How experienced individuals contribute to an improvement in collective performance in ants

Certain groups of organisms are capable of improving their collective performance with experience. In a recent study, we demonstrated that, over successive emigrations, colonies of the ant Temnothorax albipennis are able to improve their collective performance by reducing the time taken to complete an emigration (Langridge et al., Behav Ecol Sociobiol 56:523–529, 2004). In this paper, by recording the performance of individually marked workers during repeated emigrations, we were able to analyse some of the ways in which time gains are achieved. We found that: (1) those transporters that also transported in the preceding emigration began to transport earlier in the current emigration and, in the majority of emigrations, transported more items than those transporters that had not transported in the preceding emigration; (2) the time that elapsed before the first item was transported into the new nest reduced over successive emigrations, and this first item was, in the majority of emigrations, carried by a transporter that had also transported in the preceding emigration; (3) the number of adults that were transported reduced over successive emigrations. Our results strongly suggest that the behaviour of transporters that also transported in a preceding emigration may be modified as a result of their experience and that, consequently, their efforts in the next emigration make a major contribution to the improved performance of the colony as a whole.     

A South East Asian ponerine ant of the genus Leptogenys (Hym., Form.) with army ant life habits

Summary The emigration and raiding behavior of the SE Asian ponerine ant Leptogenys sp. 1, which resembles L. mutabilis, were observed in the field (Ulu Gombak, Malaysia). The ants formed monogynous colonies that consisted of up to 52 100 workers. The bivouac sites of this species were found in leaf litter, rotten logs, ground cavities, etc., and were rarely modified by the ants. The colonies stayed in these temporary nests for several hours to 10 days; afterwards, they moved to a new nest site. The emigration distances ranged from 5–58 m. Since nest changing takes place at irregular intervals, and pupae and larvae are always present in the nest relocations of Leptogenys sp. 1, the emigration behavior is not linked to a synchronized brood development. Leptogenys sp. 1 is a nocturnal forager; in our study, up to 42 600 workers participated in each raid. The ants move forward on a broad front; behind the swarm a fan-shaped network of foraging columns converges to form a main trunk trail. A new system of foraging trails is developed in each raid. The workers search for their prey collectively; they attack and retrieve the booty together. The diet of Leptogenys sp. 1 consists mainly of arthropods. Army ant behavior is characterized by (1) formation of large monogynous colonies, (2) frequent emigrations, and (3) mass raids in which all foraging activities are carried out collectively. Since Leptogenys sp. 1 performs these typical army ant behavior patterns, this species represents the army ant ecotype. However, this species differs considerably from army ant species that have synchronized broods and huge colonies with dichthadiiform queens.Dedicated to Professor Dr. M. Lindauer on the occasion of his 70th birthday     

Why do not all workers work? Colony size and workload during emigrations in the ant Temnothorax albipennis

Here, we study distribution of workload and its relationship to colony size among worker ants of Temnothorax albipennis, in the context of colony emigrations. We find that one major aspect of workload, number of items transported by each worker, was more evenly distributed in larger colonies. By contrast, in small colonies, a small number of individuals perform most of the work in this task (in one colony, a single ant transported 57% of all items moved in the emigration). Transporters in small colonies carried more items to the new nest per individual and achieved a higher overall efficiency in transport (more items moved per transporter and unit time). Our results suggest that small colonies may be extremely dependent on a few key individuals. In studying colony organisation and division of labour, the amount of work performed by each individual, not just task repertoire (which tasks are performed at all), should be taken into account.     

Division of labour in a crisis: task allocation during colony emigration in the ant Leptothorax unifasciatus (Latr.)

Division of labour during colony emigration is widespread in ants. An important problem is how tasks are allocated during colony movement from one nest site to another. The generally favoured view is that emigrations are organised by a minority group of individuals, which either work unusually hard at tasks (elites) or have the exclusive task of carrying out the emigration (moving specialists). Five consecutive emigrations of a Leptothorax unifasciatus (Latr.) colony showed that the number of transporters, i.e. the individuals that took an active part in the emigration by transporting brood and ants, was smaller than it would have been if allocation of this task was random during each emigration. However, single emigrations of another three colonies, for which the spatial distribution and behaviour of the workers had been observed for a week prior to the emigration, demonstrated that the transporters did not form a homogeneous group. They differed in their spatial positions and tasks before the emigration. There was also no evidence that transporters worked harder or less hard than their nestmates before the emigration. Therefore, the individuals which carry out emigrations in L. unifasciatus colonies appear to be neither moving specialists nor elites. We propose that task allocation during emigrations of L. unifasciatus colonies is based on a feedback mechanism that involves learning.     

Predatory behavior and prey selection by army ants in a desert-grassland habitat

Summary Colonies of Neivamyrmex nigrescens conduct extensive nocturnal raids on other ants and termites in the desert-grassland of Arizona-New Mexico. We collected quantitative data on several aspects of raiding to pinpoint differences due to colony size and behavioral phase. In the nomadic phase, colonies began raiding at sunset and continued until dawn. Larger colonies covered more area, discovered more prey sites, and collected more booty than smaller colonies, but there were no systematic changes in raid intensity over the course of the nomadic phase. In the statary phase, raiding occurred less frequently and was less intense when it occurred; however, at the end of this phase, raiding was similar to nomadic phase raids in extent, duration, and booty captured. N. nigrescens preyed exclusively on termites and ants, and appeared to select certain species of Pheidole in preference to other ants. Pheidole was the most abundant genus, but was preyed upon twice as often as expected based on relative colony density. Pheidole attempted to avoid predation by fleeing or defending their nest, but rarely succeeded. Because they are about the same size as army ants and lack defensive chemicals, Pheidole made comparatively easy prey. N. nigrescens ignored or was repelled by other ants (Pogonomyrmex, Novomessor, Iridomyrmex, Myrmecocystus) during the early summer, when Pheidole was abundant; however, in late summer when Pheidole was less available, the army ants preyed upon Novomessor cockerelli. N. harrisi raided in close proximity to N. nigrescens, but preyed exclusively on Solenopsis xyloni. Selection of prey and partitioning of resources are now indicated in several army ant species; these processes have probably been important factors in the evolution of the ants' predatory behavior.     

Predatory behavior and chemical communication in two Metapone species (Hymenoptera:Formicidae)

Summary. Colonies of two species of Metapone (M. madagascarica, M. new species.) were collected in Madagascar and established in laboratory nests. It could be demonstrated that both species are specialist predators of termites (Cryptotermes kirbyi). During hunting the ants sting the termites and thereby paralyze and preserve the prey alive. In this way prey can be stored in the ant nest for extended periods. During foraging and colony emigrations the ants lay chemical trails with poison gland secretions. Among the seven compounds identified in the venom only methyl pyrrole-2-carboxylate elicits trail following behavior in both Metapone species. Received 11 February 2002, accepted 23 February 2002.     

High social density increases foraging and scouting rates and induces polydomy in Temnothorax ants

Many organisms live in crowded groups where social density affects behavior and fitness. Social insects inhabit nests that contain many individuals where physical interactions facilitate information flow and organize collective behaviors such as foraging, colony defense, and nest emigration. Changes in nest space and intranidal crowding can alter social interactions and affect worker behavior. Here, I examined the effects of social density on foraging, scouting, and polydomy behavior in ant colonies—using the species Temnothorax rugatulus. First, I analyzed field colonies and determined that nest area scaled isometrically with colony mass—this indicates that nest area changes proportionally with colony size and suggests that ants actively control intranidal density. Second, laboratory experiments showed that colonies maintained under crowded conditions had greater foraging and scouting activities compared to the same colonies maintained at a lower density. Moreover, crowded colonies were significantly more likely to become polydomous. Polydomous colonies divided evenly based on mass between two nests but distributed fewer, heavier workers and brood to the new nests. Polydomous colonies also showed different foraging and scouting rates compared to the same colonies under monodomous conditions. Combined, the results indicate that social density is an important colony phenotype that affects individual and collective behavior in ants. I discuss the function of social density in affecting communication and the organization of labor in social insects and hypothesize that the collective management of social density is a group level adaptation in social insects.     

Improvement in collective performance with experience in ants

We show that entire ant colonies can improve their collective performance progressively when they repeat the same process. Colonies of Leptothorax albipennis can reduce their total emigration times over successive emigrations. We show that this improvement is based on experience and some memory-like process, rather than a coincidental developmental change or an increased general level of arousal. We demonstrate that the benefits of experience can be lost (i.e. forgotten) if the interval between successive emigrations is too long. We also show that the benefits of experience are more likely to be retained over a longer period if the collective performance has been repeated several times. This is a new demonstration of a process akin to learning in ants and we briefly discuss how it may involve not only improvements in individual performance but also improvements in the ways in which the ants interact with one another.Communicated by L. Sundström     

Increased grooming after repeated brood care provides sanitary benefits in a clonal ant

Repeated pathogen exposure is a common threat in colonies of social insects, posing selection pressures on colony members to respond with improved disease-defense performance. We here tested whether experience gained by repeated tending of low-level fungus-exposed (Metarhizium robertsii) larvae may alter the performance of sanitary brood care in the clonal ant, Platythyrea punctata. We trained ants individually over nine consecutive trials to either sham-treated or fungus-exposed larvae. We then compared the larval grooming behavior of naive and trained ants and measured how effectively they removed infectious fungal conidiospores from the fungus-exposed larvae. We found that the ants changed the duration of larval grooming in response to both, larval treatment and their level of experience: (1) sham-treated larvae received longer grooming than the fungus-exposed larvae and (2) trained ants performed less self-grooming but longer larval grooming than naive ants, which was true for both, ants trained to fungus-exposed and also to sham-treated larvae. Ants that groomed the fungus-exposed larvae for longer periods removed a higher number of fungal conidiospores from the surface of the fungus-exposed larvae. As experienced ants performed longer larval grooming, they were more effective in fungal removal, thus making them better caretakers under pathogen attack of the colony. By studying this clonal ant, we can thus conclude that even in the absence of genetic variation between colony members, differences in experience levels of brood care may affect performance of sanitary brood care in social insects.     

Contrasting foraging tactics by northern gannets (<Emphasis Type="Italic">Sula bassana</Emphasis>) breeding in different oceanographic domains with different prey fields

In order to forage and to provision offspring effectively, seabirds negotiate a complex of behavioural, energetic, environmental and social constraints. In first tests of GPS loggers with seabirds in North America, we investigated the foraging tactics of free-ranging northern gannets (Sula bassana) at a large and a medium-sized colony that differed in oceanography, coastal position and prey fields. Gannets at Low Arctic colony (Funk Island) 50 km off the northeast coast of Newfoundland, Canada provisioned chicks almost entirely with small forage fish (capelin Mallotus villosus, 89%), while at boreal colony (Bonaventure Island) 3 km from shore in the Gulf of St. Lawrence, Quebec, Canada, large pelagic fish dominated parental prey loads (Atlantic mackerel Scomber scombrus 50%, Atlantic herring Clupea harengus 33%). Mean foraging range and the total distance travelled per foraging trip were significantly greater at the larger inshore colony (Bonaventure) than at the smaller offshore colony (Funk Island; 138 and 452 km vs. 64 and 196 km, respectively). Gannets from Funk Island consistently travelled inshore to forage on reproductive capelin shoals near the coast, whereas foraging flights of birds from Bonaventure were much more variable in direction and destination. Birds from the Low Arctic colony foraged in colder sea surface water than did birds from the boreal colony, and dive characteristics differed between colonies, which is concordent with the difference in prey base. Differences between the colonies reflect oceanographic and colony-size influences on prey fields that shape individual foraging tactics and in turn generate higher level colony-specific foraging “strategies”.     

Effects of prey quantity on predatory wasps (<Emphasis Type="Italic">Polistes dominulus</Emphasis>) when patch quality differs

To determine the effects of prey quantity on central-place foraging of predatory wasps ( Polistes dominulus), prey of varying quality were distributed in patches. A field experiment was conducted, which controlled the amount and quality of prey available. 'Low-fed' colonies were provided with one-third the quantity of prey as that of 'high-fed' colonies. Both were provided with a 1:1 ratio of palatable:unpalatable prey. Experienced wasps of the high-fed colonies never selected unpalatable prey as their first choice of the day, whereas those of the low-fed colonies selected unpalatable prey as their first choice about 1:4 times. In general, wasps from the high-fed colonies reduced palatable patches to zero prey before exploiting unpalatable patches. Foundresses of high-fed colonies captured disproportionately more palatable prey than those of low-fed nests, but there was no correlation between ratio of palatable to unpalatable prey taken and the number of offspring produced. Wasps from low-fed colonies attacked unpalatable prey sooner, but not without considerable effort to avoid use of those patches. Foundresses of low-fed colonies also spent a greater proportion of time overall in unsuccessful search, which may explain why only wasps from low-fed colonies foraged on cool days. High-fed colonies produced more cells and more and heavier offspring than low-fed colonies. But the productivity of the low-fed colonies was greater than that of the 'natural' colonies, which had to find their own prey in a field-woodland area. These results indicate that prey scarcity changes foraging behavior and affects prey choice. These changes may not totally alleviate negative effects of unpalatable prey on colony development and offspring production. The results of this study increase understanding of the central-place foraging behavior of paper wasps, which are important biocontrol agents in natural and agricultural settings.     

Larval hemolymph feeding: a nondestructive parental cannibalism in the primitive ant Amblyopone silvestrii Wheeler (Hymenoptera: Formicidae)

Summary The queens of larger colonies of the primitive ant Amblyopone silvestrii are exclusively dependent on the hemolymph of their own larvae as a nutrient, even when prey feeding is possible. On the other hand, the foundresses suppress larval hemolymph feeding (LHF) when prey is available, allowing them to rear the first workers more swiftly. The nondestructive form of cannibalism can be regarded as a nutritive adaptation related to: (1) the lack of social food transfer in this species, and (2) its specialized predation on large sporadic prey (centipedes). LHF similar to that in Amblyopone was found in Proceratium and another type of LHF, with a larval specialized exudatory organ, in Leptanilla.     

Habitat structure helps guide the emergence of colony-level personality in social spiders

Studying the environmental factors that guide the emergence of collective behaviors is instrumental to understanding the ecology and evolution of animal societies. Although recent work has provided insights into the demographic factors that influence inter-colony variation in collective behavior (i.e., colony-level personality or collective personality), relatively few studies have investigated how the physical environment (e.g., habitat structure) affects colony-level personality. Here, we study the emergence of collective personality in prey capture behavior in the social spider, Stegodyphus dumicola. We measured collective prey capture behavior four times over 36 days in a classic repeated measures design. We used four different artificial habitat (web support) structures in three different treatments: habitat structure was either (1) fixed and undisturbed, (2) disturbed with a complete removal of webbing between each measurement, or (3) disturbed with changes of habitat structure between each measurement. Our results revealed that repeatability in colony-level personality was retained as long as habitat structure was not altered. However, the repeatability of colony-level personality declined precipitously when groups were forced to build their webs on novel habitat structures. Furthermore, habitat structure affected collective capture behavior, that is, latency to attack and the number of attackers differed among colonies on different habitat structures. Collectively, our data demonstrate that habitat structure is instrumental in shaping both the mean and repeatability of the collective behavior of colonies and may influence overall foraging success.     

How colony growth affects forager intrusion between neighboring harvester ant colonies

Summary Colonies of the harvester ant, Pogonomyrmex barbatus, adjust the direction and length of foraging trails in response to the foraging behavior of their conspecific neighbors. In the absence of any interaction with its neighbor, a mature colony expands its foraging range at a rate of 0.85 ± 0.15 m per day. Exclusion experiments show that if a colony is prevented from using its foraging trails, the neighbors of that colony will enter its foraging range within 10 days. Exclusion experiments were performed with three age classes of colonies: young (1 year old), intermediate (3–4 years old), and old (5 years old or more). Colonies 3–4 years old are most likely to expand foraging ranges, and to retain newly-gained areas. To examine the relation of colony age (in years) and colony size (in numbers of workers), colonies of known age were excavated. Colonies increase greatly in size in years 3 and 4. Foraging area may be of greater current or prospective value for younger, smaller, quickly growing colonies than for older, larger ones of stable size. Correspondence to the second address     

Locomotion,feeding, grooming and the behavioural responses to gravity,light and hydrostatic pressure in the stage I zoea larvae of Ebalia tuberosa (Crustacea: Decapoda: Leucosiidae)

The stage I zoeae of Ebalia tuberosa swam by sculling with the exopodites of the 1st and 2nd maxillipeds and flexed the abdomen to brake or change direction. The larvae gained depth by stopping all natatory movements and sinking passively at rates of 6 mm s^-1. The zoeae refused both living and dead nauplii of Artemia spp., as well as two species of diatoms, but fed readily on detritic material on the bottom which they scooped up using the endopodites of the maxillipeds and pressed against the mouthparts using the telson. The setae on the posterior border of the telson were used for grooming the maxillipeds and the anterior mouthparts. Day-old stage I zoeae were negatively geotactic, positively phototactic and responded to pressure increases by swimming upwards and by high barokinesis. By the third day some larvae had become positively geotactic but were photopositive, and the majority responded to pressure increases as in the day-old larvae. Five-day old larvae were still photopositive but the majority had become positively geotactic and fewer himbers responded to pressure. Seven-day old larvae failed to respond to any of the stimuli used and assumed a predominantly benthic lifestyle. It is suggested that this anomalous behaviour is related to the dispersal of the larvae and to the specialized habitat requirements of the adults while the rather unusual morphology of the larvae is related to their feeding behaviour and semi-benthic lifestyle.     

Larval behavior of predacious sister-species: orientation,molting site,and survival in Chrysopa

Field and laboratory studies compared two features of larval behavior in a pair of predacious sisterspecies of green lacewings: one (Chrysopa slossonae) a specialist on a single species of colonial aphids (the woolly alder aphid) that occur on branches and trunks of alder trees, the other (C. quadripunctata) a general aphid feeder whose primary prey is dispersed on foliage of diverse types of trees. First, a few hours after hatching, larvae of the two species develop significantly different phototactic responses; the differences correspond well with the spatial distributions of their prey. Most C. slossonae exhibited negative phototaxis, a response that helps move hatchlings inward on alder trees toward the woolly alder aphid colonies, whereas most C. quadripunctata hatchlings showed positive orientation to light, a response that tends to keep them in tree canopies with their prey. Second, in greenhouse experiments, a significantly greater proportion of C. slossonae larvae (second instars) molted within woolly alder aphid colonies and remained with the aphids than did C. quadripunctata larvae. These differences indicate that the specialist larvae have evolved a high degree of behavioral fidelity to their prey. However, larvae (second instars) of the two species that were released near ant-tended woolly alder aphid colonies in the field had similar recovery (= survival) rates. Consequently, natural selection may not act on behavioral traits that influence larval fidelity to prey during the late second and early third instars.     

Macro-habitat patch structure,environmental harshness,and Marmota flaviventris

Summary Social behavior of 15 colonies of Yellow-bellied Marmots was studied at sites differing in both elevation (plant growing season length) and patch structure (density and spacing of suitable colony sites in large blocks of habitat) from 1976 through 1978. Colonies were typically composed of family groups. In low elevation colonies, offspring dispersed at the end of the juvenile year. In all high elevation areas, offspring dispersed as yearlings, and parent-offspring and sibling interactions during the juvenile year were highly amicable. In high elevation areas in continuous habitat, dispersal by yearlings occurred with no agonistic interactions. In high elevation areas with patchy habitat, however, brief but intense periods of extremely agonistic sibling and parent-offspring interactions occurred immediately prior to dispersal of yearlings.Social behavior and dispersal are uncorrelated with elevation, plant growing season length, or available foraging time. Social interaction among relatives in marmot colonies is amicable when other suitable colony sites are located nearby; in these areas, dispersal occurs without any antagonism. Agonistic behavior occurs only in areas with a patchy distribution of suitable colony sites. At these sites, dispersal occurs only after periods of extreme antagonism. Further, those offspring who do not disperse from the colony site are the ones who dominate and initiate agonistic interactions with their siblings. The large-scale structure of the habitat influences the propensity of the individual to disperse. When the animal is reluctant to disperse because of a combination of high transit difficulty and low desirability of a new site, angonistic social interactions with relatives force dispersal.     

Behavioral ecology of the neotropical termite-hunting ant Pachycondyla (= Termitopone) marginata: colony founding,group-raiding and migratory patterns

This study provides the first detailed field account of colony founding, group-raiding and migratory habits in the neotropical termite-hunting ant rPachycondyla marginata, in a semi-deciduous forest in south-east Brazil. New colonies can originate by haplometrosis, pleometrosis, or colony fission. Incipient colonies with multiple foundresses persisted longer in the field, and most excavated nests contained more than one dealated female. A total of 202 group raids by P. marginata were registered, and in all cases the raided termite species was Neocapritermes opacus. Nearly 20% of the workers within a colony engage with raiding activity. Colonies of P. marginata hunt for termites approximately every 2–3 weeks, and group-raids may last for more than 24 h. Target termite nests are up to 38 m from the ant colony, and occasionally two nests are simultaneously raided by one ant colony. Raiding ants carry 1 or 2 paralysed prey, and nearly 1600 termites can be captured during a 9-h raid. Migration by P. marginata colonies lasted over 2 days and covered distances of 2-97 m (n = 48). Average residence time at a given location was 150 days. Three basic migratory patterns were noted: colony fission (only part of the colony moves), long-distance migrations, and short-distance migrations. Both raiding and migratory activities appeared to be strongly affected by seasonal factors. The group raiding and migratory patterns of P. marginata are compared with other ant taxa with similar habits. It is concluded that P. marginata presents a rudimentary form of the so-called army ant behavior, which is highly developed in the subfamilies Dorylinae and Ecitoninae. The extremely specialized diet of P. marginata and the associated high costs of migration are features likely to prevent it from evolving a full army ant life pattern.     

Population structure of the Spanish sardine Sardinella aurita: natural morphological variation in a genetically homogeneous population

The importance of larval dispersal in determining the distribution and abundance of benthic marine organisms is well recognized; however, the contribution of post-larval dispersal has not been measured. I compared the dispersal of swimming larvae with that of rafting colonies in a population of compound ascidians, Botrylloides sp., living attached to leaves of the eelgrass Zostera marina in Tomales Bay, California, USA in 1990–1992. Colonies rafting on broken eelgrass traveled over 200 times farther and had comparable recruitment success relative to swimming larvae. The recruitment of rafting colonies into new habitats was facilitated by the ability of thesecolonial animals to grow asexually onto surrounding substrata. Rafting colonies brooded larvae that were subsequently relased after the colony settled into a new habitat. These results suggest that colonization of new habitats can occur by post-larvae as well as larvae, and that long-range dispersal by species with short-lived larvae may occur by post-larval rafting.     

Parent-offspring aggression in moorhens

Summary Colonial orb-weaving spiders from Mexico were studied to test predictions of risksensitive foraging theory: 1. group foraging increases prey capture/individual, and reduces prey variance; 2. spiders should be expected to exhibit risk-averse behavior (forage in groups) when the average level of prey exceeds individual needs, and exhibit risk-prone behavior (forage solitarily) when prey are searce. Laboratory and field studies show that group foraging increases capture efficiency and reduces variability in prey captured per spider. In desert/mesquite grassland habitat, where prey availability is low, M. atascadero forage solitarily in most cases. In tropical rainforest/agriculture sites, M. increassata forage in large colonies of thousands of webs. In intermediate habitats, M. spinipes forages solitarily or in groups, depending on prey availability. Over a range of sites with varying levels of prey, M. spinipes shifts from a risk-prone to a risk-averse group foraging strategy as prey increases.Group foraging behavior observed in colonial Metepeira fits the predictions of risk-sensitive foraging models. These findings explain why spiders tend to group webs together only in areas of superabundant prey. The role of risk-sensitivity in the evolution of coloniality in spiders is discussed.