Macronutrient regulation in the Rasberry crazy ant (<Emphasis Type="Italic">Nylanderia</Emphasis> sp. nr. <Emphasis Type="Italic">pubens</Emphasis>)

Animals grow and optimize performance when they collect foods in amounts and ratios that best meet their species-specific nutritional requirements. For eusocial organisms like ants, where only a small fraction of the colony members collect food, increasing evidence demonstrates that strong macronutrient regulation occurs at the colony level. Here, we explored regulation of protein and carbohydrate (p:c) intake in the Rasberry crazy ant, Nylanderia sp. nr. puben. We did this using dry artificial foods (14–42% total macronutrient content) having a range of fixed p:c ratios in a series of choice and no-choice laboratory experiments and used worker mortality to gauge colony-level costs associated with active nutrient regulation. Choice experiments revealed that colonies preferred carbohydrate-rich foods and self-selected a diet having a p:c ratio ~1:2. No-choice experiments demonstrated that food p:c ratio only moderately affected worker food collection behavior, likely because colonies regulated the intake of only the non-limiting nutrients. Absolute worker mortality was generally high, but lowest in colonies feeding on the food having a p:c ratio of 1:2 (the p:c ratio ants self-selected in the choice experiment), although mortality was not significantly affected by food p:c ratio. The self-selected p:c ratio in our study is consistent with that observed in other recent ant nutrient regulation studies. Taken together, the results from these combined studies reveal emerging commonalities among ants in macronutrient regulation strategies, and similarities in foraging behaviors and costs associated with macronutrient regulation. Finally, from a methodological perspective, the high mortality observed in our study, when compared with other recent studies, suggests that ant nutrient regulation studies should be conducted using foods having high moisture and total macronutrient content.     

Genetic variation and task specialization in the desert leaf-cutter ant, Acromyrmex versicolor

The genetic diversity in social insect colonies that is generated by multiple mating or multiple queens has been hypothesized to promote worker task specialization and therefore facilitate division of labour. However, few studies have actually examined the mechanisms by which genotype may influence individual worker behaviour. In this study, we dissect possible genetic effects on worker task performance in the desert leaf-cutter ant. We hypothesize that genotype could affect worker behaviour via (1) the rate of age-related task switching (age polyethism schedule), (2) individual task preference, and/or (3) task performance rates. To discriminate among these possible mechanisms, we generated composite colonies of workers from different genetic sources and followed the behaviour of individually marked workers over their lifetimes. We found significant differences among matrilines (offspring of different queens) in overall task performance. In particular, we found a negative covariance in likelihood of foraging versus tending fungus inside the nest. Workers of different matrilines also varied in the age of transition from inside the nest to foraging, but did not vary in task performance rates. Our results suggest that division of labour in this system is affected by genetic influences on individual task preference and age-related task choice, but not on variation in activity level.     

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.     

Task-specific expression of the foraging gene in harvester ants

In social insects, groups of workers perform various tasks such as brood care and foraging. Transitions in workers from one task to another are important in the organization and ecological success of colonies. Regulation of genetic pathways can lead to plasticity in social insect task behaviour. The colony organization of advanced eusocial insects evolved independently in ants, bees, and wasps and it is not known whether the genetic mechanisms that influence behavioural plasticity are conserved across species. Here we show that a gene associated with foraging behaviour is conserved across social insect species, but the expression patterns of this gene are not. We cloned the red harvester ant (Pogonomyrmex barbatus) ortholog (Pbfor) to foraging, one of few genes implicated in social organization, and found that foraging behaviour in harvester ants is associated with the expression of this gene; young (callow) worker brains have significantly higher levels of Pbfor mRNA than foragers. Levels of Pbfor mRNA in other worker task groups vary among harvester ant colonies. However, foragers always have the lowest expression levels compared to other task groups. The association between foraging behaviour and the foraging gene is conserved across social insects but ants and bees have an inverse relationship between foraging expression and behaviour.     

Mechanisms of dispersed central-place foraging in polydomous colonies of the Argentine ant

Many species of ants occupy multiple nests, a condition known as polydomy. Because of their decentralized structure, polydomous colonies may be removed from some of the constraints associated with classic central-place foraging. We used laboratory and field experiments to assess the mechanisms involved in dispersed central-place foraging in polydomous colonies of the Argentine ant Linepithema humile, a widespread invasive species. Both in the laboratory and in the field, Argentine ants established new nests at sites located near food. Laboratory colonies of L. humile redistributed workers, brood and resources among nests in response to the spatial heterogeneity of food resources. In addition, laboratory colonies formed recruitment trails between nests in the context of foraging, providing a mechanism for the transport of material between nests. This highly flexible system of allocating nests, workers and brood throughout a colony's foraging area potentially increases foraging efficiency and competitive ability. The importance of polydomy as a determinant of competitive ability is underscored by its prevalence among ecologically dominant ants, including most, if not all, highly invasive species. Copyright 2000 The Association for the Study of Animal Behaviour.     

Foraging decisions of individual workers vary with colony size in the greenhead ant Rhytidoponera metallica (Formicidae, Ectatomminae)

Summary. The ability of worker ants to adapt their behaviour depending on the social environment of the colony is imperative for colony growth and survival. In this study we use the greenhead ant Rhytidoponera metallica to test for a relationship between colony size and foraging behaviour. We controlled for possible confounding ontogenetic and age effects by splitting large colonies into small and large colony fragments. Large and small colonies differed in worker number but not worker relatedness or worker/brood ratios. Differences in foraging activity were tested in the context of single foraging cycles with and without the opportunity to retrieve food. We found that workers from large colonies foraged for longer distances and spent more time outside the nest than foragers from small colonies. However, foragers from large and small colonies retrieved the first prey item they contacted, irrespective of prey size. Our results show that in R. metallica, foraging decisions made outside the nest by individual workers are related to the size of their colony.Received 23 March 2004; revised 3 June 2004; accepted 4 June 2004.     

Division of labor inPonera pennsylvannica (Formicidae: Ponerinae)

Summary We examined division of labor and colony demography in the antPonera pennsylvannica. Observation of three colonies with individually marked workers revealed a high degree of interindividual behavioral variation and a rough but consistent division of labor between brood tenders and foragers. This division was present both in colonies consisting entirely of workers produced in the previous summer and in colonies containing freshly eclosed ants. Two colonies showed typical age-based polyethism, with young ants focusing on brood care and overwintered ants on foraging. No such age basis was detected in the third colony. This difference may relate to variability in brood production schedules. Colonies showing temporal polyethism had two peaks of brood production and thus had relatively large brood populations when the first young workers eclosed, while the third colony had only one peak and little brood for the young workers to tend. Even if young ants have a lower threshold for brood care, it may have been concealed in the latter situation. Demographic data indicate that natural colonies produce one brood per year and that workers typically eclose into colonies with relatively low brood care demands. This suggests that overwintered workers do most of a colony's work and that the division of labor among overwintered ants is the more important one under natural conditions. The basis of this division is as yet unknown. These results also suggest that small colony size, univoltine brood schedule and a close association between foraging and brood care do not preclude division of labor among specialized castes, as has been suggested for another ponerine species (Traniello 1978).     

Worker thermal tolerance in the thermophilic ant Cataglyphis cursor (Hymenoptera, Formicidae)

In the thermophilic ant genus Cataglyphis, species differing in their physical caste system have developed alternative mechanisms to face extreme heat by physiological and/or behavioural adaptations. In this study, we tested whether thermal tolerance is related to worker size in the ant Cataglyphis cursor that presents intermediate worker size compared with previously studied species (size range 3.5–10 mm). Thermal tolerance at two temperatures was tested in the laboratory on colonies originating from two habitats (seaside versus vineyard), known to differ in average worker size. As expected large workers were more resistant to high temperature than small workers, but the effect of worker size on thermal resistance was less pronounced under the more extreme temperatures. The pattern of thermal tolerance was similar in the two habitat types. After controlling for worker size, worker thermal tolerance significantly varied amongst colonies, but this variation was not related to colony size. Our results suggest that a higher thermal tolerance can confer an advantage to larger workers especially during foraging and are discussed in the context of the evolution of worker size in ants.     

The effects of colony characteristics on life span and foraging behavior of individual wasps (Polybia occidentalis,Hymenoptera: Vespidae)

Summary We studied the effects of intrinsic colony characteristics and an imposed contingency on the life span and behavior of foragers in the swarm-founding social waspPolybia occidentalis. Data were collected on marked, known-age workers introduced into four observation colonies.To test the hypothesis that colony demographic features affect worker life span, we examined the relationships of colony age and size with worker life span using survivorship analysis. Colony age and size had positive relationships with life span; marked workers from two larger, older colonies had longer life spans (¯X = 24.7 days) than those from two smaller, younger colonies (¯X = 20.1 days).We quantified the effects of experimentally imposed nest damage on forager behavior, to determine which of three predicted behavioral responses by foragers to this contingency (increased probability of foraging for building material, increased rate of foraging, or decrease in age of onset of foraging) would be employed. Increasing the colony level of need for materials used in nest construction (wood pulp and water) by damaging the nests of two colonies did not cause an increase in either the proportion of marked workers that gathered nest materials or in foraging rates of marked individuals, when compared with introduced workers in two simultaneously observed control colonies. Instead, nest damage caused a decrease in the age at which marked workers first foraged for pulp and water. The response to an increase in the need for building materials was an acceleration of behavioral development in some workers.     

Division of labour and specification of castes in the red imported fire ant Solenopsis invicta buren

Division of labour in Solenopsis invicta follows a familiar pattern: younger, smaller ants tend toward brood care while older, larger ants tend toward foraging. However, long-term observations of marked individuals reveal that length of nursing and foraging ‘careers’ and the age of transition between these activities vary considerably between and within size groups, and are related to length of life. Experiments with entire colonies show that larger ants are more likely than smaller ants to forage for insect prey. There are two main worker castes, ‘nurses’ and ‘foragers’, whose members span a wide age-size range, and a large ‘reserve’ subcaste, heterogeneous in age, size, and behaviour: reserves may nurse, forage, store liquid food, or relay food from nurses to foragers. The proportion of ants engaged in foraging decreases with colony size because many ants in large colonies are not exposed to recruitment signals.     

Foraging for Work and Age-Based Polyethism: The Roles of Age and Previous Experience on Task Choice in Ants

In social insects, colonies commonly show temporal polyethism in worker behavior, such that a worker follows a predictable pattern of changes between tasks as it ages. This pattern usually leads from workers first doing a safe task like brood care, to ending their lives doing the most dangerous tasks like foraging. Two mechanisms could potentially underlie this pattern: (1) age‐based task allocation, where the aging process itself predisposes workers to switch to more dangerous tasks; and (2) foraging for work, where ants switch to tasks that need doing from tasks which have too many associated workers. We tested the relative influence of these mechanisms by establishing nests of Camponotus floridanus with predetermined combinations of workers of known age and previous task specialization. The results supported both mechanisms. Nests composed of entirely brood‐tending workers had the oldest workers preferentially switching to foraging. However, in nests initially composed entirely of foragers, the final distribution of tenders and foragers was not different from random task‐switching and therefore supportive of foraging for work. Thus, it appears that in C. floridanus there is directionality to the mechanisms of task allocation. Switching to more dangerous tasks is age‐influenced, but switching to less dangerous tasks is age‐independent. The results also suggest that older workers are more flexible in their task choice behavior. Younger workers are more biased towards choosing within‐nest tasks. Finally, there are effects of previous experience that tend to keep ants in familiar tasks. Task allocation based on several mechanisms may balance between: (1) concentrating the most worn workers into the most dangerous tasks; (2) increasing task performance levels; and (3) maintaining behavioral flexibility to respond to demographic perturbations. The degree to which behavior is flexible may correlate to the frequency of such perturbations in a species.     

Task allocation in ant colonies within variable environments (A study of temporal polyethism: Experimental)

We briefly review the literature on the division of labour in ant colonies with monomorphic worker populations, and show that there are anomalies in current theories and in the interpretation of existing data sets. Most ant colonies are likely to be in unstable situations and therefore we doubt if an age-based division of labour can be sufficiently flexible. We present data for a type of small ant colony in a highly seasonal environment, concentrating on individually marked older workers. We show that contrary to expectation such workers undertake a wide variety of tasks and can even retain their ability to reproduce, even whilst younger workers are actively foraging. Our analysis shows that old workers occupy four distinct spatial stations within the nest and that these are related to the tasks they perform. We suggest that correlations between age and task in many ant colonies might simply be based on ants foraging for work, i.e. actively seeking tasks to perform and remaining faithful to these as long as they are profitably employed. For this reason, employed older workers effectively displace unemployed younger workers into other tasks. In a companion paper, Tofts 1993,Bull. math. Biol. develops an algorithm that shows how foraging for work can be an efficient and flexible mechanism for the division of labour in social insects. The algorithm creates a correlation between age and task purely as a by-product of itsmodus operandi.     

Reproductive Caste Performs Intranidal Tasks Instead of Workers in the Ant Mystrium oberthueri

Division of labour improves the efficiency of animal societies. Efficiency is further improved in many social insects where morphologically specialized adults perform different tasks. In ants, the wingless worker caste performs non‐reproductive activities and sometimes exhibits multiple phenotypes when requirements between brood care and expert foraging diverge. Mystrium rogeri from Madagascar is a specialist predator on large centipedes, and the worker caste is highly polymorphic in size. In contrast, M. oberthueri has only large workers. The replacement of the queen caste by wingless intermorphs much smaller than workers explains this evolutionary shift in M. oberthueri. Many intermorphs occur in each colony but only a few mate and reproduce. In order to determine their contribution to non‐reproductive tasks, we performed multivariate analyses on behavioural data recorded by scan sampling from four M. oberthueri colonies in the laboratory. In unmanipulated colonies, workers and intermorphs exhibited two distinct behavioural profiles. Workers focused on guarding and foraging, while intermorphs performed brood care and nest cleaning, regardless of whether they reproduced or not. This pattern of polyethism where the reproductive caste completely takes charge of some non‐reproductive tasks is novel, as confirmed by our observations of one colony of M. rogeri where non‐reproductive tasks were restricted to workers, as in most ants. When isolated from one another, M. oberthueri workers and intermorphs developed less distinctive behavioural patterns. Some workers cared for the brood, but the intermorphs could not hunt because of their small mandibles. Such plasticity in polyethism at the colony level confers the ability to react to unexpected changes, including variable proportions of workers and intermorphs.     

Foraging arena size and structural complexity affect the dynamics of food distribution in ant colonies

Food acquisition by ant colonies is a complex process that starts with acquiring food at the source (i.e., foraging) and culminates with food exchange in or around the nest (i.e., feeding). While ant foraging behavior is relatively well understood, the process of food distribution has received little attention, largely because of the lack of methodology that allows for accurate monitoring of food flow. In this study, we used the odorous house ant, Tapinoma sessile (Say) to investigate the effect of foraging arena size and structural complexity on the rate and the extent of spread of liquid carbohydrate food (sucrose solution) throughout a colony. To track the movement of food, we used protein marking and double-antibody sandwich enzyme-linked immunosorbent assay, DAS-ELISA. Variation in arena size, in conjunction with different colony sizes, allowed us to test the effect of different worker densities on food distribution. Results demonstrate that both arena size and colony size have a significant effect on the spread of the food and the number of workers receiving food decreased as arena size and colony size increased. When colony size was kept constant and arena size increased, the percentage of workers testing positive for the marker decreased, most likely because of fewer trophallactic interactions resulting from lower worker density. When arena size was kept constant and colony size increased, the percentage of workers testing positive decreased. Nonrandom (clustered) worker dispersion and a limited supply of food may have contributed to this result. Overall, results suggest that food distribution is more complete is smaller colonies regardless of the size of the foraging arena and that colony size, rather than worker density, is the primary factor affecting food distribution. The structural complexity of foraging arenas ranged from simple, two-dimensional space (empty arenas) to complex, three-dimensional space (arenas filled with mulch). The structural complexity of foraging arenas had a significant effect on food distribution and the presence of substrate significantly inhibited the spread of food. Structural complexity of foraging arenas and the resulting worker activity patterns might exert considerable influence on socioecological processes in ants and should be considered in laboratory assays.     

Effect of temperature and social environment on worker size in the ant Temnothorax nylanderi

Warm temperatures decrease insect developmental time and body size. Social life could buffer external environmental variations, especially in large social groups, either through behavioral regulation and compensation or through specific nest architecture. Mean worker size and distribution of worker sizes within colonies are important parameters affecting colony productivity as worker size is linked to division of labor in insect societies. In this paper, we investigate the effect of stressful warm temperatures and the role of social environment (colony size and size of nestmate workers) on the mean size and size variation of laboratory-born workers in the small European ant Temnothorax nylanderi. To do so, we reared field-collected colonies under medium or warm temperature treatments after having marked the field-born workers and removed the brood except for 30 first instar larvae. Warm temperature resulted in the production of fewer workers and a higher adult mortality, confirming that this regime was stressful for the ants. T. nylanderi ants followed the temperature size rule observed in insects, with a decreased developmental time and mean size under warm condition. Social environment appeared to play an important role as we observed that (i) larger colonies buffered the effect of temperature better than smaller ones (ii) colonies with larger workers produced larger workers whatever the rearing temperature and (iii) the coefficient of variation of worker size was similar in the field and under medium laboratory temperature. This suggests that worker size variation is not primarily due to seasonal environmental fluctuations in the field. Finally, we observed a higher coefficient of variation of worker size under warm temperature. We propose that this results from a disruption of social regulation, i.e. the control of nestmate workers over developing larvae and adult worker size, under stressful conditions.     

Reproductive conflict in social insects: male production by workers in a slave-making ant

In insect societies, workers cooperate but may also pursue their individual interests, such as laying viable male eggs. The case of obligatory slave-making ants is of particular interest because workers do not engage in maintenance activities and foraging. Therefore, worker egg laying is expected to be less detrimental for colony efficiency than in related, nonparasitic species. Furthermore, as slave-making workers usually do not perform brood care and thus might have little power in manipulating sex allocation, they might be more strongly selected to increase their direct fitness by producing their own sons than workers in nonparasitic species. In this study we investigated worker reproduction in four natural colonies of the slave-making ant Polyergus rufescens, using highly variable microsatellite markers. Our results show that workers produce up to 100% of the males. This study thus presents the first direct evidence of an almost complete takeover of male reproduction by workers in ants.     

Foraging preferences of ants on a heterogeneous Brazilian sandy shore habitat

1. Ants may select their food in response to nutritional needs of the colony and forage in a way that optimises a complementary nutrition. Even though resource availability is known to affect ant colony and individual health, there is still no study that has investigated the plastic preferences of ants according to spatial resource availability in naturally heterogeneous conditions. 2. Beaches are great biomes to test spatial foraging preference because a complete absence of nectaries can be found. Dorymyrmex nigra Pergande 1896 was found inhabiting a beach in southeastern Brazil, in which nectar sources are heterogeneously distributed. This study tested whether the foraging preference to sugar baits depended on the availability of nectar sources surrounding the nests. 3. We found that more D. nigra workers foraged on sugar baits when the colonies lacked naturally occurring nectar in their vicinity compared with colonies with abundant nectar nearby. 4. These results show that the foraging preference of ants depends upon resource availability. This is the first study to use a natural mosaic of resource availability to show that resource preference of ants is plastic and varies spatially.     

Nest raiding by the invasive Argentine ant on colonies of the harvester ant, Pogonomyrmex subnitidus

Invasive ants often displace native ants, and published studies that focus on these interactions usually emphasize interspecific competition for food resources as a key mechanism responsible for the demise of native ants. Although less well documented, nest raiding by invasive ants may also contribute to the extirpation of native ants. In coastal southern California, for example, invasive Argentine ants (Linepithema humile) commonly raid colonies of the harvester ant, Pogonomyrmex subnitidus. On a seasonal basis the frequency and intensity of raids vary, but raids occur only when abiotic conditions are suitable for both species. In the short term these organized attacks cause harvester ants to cease foraging and to plug their nest entrances. In unstaged, one-on-one interactions between P. subnitidus and L. humile workers, Argentine ants behaved aggressively in over two thirds of all pair-wise interactions, despite the much larger size of P. subnitidus. The short-term introduction of experimental Argentine ant colonies outside of P. subnitidus nest entrances stimulated behaviors similar to those observed in raids: P. subnitidus decreased its foraging activity and increased the number of nest entrance workers (many of which labored to plug their nest entrances). Raids are not likely to be the result of competition for food. As expected, P. subnitidus foraged primarily on plant material (85% of food items obtained from returning foragers), but also collected some dead insects (7% of food items). In buffet-style choice tests in which we offered Argentine ants food items obtained from P. subnitidus, L. humile only showed interest in dead insects. In other feeding trials L. humile consistently moved harvester ant brood into their nests (where they were presumably consumed) but showed little interest in freshly dead workers. The raiding behavior described here obscures the distinction between interspecific competition and predation, and may well play an important role in the displacement of native ants, especially those that are ecologically dissimilar to L. humile with respect to diet. Received 15 July 2005; revised 19 October 2005; accepted 26 October 2005.     

Worker life tables, survivorship, and longevity in colonies of Bombus (Fervidobombus) atratus (Hymenoptera: Apidae)

Survivorship curves and longevity of workers were studied in two queenright and two queenless colonies of Bombus (Fervidobombus) atratus. Survivorship curves for workers of all colonies were, in general, convex, indicating an increasing mortality rate with increasing age. The mean longevity for the workers from queenright colonies, 24.3 days and 17.6 days, was not significantly different from that in queenless colonies, 21.2 days and 20.2 days. In all colonies workers started foraging activities when aged 0-5 days, and the potential forager rates rose progressively with increasing age. Mortality rates within each age interval were significantly correlated with the foraging worker rates in all colonies. Only in two of the colonies (one queenright and one queenless) longevity was significantly correlated with worker size. The duration of brood development period seems to be one of the most important factors influencing adult worker longevity in bumble bee species.