Reproductive Competition in Colonies of the Ant Leptothorax gredleri

Avoid mistakes when choosing a new home: Nest choice and adoption of Leptothorax ant queens

In ants, mating and colony founding are critical steps in the life of ant queens. Outside of their nests, young queens are exposed to intense predation. Therefore, they are expected to have evolved behavior to accurately and quickly locate a nesting place. However, data on the early life history of female reproductives are still lacking. Leptothorax gredleri is a suitable model organism to study the behavior of young queens. Reproductives can be reared under artificial conditions and readily mate in the laboratory. After mating, L. gredleri queens have the options to found solitarily, seek adoption into another colony, or return into their natal nest. In this study, we investigated the decision-making processes of female sexuals before and after mating. In particular, we tested whether female sexuals use chemical cues to find their way back to the nest, studied if they prefer their own nest over other nesting sites and followed the adoption dynamics of mated queens over 8 weeks (plus hibernation and spring). We showed that female sexuals and freshly mated queens spent more time on substrate previously used by workers from their own colony and from another colony than on a blank substrate. This discriminatory capability of queens appears to be lost in old, reproductive queens. Nest choice experiments showed that female sexuals and freshly mated queens can distinguish their own nest while old mated queens do not. When reintroduced in their maternal colony, young queens were readily adopted, but a few weeks later aggression against young queens led to their emigration from the maternal nest and eventually also death.

Behavioral plasticity in ant queens: environmental manipulation induces aggression among normally peaceful queens in the socially polymorphic ant Leptothorax acervorum

The behavioral traits that shape the structure of animal societies vary considerably among species but appear to be less flexible within species or at least within populations. Populations of the ant Leptothorax acervorum differ in how queens interact with other queens. Nestmate queens from extended, homogeneous habitats tolerate each other and contribute quite equally to the offspring of the colony (polygyny: low reproductive skew). In contrast, nestmate queens from patchy habitats establish social hierarchies by biting and antennal boxing, and eventually only the top-ranking queen of the colony lays eggs (functional monogyny: high reproductive skew). Here we investigate whether queen-queen behavior is fixed within populations or whether aggression and high skew can be elicited by manipulation of socio-environmental factors in colonies from low skew populations. An increase of queen/worker ratio and to a lesser extent food limitation elicited queen-queen antagonism in polygynous colonies from Nürnberger Reichswald similar to that underlying social and reproductive hierarchies in high-skew populations from Spain, Japan, and Alaska. In manipulated colonies, queens differed more in ovarian status than in control colonies. This indicates that queens are in principle capable of adapting the magnitude of reproductive skew to environmental changes in behavioral rather than evolutionary time.

Workers influence royal reproduction

Understanding which parties regulate reproduction is fundamental to understanding conflict resolution in animal societies. In social insects, workers can influence male production and sex ratio. Surprisingly, few studies have investigated worker influence over which queen(s) reproduce(s) in multiple queen (MQ) colonies (skew), despite skew determining worker-brood relatedness and so worker fitness. We provide evidence for worker influence over skew in a functionally monogynous population of the ant Leptothorax acervorum. Observations of MQ colonies leading up to egg laying showed worker aggressive and non-aggressive behaviour towards queens and predicted which queen monopolized reproduction. In contrast, among-queen interactions were rare and did not predict queen reproduction. Furthermore, parentage analysis showed workers favoured their mother when present, ensuring closely related fullsibs (average r = 0.5) were reared instead of less related offspring of other resident queens (r ≤ 0.375). Discrimination among queens using relatedness-based cues, however, seems unlikely as workers also biased their behaviour in colonies without a mother queen. In other polygynous populations of this species, workers are not aggressive towards queens and MQs reproduce, showing the outcome of social conflicts varies within species. In conclusion, this study supports non-reproductive parties having the power and information to influence skew within cooperative breeding groups.

Polymorphic social organization in an ant

Identifying species exhibiting variation in social organization is an important step towards explaining the genetic and environmental factors underlying social evolution. In most studied populations of the ant Leptothorax acervorum, reproduction is shared among queens in multiple queen colonies (polygyny). By contrast, reports from other populations, but based on weaker evidence, suggest a single queen may monopolize all reproduction in multiple queen colonies (functional monogyny). Here we identify a marked polymorphism in social organization in this species, by conclusively showing that functional monogyny is exhibited in a Spanish population, showing that the social organization is stable and not purely a consequence of daughter queens overwintering, that daughter queen re-adoption is frequent and queen turnover is low. Importantly, we show that polygynous and functionally monogynous populations are not genetically distinct from one another based on mtDNA and nDNA. This suggests a recent evolutionary divergence between social phenotypes. Finally, when functionally monogynous and polygynous colonies were kept under identical laboratory conditions, social organization did not change, suggesting a genetic basis for the polymorphism. We discuss the implications of these findings to the study of reproductive skew.

Mating is associated with immediate changes of the hydrocarbon profile of Leptothorax gredleri ant queens

While sexual communication is often characterized by attempted manipulation, both sexes agree about females reliably signalling their receptivity. Female sexuals of the ant Leptothorax gredleri quickly became unattractive to males after their first copulation. This loss of attractiveness coincided with almost immediate changes in their cuticular hydrocarbon (CHC) profiles. Already 30 min after mating, the CHC profiles of female sexuals had significantly lower relative amounts of branched alkanes and higher amounts of linear alkanes than those of unmated and freshly mated female sexuals. Discriminant analysis did not distinguish between the profiles of freshly mated and unmated female sexuals, suggesting that the extremely rapid modification of CHC profiles is not caused by males marking females with anti-aphrodisiac CHCs. Instead, the new profile is produced by the female sexuals themselves. In addition to making them unattractive to males, this change may also help mated female sexuals when seeking adoption into established colonies.

Aging and reproduction in social insects--a mini-review

Perennial social insects are characterized by the extraordinarily long lifespan of their reproductive females, which may be tens or hundreds of times larger than that of non-social insects of similar body size and also greatly surpasses that of conspecific non-reproductives. Evolutionary theories of aging explain this phenomenon from the low extrinsic mortality queens experience once they have successfully established their colony. The aim of our review is to summarize recent findings on the ultimate and proximate causes of increased queen longevity in social insects, in particular ants and honey bees. While progress is being made in elucidating the interrelations between the vitellogenin, juvenile hormone, fecundity, and senescence, we feel that the explanation for the comparatively short lifespan of queens in multi-queen societies is as yet not satisfactory and needs further attention, both concerning its proximate and ultimate basis.

Dynamics of sperm transfer in the ant Leptothorax gredleri

Mating tactics differ remarkably between and within species of social Hymenoptera (bees, wasps, ants) concerning, e.g., mating frequencies, sperm competition, and the degree of male sperm limitation. Although social Hymenoptera might, therefore, potentially be ideal model systems for testing sexual selection theory, the dynamics of mating and sperm transfer have rarely been studied in species other than social bees, and basic information needed to draw conclusions about possible sperm competition and female choice is lacking. We investigated sperm transfer in the ant Leptothorax gredleri, a species in which female sexuals attract males by "female calling." The analysis of 38 female sexuals fixed immediately or up to 7 days after copulation with a single male each revealed that the sperm is transferred into the female bursa copulatrix embedded in a gelatinous mass, presumably a spermatophore. Sperm cells rapidly start to migrate from the tip of the spermatophore towards the spermatheca, but transfer is drastically slowed down by an extreme constriction of the spermathecal duct, through which sperm cells have to pass virtually one by one. This results in the spermatheca being filled only between one and several hours after mating. During this time, the posterior part of the spermatophore seals the junction between bursa copulatrix and spermathecal duct and prevents sperm loss. The prolonged duration of sperm transfer might allow female sexuals to chose between ejaculates and explain previously reported patterns of single paternity of the offspring of multiply mated queens.

Worker age, size and social status in queenless colonies of the ant Leptothorax gredleri

Workers in queenless colonies of the ant Leptothorax gredleri form dominance hierarchies by antennation bouts and biting. The frequency of aggressive interactions peaked when the ants became active again after hibernation, a second time in summer, when new workers eclosed from overwintered brood, and a third time when we experimentally removed the queen. High-ranking individuals were more active and had greater ovarian development than their low-ranking nestmates. We investigated what proximately determines a worker's social status. Neither age, previously shown to affect hierarchy rank in several other social insects, nor levels of fluctuating asymmetry appeared to be important in worker rank orders in L. gredleri. High-ranking workers were on average larger than low-ranking individuals and body size and rank were correlated in four of five colonies investigated. However, individuals of similar age but different social status did not differ in size. Copyright 1999 The Association for the Study of Animal Behaviour.

Worker reproduction and social hierarchies in Leptothorax ants

Kin selection theory predicts the existence of potential conflict between queen and workers and among workers concerning the production of males in insect societies with a single, once-mated queen. We investigated the occurrence of reproductive conflict and worker reproduction in both single- and multi-queen colonies of 10 species of the ant genus LeptothoraxIn contrast to previous observations in related species, workers only infrequently engaged in aggressive interactions and did not lay large numbers of eggs in colonies containing queens. Allozyme analyses of queens, workers and males suggest that the contribution of workers to the males produced in colonies with queens is indeed minimal, at least in L. unifasciatusWhen the queens died or were experimentally removed from the colonies, in most species dominance interactions among workers became significantly more frequent and one or several high-ranking workers started to lay eggs. Workers with an increased number of ovarioles per ovary apparently had a reproductive advantage over workers with normal ovaries.Copyright 1997 The Association for the Study of Animal Behaviour1997The Association for the Study of Animal Behaviour

Paternity in eusocial Hymenoptera

Variation in paternity frequency in colonies of eusocial insects has profound effects on the relatedness among offspring and on the genetic diversity of colonies. Data on queen ‘mating-frequency’ in eusocial Hymenoptera vary in both quality and the phase of the ‘mating’ process they address. Some are observational studies of the range or maximum number of copulations; others are derived from estimates of the number of sperm in males and queens; others use genetic techniques to determine the paternity of different males among female offspring. Only the latter data can be used to calculate relatedness among offspring females. Previous reviews drew attention to these problems, but their results have established the impression that high paternity frequencies are common, largely because multiple copulations are frequently observed. For ants, we show that: (i) the range of observed copulations overestimates effective paternity frequency; and (ii) the mean effective paternity frequency in 19 species, for which accurate data based on allozyme analysis of mother — offspring combinations are available, is only 1.16 (range 1-1.48). Over one third of these species have queens in which only one male contributes to paternity. Data from 34 species, which include less detailed genetic studies and four species studied using sperm counts, give similar results. Only two species, bothAttaleaf cutter ants and both studied using data on sperm stored in queen spermathecas, appear to have effective insemination frequencies above two. Data on bees and wasps show a similar trend. We conclude that reliably documented high paternity or insemination frequencies (> 2) are currently restricted to one phylogenetically isolated and highly eusocial taxon each in ants, eusocial bees and wasps (Atta, ApisandVespula, respectively). This pattern justifies the working hypothesis that multiple mating, by lowering the relatedness between female offspring and thereby the benefits of reproductive helping behaviour, has not been a general constraint for the evolution of eusociality in the Hymenoptera. Using reliable data on paternity frequency and insemination, we re-analyse two factors that it has been suggested correlate with mating frequency: colony population and number of egg-laying queens per colony. We find the following. 1. There is a significant positive correlation between paternity/insemination frequency and colony size for monogynous ants, but not for polygynous ants. This result seems to support the ‘sperm limitation’ hypothesis, that queens which need to be highly fecund copulate multiply to store sufficient sperm. We note, however, that the same trend is expected when large and/or long-lived colonies profit more from having genetically diverse offspring. 2. There is no significant negative correlation between paternity/insemination frequency and number of queens per colony. However, when the analysis is restricted to species with large colonies and no intranidal mating, the correlation between paternity frequency and queen number becomes marginally significant. Several previous reviews have addressed the possible adaptive significance of multiple paternity. In contrast, and in keeping with the data that show single paternity to be frequent, we discuss selective reasons for single or low paternity. We compare the relative effects of multiple paternity and multiple maternity on genetic diversity within colonies and show that they are not equivalent, and we also discuss directions for future research on paternity issues in social insects.

Habitat structure, dispersal strategies and queen number in two boreal Leptothorax ants

In two nearctic ants, Leptothorax canadensis and Leptothorax sp. A, young queens may either found their own nest solitarily after mating or seek adoption into an established colony. Whether a queen disperses or not is associated with genetically determined queen morphology in Leptothorax sp. A. Whereas a majority of winged queens attempt solitary colony founding after mating, most wingless, intermorphic queens return to their maternal nests and new colonies are founded by budding after hibernation. The latter strategy appears to be correlated with patchy, isolated habitats, whereas in extended boreal forests dispersal on the wing is probably more common. Alternative dispersal strategies strongly affect the average number of queens per colony and seasonal fluctuations of colony structure.