Rainer Rosengren

Ortstreue in foraging ants of the Formica rufa group - Hierarchy of orienting cues and long-term memory

SummaryRoute fidelity and site allegiance (Ortstreue) ofFormica rufa group ants are analysed with repect to orienting cues and preservation through periods of isolation or winter dormancy. Colour-marked workers showed certain site allegiance to different parts of a moundnest, but the phenomenon was too weak to explain the retention of extranidal Ortstreue measurad in nature. The same applies to kinesthetic and other cues based on intranest architecture. Evidence is presented that extranidal Ortstreue is based on long-lasting individual memory of spatially organized visual cues. The engram stored through several months of winter dormancy can be indirectly identified with a representation of landmark/canopy patterns. The experiments carried out in large openair arenas show, however, that olfactory cues, probably identical with scent markings of the home range, are a cause of idiosyncratic Ortstreue during the dark period. The latter orienting cues, which in the main are switched off in the presence of visual cues, may also be stored in the memory, but preservation through winter dormancy could not be confirmed. The possibility that the ants may use geomagnetic cues was tested, but the result was negative. The ecological implications of the findings are discussed.ZusammenfassungDas Phänomen der Ortstreue von Ameisen derFormica rufa Gruppe wurde analysiert im Hinblick auf die zugrundeliegenden Orientierungsreize und die Erinnerung über Perioden der Isolation und der Winterruhe. Farbmarkierte Arbeiter zeigten eine gewisse Ortstreue zu verschiedenen Teilen eines Hügelnestes, doch war das Phänomen zu schwach, um die Erhaltung der Ortstreue außerhalb des Nestes zu erklären, die in freier Natur gemessen wurde. Dasselbe gilt für kinästhetische und andere Reize, die auf der Architektur innerhalb des Nestes beruhen. Es wird gezeigt, daß Ortstreue außerhalb des Nestes auf extrem langlebiger individueller Erinnerung räumlich organisierter visueller Reize beruht. Das Engramm wird über mehrere Monate der Winterruhe bewahrt. Es kann auf indirektem Wege als Repräsentation eines Landmarken/Baumkronen-Musters identifiziert werden. In großen Freiluftarenen angestellte Versuche zeigen jedoch, daß olfaktorische Reize, die wahrscheinlich mit Duftmarkierungen des Aktionsrammes identisch sind, eine Ursache für idiosynkratische Ortstreue während der Dunkelperiode darstellen. Diese Orientierungsreize, die im allgemeinen in Gegenwart visueller Reize ausgeschaltet sind, mögen ebenfalls im Gedächtnis gespeichert werden, doch konnte ihre Erhaltung über die Winterruhe hinweg nicht bestätigt werden. Die Möglichkeit, daß die Ameisen geomagnetische Reize verwenden könnten, wurde geprüft, doch mit negativem Resultat. Die ökologischen Implikationen der Befunde werden diskutiert.

Diploid males and colony-level selection in Formica ants

It is suggested that the evolution of polyandry by social hymenopteran queens is caused by colony-level selection, either because polyandry affects the distribution of non-functional diploid males in colonies (the load hypothesis) or because it increases the genetic diversity of the worker force (the diversity hypothesis). Diploid males that arise from fertilized eggs thai are homozygous at the sex-determining locus (or loci) are inviable or infertile. Models of the load hypothesis analysed in this study suggest that slow growth and high mortality of colonies with diploid males favour single mating by queens. The longer the period ol colony growth (the period with selective differences) and the heavier the mortality, the stronger is the selection for monandry. The load hypothesis also predicts an association between monogyny and monandry. In contrast, the diversity hypothesis predicts an association between monogyny and polyandry, as multiple mating offers a way by which a monogynous colony could increase...

Seasonal polydomy and unicoloniality in a polygynous population of the red wood ant Formica truncorum

Ant colonies may have a single or several reproductive queens (monogyny and polygyny, respectively). In polygynous colonies, colony reproduction may occur by budding, forming multinest, polydomous colonies. In most cases, budding leads to strong genetic structuring within populations, and positive relatedness among nestmates. However, in a few cases, polydomous populations may be unicolonial, with no structuring and intra-nest relatedness approaching zero. We investigated the spatial organisation and genetic structure of a polygynous, polydomous population of Formica truncorum in Finland. F. truncorum shifts nest sites between hibernation and the reproductive season, which raises the following question: are colonies maintained as genetic entities throughout the seasons, or is the population unicolonial throughout the season? Using nest-specific marking and five microsatellite loci, we found a high degree of mixing between individuals of the population, and no evidence for a biologically significant genetic structuring. The nestmate relatedness was also indistinguishable from zero. Taken together, the results show that the population is unicolonial. In addition, we found that the population has undergone a recent bottleneck, suggesting that the entire population may have been founded by a very limited number of females. The precise causes for unicoloniality in this species remain open, but we discuss the potential influence of intra-specific competition, disintegration of recognition cues and the particular hibernation habits of this species.

Worker reproduction in ants — a genetic analysis

Workers of social insects may enhance their inclusive fitness by laying unfertilized eggs that develop into males. In particular, workers may gain from rearing worker-produced males if their average relatedness to them exceeds their relatedness to queen-produced males. These relatedness values depend both on the queen mating frequency and on the number and relatedness of nestmate queens. We examined the occurrence of worker reproduction in field colonies of four ant species of the genera Formica and Myrmica. Based on relatedness arguments alone, worker reproduction was expected in all species because of low queen mating frequency, or low effective queen numbers. Nevertheless, genotype matching of workers and males showed that worker reproduction was absent or rare in two of the three Formica species studied here. In M. ruginodis, queens may have been the sisters of the workers in many cases, which means that workers of this species may regularly rear nephews. In the three species in which worker reproduction was not found, workers bias colony sex ratios to enhance their inclusive fitness. We therefore hypothesize that sex ratio biasing and male production may be mutually exclusive strategies for workers.

Queen Recruitment in a Highly Polygynous Supercolony of Formica lugubris (Hymenoptera, Formicidae)

This work examines behavioural relationships between young females (potential queens) and workers, in a multi-nest population (supercolony), of Formica lugubris. Each nest contains hundreds of functional queens but the colony is initiated by a single foundress (secondary polygyny). Thus, recruitment of new queens into the nests is part of the population dynamics. Substantial variation in worker response towards introduced female sexuals, ranging from execution to complete acceptance, is demonstrated. The mating status of the introduced females has a clear effect on the worker response: virgin females are accepted with about twice the probability of inseminated females. When native alates are present in a nest, all introduced females are accepted with higher probability than when the native alates are absent, later in the season. No effect of distance (between donor and recipient nests) on the worker reaction was found, within the supercolony borders. Proximate mechanisms and selective forces regulating the recruitment process are discussed in light of these findings.

Identification of the sex-pheromone of an ant, Formica lugubris (Hymenoptera, Formicidae)

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The allocation of foragers in red wood ants

Abstract. 1. We studied how colonies of the red wood ant, Formica polyctena, adjust the numbers of foragers allocated to different foraging trails. In a series of field experiments, foragers were marked and transferred from one nest to another, related nest, where they joined the foraging force. Transferred workers acted as a reserve of uncommitted, available foragers.

Alternative reproductive strategies in Formica lugubris Zett. (Hymenoptera Formicidae)

A long term study on the reproductive strategies of Formica lugubris (Formica rufa group) has revealed many new important facts. Within a supercolony the sex-ratio is strongly female biased. Some of the sexuals mate on the nest surface, whereas others fly from the nest. Those sexuals participating in flights aggregate in particular meadow sites (mating places). On these places females attract males by releasing a sex pheromone.

Attraction of the sexes inFormica lugubris Zett (Hymenoptera: Formicidae)

SummarySexuals ofFormica lugubris fly to mating places, where females attract males by using a sex pheromone. Females collected on the nest surface before departing on a mating flight are much less attractive than those collected on the mating place after the mating flight, suggesting that the mating flight triggers the release of the sex pheromone. Olfactory cues are essential for males to locate females while they patrol. Males probably use visual cues to locate females once they have alighted nearby them. Males are also attracted by aggregations of other males on the ground, probably because one or several females are likely to be close to male aggregations.