Breeding system, colony structure, and genetic differentiation in the Camponotus festinatus species complex of carpenter ants

All social insects live in highly organized societies. However, different social insect species display striking variation in social structure. This variation can significantly affect the genetic structure within populations and, consequently, the divergence between species. The purpose of this study was to determine if variation in social structure was associated with species diversification in the Camponotus festinatus desert carpenter ant species complex. We used polymorphic DNA microsatellite markers to dissect the breeding system of these ants and to determine if distinct C. festinatus forms hybridized in their natural range. Our analysis of single-queen colonies established in the laboratory revealed that queens typically mated with only a single male. The genotypes of workers sampled from a field population suggested that multiple, related queens occasionally reproduced within colonies and that colonies inhabited multiple nests. Camponotus festinatus workers derived from colonies of the same form originating at different locales were strongly differentiated, suggesting that gene flow was geographically restricted. Overall, our data indicate that C. festinatus populations are highly structured. Distinct C. festinatus forms possess similar social systems but are genetically isolated. Consequently, our data suggest that diversification in the C. festinatus species complex is not necessarily associated with a shift in social structure.     

Association between caste and genotype in the termite Mastotermes darwiniensis Froggatt (Isoptera: Mastotermitidae)

Abstract Termite workers and soldiers differ markedly in their morphology and behaviour. We sought evidence for genetic influences on caste determination in the giant northern termite, Mastotermes darwiniensis , by investigating if workers and soldiers from the same colony differed genetically. The genotypes of 795 termites from 11 distinct colonies were assayed at six polymorphic microsatellite loci. We found that the multilocus genotypes of workers and soldiers from 8 of the 11 colonies did not differ significantly. Thus, the majority of the data provided no evidence for a genetic association with caste in workers and soldiers of M. darwiniensis . However, the genotype frequencies of workers and soldiers from three colonies differed, suggesting that genotype is occasionally associated with caste in this species. The genetic differentiation of castes within these colonies could reflect differences in the propensities of termites with distinct genotypes to develop into particular castes and provide a selective advantage to colonies headed by multiple reproductives.     

Alate susceptibility in ants

Pathogens are predicted to pose a particular threat to eusocial insects because infections can spread rapidly in colonies with high densities of closely related individuals. In ants, there are two major castes: workers and reproductives. Sterile workers receive no direct benefit from investing in immunity, but can gain indirect fitness benefits if their immunity aids the survival of their fertile siblings. Virgin reproductives (alates), on the other hand, may be able to increase their investment in reproduction, rather than in immunity, because of the protection they receive from workers. Thus, we expect colonies to have highly immune workers, but relatively more susceptible alates. We examined the survival of workers, gynes, and males of nine ant species collected in Peru and Canada when exposed to the entomopathogenic fungus Beauveria bassiana. For the seven species in which treatment with B. bassiana increased ant mortality relative to controls, we found workers were significantly less susceptible compared with both alate sexes. Female and male alates did not differ significantly in their immunocompetence. Our results suggest that, as with other nonreproductive tasks in ant colonies like foraging and nest maintenance, workers have primary responsibility for colony immunity, allowing alates to specialize on reproduction. We highlight the importance of colony‐level selection on individual immunity in ants and other eusocial organisms.     

Respiration,worker body size,tempo and activity in whole colonies of ants

Ants are social, and their metabolism should be measured on at least two levels: (i) the individual workers and brood of which the colony is composed and (ii) the colony in its entirety. Whole colony respiration, tempo (size‐free running speed in body lengths per second) and whole colony activity were simultaneously measured for 15 species of ants in four subfamilies, and these data are related to average worker and whole‐colony weight, activity, percentage brood and percentage fat. Across all 15 species, whole colony respiration rate (μL CO₂ h^?1) is linearly related to whole colony live weight (log–log slope = 1.0). Colonies composed of large workers respire less than colonies composed of an equal live weight of small workers, and colonies with high tempos respire more than lower tempo colonies of equal weight. The tempos and respiration rates of smaller ants tend to be higher, and a higher tempo exacts a cost in higher respiration independent of the effect of small body size. Individual worker respiration (μL CO₂ h^?1) scales to worker live weight with an exponent of 0.76. Whole colony specific respiration rate (μL CO₂ g^?1 h^?1) is unrelated to colony live weight. The regressions of respiration rates against colony and worker dry weight, lean weight and metabolic weight have similar slopes to those of live weight but different intercepts. Respiration is not related to worker percentage fat, percentage brood or activity. Ant ecology, tempo, body size, polymorphism and colony size are discussed in relation to respiration.     

Energetics of newly-mated queens and colony founding in the fungus-gardening ants Cyphomyrmex rimosus and Trachymyrmex septentrionalis (Hymenoptera: Formicidae)

Abstract.  The energetics of colony founding is investigated in the fungus gardening ants (Attini) Trachymyrmex septentrionalis and Cyphomyrmex rimosus . Similar to most ants, inseminated queens of these two species found nests independently unaccompanied by workers (haplometrosis). Whereas most ant founding queens seal themselves in a chamber and do not feed when producing a brood entirely from metabolic stores (claustral founding), the majority of fungus gardening ants must forage during the founding phase (semiclaustral founding). Laboratory-reared T. septentrionalis individuals comprise 84 dealate females collected after mating flights in June 2004. Twenty are immediately killed to obtain values for queen traits and another 20 after worker emergence for queen, fungus garden and worker traits. Cyphomyrmex rimosus comprise 22 dealate females collected in June 2005; ten of which are immediately killed and similarly prepared. Newly-mated T. septentrionalis queens have 25% of their dry weight as fat; whereas newly-mated C. rimosus queens contain 11% fat. These amounts are 50–75% less than most independently founding ant species. Trachymyrmex septentrionalis queens lose merely 5% of their energetic content during colony founding, whereas the total energetic content of their brood is more than three-fold the amount lost by the queen. Incipient T. septentrionalis colonies produce approximately half as much ant biomass per gram of fungus garden as do mature colonies. Similar to most ants, T. septentrionalis produces minim workers that are approximately 40% lighter than workers from mature colonies. Regardless of their size, T. septentrionalis workers contain much lower fat than do workers of claustral species. These data indicate that fungus gardening is adaptive because colonies can produce much cheaper offspring, making colony investment much lower.     

Tradeoffs in the Evolution of Caste and Body Size in the Hyperdiverse Ant Genus Pheidole

The efficient investment of resources is often the route to ecological success, and the adaptability of resource investment may play a critical role in promoting biodiversity. The ants of the “hyperdiverse” genus Pheidole produce two discrete sterile castes, soldiers and minor workers. Within Pheidole, there is tremendous interspecific variation in proportion of soldiers. The causes and correlates of caste ratio variation among species of Pheidole remain enigmatic. Here we test whether a body size threshold model accounts for interspecific variation in caste ratio in Pheidole, such that species with larger body sizes produce relatively fewer soldiers within their colonies. We evaluated the caste ratio of 26 species of Pheidole and found that the body size of workers accounts for interspecific variation in the production of soldiers as we predicted. Twelve species sampled from one forest in Costa Rica yielded the same relationship as found in previously published data from many localities. We conclude that production of soldiers in the most species-rich group of ants is regulated by a body size threshold mechanism, and that the great variation in body size and caste ratio in Pheidole plays a role in niche divergence in this rapidly evolving taxon.     

Effect of food restriction on survival and reproduction of a termite

Food availability affects the trade-off between maintenance and reproduction in a wide range of organisms, but its effects on social insects remain poorly understood. In social insects, the maintenance-reproduction trade-off seems to be absent in individuals but may appear at the colony level, although this is rarely investigated. In this study, we restricted food availability in a termite species to test how it affects survival and reproduction, both at the individual and colony level. Using Bayesian multivariate response models, we found very minor effects of food restriction on the survival of queens, individual workers or on the colonies. In contrast, queen fecundity was significantly reduced, whereas colony-level fecundity (i.e., the number of dispersing alates, future reproductives) increased under food restriction as workers gave up cooperation within the colony and became alates that dispersed. Our study shows that life-history trade-offs can be mitigated by individuals' social behaviours in social organisms.     

Be meek or be bold? A colony-level behavioural syndrome in ants

Consistent individual variation in animal behaviour is nearly ubiquitous and has important ecological and evolutionary implications. Additionally, suites of behavioural traits are often correlated, forming behavioural syndromes in both humans and other species. Such syndromes are often described by testing for variation in traits across commonly described dimensions (e.g. aggression and neophobia), independent of whether this variation is ecologically relevant to the focal species. Here, we use a variety of ecologically relevant behavioural traits to test for a colony-level behavioural syndrome in rock ants (Temnothorax rugatulus). Specifically, we combine field and laboratory assays to measure foraging effort, how colonies respond to different types of resources, activity level, response to threat and aggression level. We find evidence for a colony level syndrome that suggests colonies consistently differ in coping style—some are more risk-prone, whereas others are more risk-averse. Additionally, by collecting data across the North American range of this species, we show that environmental variation may affect how different populations maintain consistent variation in colony behaviour.     

Foraging behaviour of Atta cephalotes (leaf-cutting ants): an examination of two predictions for load selection

Two mechanisms have been proposed to explain how colony-level foraging performance of leaf-cutting ants can be maximized when workers harvest leaf fragments of a size that does not maximize their individual performance. Each mechanism predicts that ants will adjust the size of leaf fragments between starting a foraging bout and establishing full traffic between the nest and foraging site, but the two models predict shifts in opposite directions. I examined fragment sizes at the start of daily foraging in five field colonies of Atta cephalotes in Costa Rica and detected an obvious shift in only one case. More shifts were detected when the small and large ends of the worker body size range were considered separately, but the direction was inconsistent among colonies. I also examined the role of returning laden workers in recruitment of nestmates by intercepting all laden workers for the first 2 h of foraging, and measuring the effect on the arrival of recruits at the foraging site. In two cases, the flow of recruits was not diminished by the interception of returning workers. The results suggest that neither mechanism correctly and consistently accounts for load size selection by leaf-cutting ants. Copyright 2000 The Association for the Study of Animal Behaviour.     

The evolution of worker caste diversity in social insects

Morphological diversification of workers is predicted to improve the division of labor within social insect colonies, yet many species have monomorphic workers. Individual-level selection on the reproductive capacities of workers may counter colony-level selection for diversification, and life-history differences between species (timing of caste determination, colony size, genetic variation available) may mediate the strength of this selection. We tested this through phylogenetically independent contrast analyses on a new data set for 35 ant species. Evidence was found that early divergence of queen-worker developmental pathways may facilitate the evolution of worker diversity because queen-worker dimorphism was strongly positively associated with diversity. By contrast, risks for colonies that invest in specialized workers and colony size effects on costs of worker reproduction seem unlikely to strongly affect the evolution of worker diversity because there was no significant association between colony size and diversity when controlling statistically for queen-worker dimorphism. Finally, worker diversity was greater in species with multiple lineages per colony, and it was negatively associated with relatedness in monogynous species. This could be due to high intracolonial genetic variance favoring the expression and evolution of great worker diversity or to diversity evolving more easily when there is selection for repression of worker reproduction (worker policing).     

Storage function and ultrastructure of the adult fat body in workers of the ant Camponotus festinatus (Buckley) (Hymenoptera : Formicidae)

The presence of storage proteins in ants is perhaps most remarkable in its abundance in some species in the adult stage. The ultrastructure of fat body in workers of Camponotus festinatus (Hymenoptera : Formicidae) confirms that they do indeed store large quantities of protein, as well as lipid and carbohydrate, under some conditions. Rounded electron-dense granules, which are abundant in workers maintained in groups isolated from the parent colony, probably contain an arylphorin-like protein. Irregularly shaped electron-dense granules present in all workers regardless of age, caste or social environment, resemble primarily lysosomes, but lack acid phosphatase activity. Peroxisomes were also identified but were not associated with either type of dense granule. Lipid analysis showed that lipid storage followed similar patterns to protein storage, with isolated workers, especially soldiers, accumulating huge quantities of triglycerides. The relationship between storage of nutrient reserves and the presence of larvae suggests that the stores may function in regulating seasonal brood production.     

Worker reproduction in Formica ants

A potential tragedy of the commons arises in social-insect colonies where workers are fertile if egg-laying workers decrease their contribution to other tasks. We studied worker ovary development and egg laying in relation to kin structure, colony size, and the presence of a queen in nine species (11 populations) of Formica ants. Workers were highly fertile and laid eggs in the presence of a queen in five out of the seven species where egg samples were obtained. Worker fertility correlated neither with colony size nor with kin structure, which suggests that colony-level costs and efficiency of policing precede relatedness as the most important conflict determinant. We conclude that careful quantification of the costs of worker reproduction and policing is essential for inferences about the tragedy of the commons.     

Cuticular permeability of two species of Coptotermes Wasmann (Isoptera: Rhinotermitidae)

Cuticular permeability (CP) values of worker and soldier castes of Coptotermes formosanus Shiraki and Coptotermes vastator Light were determined using gravimetric techniques. Comparisons were made between castes, species, and between colonies of C. formosanus and C. vastator. CP values did not differ among either species or colony, however they did differ between castes. Data reported here and that of established data in the literature for C. formosanus are inconsistent, with the current report having values as small as half that of previously published data. Examining mean CP data by colony indicated that there is a range for Coptotermes spp. termites, from 6.84+/-1.2 to 25.61+/-1.49 microg H(2)Ocm(-2)mmHg(-1)h(-1) dependent upon caste. A novel method of examining intercaste CP variability is the use of worker:soldier CP ratios. Ratios for C. formosanus were consistent between the current study and previous reports. Mean percentage total body water (%TBW) lost at the time of death was 33.75+/-1.15% (soldiers) and 54.24+/-1.43% (workers) for C. formosanus, and 37.69+/-3.31% (soldiers) and 52.87+/-3.65% (workers) for C. vastator. Consistently, %TBW (and fresh mass) was greater in worker than in soldier castes. These data suggest the use of water storage mechanisms in worker termites, as shown for other rhinotermitids.     

Nutritional function of replete workers in the pharaoh's ant, Monomorium pharaonis (L.)

Summary: Queens of the pharaoh's ant Monomorium pharaonis (L.), like several other ant species, feed on larval secretions as their main nourishment and their fecundity is positively correlated with the number of large larvae present in the nest. The surplus of secretions produced by larvae is stored in a temporary caste of replete workers, which comprises young workers who remain in the nest and store liquid nourishment. Repletes are characterised by a conspicuously large gaster, caused by large amounts of liquid food stored in the crop, from which it may be regurgitated and distributed among colony members. In this study, repletes of pharaoh's ants were demonstrated to be functioning as buffers, smoothing fluctuations in availability of high quality food to the reproductive queens when larvae are scarce or missing, thus temporarily keeping up the egg production of queens.¶In undisturbed two-queen colonies with 20 large worker larvae and 30 workers (15 young and 15 old workers), approximately 10 repletes developed (one replete per two larvae). Development of older workers into repletes, when some or all repletes had been removed from the colonies, demonstrated that their temporal polyethism exhibits great plasticity in this trait.¶This study confirmed that, in pharaoh's ants, the regulation of fecundity depends not only on the food flow to the queen from larvae or from repletes but also on an unknown larval stimulus.¶The term crop repletes is suggested for replete workers which use their crop to store nourishment, as opposed to fat-body repletes, which store nourishment in their fat body.¶The presence of brood tending crop repletes in nests in several European ant species of Leptothorax, Myrmica, and Lasius, show that repletism is a common trait in ants, and that it may play an important role in regulation of nutrition in ant colonies, as demonstrated in Monomorium pharaonis.     

Shift in colonial reproductive strategy associated with a tropical-temperate gradient in Rhytidoponera ants

Offspring quality and quantity are subject to a trade-off. Depending on species, ant colonies can produce either numerous winged queens that start new colonies alone (independent colony foundation [ICF]) or one propagule consisting of reproductives helped by workers (fission). Some species do both. In the Rhytidoponera impressa group, colonies with winged queens use ICF, whereas colonies with mated workers use fission. We assessed the effect of latitude and environment on colonial reproduction by collecting 79 colonies along the Australian east coast. A population dynamical model predicted that from tropical north to temperate south, seasonal fluctuations and harsher conditions should lead to a decrease in ICF success (-48%), making fission a suitable alternative. Yet, our empirical data showed that ICF persists at a higher rate than expected, presumably because it allows aerial dispersal, unlike fission. Investigation of colony demography and measurement of lean and fat weights in virgin winged queens and workers showed that, from north to south, colonies increase quality of queens relative to workers (+50%) at the cost of quantity (-86%). This modification limits the decrease in ICF success (-34%). A tremendous range of offspring phenotypes associated with quality-quantity trade-offs make ants competitive in diverse habitats.     

The relative availabilities of complementary resources affect the feeding preferences of ant colonies

Theory predicts that consumers selecting among complementaryresources will show stronger preferences for items that becomerelatively less available. I tested this hypothesis in a fieldstudy that compared the preferences of ant colonies given simultaneousaccess to experimental foods differing in carbohydrate and proteincontent. In the first part of the study, I examined the effectof nutrient supplementation on colony-level preference in theant Dorymyrmex smithi. Colonies that had received a proteinsolution for 24 h consumed proportionally more carbohydratesthan control colonies that had been given access to water, suggestingthat colonies preferred nutrients when they became relativelyrare. In the second part of the study, I compared colony-levelpreference among eight species of ants that differ in theirrelative access to carbohydrates and protein in the field. Ifound that species with relatively easy access to carbohydratespreferred protein, whereas species with greater access to proteinpreferred carbohydrates. These results suggest that the benefitsof a nutritionally mixed diet coupled with differences in therelative availability of nutrients may explain variation infeeding decisions both within and among ant species.     

Major Quantitative Trait Loci Affecting Honey Bee Foraging Behavior

We identified two genomic regions that affect the amount of pollen stored in honey bee colonies and influence whether foragers will collect pollen or nectar. We selected for the amount of pollen stored in combs of honey bee colonies, a colony-level trait, and then used random amplified polymorphic DNA (RAPD) markers and interval mapping procedures with data from backcross colonies to identify two quantitative trait loci (pln1 and pln2, LOD 3.1 and 2.3, respectively). Quantitative trait loci effects were confirmed in a separate cross by demonstrating the cosegregation of marker alleles with the foraging behavior of individual workers. Both pln1 and pln2 had an effect on the amount of pollen carried by foragers returning to the colony, as inferred by the association between linked RAPD marker alleles, D8-.3f and 301-.55, and the individual pollen load weights of returning foragers. The alleles of the two marker loci were nonrandomly distributed with respect to foraging task. The two loci appeared to have different effects on foraging behavior. Individuals with alternative alleles for the marker linked to pln2 (but not pln1) differed with respect to the nectar sugar concentration of their nectar loads.     

Mixed colonies and hybridisation of <Emphasis Type="Italic">Messor</Emphasis> harvester ant species (Hymenoptera: Formicidae)

The Mediterranean harvester ant species Messor minor, M. cf. wasmanni, and M. capitatus can co-occur in the same habitat. In Italian populations, we encountered colonies that contained workers from more than one species as identified via standard morphology, as well as colonies with workers that appeared to be morphologically intermediate between species. This unusual finding required further analysis. We analysed such colonies using microsatellites, mitochondrial DNA and refined morphometrics, and a simple inference key for the colony-level interpretation of data from the three sources combined. We infer that Messor minor and M. cf. wasmanni engage in bidirectional interspecific gene flow. Hybrids between these two species are inferred to produce fertile offspring, which would indicate that barriers to hybridisation do not exist or can be completely overcome. This is unexpected, given that they are non-sister species and broadly sympatric in nature. Our findings also indicate the possible occurrence of hybrid-hybrid crosses, a phenomenon rarely observed in ants. We cautiously interpret the data at hand as in support of the interspecific gene flow considerably shaping the genetic makeup of populations, raising the question about a potential adaptive value of this hybridisation. Messor capitatus mixes with hybrids of the other two species, but we found no indication of hybridisation involving this species. We discuss various hypotheses on the causations of colony mixing and hybridisation in the three Messor species at the proximate and ultimate level.     

Lack of conflict during queen production in the social wasp Vespula maculifrons

Social insects display extreme cooperative and helping behaviours. However, social insect colonies are also arenas of intense competitive interactions. One particularly important matter over which colony members may compete centres on the development of sexual offspring. Specifically, colony members may engage in selfish behaviours leading to reproductive competition, whereby individuals either strive to develop as sexuals or assist kin so that close relatives emerge as new reproductives. We investigated whether reproductive competition occurred in the polyandrous social wasp Vespula maculifrons. We genotyped V. maculifrons workers and new queens at eight polymorphic microsatellite loci to determine if larvae of particular genotypes were reared as gynes more frequently than expected by chance. However, we found no significant evidence of reproductive competition in this species. The proportional contributions of males to workers and new queens did not vary within colonies. Moreover, male reproductive skew did not differ between workers and new queens. Finally, novel statistical techniques uncovered no evidence of patriline reversal, the phenomenon whereby males that contribute little to worker production contribute substantially to new queen production. Consequently, we conclude that individual level selection operating to increase the frequency of selfish behaviours that would lead to reproductive competition has been nullified by colony-level selection acting to maintain colony efficiency and cooperation.     

The social regulation of physical caste: the superorganism revived

Among the ants, the population of worker phenotypes made by a colony is closely regulated. For the first time, a model of the regulation of a particular size class, that of major workers, is developed within a new theory of the superorganismic activity of social insect colonies. A remarkable linkage is found to exist between caste homeostasis in a model polymorphic society and three well-known but previously unrelated social insect colony properties: the form of the caste distribution, the short-term nature of a colony's “memory”, and the existence of co-operative interactions among nestmates. The linkage consists of mass social processes and takes the form of a colony-level, negative feedback circuit controlling the production of the major workers. The implications of this case study for rigorous, substantive theories of social insect mass phenomena are discussed.