Overdispersion of ant colonies: a test of hypotheses

Summary The nest locations of two ant species in the Colorado Desert are intraspecifically overdispersed. Intraspecific overdispersion has been thought to represent strong intraspecific competition. Here we consider this hypothesis along with three competing hypotheses: microhabitat selection by foundress queens, predation on foundress queens, and predation on established colonies. To test these hypotheses five types of data were collected: (1) the forager population sizes of Veromessor pergandei and Pogonomyrmex californicus, (2) the response of the territory use of V. pergandei to varying levels of food, (3) the encounter rates of conspecifics and other ant species to foundress queens artificially placed near and far from conspecific colonies, (4) predation on colonies as a function of colony spacing, and (5) the relationship between the plant microhabitat at the nest and colony spacing. The results show that established colonies have no apparent selectivity for a particular type of plant microhabitat nor do foundress queens show avoidance or attraction toward conspecifics. V. pergandei workers show only a slight ability to find V. pergandei queens that are artificially placed near their entrances. Certain spiders are the most common ant predators on our study area. Direct observations on spiders indicate that colonies with closer neighbors are not prone to a higher risk of predation. In addition, the estimates of the death rate of workers from a mark-recapture technique indicate that colonies with closer neighbors lose similar numbers of workers as compared to colonies with further neighbors. In favor of the competition hypothesis, the summed size of intraspecific nearest neighbor pairs are larger for colonies that are spaced further apart than those colonies that are spaced closer together. We also develop an index of foraging directionality for the column foraging species V. pergandei. Using this measure, we find that nearest neighbors tend to avoid foraging toward each other. The response of territory use to food levels was tested with experiments involving patches of cracked wheat. These experiments showed that patches away from nearest neighbors were found significantly sooner than patches toward nearest neighbors. The above five sets of data together suggest that resource competition and perhaps queen predation by established colonies account for the intraspecific spatial patterns of these species.     

Reproductive behavior of the antLeptothorax (Dichothorax) pergandei

Summary Leptothorax pergandei, the only member of the subgenusDichothorax, is a common ant in Chihuahuan desert habitats in southwestern Arizona. According to colony censuses and relatedness estimates, colonies are mostly monogynous and queens typically mate only once. Mating takes place in local swarming flights; virgin queens attract males with secretions from the poison gland. Some dealate queens running on the soil surface directly after the mating flight had well-developed ovaries containing mature eggs and yellow bodies.     

Stay or drift? Queen acceptance in the ant <Emphasis Type="Italic">Formica paralugubris</Emphasis>

The acceptance of new queens in ant colonies has profound effects on colony kin structure and inclusive fitness of workers. Therefore, it is important to study the recognition and discrimination behaviour of workers towards reproductive individuals entering established colonies. We examined the acceptance rate of queens in populations of the highly polygynous ant F. paralugubris, where the genetic differentiation among nests and discrimination ability among workers suggest that workers might reject foreign queens. We experimentally introduced young queens in their natal nest and in foreign nests. Surprisingly, the survival rate of mated queens did not differ significantly when introduced in a foreign male-producing nest, a foreign female-producing nest, or the natal nest. Moreover, the survival of virgin queens in their natal nest was twice the one of mated queens, suggesting that mating status plays an important role for acceptance. The results indicate that other factors than queen discrimination by workers are implicated in the limited longdistance gene flow between nests in these populations. Received 8 April 2008; revised 16 June 2008; accepted 1 July 2008.     

The reproductive phenology of an Amazonian ant species reflects the seasonal availability of its nest sites

In saturated tropical ant assemblages, reproductive success depends on queens locating and competing for scarce nest sites. Little is known about how this process shapes the life histories of tropical ants. Here I investigate the relationship between nest site availability and an important life history trait, reproductive phenology, in the common Amazonian ant species Allomerus octoarticulatus. A. octoarticulatus is a plant-ant that nests in the hollow, swollen stem domatia on Cordia nodosa. I provide evidence that nest sites are limiting for A. octoarticulatus. Most queens produced by A. octoarticulatus colonies died before locating suitable host plants, and most queens that located hosts died before founding colonies, probably from intraspecific competition among queens for control of host plants. I further show that the reproductive phenology of A. octoarticulatus closely matches the seasonal availability of its nest sites, domatia-bearing C. nodosa saplings. Both the production and flight of A. octoarticulatus reproductives, and the number of C. nodosa saplings available for colonization by ants, peaked from March to May. There was correlative evidence that A. octoarticulatus colonies use temperature as a cue to synchronize their reproduction to the availability of C. nodosa saplings: both the production of reproductives by ant colonies and the number of C. nodosa saplings available for colonization were correlated with temperature, and not with rainfall. All of these results suggest that nest site limitation constrains the reproductive phenology of A. octoarticulatus.     

Dietary similarity and foraging range of two seed-harvester ants during resource fluctuations

Summary Diets of desert seed-harvester ants Veromessor pergandei and Pogonomyrmex rugosus were studied for 3 years at two habitats where they are common and sympatric. Diets of the two species were similar, consisting mainly (87% of 23,913 seeds) of three annual plant species (Schismus arabicus, Plantago insularis, and Pectocarya recurvata). Diets converged following a drought in Winter/Spring 1984 which reduced seed production during this time. Foraging range of P. rugosus almost doubled following the drought while foraging range of V. pergandei remained constant. Neither ant species move their nests once established rendering them effectively sessile granivores. This limits the dietary options of a given colony and may explain interspecific dietary convergence.     

Unusual Behavior of Polygyne Fire Ant Queens on Nuptial Flights

This study reports previously undescribed behavior of fire ant queens (Solenopsis invicta) on their nuptial flights. We captured large numbers of alate (winged) queens flying at low altitudes in dense swarms that were virtually devoid of males. We assayed the genotypes of these alate queens at the locus Gp-9, which exhibits strong genotype frequency differences between monogyne (single-queen) and polygyne (multiple-queen) populations, and found that almost all of these low-flying queens originated from polygyne colonies. Comparisons of mtDNA haplotype distributions of these queens to those of alates leaving polygyne nests suggest that the flying queens had not dispersed more than a few hundred meters. Moreover, the proportion of flying queens that were mated did not differ significantly from the proportion of reproductive queens that were mated within the same sites. Thus the flight behavior appears to occur subsequent to mating. We suggest that the flying queens are sampling the local environment in order to select a suitable landing site. Such a site would contain established polygyne nests into which the queens may be adopted as new reproductives.     

Three queen morphs with alternative nest-founding behaviors in the ant, Temnothorax longispinosus

In many ant species, multiple modes of founding new colonies occur in the same population. These modes include dependent founding, independent founding by haplometrosis (single queen), and independent founding by pleometrosis (multiple queens). In several cases, a dimorphism in queen size has been found, such that each morph specializes in a particular nest-founding behavior. I investigated queen size in the ant Temnothorax longispinosus in several southern Wisconsin populations and found three distinct queen morphs: small queens with very low fat content and short wings, large queens with low fat and long wings, and large queens with high fat and very long wings. Several traits associated with founding behavior correlated with these queen sizes. Small queens were produced in lower numbers, were more common in polygynous nests, and returned to the nest in higher proportions than both large queen morphs. The size ranges and fat levels of each queen morph were similar to those of other species that specialize in either haplometrosis (very large, high fat), pleometrosis (large, low fat), or dependent founding (small and low fat). However, there was extensive overlap in several of the founding behaviors, suggesting that the morphs in these populations have some flexibility in founding behavior. The queen morphs in these populations of T. longispinosus may resemble early stages in the evolution of more specialized dispersal polymorphisms found in other ant species. Received 11 January 2006; revised 15 September 2006; accepted 18 September 2006.     

Ant colonization of Maieta guianensis seedlings,an Amazon ant-plant

The ants Pheidole minutula and Crematogaster sp. are obligate inhabitants of the ant-plant Maieta guianensis. They nest and reproduce exclusively in this and a few other Amazon melastome ant-plants. Experimental transplants of uncolonized M. guianensis seedlings to sites at different distances from established colonies of these two ant species, which are sources of founding queens, have shown that distance is an important factor affecting seedling colonization by ants. The proportion of colonized seedlings and the average number of colonizations per seedling, both by Crematogaster sp. and P. minutula, decreased as distance from established colonies increased. Seedling colonization was also affected by rainfall and fewer seedlings were colonized during the dry season, especially by P. minutula, than during the rainy season. P. minutula queens usually cooperate with each other during colony foundation, a behaviour not observed among founding queens of Crematogaster sp. Competition between these two ant species for host-plants appears to be strong as 63.6% of the seedlings colonized by Crematogaster sp. were also colonized by P. minutula during a 15-month observational period. However, there was generally an interval of a few months between colonizations, possibly giving the first species to colonize the plant a better chance of domination on this plant.     

First Record of Polydomy in a Monogynous Ponerine Ant: A Means to Allow Emigration Between Pachycondyla goeldii Nests

The ponerine ant Pachycondyla goeldii is a monogynous (i.e. one queen per colony) arboreal species that colonizes pioneer areas. Founding queens and first generation workers initiate their own ant garden by building a cardboard-like structure into which epiphyte seeds are integrated. Following the growth of the epiphyte, the colony establishes its nest within the root system. This particular nest-building behavior is crucial in an environment where suitable nest sites are rare. Nevertheless, the slow growing process of ant gardens does not allow this species to readily evacuate and find another refuge in the advent of an attack by a predator or worsening climatic conditions. Previous field studies of P. goeldii were performed after forest destruction and subsequent colonization by P. goeldii. As a result, the colonies studied where relatively young and monodomous (i.e. one nest per colony). Our study of nest composition, worker exchanges between ant gardens in the field, and dyadic encounters shows that mature colonies of P. goeldii are polydomous (i.e. multiple nests per colony). In ants, the association of polydomy with monogyny has infrequently been reported. To our knowledge, P. goeldii represents the first record of a Ponerinae exhibiting both these particular characteristics. Our field and laboratory experiments suggest that polydomy is adaptively advantageous in coping with the microclimatic instability of pioneer areas by providing colonies with easily accessible nests.     

Seasonal cycle of colony structure in the Ponerine ant Pachycondyla chinensis in western Japan (Hymenoptera, Formicidae)

Investigation of reproductive strategies of ants in the subfamily Ponerinae is important for understanding of the evolution of social structure and of the significance of caste dimorphisms. The biology of species with mated and egg-laying workers (gamergates) has been studied for many species, however, little attention has been paid to species that reproduce via alate queens only. We investigated the seasonal cycle of changes in the colony structure of Pachycondyla chinensis reproduced by alate queens in western Japan, and found the following novel biological characteristics of this species. P. chinensis showed a remarkable caste dimorphism in ovariole numbers: workers had no ovaries while queens had 18 to 36 ovarioles in their ovaries. The nesting system seemed to be polydomous: 266 of 400 nests collected were queenless. The number of queenless nests increased during the reproductive season. Among the 134 queenright nests, 38 had several mated-queens without significant differences in ovary activation and the remaining 96 nests were monogynous. During winter to early spring, most nests were polygynous. After alate production, most of the old queens seemed to die or be expelled and replaced by new queens. Virgin dealated queens were often found and they seemed to have laid eggs. Received 3 August 2007; revised 19 December 2007; accepted 20 December 2007     

Ecological barriers to early colony establishment in three coexisting acacia-ant species in Kenya

In black cotton uplands in East Africa, four symbiotic acacia-ant species compete for possession of a single swollen thorn tree species, Acacia drepanolobium, and yet coexist at fine spatial scales. Three of the four ant species produce independent foundress queens that establish colonies claustrally within swollen thorns, most often on small saplings. We conducted surveys of such saplings at two sites in 2001 and 2004, and examined foundresses and incipient colonies within their swollen thorns to determine what factors influence their success. Competition among foundresses for nest initiation sites was intense, with an average of over one founding attempt per swollen thorn in all samples, and with living and dead queens significantly hyper-dispersed among available thorns. Combat with other foundresses was the most common cause of death among claustral queens, especially for Tetraponera penzigi. In interspecific battles for nest initiation sites, T. penzigi was dominant over Crematogaster nigriceps and C. mimosae, and C. nigriceps won over 80% of its contests with C. mimosae foundresses. For singleton foundresses, brood parasitism by the braconid wasp Trigastrotheca laikipiensis typically results in the death of the entire ant brood. Host queens defend parasite larvae, pupae and eclosed adults, apparently unable to distinguish the wasps from legitimate offspring. Rates of brood parasitism were as high as 15–20% for incipient colonies of both Crematogaster species, but were extremely low for T. nigriceps in all samples. Although T. penzigi and C. nigriceps foundresses are always solitary, approximately 18% of claustral C. mimosae colonies contain cooperating pleometrotic queens. For unparasitized, claustral C. mimosae colonies, brood production per queen did not differ between solitary and cooperating foundresses. However, the per-capita risks associated with parastitism were reduced for pleometrotic queens. Received 8 March 2005; revised 23 May 2005; accepted 3 June 2005.     

Queen phenotype and behaviour during cooperative colony founding in Polyrhachis moesta

Summary. Founding queens of the arboreal ant Polyrhachis moesta aggregate independently of kinship and cooperate in caring for their brood. In field studies, the number of queens in a founding nest varied from 1 to 8. The number of queens in the nests with multiple queens decreased significantly with time after the nuptial flight, resulting in monogynous or oligogynous nests. Single- and multiple-founding queens did not differ in characteristics representing nutritional states or body size immediately after the nuptial flight. Wet body weight decreased as days passed, whereas head width of founding queens who overwintered successfully were relatively larger. In laboratory studies, founding queens performed liquid food exchanges more frequently with queens from other founding nests or immature colonies than with those from the same nests. Queens in founding nests and immature colonies were observed to show no aggression against non-nestmate queens, whereas queens in established colonies showed aggressive behaviours against non-nestmates. This indicates that founding queens change drastically in their aggression levels before and after colony establishment. Multiple-founding queens started laying eggs earlier than single-founding queens under laboratory conditions. Higher brood productivity and lower brood mortality were observed in multiple-queen nests. These potential advantages in multiple-queen founding may support the cooperative association among unrelated founding queens.Received 1 December 2003; revised 25 March and 20 May 2004; accepted 3 June 2004.     

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.     

Lethal sibling rivalry for nest inheritance among virgin ant queens

Fights among conspecific animals are normally restricted to ritualized interactions. They may escalate to serious injury and death when the contested resource has a very high value and the chances of finding alternative resources later in life are low. This appears to be the case in young queens of the ant Cardiocondyla “latifrons”, a species that builds its nests in rather stable rock crevices. Young queens in small colonies without a mature queen fought violently for inheritance of the nest site and its workers. Queen aggression and subsequent attacks by workers killed about 50% of the young queens in our experimental nests. Queen killing differs qualitatively from previously reported dominance interactions among prospective female reproductives in other ants, which are mostly ritualized and allow losers to pursue alternative ways of increasing their direct or indirect fitness. Our observations add a novel case of siblicide to the previously reported lethal aggression among nestling birds, parasitoid larvae, and honey bee queens. They corroborate the hypothesis that relatedness does not play a significant role in local competition for highly limited resources.     

Nestmate discrimination and cuticular profiles of a temporary parasitic ant Lasius sp. and its host L. fuliginosus (Hymenoptera, Formicidae)

Workers of the temporary parasitic ant Lasius sp. were perfectly compatible with their hosts Lasius fuliginosus. Aggression was never seen between allospecific nestmates in either field or laboratory. In the laboratory, trophallaxis and allogrooming between allospecific nestmates were statistically more frequent than that between conspecifics. These ants were highly aggressive toward individuals of either species from another mixed colony located 1 km away, indicating that they discriminated nestmates and nonnestmates regardless of the species to which they belonged. No aggressive actions, however, were observed between two neighboring mixed nests located 5 m apart. We have not considered, however, the relationship of the two neighboring nests: the two nests may have been two subunits of the same family or sister colonies, or may have been different colonies but discriminated from distant aliens. Gas chromatography analysis on the total cuticular extracts showed that Lasius sp. and L. fuliginosus possess distinct cuticular profiles, even though they were collected from the same mixed colony. Received: December 13, 1999 / Accepted: April 25, 2000     

Colony growth and reproductive ability of feral nests of the introduced bumblebee Bombus terrestris in northern Japan

The introduced Bombus terrestris has been blamed for the decline of native bumblebee populations in Japan. To control this invasive species, it is necessary to understand its ecological traits in its newly established range. Here, we investigated the colony growth pattern and reproductive ability of feral nests of B. terrestris in northern Japan from 2003 to 2006. Nests collected at various times showed initially slow growth followed by rapid development. This pattern is consistent with findings in previous studies using laboratory-raised colonies. Our results also suggested that protandrous colonies tend to specialize in male production, whereas protogynous colonies specialize in gyne production, producing a split sex ratio in feral colonies of B. terrestris. The numerical population sex ratio was 1.40 gynes per male, calculated from the numbers of pre-emergence cocoons for gynes and males, showing a female-biased sex ratio at the population level. Mature colonies produced a mean of 376.5 cocoons and 90.2 gyne cocoons (22.1% of the total). The proportion of gynes produced by B. terrestris nests exceeded both those of conspecifics observed in other countries and those of consubgeneric native Japanese species. The propagule pressure hypothesis appears to explain the probability of establishment of this invasive species. Suitable nest sites for B. terrestris queens appeared to be in short supply, and B. terrestris may win usurpation contests against native species due to its large size, resulting in the decline of native bumblebee species.     

Breeding behaviour, relatedness and sex-investment ratios inLeptothorax tuberum Fabricius

The ant speciesLeptothorax tuberum was shown to be predominantly monogynous. Queens usually mate once only but some nests may have a multiply-mated queen or are partially or serially polygynous. As expected from these results, within nest relatedness between workers and between workers and alate queens was found to be high. Almost fifty percent of nests had no nest queen which may indicate high queen mortality, queens leaving to found new nests or nest fragmentation. Observed female investment frequencies (IF _o ) were not significantly different from those expected on the basis of worker control of sexual production and the relatedness estimates of workers to alate queens and workers to males calculated from isozyme data (IF _E ). These values were not consistent with queen control. There was no evidence for lower IF _o s in queenless nests nor for higher IF _o s in larger nests classified by worker number. When nests were classified by sexual productivity, however, there was a strong rank correlation between productivity and female bias. This is the first study of an ant species to test observed IF against expected IF calculated without inferring between caste relatedness from worker data or pooling of data from different castes.     

Impact of plant cover on the cavity‐nesting ant Temnothorax crassispinus

1. Plant communities influence the availability of important resources for ants, such as nest sites and food, as well as environmental conditions. Thus, plants affect the abundance and distribution of ants. 2. In a field experiment, the influence of plant cover on the settlement of nest sites and per‐capita productivity of sexual individuals by the ant Temnothorax crassispinus was analysed. In July 2014, in five areas with patches of alien balsam Impatiens parviflora, and another five of native balsam I. noli‐tangere, transects composed of artificial nests were established; the nest sites were situated inside patches of balsams, and outside of them. Four hundred and forty artificial nests were used. One year later, the nests were collected. 3. Colonies of the ants three times more often inhabited nest sites outside the patches of both balsams. Besides, colonies with queens were more frequently found in nest sites located away from balsams. The per‐capita productivity of sexual individuals was higher in nests collected in patches of balsam, and the colonies from patches of alien balsam produced a more female‐biased sex ratio. 4. In terms of the impact on the ant, no clear differences were found between the alien balsam and the native one. The most important factor affecting the fitness of ants in areas dominated by balsams is the presence of herbaceous plant cover rather than whether the plant is alien or native.     

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.     

Dispersed central-place foraging in the polydomous odorous house ant, Tapinoma sessile as revealed by a protein marker

The odorous house ant, Tapinoma sessile, is a native ant species common throughout North America. In urban areas, this ant is classified a pest species and exhibits several attributes characteristic of invasive “tramp” ants (sensu Passera, 1994). These include: extreme polygyny, colony reproduction by budding, reduced internest aggression, generalist diet, and polydomy. Here we explore the organization of foraging and the pathways of food distribution in polydomous colonies of T. sessile in the laboratory and field using a novel marking technique (rabbit IgG protein) and enzyme linked immunosorbent assay (ELISA). Laboratory assays revealed patterns of food allocation from foragers to other castes and developmental stages. Foragers distributed the IgG- labelled sucrose to the majority of workers within 24 h, and workers retained significantly more sucrose than either queens or larvae. Approximately 50% of queens tested positive for the IgG marker and some queens received significantly more sucrose than others, indicating a possible reproductive dominance hierarchy. Larvae received little sucrose demonstrating their minor reliance on carbohydrates. The results of field experiments showed that odorous house ants are dispersed central-place foragers whereby ants from individual nests exhibit high foraging site fidelity, travel along well-established trails, and forage on a local scale. Dispersed central-place foraging most likely allows the odorous house ant to more efficiently secure both clumped and dispersed food sources and possibly increases its competitive ability. As a result, colonies become numerically large and ecologically dominant. The results of our study contribute to our understanding of the social behavior and colony organization in T. sessile. In addition, they provide a framework for designing more effective ant control programs based on liquid baits. Received 13 December 2005; revised 28 February 2006; accepted 3 March 2006.