Colony structure and sex allocation ratios in the ant <Emphasis Type="Italic">Temnothorax crassispinus</Emphasis>

Summary. We analyzed the impact of ecological parameters, such as nest density and nest site availability, on colony organization and investment patterns in two populations of the ant Temnothorax crassispinus, a parapatric sibling species of the well-studied T. nylanderi (Temnothorax was until recently referred to as Leptothorax (Myrafant); Bolton, 1993). As in T. nylanderi, sex allocation ratios were strongly associated with total sexual reproduction, i. e., nests with large sexual investment produced mainly female sexuals. Furthermore, nest site quality affected sex allocation ratios, with colonies from ephemeral nest sites producing a more male-biased sex allocation ratio than colonies from sturdy nest sites. In contrast to T. nylanderi, workers in colonies of T. crassispinus were mostly fullsisters both in a dense and a sparsely populated area, suggesting that colony fusion and colony usurpation are rare in this species. In addition, the presence of a queen in a local nest unit strongly influenced sex ratio decisions, in that these nests raised a more male biased allocation ratio compared to queenless nests. This also suggests that colony structure is more stable in T. crassispinus than in T. nylanderi. We conclude that sibling species, though often very similar in their morphology and ecological requirements, may nevertheless react very differently to ecological variation.Received 11 December 2003; revised 4 March 2004; accepted 19 April 2004.     

Apparent Dear-enemy Phenomenon and Environment-based Recognition Cues in the Ant Leptothorax nylanderi

Inter- and intraspecific competition was investigated in ants of the myrmicine genus leptothorax in a deciduous woodland near Würzburg, Germany. The most common species, A. (Myrafant) nylanderi, lives in rotting pine, oak, and elder sticks and may locally reach densities of 10 nests per m². In the studied sites, only a small fraction of colonies were polydomous, i.e. single colonies typically did not inhabit several nest sites. The home ranges of nylanderi colonies overlap the ranges of other conspecific colonies and colonies of other species, especially L. (s.str.) gredleri. Foragers from different colonies encountering one another in the field back off without exhibiting strong aggression, suggesting that colonies do not defend absolute foraging territories. In laboratory experiments, the frequency and severity of agonistic interactions among workers from different colonies, all living in pine sticks, increased significantly with the distance between their nests. Workers from colonies nesting in different types of wood exhibited significantly more aggression. Experiments in which we transferred colonies from pine sticks into artificial pine or oak nests corroborate the hypothesis that nesting material strongly influences colony odour in L. nylanderi. The evolutionary significance of this apparent dear-enemy phenomenon is discussed.     

Nest takeover by queen and its positive impact on colony development in the Japanese bumblebee Bombus ignitus (Apidae: Hymenoptera)

Lone Bombus ignitus queens are known to start nests in small underground cavities. To examine the nest‐site preference of post‐hibernating queens, choice tests were carried out by providing queens with orphan colonies and empty cavities as possible nesting sites within an experimental box. Our results showed that the queens had a strong preference for takeover of an orphan colony, suggesting that nest takeover (usurpation) could occur in nature even with the presence of possible empty cavities for nesting. We compared the colony‐growth process and final production of sexuals between non‐takeover and takeover colonies. The increase in the number of egg cups was faster in the takeover colonies, suggesting that orphan broods elicit earlier oviposition by the usurping queen. Reproductives emerged earlier (significant for new queens) and in greater numbers (males) from takeover colonies than from non‐takeover colonies. Thus, post‐hibernating B. ignitus queens would search for and take over small orphan colonies to increase their fitness.     

Reproductive allocation within a polygyne, polydomous colony of the ant Myrmica rubra

1. Ant colonies commonly have multiple egg‐laying queens (secondary polygyny). Polygyny is frequently associated with polydomy (single colonies occupy multiple nest sites) and restricted dispersal of females. The production dynamics and reproductive allocation patterns within a population comprising one polygyne, polydomous colony of the red ant Myrmica rubra were studied. 2. Queen number per nest increased with nest density and the number of adult workers increased with the number of resident queens and with nest density. This suggests that nest site limitation promotes polygyny and that workers accumulate in nest units incapable of budding. 3. Nest productivity increased with the number of adult workers and production per queen was independent of queen number. Productivity increased with nest density, suggesting local resource enhancement. This shows that productivity can be a linear function of queen numbers and that the limiting factor is not the egg‐laying capacity of queens. 4. The total and per capita production of reproductives decreased towards the periphery of the colony, suggesting that the spatial location of nest units affects sexual production. Thus nests at the periphery of the colony invested more heavily in new workers. This is consistent with earlier observations in plants and could either represent investment in future budding or increased defence. 5. The colony produced only five new queens and 2071 males, hence the sex ratio was extremely male biased.     

Spatial patterns, temporal variability, and the role of multi-nest colonies in a monogynous Spanish desert ant

Abstract 1. The colonies of the Spanish desert ant Cataglyphis iberica are polydomous. This study describes the temporal and spatial patterns of the polydomy in this species at two different sites, and presents analyses of its role in reducing the attacks of the queen over sexual brood, and in allowing better habitat exploitation.
2. The spatial distribution of nests was clumped while colonies were distributed randomly. Mean nearest neighbour distance ranged from 3.4 to 7.0 m for nests and from 12.3 to 14.1 m for colonies. Distance of foragers searching for food varied among nests: mean values were between 6.1 and 12.6 m.
3. At both sites, the maximum number of nests per colony occurred in summer, during the maximum activity period of the species. Colonies regrouped at the end of this period but overwintered in several nests.
4. Nest renewal in C. iberica colonies was high and showed great temporal variability: nests changed (open, close, re-open) continuously through the activity season and/or among years. The lifetime of up to 55% of nests was only 1–3 months.
5. Polydomy in C. iberica might decrease the interactions between the queen and the sexual brood. In all colonies excavated just before the mating period, the nest containing the queen did not contain any virgin female. Females were in the queenless nests of the colony.
6. The results also suggest that polydomous C. iberica colonies may enhance habitat exploitation because foraging activity per colony increases with nest number. The relationship between total prey input and foraging efficiency and number of nests per colony attains a plateau or even decreases after a certain colony size (four to six nests). This value agrees with the observed mean number of nests per colony in C. iberica .     

Nest site limitation and colony takeover in the ant Leptothorax nylanderi

Ecological constraints on the success of independent coloniesare thought to strongly shape the organization of ant societies.One of the most important factors is probably the availabilityof suitable empty nest sites. By population censuses, laboratoryexperiments, and microsatellite analyses, we investigated thecolony and population structure of the small, myrmicine antLeptothorax (Myrafant) nylanderi in a deciduous forest nearWürzburg, Germany, where nest sites appear to be stronglylimited, especially in late summer. Colonies of L. nylanderiinhabit cavities in rotting branches, hollow acorns, grass stems,etc. After hibernation, a temporary overabundance of empty nestsites facilitates the fragmentation of larger colonies intosmaller buds, which, because the species is monogynous, arein part queenless. Nest sites become scarce in summer due torapid deca and both established colonies and young foundingqueens face a severe shortage of suitable nest sites. This leadsto the fusion of established, unrelated colonies, which afterinitial fighting permanently merge and live together. Typicalyonly one queen survives after fusion. Similarly, young matedqueens may seek adoption in alien nests instead of foundingtheir own colonies solitarily, and here again only a singlequeen survives. This temporary intraspecific parasitism maybe an important first step in the evolution of obligatory permanentparasitism, which is widespread in the genus Leptothorax.     

Effect of habitat saturation on the number and turnover of queens in the polygynous ant,Myrmica sulcinodis

In polygynous social insects more than one queen reproduces in a colony. In such populations ecological factors affecting survival and reproduction of queens are likely to be of prime importance for social organization. In particular, habitat saturation leading to severe limitations in the availability of nest sites has been suggested to promote high queen number. In this study we examine the social and genetic structure of colonies in the polygynous ant Myrmica sulcinodis. We investigated a single breeding population in two adjacent habitats which differed markedly in the availability of nest sites. In the main habitat M. sulcinodis occupied almost all suitable nest sites, whereas in the other (marginal) habitat most sites were unoccupied by ants, due to a recent fire. In support of the habitat saturation hypothesis, the number of queens per colony which could explain the estimated relatedness among workers was almost five times higher for the main habitat than for the marginal habitat. This is the first demonstration that the kin structure of a social insect population is plastic and responds adaptively to short-term changes in ecological constraints such as nest site availability. Based on combined genetic and demographic data we discuss queen reproductive strategies and suggest that a special class of queen ‘floaters’ only stays ephemerally in the colonies, thus causing a substantial turnover of reproducing queens across years.     

Colony stage and not facultative policing explains pattern of worker reproduction in the Saxon wasp

Inclusive fitness theory predicts that in colonies of social Hymenoptera headed by a multiple‐mated queen, workers should benefit from policing eggs laid by other workers. Foster & Ratnieks provided evidence that in the vespine wasp Dolichovespula saxonica, workers police other workers’ eggs only in colonies headed by a multiple‐mated queen, but not in those headed by a single‐mated one. This conclusion, however, was based on a relatively small sample size, and the original study did not control for possible confounding variables such as the seasonal colony progression of the nests. Our aim, therefore, was to reinvestigate whether or not facultative worker policing occurs in D. saxonica. Remarkably, our data show that in the studied Danish population, there was no correlation between worker–worker relatedness and the percentage of worker‐derived males. In addition, we show that variability in cuticular hydrocarbon profiles among the workers did not significantly correlate with relatedness and that workers therefore probably did not have sufficient information on queen mating frequency from the workers’ cuticular hydrocarbon profiles. Hence, there was no evidence that workers facultatively policed other workers’ eggs in response to queen mating frequency. Nevertheless, our data do show that the seasonal progression of the nest and the location in which the males were reared both explain the patterns of worker reproduction found. Overall, our results suggest that the earlier evidence for facultative worker policing in D. saxonica may have been caused by accidental correlations with certain confounding variables, or, alternatively, that there are large interpopulation differences in the expression of worker policing.     

An alternative strategy for maintenance of eusociality after nest destruction: new nest construction in a primitively eusocial wasp

Although nests are central to colonial life in social insects, nests are sometimes damaged by predators or natural disasters. After nest destruction, individuals usually construct new nests. In this case, a sophisticated mechanism like the scent trail pheromone used in large insect colonies that recruit individuals to new nest sites would be important for the maintenance of eusociality. In independent-founding Polistes wasps, it is well known that queens enforce workers physiologically on the natal nests even if evidence of trail pheromone use has not been exhibited. We investigated the effect of the queen on an alternative strategy for the maintenance of eusociality by first females after nest destruction in the primitively eusocial wasp Polistes chinensis. We predicted that the first females in queen-absent colonies have various behavioral options after nest destruction. Even if the females construct new nests cooperatively with other individuals, the new nest construction should be conducted more smoothly in queen-present colonies because the queens regulate the behavior of wasps. We made wasps construct new nests by removing the entire brood from existing nests. The presence of the queen did not cause variation in the alternative strategy of the first females, as the first females (workers) usually constructed new nests cooperatively irrespective of the queen-presence. Thus, the workers in the queenpresent colonies affiliated to the new nest construction more smoothly and constructed new nests more efficiently than workers in the queen-absent colonies. Our results suggest that the presence of the queen is important for maintaining eusociality in primitively eusocial wasps after nest destruction. Received 8 February 2005; revised 5 October 2005; accepted 17 October 2005.     

Reproductive allocation in multinest colonies of the ponerine ant Pachycondyla goeldii

Abstract.  1.  Pachycondyla goeldii constitutes the only recorded case of a monogynous (i.e. one queen per colony) polydomous (i.e. several nests per colony) species in the Ponerinae subfamily. This study examines the impact of polydomy on reproductive allocation between nests (also called 'calies' in polydomous society) in Pachycondyla goeldii Forel, by: (i) recording the number of workers and sexuals in 67 nests belonging to 21 colonies; (ii) dissection of workers in nine nests containing a queen (QR nests), nine nests without a queen but associated to a QR nest (QL nests) and five nests belonging to colonies that permanently lost the queen (OR nests); and (iii) measuring the length of all eggs present in the nests (our laboratory study shows that queen-laid eggs were significantly longer than worker-laid eggs).
2. The number of workers was significantly higher in QR nests than in QL nests, while the number of virgin queens was significantly higher in QL nests compared with QR nests.
3. Worker ovarian activity is inversely related to queen proximity: highest in OR nests, intermediate in QL nests, and lowest in QR nests.
4. Egg length was highest in QR nests, where the queen was most likely the primary egg-layer, intermediate in QL nests, where eggs could have originated from both the queen and workers, and lowest in OR nests, where workers were the sole egg-layers.
5. We postulate that the proximal mechanism explaining differences between QR and QL nests is the pheromonal absence of the queen from QL nests and that the evolutionary reasons of these divergences between nest types are likely to originate from the different conflicts occurring in ant colonies.     

Size and composition of swarming colonies in Provespa anomala (Hymenoptera, Vespidae), a nocturnal social wasp

Summary: Ninety-eight nests of various growing stages of Provespa anomala were found in the lowlands of Sumatra, Indonesia, from August to January. Its colony cycle is considered to be annual, with nests built throughout the year. There were two types of early nests before the emergence of workers, one with a worker number of less than 63 and the other of more than 231. The former were new nests, each containing a new queen and were built by reproductive swarming, whereas each of the latter was possibly re-built by an old queen and an absconding swarm. Ten swarming colonies were attracted by an oil-lamp light; these consisted of a copulated queen with immature oocytes in the ovary and 55 workers on average. No males, or workers stylopized by Xenos provesparum were observed either in colonies before worker emergence or in swarms coming flying into the light. Differences between swarming in Provespa, polygyny and nest relocation in Vespa, and those in swarm-founding Polistinae are discussed.     

Intraspecific variation in size and density of Avocet colonies: effects of nest-distances on hatching and breeding success

In many colonial bird species there is considerable intraspecific variation in colony size and inter‐nest distance (colony density). Possible causes of this variation and its effects on hatching success (survival of eggs) and breeding success (probability of a pair raising chicks) were studied in 48 Avocet Recurvirostra avosetta colonies in Schleswig‐Holstein (Germany) between 1991 and 1996. Colony density was influenced by time of year and habitat (categories: island or mainland, close to or far from feeding grounds). Colonies on islands had the highest densities. When all available space at a colony site was used, colonies became very dense (mean nearest‐neighbour nest distance less than 1 m). Colony size (number of clutches) was influenced by time of year, but not by habitat. Hatching success was low in high density colonies and in very low density ‘colonies’ (single nests) and high over a broad range of intermediate nest densities. The low success rate of single nests was caused by a very high predation rate, whereas the low success rate in very dense colonies was caused by a high rate of nest abandonment. Nest abandonment in very dense colonies was associated with a high level of aggressiveness among Avocets during the egg‐laying period. Due to territorial behaviour, Avocets seemed to be expelled from the densest breeding sites. In very dense colonies, high frequencies of clutches of unusual size occurred due to conspecific nest parasitism. The number of Avocets taking part in attacks on potential egg predators was small and (in colonies of more than one clutch) depended neither on colony size nor on colony density. Despite a low hatching success in very dense colonies, individuals breeding in the densest colonies had significantly better chances of raising chicks than Avocets breeding in less dense colonies. Coloniality seemed to be obligatory for Avocets in order to ensure hatching success. The size and density of colonies seemed to be associated with the availability of suitable nesting habitats (islands).     

Variation in the quality of Vespula vulgaris (L.) queens (Hymenoptera: Vespidae) and its significance in wasp population dynamics

Abstract

Density‐related variation in queen quality has been proposed as a possible mechanism regulating population fluctuations in Vespula species. We investigated annual variation in the quality (size, weight, and fat content) of adult V. vulgaris queens representing four stages of their life cycle (spring, founding, developing, and emerged) taken from six sites in beech forest, South Island, New Zealand. For each queen the dry weight, head width, and thorax length was measured. For a subsample of queens, the fat content was determined by ether extraction. The size of queen cells was measured from a subsample of nests. Size, weight, and fat content of queens varied between wasp colonies and sites. The smallest juvenile queens were under‐represented in the reproductive population. There was no direct link between body size and food supply. Size and weight of developing queens increased as the number of cells in the nest increased. The size of the queen cells varied significantly among layers in a nest and among nests. The under‐representation of small queens in the reproductive population suggests that queen quality may affect survival and/or competitive ability by increasing winter fat storage, nest building activity, and/or success in usurpation disputes.     

Nest architecture and colony size of the fungus-growing ant <Emphasis Type="Italic">Mycetophylax simplex</Emphasis> Emery, 1888 (Formicidae,Attini)

Detailed measurements on nest architecture and colony size of the fungus-growing ant Mycetophylax simplex Emery, 1888 (Formicidae, Attini) are reported for the first time, based on excavations of 55 nests from two sites in southern Brazil. All nests were subterranean, with a single entrance hole. Most nests consisted of two chambers, an upper and a lower chamber, but one and three-chamber nests were also found. The chambers were more cone-shaped than rounded, and located at a depth ranging from 4.0 cm to 32.5 cm below the nest entrance. The chamber dimensions generally increased as the depth of the chambers increased, and the lower chamber was mostly wider than the upper one. The fungus garden was always found resting on the chamber floor. The average colony size was 264.1 workers, ranging from 67 to 610 workers. Colonies produced most sexuals during the summer (from December to March) and a few during the winter (July). Direct observations showed that colonies were mostly monogynous, but more than one queen was recorded in two nests, suggesting that polygyny may also occur in this species. Received 30 November 2006; revised 20 April 2007; accepted 23 April 2007.     

Social behavior of early emerging males of a Japanese paper wasp,Polistes chinensis antennalis (Hymenoptera: Vespidae)

Summary Five cases of the early emergence of males in a Japanese paper wasp,Polistes chinensis antennalis, in which male emerged together with the first group of workers, were described. In one case of the five where the queen disappeared before the emergence of male, worker(s) produced female offspring. The frequency of the nests where the early emergence of male was observed was 16.7% (5/30 nests). In two colonies, worker(s) and/or queen chased off males. But in an orphan nest where worker(s) produced female offspring, the dominance order among workers which was similar to that of colony without male was observed. The significance of the early emergence of male in the social evolution of wasps was discussed.     

Reproductive parameters vary with social and ecological factors in the polygynous ant Formica exsecta

Due to their haplo‐diploid sex determination system and the resulting conflict over optimal sex allocation between queens and workers, social Hymenoptera have become important model species to study variation in sex allocation. While many studies indeed reported sex allocation to be affected by social factors such as colony kin structure or queen number, others, however, found that sex allocation was impacted by ecological factors such as food availability. In this paper, we present one of the rare studies that simultaneously investigated the effects of social and ecological factors on social insect nest reproductive parameters (sex and reproductive allocation, nest productivity) across several years. We found that the sex ratio was extremely male biased in a polygynous (multiple queens per nest) population of the ant Formica exsecta. Nest‐level sex allocation followed the pattern predicted by the queen‐replenishment hypothesis, which holds that gynes (new queens) should only be produced and recruited in nests with low queen number (i.e. reduced local resource competition) to ensure nest survival. Accordingly, queen number (social factor) was the main determinant on whether a nest produced gynes or males. However, ecological factors had a large impact on nest productivity and therefore on a nest's resource pool, which determines the degree of local resource competition among co‐breeding queens and at what threshold in queen number nests should switch from male to gyne production. Additionally, our genetic data revealed that gynes are recruited back to their parental nests after mating. However, our genetic data are also consistent with some adult queens dispersing on foot from nests where they were produced to nests that never produced queens. As worker production is reduced in gyne‐producing nests, queen migration might be offset by workers moving in the other direction, leading to a nest network characterized by reproductive division of labour. Altogether our study shows that both, social and ecological factors can influence long‐term nest reproductive strategies in insect societies.     

Performance of Myrmica ant colonies is correlated with the presence of social parasites

1. The performance of ant colonies depends on different factors such as nest site, colony structure or the presence of pathogens and social parasites. Myrmica ants host various types of social parasites, including the larvae of Maculinea butterflies and Microdonmyrmicae (Schönrogge) hoverfly. How these social parasites affect host colony performance is still unexplored. 2. It was examined how the presence of Maculinea teleius Bergsträsser, Maculinea alcon (Denis & Schiffermüller), and M. myrmicae larvae, representing different feeding and growth strategies inside host colonies, is associated with worker survival, the number of foragers, and colony productivity parameters such as growth and reproduction. 3. It was found that the presence of social parasites is negatively associated with total colony production and the production of ant larvae and gynes. Male production was lower only in nests infested by M. teleius, whereas the number of worker pupae was significantly higher in all types of infested colonies than in uninfested colonies. Laboratory observations indicated that nests infested by Maculinea larvae are characterised by a higher number of foragers compared to uninfested nests but we did not find differences in worker survival among nest types. 4. The observed pattern of social parasite influence on colony productivity can be explained by the feeding strategies of parasitic larvae. The most negative effect was found for M. teleius, which feeds on the largest host brood and eliminates a high number of sexual forms. The strong, adverse influence of all studied parasite species on gyne production may result in low queen production in Myrmica populations exposed to these social parasites.     

Ecological studies ofRoparidia fasciata in Okinawa island I. Distribution of single- and multiple-foundress colonies

To compare between a single-foundress colony and a multiple-foundress colony at the pre-emergence state of a social wasp, R. fasciata, nest distributions and colony terminations were investigated in 8 sites with different environmental conditions. Marking experiments were also conducted in two sites at high wasp density.

1. Foundress populations were composed of single-foundress colonies in sites C, D and E, new environments where have recently suited for inhaviting, at low wasp density. In sites like A and B which were used year after year, at high wasp density, coexistence of multiple-and single-foundress colonies was observed.
2. From the marking experiment, nests initiated by a single foundress were more distant away from the nest where the original foundress emerged the fall before, compared to multiple-foundress nests which were initiated by multiple foundress.
3. Greater percentage of colony termination was observed in single-foundress nests than in multiple-foundress nests, and the colony termination in single-foundress colonies increased with the nest density.
4. Ant predation was the key factor causing the variation of the percentage of colony termination.

     

Nest demographics and foraging behavior of <Emphasis Type="Italic">Apterostigma collare</Emphasis> Emery (Hymenoptera,Formicidae) provide evidence of colony independence

Apterostigma collare Emery is a highly derived fungus-growing ant within the Tribe Attini whose small, fungal nests are found in tropical rain forests. This study focuses on determining the colony structure of A. collare, specifically searching for evidence of polydomy or independence. We surveyed and observed nests in the field, and performed foraging bioassays and dissected nests in the laboratory. We determined the size and contents of nests in field populations. Nests found near other nests were not statistically different in size compared to nests found alone. There was also no statistical difference between near and lone nests regarding the presence of a queen in the nest. Most nests contained one queen with brood and workers, regardless of their proximity to other nests. Observations also were made of foraging and trail-marking behaviors. Foraging activity observed in the field revealed that workers left the nest area and followed trails upwards into the canopy, but they did not interact with foragers from other nearby nests. In a laboratory foraging arena, foragers marked a trail to a food source by dragging the gaster. Bioassays showed that A. collare workers preferred their own foraging trails, but not those of other conspecific colonies. All results suggest that each nest represents an independent colony, supporting a previous report that nests found in close proximity do not constitute a polydomous colony. Received 19 July 2006; revised 23 March 2007; accepted 6 June 2007.     

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.