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.     

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.     

Subcaste‐specific evolution of head size in the ant genus Pheidole

An organism's morphology is constrained by its evolutionary history and the need to meet a variety of potentially competing functions. The ant genus Pheidole is the most species‐rich ant genus and almost every species has a dimorphic worker caste (a few are trimorphic). This separation of workers into two developmentally distinct subcastes (smaller minors and larger majors with distinctively large heads) may partially release individuals from functional constraints on morphology, making Pheidole an ideal genus for addressing questions on the evolution of morphology in relation to ecological specialization. Major workers can perform a variety of tasks, although they are usually specialized for defence, as well as food retrieval and processing. Pheidole species vary in their diet, although many species gather seeds. The major workers mill the seeds using large jaws powered by mandible closer muscles that occupy a large proportion of the head cavity. In the present study, we examined the relationship between seed‐harvesting and morphology in Pheidole, hypothesizing that majors of seed‐harvesting species would have larger heads relative to non‐seed‐harvesters to accommodate the powerful mandibular muscles needed to mill seeds. By taking a phylogenetically controlled comparative approach, we found that majors of seed‐associated Pheidole did not have larger heads (width and length) than majors of non‐seed‐harvesting species. However, the head length of minors (and to a lesser extent head width) was smaller in seed‐harvesters. Additionally, we found the difference in head size between majors and minors was greater in seed‐harvesting species. These morphological differences in diet, however, were not related to changes in the rate of evolution in either seed‐harvesting or non‐seed‐harvesting lineages. These findings suggest that the morphologies of worker subcastes can evolve independently of each other, allowing colonies with polymorphic workers to specialize on new resources or tasks in ways not possible in monomorphic species.     

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.     

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.     

Spatial organization in a dimorphic ant: caste specificity of clustering patterns and area marking

Living in groups constitutes the root of social organizationin animals. Likewise, the spatial aggregation between membersof insects societies plays a crucial role in social cohesionand division of labor, namely, in polymorphic ant species. Inthe present paper, we show caste-specific aggregation patternsin the strictly dimorphic Pheidole pallidula ant species. Weinvestigate the influence on the clustering of ants exertedby direct contacts between nest mates as well as by indirectcues through chemical marking. In a homogeneous environmentdeprived of chemical cues, majors show a higher aggregationlevel than minors and a centripetal behavior. By contrast, minorsare more scattered in the experimental arena and display a centrifugalbehavior. In addition, area marking laid by minors enhancestheir own aggregative behavior while contributing to the localizationof the spontaneously aggregating majors. Such differences inaggregative patterns as well as their adaptive value have tobe coupled with the mobility level and the task performanceefficiency of each worker caste. Contrary to majors that arelikely to aggregate, highly mobile minors, scattered insideand outside the nest colony, can detect colony needs and cancarry out most of the daily tasks for which they are more efficientthan majors.     

The caste system of the dolichoderine antTechnomyrmex albipes (Hymenoptera: Formicidae): morphological description of queens,workers and reproductively active intercastes

Females ofTechnomyrmex albipes consist of winged queens, intercastes and workers. In established colonies, reproduction is performed by many intercastes (wingless females which have intermediate phenotypes between queen and worker characters). Dissection and morphological examination revealed that intercastes had a spermatheca, but workers did not. Intercastes can be divided into three classes: major intercastes with three ocelli, medium intercastes with one ocellus, and minor intercases without ocelli. Workers have no ocelli. The thoracic structure of intercastes gradually becomes more complex from minors to majors. The body size of intercastes gradually increases from minors to majors, and so does the number of overioles. The body size distributions of minor intercastes and workers overlap considerably, but the distributions of ovariole numbers overlap less. Winged queens had distinctly larger body sizes, more ovarioles and larger spermathecae than intercastes. Most intercastes were inseminated with developed ovaries and appeared to reproduce. The caste system and reproductive division of labour inT. albipes is compared to those of ant species in which permanently wingless females reproduce.     

Castes in humivorous and litter-dwelling neotropical nasute termites (Isoptera,Termitidae)

Summary The developmental pathways of the neuter castes were studied in three species of Nasutitermitinae from central Panama. The humivorousSubulitermes denisae andCoatitermes clevelandi display several primitive traits: absence of sex dimorphism, representation of both sexes among workers and soldiers, and occurrence of successive worker instars. The litter-dwellingVelocitermes barrocoloradensis has a more complex caste system: female larvae are larger than males and give rise to the large workers, which constitute the bulk of the work force; male larvae proceed to soldiers through a small worker or a special larval instar. The resulting soldier caste is polymorphic. These results support previously formulated hypotheses regarding a link between humivorous diet and reduced polymorphism on the one hand, and between forest-floor foraging and large continuous size variation among soldiers on the other. Whereas the caste systems ofSubulitermes andCoatitermes probably represent a primitive condition,Velocitermes shares derived traits withNasutitermes and the other fully nasute genera previously studied. I therefore hypothesize that ancestors with these advanced features may have spread from the neotropics and be at the origin of most nasute genera, including humivorous taxa, present in other regions.     

Differential resistance and the importance of antibiotic production in Acromyrmex echinatior leaf-cutting ant castes towards the entomopathogenic fungus Aspergillus nomius

Paired exocrine metapleural glands are present in almost all ants and produce compounds with antibiotic properties towards a variety of pathogenic fungi and bacteria. In Acromyrmex leaf-cutting ants, small workers have relatively large metapleural glands compared to large workers, and thus harbour approximately half the number of gland cells of large workers, despite being only one-fifteenth their body mass. Here we present results showing that when the two worker castes of A. echinatior are treated with spores of the pathogenic fungus Aspergillus nomius in doses that correspond to the difference in metapleural gland cell numbers they do not differ in survival. However, we also show, for the first time, that small workers survive significantly longer than large workers when both are challenged with a dose of spores that corresponds to their difference in body mass. Furthermore, the time until Aspergillus nomius hyphae and spores appear on the cadavers of workers dead from infection, is significantly increased in the small worker caste. In addition to supporting previous findings that the metapleural glands have an important defence function, the results of this study indicate that the relatively large glands in small workers makes this caste particularly well adapted to preventing pathogenic microorganisms from entering the colony. Received 23 January 2006; revised 7 April 2006; accepted 11 April 2006.     

Behavioral Response of Two Subterranean Termites (Isoptera: Rhinotermitidae) to Vibrational Stimuli

Behavioral responses to vibrational stimuli were examined in two subterranean termite species, Coptotermes formosanus Shiraki and Reticulitermes flavipes (Kollar). Termites habituated to vibrational stimulation when in social groups, but failed to do so individually, indicating that habituation is a collective action. In assays on termite groups, both species demonstrated a similar pattern of behavioral responses to vibrational stimuli: evanescent cessation of activity and movement, followed by withdrawal from the vibration source. Groups of both species then gradually moved back toward the vibration source as a consequence of continuous exposure. However, it took a significantly shorter period for C. formosanus to return (57 s) to the test arena and to resume (80 s) normal foraging activities in the test arena compared with R. flavipes, which took 97 and 227 s, respectively, when exposed to the vibration frequency of 120 bmpm. High vibration frequency (240 bmpm) increased the time required to return (C. formosanus, 80 s; R. flavipes, 153 s) and to resume regular locomotion (C. formosanus, 186 s; R. flavipes, 263 s). Our experiments demonstrate that workers play a crucial part in adjusting groups of termites to distressful vibrations. Soldiers of R. flavipes demonstrated similar behavioral responses as workers, however, C. formosanus soldiers exhibited a transient positive response before withdrawal. An erratum to this article is available at .     

Colony demographics of rare soldier-polymorphic worker caste systems in <Emphasis Type="Italic">Pheidole</Emphasis> ants (Hymenoptera,Formicidae)

Nearly all species in the ant genus Pheidole have dimorphic workers, with distinct small minors and larger soldiers. The size range of both castes is typically narrow. Just seven described species are soldier-polymorphic, with a broad soldier size range. Here, we characterize worker caste allocation and demography in the soldier-polymorphic P. obtusospinosa, P. rhea, and P. tepicana, and the dimorphic P. spadonia for comparison. The head allometry of soldiers in soldier-polymorphic species is strongly positive and that of dimorphic species is negative. Among soldier-polymorphic species, the soldier castes differ from each other in the degree of positive allometry. In addition, they differ in the number of size modes: P. obtusospinosa and P. rhea have two and P. tepicana has one. During colony ontogeny, P. obtusospinosa first has one mode and develops the second mode much later, while P. rhea produces multiple modes throughout. We also characterize worker caste systems based on the biomass allocation. For all three soldier-polymorphic species, the majority of soldiers are small soldiers. Pheidole obtusospinosa and P. rhea allocate roughly equal biomass to the two soldier classes, while P. tepicana allocates little to supersoldiers based on both biomass and caste ratio. These findings illustrate the interplay among caste ratios, biomass allocation, size frequency distributions, and allometry in the evolution of different worker caste systems. We conclude that soldier-polymorphic species may have evolved convergently in response to broad-scale factors, but differences among them suggest selection pressures in small-scale environments have been different.     

Dimorphic Males Display Alternative Reproductive Strategies in the Marine Amphipod Jassa marmorata Holmes (Corophioidea: Ischyroceridae)

Discrete dimorphism of males within a species is often the result of selection for alternative reproductive strategies, and these strategies may be evolutionarily stable (ESS). An ESS may be either mixed (genetically fixed differences) or conditional (flexible differences related to varying environmental conditions) (PARKER 1984). Mature males of the marine amphipod Jassa marmorata are dimorphic. Large ‘major’ males have a greatly enlarged thumb (propodus) on their 2nd gnathopods, while small ‘minor’ males exhibit thumbs that are reduced, and CONIAN (1989) suggested that minors may exhibit a different mating strategy from majors. Ratios of males and females fluctuate seasonally (FRANZ 1989) and female body size is inversely correlated with temperature (FRANZ 1989) so male dimorphism could be a flexible response to varying environmental conditions. I sampled a natural population of J. marmorata over a 1-yr period, quantified major and minor morphology, and measured male behaviour and mating success in experimental arenas that contained varying proportions of male morphs and females. Morphology of the two morphs is discrete; female body size varies with season with significantly smaller individuals in summer and fall; body size predicts morph type; and ratios of majors, minors and females fluctuate seasonally. Finally I showed that majors and minors use different mating tactics to gain access to receptive females, and that these behaviours depend on the male's own morphology and on the environmental setting that it finds itself in. Major males fight, display and attempt to evict other males to mate with receptive females. Minors never fight with, display to or attempt to evict majors, but they infrequently display to and attempt to evict other minor males. Furthermore, mating success of the two morphs was not significantly different and may depend on whether males are with a majority or minority of their own type. These data support the conclusion that major and minor male J. marmorata display evolutionarily stable alternative reproductive strategics, but more work should address the nature of this ESS.     

Genetic Variation in Behavioral Response to Herbivore-Infested Plants in the Parasitic Wasp, Cotesia glomerata (L.) (Hymenoptera: Braconidae)

Female Cotesia glomerata (L.) relies on stimuli from herbivore-infested plants to select suitable hosts, but behavioral response to such stimuli is highly variable among individuals. This study investigates a genetic component of phenotypic variability in both short-range host-search and long-range host-location behaviors in the tritrophic system consisting of cabbage plants (Brassica oleracea L.), cabbage butterfly (Pieris brassicae L.) and the parasitoid, by comparing full-sib families established from a laboratory population and isofemale strains from a field population. Short-range host-search behaviors were examined within a Petri-dish test arena, and long-range host-location behaviors assessed in a wind tunnel. Significant differences among full-sib families were shown in the duration of walking on a plant-herbivore complex (i.e., a leaf section with two host caterpillars, their silk and feces) and searching off the complex, and the total time elapsed for wasps to locate a host larva after release into the test arena. Flight responses to and landing choices between the intact and the herbivore-infested plants were also significantly different among these families. Effects of families on both short-range host-search and long-range host-location behaviors were consistent, without significant influences of host larvae from which wasps emerged. The analysis of isofemale strains reveals that strains account for significant variation in the oriented flight response to herbivore-infested plants, and the isofemale heritability for this behavioral character is estimated as 0.447. The results suggest that genetic variation exists at different behavioral levels of the host-selection process in this parasitoid.     

Discrimination by male ring-tailed Lemurs (Lemur catta) between the scent marks of male and those of female conspecifics

Olfaction plays an important role in the social communication of all prosimians. (The experiment reported in this paper forms part of an intensive chemobehavioral study of olfaction in Lemur catta (ring-tailed lemur) being carried out in this laboratory.) Five male Lemur cattawere tested on their behavioral responses to paired scent stimuli. Responses measured were (1) total investigation time, (2) arm-marking, (3) ABO/BO rubbing, and (4) flehmen. Males showed a strong discrimination between the scent stimuli,giving higher levels of response to female scent on measures 1, 3, and 4. This response suggests an olfactory-related preference by males for female scent under controlled conditions. This preference may be a consequence of the females’ dominance over males and the brevity of estrus in L. catta,both of which would favor such choice behavior.     

Reactions to Potential Predators in Captive-Born Marmosets (Callithrix penicillata)

We describe the behavioral repertoire of captive-born black tufted-eared marmosets (Callithrix penicillata) elicited by brief exposures to three potential mounted taxidermized predators (caracara hawk, Polyborus plancus; rattlesnake, Crotalus durissus; oncilla, Leopardus tigrina), and a stuffed toy. For each of the four stimuli, we submitted the subjects to a 9-min trial divided into three consecutive intervals: a 4-min pre-exposure baseline observation, a 1-min stimulus exposure, and a 4-min postexposure observation period. We positioned stimuli in front of each subject's home cage, and video-taped trials for behavioral analysis. During exposures to the potential taxidermized predators, we heard tsik-tsik vocalization and alarm behavior. After exposures, only the cat induced these reactions. All stimuli elicited observational reaction, albeit only during exposure intervals. Further comparisons between the three trial intervals indicated a decrease in the time spent in proximity to the cat during exposures, while an increase in proximity occurred when subjects were exposed to either the hawk or snake for the same period. Taken together, the behavioral responses during and after exposures to the taxidermized oncilla suggest that this stimulus is capable of inducing strong and persistent emotional reactions in Callithrix penicillata.     

Inter- and intraspecific alarm response in the termiteMacrotermes subhyalinus (Rambur)

Summary In laboratory studies we analysed the reaction of aMacrotermes subhyalinus (Rambur) colony in contact with a small group of test termites placed at the head of a gallery system. As test termites we usedMacrotermes subhyalinus of the same nest,Macrotermes subhyalinus of a foreign nest and the related sympatric speciesMacrotermes bellicosus (Smeathman).Contact with an introduced group of foragers evokes a significant recruitment of workers and minor soldiers. The intensity of the response depends on the test groups we introduced. The highest recruitment results withM. bellicosus andM. subhyalinus from a foreign nest. During the recruitment the increase in the number of minor soldiers is greater than the increase in the number of workers, and as a result, the relative proportion of the soldiers in the population leaving the nest is significantly higher than before.Direct observations at the place of contact show thatM. bellicosus individuals are immediately attacked and killed. In the case ofM. subhyalinus from a foreign nest, not all individuals are killed; some are only intensively examined with the antennae or seized with the mandibles.M. subhyalinus from the same nest are rarely attacked. Mostly they are antennally inspected or enter the gallery system unhindered.The recruitment of termites from the nest is released by major workers which, after contact with the test termites, run back into the nest and provoke an alarm by tactile stimuli. On their way back into the nest the major workers, in all probability, lay a pheromone trail which leads the outgoing alarmed termites towards the place of contact.     

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.     

The Nesting Behavior of Dawson's Burrowing Bee, Amegilla dawsoni (Hymenoptera: Anthophorini), and the Production of Offspring of Different Sizes

Females of Dawson's burrowing bees have a well-defined brood cell cycle involving cell construction, waxing, provisioning, egg laying, and cell capping. In one study population, nesting bees built smaller brood cells for offspring of lower weight and larger ones for heavier offspring, demonstrating their ability to anticipate the desired size of an offspring at the outset of a brood cell cycle. Furthermore, individual females varied the number of provisioning trips made per brood cell cycle by a factor of two or more, apparently exercising control over the amount of brood provisions supplied to an offspring. The size distribution of emerging males at two widely separated locations in 1997 was nearly identical to that recorded in 1995. These findings suggest that the production of small males (minors) is the result of active female control rather than the product of food shortages that force females to undersupply some brood cells. Female foraging decisions resulted in a bimodal distribution of weights of mature dormant larvae at one site in 1997. However, the times required to complete brood cell cycles at this site were not distributed bimodally. This result stemmed in part from daily variation among females in the duration of their provisioning trips as well as from seasonal variation in provisioning trip times. When provisioning trips lasted longer, females tended to make fewer trips per brood cell cycle, and so were presumably more likely to produce minor sons. As a result, the weight of an offspring was not tightly linked to the time investment required to produce it, making it difficult to compare the relative costs of minors and majors in terms of maternal time investments.