less closely related. Rather than attributingthis to reproductive hierarchies among nest mate queens, weshow how this phenomenon could result from seasonal fluctuationsof colony composition and a decline of the production of femalesexuals in polygynous colonies. We suggest that by queen adoptionand emigration or budding, colonies easily switch from monogynyto polygyny and vice versa. Due to the long developmental timeof sexual larvae, colonies that have become polygynous onlyrecently will still produce the female sexual progeny of a singlequeen. In older polygynous nests, fewer and fewer female sexualsare produced, but colonies may fragment into monogynous budsin which the production of female sexuals may begin again. Relatednessestimates, dissection results, and field observations supportthis suggestion. This pattern of cyclical monogyny and polygynykeeps nest mate relatedness high and probably facilitates colonyfounding in boreal habitats. Preliminary data suggest that thepattern of the production of sexuals in colonies of L. acervorumfits the expectations of sex allocation theory.     

Intracolonial conflict in the slave-making ant <Emphasis Type="Italic">Protomognathus americanus</Emphasis>: dominance hierarchies and individual reproductive success

Summary Social group viability results from a trade-off between cooperation and conflict, driven respectively by group and individual interests. Workers of the slave-making ants are known to have a high egg-laying potential, leading to a potential conflict over male production. Queenright and queenless nests of the slave-making ant Protomognathus americanus show a near-linear dominance hierarchy, and dominance rank is correlated with reproductive activity. Genetic and behavioural analysis revealed that the queen, when present in the nest, is behaviourally dominant and monopolises reproduction. In queenless nests, the haploid (male) brood is produced primarily by a single worker. We suggest the dominance hierarchy regulates male production, between the queen and her workers as well as among workers. Comparison of our results to another study allows us to place our data in an ecological context. This slave-making ant species appears to fit the concession model of reproductive skew: where resources (i.e. host nests) are poor, there is strong skew and where resources are richer reproduction is more egalitarian.Received 31 July 2003; revised 7 October 2003; accepted 9 October 2003.     

A theoretical model of the evolution of maternal effects under parent-offspring conflict

The evolution of maternal effects on offspring phenotype should depend on the extent of parent-offspring conflict and costs and constraints associated with maternal and offspring strategies. Here, we develop a model of maternal effects on offspring dispersal phenotype under parent-offspring conflict to evaluate such dependence. In the absence of evolutionary constraints and costs, offspring evolve dispersal rates from different patch types that reflect their own, rather than the maternal, optima. This result also holds true when offspring are unable to assess their own environment because the maternal phenotype provides an additional source of information. Consequently, maternal effects on offspring diapause, dispersal, and other traits that do not necessarily represent costly resource investment are more likely to maximize offspring than maternal fitness. However, when trait expression was costly, the evolutionarily stable dispersal rates tended to deviate from those under both maternal and offspring control. We use our results to (re)interpret some recent work on maternal effects and their adaptive value and provide suggestions for future work.     

Reproductive conflict in social insects: male production by workers in a slave-making ant

In insect societies, workers cooperate but may also pursue their individual interests, such as laying viable male eggs. The case of obligatory slave-making ants is of particular interest because workers do not engage in maintenance activities and foraging. Therefore, worker egg laying is expected to be less detrimental for colony efficiency than in related, nonparasitic species. Furthermore, as slave-making workers usually do not perform brood care and thus might have little power in manipulating sex allocation, they might be more strongly selected to increase their direct fitness by producing their own sons than workers in nonparasitic species. In this study we investigated worker reproduction in four natural colonies of the slave-making ant Polyergus rufescens, using highly variable microsatellite markers. Our results show that workers produce up to 100% of the males. This study thus presents the first direct evidence of an almost complete takeover of male reproduction by workers in ants.     

Effects of experimental small-scale grassland fragmentation on spatial distribution, density, and persistence of ant nests

Abstract.  1. Grassland fragmentation is expected to influence the abundance of different invertebrate species to a different extent. Fragmentation-related effects are of particular importance in species that interact with many other species.
2. The density and spatial distribution of nests of 15 ant species in experimentally fragmented calcareous grasslands at three sites in the Northern Swiss Jura mountains were examined. Fragments of different size (0.25 m², 2.25 m², and 20.25 m²) were isolated by a 5-m wide strip of frequently mown vegetation. Control plots of corresponding size were situated in adjacent undisturbed grassland.
3. Three years after initiation of the experiment, ant nest density did not differ between fragments and control plots. Six years after initiation of the experiment, however, ant nest density and forager abundance were higher in large fragments than in large control plots. Ant nests tended to occur more frequently along the edge of fragments than in the core area. Persistence time of nests of the most abundant species, Lasius paralienus , tended to be shorter in fragments than in control plots. Furthermore, persistence time was longer in nests situated close to the fragment edge than in nests in the core area.
4. Effects on nest density, edge effects on the spatial distribution of nests, and the relationships between nest density and environmental factors were more pronounced when only nests of L. paralienus were considered. The implications of these findings for plant and other invertebrate species are discussed.     

Games among cannibals: competition to cannibalize and parent-offspring conflict lead to increased sibling cannibalism

Sibling cannibalism occurs in many species, yet understanding of sibling cannibalism as an adaptation currently lags behind understanding of other antagonistic interactions among siblings. Observed sibling cannibalism phenotypes likely reflect the interaction between competitive games among siblings and parent-offspring conflict. Using a game-theoretic approach, we derive optimal offspring cannibalism behaviour and parental modifiers that limit or facilitate cannibalism. The results are compared to contemporary frequency-independent analysis. With the addition of game interactions among siblings or parent-offspring co-evolution, our model predicts increased cannibalism (compared to the frequency-independent prediction), as offspring compete to eat siblings. When infertile eggs are present--strengthening competition--offspring risk eating viable siblings in order to gain access to infertile eggs, intensifying parent-offspring conflict. We use the results to make new predictions about the occurrence of sibling cannibalism. Additionally, we demonstrate the utility of trophic egg laying as a maternal mechanism to promote egg eating.     

Hamilton's rule, imprinting and parent-offspring conflict over seed mass in partially selfing plants

When genes in the offspring control the provisioning of the seed, the optimal seed size can be calculated exactly by applying Hamilton's rule. When seed size is a compromise between mother and offspring, we predict that outcrossing plant species produce larger seeds than selfers. This trend was found in the British flora and in a number of well-studied plant families. The analysis was extended to imprinting, a conditional strategy in which a gene in the offspring takes more resources when derived from the father than from the mother. The conditions for imprinting to be selected were rather restrictive. The analysis is relevant for the current debate about the evolution of imprinting in Arabidopsis thaliana.     

The influence of mating system on the intensity of parent-offspring conflict in primates

An evolutionary conflict of interest exists between parents and their offspring over the partitioning of parental investment (PI) among siblings. When the direct fitness benefits to offspring of increased PI, outweigh the inclusive fitness costs from lost future sibling fitness, selection should favour the evolution of offspring selfishness over altruism. In theory, this conflict is heightened when females are not strictly monogamous, as current offspring should be less altruistic towards future half-siblings than they would be towards full-siblings. Using data collected on foetal growth rate (representing prenatal PI) in primates, I test the prediction from theory that the resolution of the parent-offspring conflict will be closer to the offspring's evolutionary optima in polyandrous species than in more monandrous species. Using phylogenetic comparative analysis, and controlling for allometry, I show that offspring are able to obtain more PI when the probability of future full-siblings decreases, and that this is most pronounced in taxa where there is the opportunity for direct foetal access to the maternal bloodstream. These results support the hypothesis that the resolution of prenatal PI conflict is influenced by both a species' mating system and by its placental structure.     

A simple and nondestructive method for estimation of worker population size in Myrmica ant nests

A method to estimate the number of workers in Myrmica ant nests on abandoned meadows was developed based on removal of workers. Ant workers have a tendency to climb up on wooden sticks put into their nests, therefore, assuming that the number of workers removed on sticks is related to the total number of workers within the nests, regression models for Myrmica rubra, M. ruginodis and M. scabrinodis may be built. We used a general regression model to perform a backward stepwise elimination of explanatory variables. These were the number of workers removed on sticks, temperature at the nest and site (a categorical variable). In case of each species the final model contained only the number of workers removed as a significant variable. The method is apparently non-destructive as we did not observe decreased survival of nests surveyed as compared to control nests. The method can be a very useful tool in population studies of ants as well as in biodiversity projects, where ants are used as bioinidcators. Received 10 February 2005; revised 4 August 2005; accepted 24 August 2005.     

War of hormones over resource allocation to seeds: Strategies and counter-strategies of offspring and maternal parent

It is suggested that maternal parent and offspring have conflicting interests over the extent of resource allocation to developing seeds. While maternal parent would be selected to allocate her resources optimally among her offspring, the latter would be selected to demand more. In animals, offspring are known to demand additional resources either visibly (through intense vocal calls) or subtly through the production of hormones. In plants though parent offspring conflict over resource allocation has been invoked, the mechanism through which the parent and offspring interact in regulating resource allocation into developing seeds is not yet clear. In this paper, we propose that the strategies and counter-strategies of the offspring and mother during the development of seeds might be manifested through the production of appropriate growth hormones. Accordingly, we predict (i) hormones that mobilize resources into seeds (e.g. auxins and gibberellic acid) shall be synthesized exclusively by the offspring tissue and (ii) hormones that inhibit resource flow in to seeds (e.g. abscisic acid) be produced exclusively by the maternal tissue. We show that these predictions are supported by existing literature on the temporal dynamics and source of production of growth hormones during seed development. Finally, we suggest that such analysis viewing the production of different hormones during early seed development, as strategies and counter-strategies of mother and offspring tissue, helps ofer a meaningful interpretation of the otherwise complex dynamics of hormone fluxes     

Parental conflict in birds: comparative analyses of offspring development, ecology and mating opportunities

Parents often conflict over how much care to provide to their offspring. This conflict is expected to produce a negative relationship between male and female parental care, the strength of which may be mediated by both ecological and life-history variables. Previous studies have observed such trade-offs, but it is not known how generally they occur. Traditional views of sexual conflict place great importance on ecological factors in determining levels of parental care, whereas alternative views propose that the key determinant is mating opportunity. We carried out a broad-scale comparative study of parental conflict using 193 species from 41 families of birds. Using phylogenetic comparative analysis, we establish the generality of intersexual parental care conflict. We also show that parental conflict, as indicated by the disparity in care between the male and the female, depends on offspring development and mating opportunities, since in precocial species both males and females responded to increased mating opportunities. Altricial birds, however, failed to show these relationships. We also found little influence of breeding climate on parental conflict. Taken together, our results suggest that sexual conflict is a key element in the evolution of parental care systems. They also support the view that the major correlates of the intersexual conflict are mating opportunities for both sexes, rather than the breeding environment.     

Superparasitism of larval hosts by the walnut fly, <Emphasis Type="Italic">Rhagoletis juglandis</Emphasis>, and its implications for female and offspring performance

The oviposition-preference–offspring-performance hypothesis predicts that female insects should prefer to deposit clutches on or in hosts that maximize offspring performance. An important assumption behind this prediction is that female fitness is tightly correlated with the fitness of any one offspring. In this study, we evaluate offspring performance in the walnut fly, Rhagoletis juglandis Cresson (Diptera: Tephritidae), in relation to a previously described oviposition preference for previously exploited host fruit. In particular, we examined how superparasitism of walnut hosts influences offspring survival and weight at pupation under field conditions. We found that superparasitism was common and that increases in larval densities within fruit were associated with reduced larval survival and weight at pupation. In a laboratory experiment, female size was correlated with lifetime fecundity. In this system, oviposition preference is therefore negatively, not positively, correlated with offspring performance. We argue that patterns of female preference in this system reflect direct benefits to females that are traded off against costs in terms of offspring fitness. Because female fitness is a product not only of offspring quality but also of the total number of offspring produced, female walnut flies may be optimizing their fitness by producing many less fecund offspring. Studies examining the preference-performance hypothesis should consider the reproductive conflicts between parents and offspring as potential factors that influence the congruence between parental preference and offspring performance.     

Ecology, life history and resource allocation in the ant, Leptothorax nylanderi

We aimed at identifying the causal basis of previously shown interrelations between demographic and genetic colony structure, ecological factors and split sex ratios in the ant, Leptothorax nylanderi. Colony-level variation in sex allocation was only partly explained by annual fluctuations during eight study years and by resource availability as indicated by sexual production of colonies. Allocation ratios were highly male-biased in dense populations with ephemeral nest sites and high frequencies of colonies containing several unrelated matrilines. Field observations and experimental manipulations showed that nest site limitation leads to such heterogeneous colonies. Laboratory experiments demonstrated that genetic heterogeneity directly causes male-biased investment, although relatedness asymmetry is not influenced by invasion of unrelated queens. The influence of genetic composition on allocation strategies might either be explained by negative feedback mechanisms connected with habitat saturation or by a lower efficiency of heterogeneous colonies. Our results thus demonstrate which factors other than variation in relatedness asymmetry can explain split sex ratios in ants. An empirical test of a model on reproductive allocation revealed on-going queen-worker conflict over colony growth and sexual reproduction. Workers controlled reproductive allocation, but queen-worker conflict ceased in large colonies with a high survival rate.     

Mating frequency and mating system of the polygynous ant, Leptothorax acervorum

Multiple mating by queens (polyandry) and the occurrence of multiple queens in the same colony (polygyny) alter patterns of relatedness within societies of eusocial insects. This is predicted to influence kin-selected conflicts over reproduction. We investigated the mating system of a facultatively polygynous UK population of the ant Leptothorax acervorum using up to six microsatellite loci. We estimated mating frequency by genotyping 79 dealate (colony) queens and the contents of their sperm receptacles and by detailed genetic analysis of 11 monogynous (single-queen) and nine polygynous colonies. Results indicated that 95% of queens were singly mated and 5% of queens were doubly mated. The corrected population mean mating frequency was 1.06. Parentage analysis of adults and brood in 17 colonies (10 monogynous, 7 polygynous) showed that female offspring attributable to each of 31 queens were full sisters, confirming that queens typically mate once. Inbreeding coefficients, queen-mate relatedness of zero and the low incidence of diploid males provided evidence that L. acervorum sexuals mate entirely or almost entirely at random. Males mated to queens in the same polygynous colony were not related to one another. Our data also confirmed that polygynous colonies contain queens that are related on average and that their workers had a mixed maternity. We conclude that the mating system of L. acervorum involves queens that mate near nests with unrelated males and then seek readoption by those nests, and queens that mate in mating aggregations away from nests, also with unrelated males.     

Optimal defence, kin conflict and the distribution of furanocoumarins among offspring of wild parsnip

The factors influencing the allocation of chemical defences to plant offspring have largely been unexplored, conceptually and experimentally. Because evolutionary interactions between maternal plants and their progeny can affect resource allocation patterns among sibling offspring, we suggest that kin conflict as well as herbivore–plant interaction theories need to be considered to predict chemical defence allocation patterns. Optimal defence theory predicts that maternal plants should defend more heavily those offspring in which resources have been disproportionately invested. In contrast, kin conflict theory predicts that natural selection will favour genotypes that can compete successfully for maternal defences irrespective of their quality, even at the expense of the fitness of siblings and the maternal plant. Evidence for these defence patterns were evaluated by examining the allocation of furanocoumarins to seeds of the wild parsnip (Pastinaca sativa, Apiaceae). Furanocoumarins are toxins that are localized within the oil tubes of the maternal tissues of seeds. We evaluated the role of offspring investment (endosperm mass) and seed genotype on furanocoumarin allocation by mating an array of pollen donors with pollen recipients. Furanocoumarins were found to be positively correlated with endosperm mass on one side of the seed, a result consistent with optimal defence theory; however, on the other side of the seed, furanocoumarin content was influenced by seed genotype and was unrelated to endosperm mass. These effects varied with maternal plant. Further experiments demonstrated that nearly 80% of furanocoumarin production occurs after pollination, when fertilization products are active. Although the amount of furanocoumarin influenced by the seed genotype is small relative to the total quantity in the seed, these furanocoumarins are the first line of defence against important predators, such as the parsnip webworm, Depressaria pastinacella (Lepidoptera: Oecophoridae). We found that parsnip webworm larvae were able to discriminate among genotypes within an inflorescence. In line with previous studies, these results suggest that a genotype's ability to influence furanocoumarin defence may affect its probability of survival. We conclude that the distribution of defences among plant offspring in wild parsnip is probably influenced by competition among seed genotypes that conflicts with maternal optimal defence. This revised version was published online in July 2006 with corrections to the Cover Date.