Behavioral flexibility in age castes of the antPheidole dentata

Division of labor among size and age classes in social insect colonies is a fundamental aspect of sociality, with such classes typically showing behavioral specialization and being associated with particular tasks for prolonged periods of time (i.e., forming castes). Theoretically, such specialization and division of labor enhance efficiency and, ultimately, reproductive success, and the ratios among members of those castes are hypothesized to be under selection and adaptive. However, our recent field study of the ant Pheidole dentatashowed no correlation between ecological factors which should affect colony growth and those caste ratios but implied flexibility in the association between minor worker age castes and their tasks. In laboratory experiments on behavioral flexibility in minor age castes of P. dentataold workers in colonies without young workers competently perform both old worker (castetypical) and young worker (atypical) tasks, and vice versa. The implications of these results for ergonomie theory are considered.     

Worker Age, Ovary Development, and Temporal Polyethism in the Swarm-Founding Wasp Polybia occidentalis (Hymenoptera: Vespidae)

I measured oocyte sizes of Polybia occidentalis workers to assess whether ovary development corresponded with adult age or with individual variation in behavior. Workers exhibited temporal polyethism by first working inside the nest, then performing on-nest tasks, and later foraging and defending the nest. Individuals varied in their ages at first performance of each of these sets of behavioral acts. Ovary development corresponded strongly with adult age. Workers' ovaries declined quickly after an initial phase of partial development. The period of ovary decline overlapped with the age range at which most workers began performing tasks on the exterior nest surface. However, after accounting for age effects, individual behavioral differences (rate of temporal polyethism and performance of foraging and defense tasks) did not correspond with ovary development. These data suggest that ovary development had little or no effect on variation in task performance by P. occidentalis workers on the nest surface.     

Age- and subcaste-related patterns of serotonergic immunoreactivity in the optic lobes of the ant Pheidole dentata

Serotonin, a biogenic amine known to be a neuromodulator of insect behavior, has recently been associated with age-related patterns of task performance in the ant Pheidole dentata. We identified worker age- and subcaste-related patterns of serotonergic activity within the optic lobes of the P. dentata brain to further examine its relationship to polyethism. We found strong immunoreactivity in the optic lobes of the brains of both minor and major workers. Serotonergic cell bodies in the optic lobes increased significantly in number as major and minor workers matured. Old major workers had greater numbers of serotonergic cell bodies than minors of a similar age. This age-related increase in serotonergic immunoreactivity, as well as the presence of diffuse serotonin networks in the mushroom bodies, antennal lobes, and central complex, occurs concomitantly with an increase in the size of worker task repertoires. Our results suggest that serotonin is associated with the development of the visual system, enabling the detection of task-related stimuli outside the nest, thus playing a significant role in worker behavioral development and colony-wide division of labor.     

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).     

Regulation of Brood-Care Behavior in the Dimorphic Castes of the Ant Pheidole morrisi (Hymenoptera: Formicidae): Effects of Caste Ratio, Colony Size, and Colony Needs

This study examines the social stimuli that regulate brood-care behavior in the two physical castes of the ant Pheidole morrisi. By increasing the proportion of major workers in a colony, brood-care behavior could be induced in individuals of this caste, which do not normally care for brood. Minor and major worker brood-care rates increased with the proportion of majors in the colony or as the need for brood care increased. Changes in colony size did not significantly affect the rate of either minor or major brood care. Major workers began to care for brood when the caste ratio reached a critical threshold but did not appear to be as efficient at rearing larvae as minor workers, which normally perform this task. After a perturbation that skewed the caste ratio toward majors, minor workers increased their rate of brood care, apparently to compensate for the inefficiency of brood care provided by majors. These results suggest that caste plasticity involves a social mechanism of ldquo;coupled compensation that maintains the efficiency of labor by ensuring that tasks are completed.     

Worker connectivity: a simulation model of variation in worker communication and its effects on task performance

We develop a simulation model of worker connectivity to analyze how variation in worker communication can influence task performance. The model generates predictions about how colony demography, worker communicative behavior, and worker cognition will affect the rate of recruitment of workers to a new task. The model explores some mechanisms for modulating the recruitment of workers. Under the conditions of our model– probabilistic interactions that lower worker’s response thresholds to tasks– worker recruitment follows a logistic growth pattern. The rate of recruiting workers increases exponentially toward an inflection point when 50% of the available force has been activated, then decreases toward the upper asymptote (all workers recruited). Many relevant features of colony design and worker behavior, including group size, probability of interacting, and strength of interaction effects on receivers, show a positive but decelerating effect on the rate of worker recruitment. We also identify features of worker cognition that can influence task recruitment, focusing on the time course of worker’s memories about previous interactions. Both learning (e.g., sensitization) and forgetting about previous interactions can influence the rate of worker recruitment to a task. The model suggests that worker cognition may be shaped by natural selection on task performance at the colony level. Forgetting about interactions may be especially costly, because it leads to unpredictable patterns of worker recruitment. We also show that social inhibition, when coupled with excitatory interactions, can effectively modulate worker recruitment at the colony level. Received 9 December 2006; revised 23 May 2007; accepted 30 May 2007.     

台湾乳白蚁行为多型的研究

本文对台湾乳白蚁的打洞挖掘行为、取水行为、修饰行为和交哺行为进行了比较详细的定量分析,探讨了年龄和品级对台湾乳白蚁群体中行为多型的影响。实验结果表明大（老）龄工蚁承担了台湾乳白蚁群体内大多数的任务,对台湾乳白蚁群体的维持具有重要意义。另外品级也是影响台湾乳白蚁群体内劳动分化的一个重要因素。在营养交哺方面,若蚁尚不具备完好的取食行为,是群体中营养交哺的受益者,而兵蚁在台湾乳白蚁群体中占据着较重要的地位,享受着营养物质优先供应的特殊权利。     

Expression of methyl farnesoate epoxidase (mfe) and juvenile hormone esterase (jhe) genes and their relation to social organization in the stingless bee Melipona interrupta (Hymenoptera: Apidae)

Social organization in highly eusocial bees relies upon two important processes: caste differentiation in female larvae, and age polyethism in adult workers. Juvenile Hormone (JH) is a key regulator of both processes. Here we investigated the expression of two genes involved in JH metabolism - mfe (biosynthesis) and jhe (degradation) - in the context of social organization in the stingless bee Melipona interrupta. We found evidence that the expression of mfe and jhe genes is related to changes in JH levels during late larval development, where caste determination occurs. Also, both mfe and jhe were upregulated when workers engage in intranidal tasks, but only jhe expression was downregulated at the transition from nursing to foraging activities. This relation is different than expected, considering recent reports of lower JH levels in foragers than nurses in the closely related species Melipona scutellaris. Our findings suggest that highly eusocial bees have different mechanisms to regulate JH and, thus, to maintain their level of social organization.     

Relationship of queen number and worker size in polygyne colonies of the fire antSolenopsis invicta

Summary We examined the relationship between queen number and worker size in colonies of the fire antSolenopsis invicta. Worker size in monogyne colonies was significantly greater than in polygyne colonies; furthermore, polygyne colonies snowed a strong negative linear relationship between queen number and worker size. Higher queen pheromone level and/or decreased food availability accompanying an increase in queen number likely play important roles in producing the observed patterns.     

Colony size predicts division of labour in attine ants

Division of labour is central to the ecological success of eusocial insects, yet the evolutionary factors driving increases in complexity in division of labour are little known. The size–complexity hypothesis proposes that, as larger colonies evolve, both non-reproductive and reproductive division of labour become more complex as workers and queens act to maximize inclusive fitness. Using a statistically robust phylogenetic comparative analysis of social and environmental traits of species within the ant tribe Attini, we show that colony size is positively related to both non-reproductive (worker size variation) and reproductive (queen–worker dimorphism) division of labour. The results also suggested that colony size acts on non-reproductive and reproductive division of labour in different ways. Environmental factors, including measures of variation in temperature and precipitation, had no significant effects on any division of labour measure or colony size. Overall, these results support the size–complexity hypothesis for the evolution of social complexity and division of labour in eusocial insects. Determining the evolutionary drivers of colony size may help contribute to our understanding of the evolution of social complexity.     

Caste differences in behavioral thresholds as a basis for polyethism during food recruitment in the ant,Pheidole pallidula (Nyl.) (Hymenoptera: Myrmicinae)

During foraging, societies of the polymorphic ant, Pheidole pallidula,display several collective patterns which differ in the ratio of recruited majors. The intensity of behavioral stimuli required to induce this majors' recruitment is determined by studying trail-laying and tactile invitations for the following two food recruitments: (I) the slow and weak recruitment of minors, without majors, to a pile of small, individually retrievable fruit flies and (2) the massive recruitment of both minors and majors to large, unretrievable cockroaches. The selective mobilization of majors only to large prey such as cockroaches is due both to their preferential invitation and to their higher behavioral threshold of response to recruiting stimuli. The experimental evidence of caste behavioral thresholds allow us to reconsider behavioral elasticity in the major caste as well as principles of division of labor in ant societies.     

Worker connectivity: a review of the design of worker communication systems and their effects on task performance in insect societies

Within-group communication is a fundamental feature of animal societies. In order for animal groups to function as adaptive units, the members must share information such that group mates respond appropriately to each others’ behavior. One important function of social communication is to affect the allocation of tasks among group members. Theoretical and empirical findings on a diverse array of social insect taxa show that interactions among workers often play important roles in structuring division of labor. We review worker interactions that regulate division of labor in insect societies, which we refer to as worker connectivity. We present a framework for synthesizing and analyzing the study of worker connectivity. The widespread reliance on worker connectivity among eusocial insect taxa and the diversity of communicative mechanisms used to recruit workers suggest that the nature of worker interactions has evolved by natural selection. We suggest that colony-level selection acting on variation in task allocation has been an important force in the evolution of mechanisms for worker connectivity. We also propose that there are important links between individual worker cognition and task allocation at the colony level. Evolutionary changes in the cognitive aspects of worker responses may affect task allocation as much as changes in the communicative signals themselves. Received 9 December 2006; revised 18 May 2007; accepted 30 May 2007.     

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.     

Polyethism within hunters of the ponerine ant,Ectatomma ruidum Roger (Formicidae,Ponerinae)

Summary During laboratory experiments, two categories of prey-foraging workers were found inEctatomma ruidum: stingers and transporters. When numerous live drosophila were offered to the ants, one group of hunters specialized in killing the prey and another in transporting simultaneously the dead drosophila to the nest. Sometimes, there was a transfer of prey by a stinger towards a transporter, after an active soliciting of the transporter by antennation or by using the forelegs. We found high positive correlations between the colony size and the number of ants in each subcaste. A negative correlation existed between the colony size and the proportion of hunters. However, the proportion of workers in the two behavioral subcastes of hunters was stable in spite of differences in colony size. The phylogenetic interest of this type of cooperative predation is discussed.     

Hard working nurses rather than over-aged nurses permit <Emphasis Type="Italic">Ropalidia marginata</Emphasis> to respond to the loss of young individuals

Summary The primitively eusocial wasp Ropalidia marginata exhibits a remarkably well-developed honey bee like age polyethism. Individuals perform different tasks sequentially as they age, starting with intranidal tasks (nursing and building in that order) and ending with extranidal tasks (foraging for pulp and food in that order). As in the case of honey bees such age polyethism is rather flexible; in the absence of old individuals (in young-cohort colonies), precocious foragers forage at abnormally young ages. Here we demonstrate that the absence of young individuals (in old-cohort colonies) does not result in over-aged nurses nursing at abnormally old ages, as seen in the case of honey bees. Instead it results in hard working nurses who nurse at abnormally high rates. The possible reasons for the absence of over-aged nurses and the presence of hard working nurses are discussed.Received 30 April 2003; revised 18 November 2003; accepted 5 January 2004.     

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

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

Implications of variation in worker body size for the honey bee recruitment system

Honey bee scouts seek food from flowers, return to the colony, and may perform a dance used to recruit dance followers to the flowers. Variation in body size of workers may result in the communication of misinformation because some information acquired by the scout and signaled to recruits is affected by body size. I tested two predictions of this hypothesis. (1) Recruitment communication takes place between bees of similar size despite the withincolony size distribution. (2) There is an inverse relationship between the size variation of foraging honey bees (Apis mellifera)and the rate at which nectar is returned to the colony. A positive relationship was found between the size of a dancer and that of its dance followers, which together comprise a dance group. There was less variation in size within dance groups than among groups. These two factors effectively lower the difference in size between signal sender and signal receiver and may enhance the flow of accurate size-dependent information. Also, an inverse relationship between size variation and rate of nectar intake was detected in each of six colonies using partial correlation analysis. This may be due to communication of misinformation when size variance is higher. The relationship was statistically significant in two colonies and the combined results were significant. The results of the first study suggest the generally weak relationships found in this second study.