How brood influences caste aggregation patterns in the dimorphic ant species Pheidole pallidula

Spatial distribution of ant workers and, notably their aggregation/segregation behaviour, is a key-element of the colony social organization contributing to the efficiency of task performance and division of labour. In polymorphic species, specialized worker castes notably differ in their intrinsic aggregation behaviour. In this context, knowing the preponderant role of minors in brood care, we investigate how a stimulus such as brood can influence the spatial patterns of Pheidole pallidula worker castes. In a homogeneous area without brood, it was shown that minors display only a low level of aggregation while majors form large clusters in the central area. Here we find out that these aggregation patterns of both minors and majors can be deeply influenced by the presence of brood. For minors, it nucleates or enhances the formation of a large stable cluster. Such high sensitivity of minors to brood stimuli fits well with their role as main brood tenders in the colony. For majors, interattraction between individuals still remains the prevailing aggregation factor while brood strongly influences the localisation of their cluster. We discuss how the balance between interattraction and sensitivity to environmental stimuli determines the mobility of each worker castes and, consequently, the availability of minors and majors to participate in everyday colony tasks. Moreover, we will evoke the functional value of majors’ cluster location close to the brood, namely with respect to social regulation of the colony caste ratio. Received 30 May 2005; revised 11 January 2006; accepted 13 January 2006.     

Caste specialization in behavioral defenses against fungus garden parasites in <Emphasis Type="Italic">Acromyrmex octospinosus</Emphasis> leaf-cutting ants

Division of labor and caste specialization plays an important role in many aspects of social insect colony organization, including parasite defense. Within leaf-cutting ant colonies, worker caste specialization permeates colony tasks ranging from foraging, substrate incorporation, brood care, and chemical defenses via glandular secretions and mutualistic bacteria. Leaf-cutting ants rely on protecting a mutualistic fungus they grow for food from microfungi in the genus Escovopsis that parasitizes the ant–fungus relationship. Here, we examine whether Acromyrmex octospinosus leaf-cutter ant castes (minors and majors) display task specialization in two behavioral defenses against Escovopsis: fungus grooming (the removal of Escovopsis spores) and weeding (the removal of large pieces of Escovopsis-infected fungus garden). Using behavioral observations, we show that minors are the primary caste that performs fungus grooming, while weeding is almost exclusively performed by majors. In addition, using a sub-colony infection experimental setup, we show that at the early stages of infection, minors more efficiently remove Escovopsis spores from the fungus garden, thereby restricting Escovopsis spore germination and growth. At later stages of infection, after Escovopsis spore germination, we find that major workers are as efficient as minors in defending the fungus garden, likely due to the increased importance of weeding. Finally, we show, using SEM imaging, that the number of sensory structures is similar between minor and major workers. If these structures are invoked in recognition of the parasites, this finding suggests that both castes are able to sense Escovopsis. Our findings support that leaf-cutter ant behavioral defense tasks against Escovopsis are subject to caste specialization, likely facilitated by worker sizes being optimal for grooming and weeding by minors and majors, respectively, with important consequences for cultivar defense.     

Seasonality, division of labor, and dynamics of colony-level nutrient storage in the ant Pheidole morrisi

Nutritional provisioning is a critical component of life history strategies, and of particular interest in social insect colonies because of the role that division of labor plays in resource allocation. To explore the mechanisms that underlie colony nutritional strategies, I examined three populations of the ant Pheidole morrisi across a gradient of overwinter food scarcity over two seasons. P. morrisi colonies were found to employ amixed strategy of fat storage with regard to a longer overwinter period: members of both worker castes increase their percent-fat in a graded manner, while the proportion of a specialized subcaste of majors known as “repletes”, also increased within the colony. Geographic variation in other colony traits such as mean colony size, mean worker size, and minor/major caste ratio were also found, although not always in a manner clearly relating to fat storage. These results indicate that colony demography responds to seasonal fluctuations in food availability through behavioral alterations (increased fat stores and recruitment of replete workers) rather than physical alterations (changes in lean body sizes or caste ratio). The findings illustrate the dynamic role division of labor plays in the success of insect colonies confronting environmental variability. Received 9 May 2006; revised 19 July 2006; accepted 24 July 2006.     

Between-species differences of behavioural repertoire of castes in the ant genus <Emphasis Type="Italic">Pheidole</Emphasis>: a methodological artefact?

Summary This study highlights the influence of sampling size on the interpretation of between castes division of labour in the dimorphic ant genus Pheidole. We show that data analyses based on rarefaction curves provide better estimates of caste repertoire sizes. Weighted observations of the two worker castes of Pheidole pallidula reveals that the behavioural repertoire of majors is far more extended than expected. Indeed, majors are not restricted to defence, seed milling or food storage but can additionally participate to within nest activities by carrying out 69% of the minors behavioural repertoire including brood care. Besides, we show that inter-specific variation in the size of majors behavioural repertoire could simply result from differences in the number of majors observed. Therefore, the ergonomic prediction that the repertoire size of one caste should be correlated to its numerical representation in the colony needs to be re-examined considering between-castes differences in the sampling effort.Received 23 April 2003; revised 28 July 2003; accepted 1 August 2003.     

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.     

Sociality in the Australian Allodapine Bee Brevineura elongata: Small Colony Sizes Despite Large Benefits to Group Living

Allodapine bees (family Apidae, subfamily Xylocopinae) provide substantial material for investigating the evolution of sociality because of their wide variation in colony size, life history traits, and caste differentiation. Two recent studies have shown that the Australian allodapine genus Brevineura exhibits a strong increase in per capita brood production (PCBP) in social colonies compared to single-female nests. However both species previously examined, B. xanthoclypeata and B. froggatti, show relatively few multi-female nests, which is puzzling considering the apparently large advantages for group-living in these species. Here we show that in a third species, B. elongata, there are also substantial benefits for group living, involving increased PCBP and a greatly reduced likelihood of nests without brood. As expected from these observations, we also found strongly female biased sex allocation. Nevertheless only a small percentage of nests contained more than one adult female, similar to the other two Brevineura species, raising the question of why multifemale colonies are not more common in this genus. Solving this puzzle will throw light on conditions that constrain sociality when group living apparently provides major advantages.     

Colony social organization of <Emphasis Type="Italic">Lasioglossum malachurum</Emphasis> Kirby (Hymenoptera,Halictidae) in southern Greece

Summary We studied the nesting and social biology of two aggregations of the obligately eusocial halictine bee Lasioglossum malachurum at Agios Nikolaos Monemvasias (ANM) in southern Greece. Observations and nest excavations carried out from May to June 2000, revealed social and demographic variation between aggregations and years at ANM, as well as notable differences between these and other European populations. In southern Greece, the colony cycle includes multiple broods: the first two broods comprise only workers, whereas the third brood comprises workers, males, and gynes. Although pleometrosis is unknown in other populations, in the ANM region, as many as 10% of nests have more than one foundress. Newly emerged workers and gynes exhibit non-overlapping size distributions, but a few queens are worker-sized, indicating that workers occasionally overwinter and become foundresses. Although the vast majority of workers are unmated and most exhibit no ovarian development, an increase in worker ovarian development at the time of male production suggests that many males may develop from worker-laid eggs. Worker reproduction seems to be inhibited by the presence of queens, and annual variation in queen mortality may underlie annual variation in worker ovarian development. Across Europe, the major demographic and social differences among L. malachurum populations are in the number of worker broods and the extent of worker ovarian development. This contrasts with the results of a principal components analysis of social traits among 15 social L. (Evylaeus) populations, which shows that interspecific social variation is defined by the proportion of males in the early brood, the proportion of workers mated, queen-worker size dimorphism, gyne overwintering locale, and the proportion of workers with developed ovaries.Received 17 June 2002; revised 16 January 2002; accepted 27 January 2003.     

Potential and realized reproduction by different worker castes in queen-less and queen-right colonies of <Emphasis Type="Italic">Pogonomyrmex badius</Emphasis>

Workers of the Florida harvester ant (Pogonomyrmex badius), the only North American Pogonomyrmex with a polymorphic worker caste, produce males when colonies are orphaned. In this study,we assessed the reproductive potential of workers of each caste group, minors and majors, in the presence and absence of the queen, and tested whether males produced in natural queen-right colonies are derived from workers. Worker size was positively correlated with ovariole number such that major workers had approximately double the number of ovarioles as minor workers. The number of vitellogenic oocytes, a measure of reproductive potential, was greater in major compared to minor workers and increased in both worker castes when queens were removed. Major workers have greater reproductive potential than minors although they represent a minority within the colony (~5% of workers are majors). Worker produced eggs were visible in colonies 28 – 35 days after queen removal. This time lag, from queen removal to egg production, is similar to other ants and bees. Though workers are capable of producing viable eggs, we found no evidence that they do so in queen-right colonies, suggesting that worker reproduction is controlled via some social mechanism (self restraint, policing, or inhibition). This result supports predictions of kin selection theory – that due to multiple mating by the queen workers are more related to queen-produced males than most worker-produced males and should thus favor reproduction by the queen and inhibit reproduction by other workers. Received 25 January 2007; revised 1 May 2007; accepted 21 May 2007.     

Colony dynamics in a primitively eusocial halictine beeLasioglossum (Dialictus) zephyrum (Hymenoptera: Halictidae)

Summary Proximate control of colony dynamics was studied in the primitively eusocial halictine beeLasioglossum (Dialictus) zephyrum using allozyme markers. The results indicate that workers produce on average 15% of the male brood (range=0–50%) in small laboratory colonies made up of unrelated, single-generation, uninseminated females. This proportion is not influenced by colony size, but is influenced by the relative size of the queen. Large queens are more successful in dominating their workers than are small queens, the queen being defined as the female that is the mother of most of the brood produced in the colony. Older and larger females tend to become queens. Thus, while small differences in age (up to 4 days) influence which female becomes a queen, her ability to control her workers is primarily influenced by her relative size. The proportion of reproduction that is co-opted by the queen is negatively correlated with colony reproductivity (the number of males/day/female). Colony reproductivity is also negatively correlated with the standard deviation in size among females.     

Both female castes contribute to colony emigration in the polygynous ant Mystrium oberthueri

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Experimental Conversion of Colony Social Organization in Fire Ants (<Emphasis Type="Italic">Solenopsis invicta</Emphasis>): Worker Genotype Manipulation in the Absence of Queen Effects

Colony social organization in the fire ant Solenopsis invicta appears to be under strong genetic control. In the invasive USA range, polygyny (multiple queens per colony) is marked by the presence of the Gp-9 ^b allele in most of a colony’s workers, whereas monogyny (single queen per colony) is associated with the exclusive occurrence of the Gp-9 ^B allele. Ross and Keller, Behav Ecol Sociobiol 51:287–295 (2002) experimentally manipulated social organization by cross-fostering queens into colonies of the alternate form, thereby changing adult worker Gp-9 genotype frequencies over time. Although these authors showed that social behavior switched predictably when the frequency of b-bearing adult workers crossed a threshold of 5–10%, the possibility that queen effects caused the conversions could not be excluded entirely. We addressed this problem by fostering polygyne brood into queenright monogyne colonies. All such treatment colonies switched social organization to become polygyne, coincident with their proportions of b-bearing workers exceeding 12%. Our results support the conclusion that polygyny in S. invicta is induced by a minimum frequency of colony workers carrying the b allele, and further confirm that its expression is independent of queen genotype or history, worker genotypes at genes not linked to Gp-9, and colony genetic diversity.     

Colony size,social complexity and reproductive conflict in social insects

The broad limits of mature colony size in social insect species are likely to be set by ecological factors. However, any change in colony size has a number of important social consequences. The most fundamental is a change in the expected reproductive potential of workers. If colony size rises, workers experience a fall in their chances of becoming replacement reproductives and, it is shown, increasing selection for mutual inhibition of one another's reproduction (worker policing). As workers’ reproductive potential falls, the degree of dimorphism between reproductive and worker castes (morphological skew) can rise. This helps explain why small societies have low morphological skew and tend to be simple in organization, whereas large societies have high morphological skew and tend to be complex. The social consequences of change in colony size may also alter colony size itself in a process of positive feedback. For these reasons, small societies should be characterized by intense, direct conflict over reproduction and caste determination. By contrast, conflict in large societies should predominantly be over brood composition, and members of these societies should be relatively compliant to manipulation of their caste. Colony size therefore deserves fuller recognition as a key determinant, along with kin structure, of social complexity, the reproductive potential of helpers, the degree of caste differentiation, and the nature of within-group conflict.     

Behavioral Repertoire, Age Polyethism and Adult Transport in Ectatomma opaciventre (Formicidae: Ponerinae)

This study describes the behavioral repertoire of a mature colony of Ectatomma opaciventre Roger. Twenty-eight different behavioral acts were recorded from 4538 individual observations. There was evidence of age polyethism, with young workers performing significantly more brood care and allogrooming than older workers. Experiments were also performed to investigate the behavioral organization of nest relocation in E. opaciventre. This complex social behavior was regulated by a few workers which specialized in transporting nestmates, including other workers, winged gynes, males, and the queen. Winged gynes were also observed performing adult transport. The carrying posture utilized in E. opaciventre is similar to that in other ponerines, although some idiosyncratic features were detected.     

Polymorphism and Division of Labor During Foraging Cycles in the Leaf-cutting Ant <Emphasis Type="Italic">Acromyrmex octospinosus</Emphasis> (Formicidae; Attini)

While division of labor within leaf-cutting ant nests has been well-characterized in the context of the collection and processing of leaf material, environmental factors such as day-night cycles and heavy rainfall limit the time during which leaf-cutting ant workers leave the nest to gather forage. Using a novel “flat panel” nest design, we studied how patterns of within-nest task performance changed when a colony of the leaf-cutting ant Acromyrmex octospinosus was and was not provided access to forage. We conducted scan samples of individuals working within the nest under both conditions and compared task allocation patterns across provisioning regimes and between workers of different sizes. When labor was compared between worker size groups, “minor” workers (head width ≤2.0 mm) and “major” workers (head width >2.0 mm) showed significantly different task performance patterns when forage was available: minors performed mostly brood-care and garden maintenance, while majors were mostly involved in the handling of freshly-cut leaf fragments. In contrast, when the colony was deprived of forage, the task performance patterns of minor and major workers converged and did not significantly differ. Marked major workers known to be foragers tended to remain idle within the nest when the colony was deprived of forage, while non-foragers of similar head width engaged in a variety of within-nest tasks, suggesting polyethism in majors may be based on factors other than size.     

Effect of juvenile hormone on caste determination and colony processes in the bumblebee Bombus terrestris

To investigate the role of juvenile hormone (JH) in caste determination, first and second instar larvae of Bombus terrestris were topically treated with one of three doses (2, 10, or 20 g/larva) of JH dissolved in acetone. Treated larvae belonged to very young colonies that had just been started by their queen. Therefore, all larvae were supposedly destined to develop into workers. Our study demonstrates that a single application of JH in the first or second instar can lead to the development of queens and that this effect is dose- and instar-dependent. The brood (second, or third brood of the colony) to which the larvae belonged also had an effect. A clear size dimorphism of the female castes exists in B. terrestris. In this study, however, intermediates also developed from treated larvae. In fact, even untreated larvae in a treated colony developed into queens and intermediates, depending on the total dose of JH applied to the colony. There are no indications that a larva, once determined to become a queen, can be forced to become a worker by means of malnutrition. Treatment with JH also had an influence on colony processes. For instance, the appearance of queen larvae resulted in the advanced start of reproduction by workers and egg robbery, the so called competition point. This indicates again the close relationship between queen rearing and the queen-worker conflict. However, the ultimate function of this casual connection is still unclear. Further, the queen reacted to the treatment by switching to the laying of haploid eggs at an earlier date in the colony development compared to untreated colonies.     

Caste determination in a polymorphic social insect: nutritional, social, and genetic factors

We examined how dietary, social, and genetic factors affect individual size and caste in the Florida harvester ant Pogonomyrmex badius, which has three discrete female castes. The diet that a larva consumed, as indicated by delta(13)C, delta(15)N, and C:N, varied with caste. Both N content and estimated trophic position of dietary input was higher for major than for minor workers and was highest for gynes (reproductive females). The size and resources of a colony affected the size of only minor workers, not that of gynes and major workers. Approximately 19% of patrilines showed a bias in which female caste they produced. There were significant genetic effects on female size, and the average sizes of a major worker and a gyne produced by a patriline were correlated, but neither was correlated with minor worker size. Thus, genetic factors influence both caste and size within caste. We conclude that environmental, social, and genetic variation interact to create morphological and physiological variation among females in P. badius. However, the relative importance of each type of factor affecting caste determination is caste specific.     

Caste specialization in food storage in the dimorphic antColobopsis nipponicus (Wheeler)

Summary Food-storage abilities of both minor and major workers of the dimorphic antColobopsis nipponicus were examined to prove the hypothesis that major workers of this species have a trophic role in addition to a defensive one. Both worker subcastes accumulated water, that was supplied with water-soluble food, as well as fat, that probably originated from food given. However, the major workers accumulated much larger amounts of water and fat than did the minor workers. Difference in water accumulation increased as food supply increased. In spite of their difference in body weight, the residual amount of fat after starvation did not differ between the two subcastes. Although the amount of stored fat increased with colony size in both subcastes, the stored fat per dry weight of the major worker was significantly larger than that of the minor worker regardless of the colony size. When workers of a colony were starved, minor workers with a single major worker survived significantly longer than those without a major worker. In addition, about half the behavioral acts of major workers were regurgitation for minor workers. These results demonstrate that major workers ofC. nipponicus functioned as a trophic caste. On the basis of the results, quantitative relationships of storage abilities between the two worker subcastes are discussed.     

中华红林蚁形态描述、行为观察及社群结构

调查显示,北京百花山上"蚂蚁山"的蚂蚁为我国林区特有的种类——中华红林蚁Formica sinensis。该蚂蚁分布海拔相对较高,通常在针叶林或针阔叶混交林中筑巢,进行社会性生活。成虫有4个品级:大型工蚁、小型工蚁、蚁后和雄蚁。每个成熟蚁巢中有2～4个蚁后,为多蚁后社群结构。夏季蚁巢中同时存在着卵、幼虫、蛹和成虫。目前还只发现工蚁和蚁后休眠越冬。对中华红林蚁的各个虫态和品级进行形态描述和测定,并记述工蚁的觅食、建巢及清巢、防御与进攻等行为习性。首次发现雄蚁具有明显的护巢行为。同时,还发现在工蚁的外出活动中,标记信息素的作用不是很明显。     

Larval cannibalism and worker-induced separation of larvae in <Emphasis Type="Italic">Hypoponera</Emphasis> ants: a case of conflict over caste determination?

Reproductive altruism and cooperative brood care are key characteristics of eusocial insects and reasons for their ecological success. Yet, Hymenopteran societies are also the stage for a multitude of intracolonial conflicts. Recently, a conflict between adult and larval colony members over caste fate was described and evidence for overt conflict was uncovered in several bee species. In theory, diploid larvae of many Hymenopteran species should experience strong fitness benefits, if they would be able to change their developmental pathway towards the queen caste. However, larval self-determination potential is low in most advanced eusocial Hymenopterans, because workers often control larval food intake and queenworker caste dimorphisms are generally high. In the ant genus Hypoponera, larvae actively feed on food provided by workers and here we show extremely low queenworker size differences in these ants: the lowest in H. opacior, where fertile wingless (intermorphic) queens weigh on average only 13% more than workers. Thus, slightly better nutrition during development might change the fate of a Hypoponera larva from a completely sterile worker to a fertile queen. One possibility to obtain extra food for Hypoponera larvae with their well-developed mandibles would be to cannibalise adjacent larvae. Indeed, we observed frequently larval cannibalism in ant nests. Yet, adult workers apparently try to prohibit larval cannibalism by carefully separating larvae in the nest. Larvae, which were experimentally brought into close contact, were rapidly set apart. Workers further sorted larvae according to size and responded swiftly to decreasing food levels, by increasing inter-larval distance. Still, an experimental manipulation of the larval cannibalism rate in H. schauinslandi failed to provide conclusive evidence for the link between larval cannibalism and caste development. Hence, further experiments are needed to determine whether the widespread larval cannibalism in Hypoponera and the untypical brood distribution can be explained by an overt caste conflict. Received 18 December 2006; revised 2 August and 20 September 2007; accepted 21 September 2007.