Trail following in ants: individual properties determine population behaviour

This paper deals with the purposeful marking of trails as a mechanism for coordinating movement. Patterns of motion are adapted to the environmental conditions, the functions to be carried out, and the condition of the organism; therefore, the networks of trails must change both quantitatively and qualitatively over time. The nature of such changes, and how they are controlled at the individual level are discussed. In particular, we show that slight modulations in individual traits, in the trail marker, or in the size of the group can account for major changes in movement patterns at the population level such as abrupt transitions from diffuse area-covering networks to focused trunk trails. Using a mathematical model and computer (cellular automata) simulation we show that trunk trails carrying a high density of traffic can form spontaneously under suitable conditions from an initially randomly distributed group. The key to this self-organizing property stems from interactions between individuals that lead to a collective effect in recruitment to trails: the influence of small groups of individuals increases rapidly with group size. The dichotomy between high traffic (strong) trunk trails versus diffuse (weak) networks is discussed.     

Hierarchical perception of fertility signals and nestmate recognition cues in two dolichoderine ants

In social insects, queens are likely to “honestly” inform their nestmates of their presence and fertility status through pheromonal communication. Cuticular hydrocarbons (CHCs) have been reported to be effective nestmate discriminators and strongly suspected to act as fertility signals, at least in some species. The use of the same chemical bouquet (i.e., the CHC profile) to convey two fundamentally different information seems puzzling. However, a recent threshold model proposes a hierarchy in the discriminating processes, i.e., fertility signals can only be perceived if nestmate recognition has been reached (Le Conte and Hefetz, Annu Rev Entomol 53:523–542, 2008). Here, we developed a simple behavioral bioassay based on chemical recruitment toward a queen placed outside the nest in two dolichoderine ants (Linepithema humile and Tapinoma erraticum), which allowed us to investigate the interplay between fertility signaling and colonial recognition. Using queen corpses of various origins (nestmates or aliens) and physiological states (fertile or infertile; mated or unmated), we demonstrated that nestmate recognition cues clearly override fertility signals under our experimental conditions. Indeed, while nestmate infertile queens were largely ignored by the workers, nestmate fertile queens (mated or not) induced worker recruitment, whereas alien fertile queens did not and were aggressed by the workers.     

Male competition in Cardiocondyla ants

The two types of males in the ant genus Cardiocondyla differ remarkably in morphology and behavior. Ergatoid males are wingless fighters whose spermatogenesis continues throughout their entire adult lives and which therefore have an “unlimited” sperm supply. They attempt to kill all eclosing ergatoid rivals and thus to increase their share in copulations with the virgin queens reared in their nests. Winged males, on the other hand, are docile and emigrate from the nests a few days after eclosion, probably to mate with queens from other colonies. By this time, their testes have fully degenerated and all sperm is stored in the seminal vesicles. Before emigration, winged males may mate with virgin queens in their maternal nests, but they are nevertheless rarely attacked by ergatoid males. In the laboratory, the life expectancy of ergatoid males is only slightly higher than that of winged males, but because of the emigration of the latter the difference is likely to be more pronounced in the field. Both male morphs are capable of inseminating more than 35 virgin queens. However, winged males older than 14 days mate less often than ergatoid males of similar age, probably due to sperm depletion in later life. The spermathecae of queens inseminated by ergatoid males contained significantly more sperm than those of queens which mated with winged males. We discuss the evolution of intranidal mating and male polymorphism in ants. Received: 8 August 1997 / Accepted after revision: 6 October 1997     

The red and the black: habituation and the dear-enemy phenomenon in two desert Pheidole ants

Many species of territorial animals are more aggressive toward strangers than neighbors, a pattern of aggression referred to as the ’dear-enemy phenomenon.’ In many cases, the mechanism by which neighbors are discriminated from strangers and the function of neighbor-stranger discrimination remain controversial. We investigated the spatial patterns of inter-colony aggression within and between two Pheidole species of seed-harvesting ants in the Mojave Desert of California by quantifying aggression between colonies in standardized staged encounters. We also tested whether the level of fighting between workers of two colonies is affected by previous exposure to each other. We show that neighbors (i.e., colonies less that 2.6 m away) of either species are treated less aggressively than more distant colonies and that habituation may be a mechanism by which this discrimination is achieved. The variation in aggression among spatially distant colonies also suggests that additional genetic or environmental factors are involved in recognition. The function of the dear-enemy phenomenon in these ant species may be related to the greater risk to the resources of a colony presented by strange workers than workers from a neighboring colony. Received: 18 November 1999 / Received in revised form: 3 April 2000 / Accepted: 3 May 2000     

Policing in queenless ponerine ants

Potential reproductive conflicts are common in insect societies. One process that can reduce or suppress conflict is policing. We review worker and "queen" policing in queenless ponerine ants. Queenless ants are an important model system for the study of intracolony reproductive conflict. Policing is widespread in queenless ants because workers are totipotent, so that additional potential conflicts occur in comparison to species where workers cannot mate, and these additional conflicts are frequently reduced by policing. Policing is more diverse than suggested by the examples known in other social insects. In almost all species of social Hymenoptera it can include preventing workers from reproducing by killing worker-laid eggs, but in queenless ants it can additionally include immobilisation or mutilation of workers attempting to reproduce by replacing the gamergate (i.e. mated worker with a queen-like role) or by becoming an extra gamergate. Policing by both workers and by the gamergate are important. Policing can be facultative. Depending on the age of the gamergate, workers can prevent her replacement by immobilising challenging workers or they can favour replacement by immobilising the gamergate. The initial definition of policing was inspired by species in which workers retain ovaries but cannot mate. We broaden the definition to include species, such as queenless ants, where females are totipotent, thereby including not only conflict over male production but also over gamergate replacement and gamergate number. Finally, we compare policing with punishment and dominance hierarchy. Policing is not always punishment and it does not necessarily entail dominance relationships.     

Recognition of caste and mating status maintains monogyny in the ant Aphaenogaster senilis

In ants dispersing through colony fission, queens mate near their natal nest and found a new society with the help of workers. This allows potential future queens to challenge the mother queen’s reproductive monopoly. Conflicts might be resolved if the mated queen signals her presence and the workers control the developmental fate of the diploid larvae (whether they develop to worker or queen). In this study we sought to determine whether, in the fission-performing ant Aphaenogaster senilis, conflicts between queens for control of the colony are resolved by the resident queen signalling her mating status. Virgin queens were less effective than newly mated queens in inhibiting queen rearing. Moreover, potential challenger queens were recognized and heavily aggressed independent of mating status. Chemical analyses showed that mating status was associated with changes in cuticular hydrocarbon and poison gland composition, but not in Dufour’s gland composition. Cuticular dimethylalkanes were identified as potential constituents that signal both caste (present in queens only) and mating status (mated queens have higher amounts). We hypothesised that pheromone emission by virgin queens did not reach the threshold needed to fully inhibit larval development into queens but was sufficiently high to stimulate overt aggression by mated queens. These findings provide evidence for the complexity of chemical communication in social insects, in which a small number of signals may have a variety of effects, depending on the context.     

An experimental study of competition between fire ants and Argentine ants in their native range

An understanding of why introduced species achieve ecological success in novel environments often requires information about the factors that limit the abundance of these taxa in their native ranges. Although numerous recent studies have evaluated the importance of natural enemies in this context, relatively few have examined how ecological success may result from differences in the magnitude of interference competition between communities in the native and introduced ranges of nonnative species. Here we examine how native-range competitive environments may relate to invasion success for two important invasive species, the red imported fire ant (Solenopsis invicta) and the Argentine ant (Linepithema humile), in a region of native-range sympatry. At two study sites in northern Argentina, we used stable-isotope analysis, a variety of observational approaches, and two different reciprocal removal experiments to test (1) whether S. invicta competes asymmetrically with L. humile (as suggested by the 20th century pattern of replacement in the southeastern United States) and (2) the extent to which these two species achieve behavioral and numerical dominance. Stable-isotope analysis and activity surveys indicated that S. invicta and L. humile are both omnivores and forage during broadly overlapping portions of the diel cycle. Short-term removal experiments at baits revealed no competitive asymmetry between S. invicta and L. humile. Longer-term colony removal experiments illustrated that S. invicta and L. humile experience an approximately equal competitive release upon removal of the other. Our results indicate that neither S. invicta nor L. humile achieves the same degree of behavioral or ecological dominance where they co-occur in native populations as they do in areas where either is common in their introduced range. These results strongly suggest that interspecific competition is an important limiting factor for both S. invicta and L. humile in South America.     

Brooding behaviour and reproductive success in two species of free-living simultaneous hermaphrodites

Although polyclads are amongst the most structurally simple of the triploblastic metazoans, they adopt a wide range of reproductive strategies. Parental care behaviour in this group is yet to be quantified for any species. We assessed the significance of brooding behaviour to the reproductive success of two free-living marine flatworms. Echinoplana celerrima and Stylochus pygmaeus were collected from the field and placed in pairs in containers of filtered seawater where they laid batches of eggs. Both parents were then removed from half of the containers and the brooding behaviour and hatching success of eggs were quantified. There were interspecific differences in brooding behaviour. Egg masses were covered by one E. celerrima parent for 12 ± 2% of time, whereas egg masses of S. pygmaeus were covered by one or both parents simultaneously for 85 ± 8% of time. Egg batches were abandoned by both species immediately prior to the onset of hatching (10–12 days). Hatching success was generally high (~90%) and brooding did not enhance the hatching success of eggs. We assessed the significance of parental care to hatching success of E. celerrima egg masses in the presence of three potential egg predators; in the presence of other organisms. E. celerrima devoted less time to brooding; however, hatching success was not affected. The amount of time spent brooding eggs differed greatly between the two polyclad species but was not essential to their reproductive success under benign conditions. Parental care may be of adaptive value under more stressful environmental conditions commonly experienced in estuarine environments such as lowered salinity, increased hypoxia or turbidity. Covering egg batches may play an additional role of advertising sexual status and a willingness to care for eggs.     

Patterns of paternity skew in Formica ants

Reproductive skew among cooperatively breeding animals has recently attracted considerable interest. In social insects reproductive skew has been studied in females but not in males. However, cooperative breeding of males occurs when two males mate with the same queen and father offspring. Here we present the first analysis of comparative data on paternity skew in ants. We show that, across seven species of Formica ants, the average skew in paternity among worker offspring of doubly mated queens is negatively correlated with the population-wide frequency of multiple (mostly double) mating. We also demonstrate that this trend is relatively robust in additional analyses taking phylogenetic relationships between species into account. The observed trend is opposite to the one normally found in non-social insects with second-male precedence through sperm displacement, but agrees with predictions based on queen-male conflict over sperm allocation as a consequence of facultative, worker controlled, sex allocation – an interpretation which assumes first-male precedence. However, alternative (but not mutually exclusive) explanations are possible and further studies will be needed to discriminate between these alternatives. Received: 16 May 1997 / Accepted after revision: 26 September 1997     

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     

Sex allocation and queen-worker conflict in polygynous ants

Summary Sex allocation theory is developed for polygynous eusocial Hymenoptera in which nests recruit their own daughters as new reproductive queens. Such restricted dispersal of females leads to the expectation of male-biased investment ratios. The expectation depends on the parameter q telling what proportion of the total contribution in the gene pool by all new queens is due to those dispersing. Under queen control the expected sex allocation, expressed as the proportion of resources invested in males, is IM =1/(1 + q). Under worker control, IM depends on the relatedness of old queens, on the number of males they have mated with, and on the proportion of males produced by workers. With single mating and no worker reproduction, the approximate predictions for IM are 1/(1 + q) when the nests have many highly related queens, 1/(1 + 2 q) when the old queens are as related as average worker nest mates, and 1/(1 + 3q) when the old queens are not related to each other at all. The observed investment ratios in polygynous ants would, on average, match values of the parameter q between 0.4 and 0.5. Values of q have not been estimated in nature. If q is smaller than 0.4, which may well be true, the observed sex allocation in polygynous ants is in fact more female-biased than predicted by the theory. This indicates that the female bias found in monogynous ants may not be exceptional and could be due to factors other than worker control of sex allocation. Because the value of q is likely to vary among species, testing the predictions of the theory requires thorough single-species studies.     

Encounter rate and task allocation in harvester ants

As conditions change, social insect colonies adjust the numbers of workers engaged in various tasks, such as foraging and nest work. This process of task allocation operates without central control; individuals respond to simple, local cues. This study investigates one such cue, the pattern of an ant's interactions with other workers. We examined how an ant's tendency to perform midden work, carrying objects to and sorting the refuse pile of the colony, is related to the recent history of the ant's brief antennal contacts, in laboratory colonies of the red harvester ant, Pogonomyrmex barbatus. The probability that an ant performed midden work was related to its recent interactions in two ways. First, the time an ant spent performing midden work was positively correlated with the number of midden workers that ant had met while it was away from the midden. Second, ants engaged in a task other than midden work were more likely to begin to do midden work when their rate of encounter per minute with midden workers was high. Cues based on interaction rate may enable ants to respond to changes in worker numbers even though ants cannot count or assess total numbers engaged in a task. Received: 1 July 1998 / Accepted: 15 November 1998     

Patterns of male parentage in the fungus-growing ants

Ant queens from eight species, covering three genera of lower and two genera of higher attine ants, have exclusively or predominantly single mating. The ensuing full-sib colonies thus have a strong potential reproductive conflict between the queen and the workers over male production. This is because, all other things being equal, relatedness incentives should favour traits expressed in both workers and the queen to monopolise the production of the colony's male offspring. Microsatellite genotyping of males from these attine species shows that workers in queenless colonies are able to produce males, but that no worker-produced males were found in queenright colonies. Our results suggest that worker reproduction is rare or even absent in colonies with a fertile queen. This indicates that either the queen directly prevents the workers from raising their own sons, or that worker reproduction is absent in the presence of a fertile queen due to high ergonomic costs of this behaviour.     

Collective decisions in ants when foraging under crowded conditions

In this paper we examine the effect of crowding on the selection of a path in the mass-recruiting ant Lasius niger. In our experiment, ants had to go from their nest to a food source by crossing a diamond-shaped bridge, giving the choice between two paths. Two types of bridges were used: the first had two branches of equal length but different width while the second had two branches of different length and width. Experiments at high traffic volume always ended up with the selection of the wider branch, even if it was longer. This result shows that overcrowding on the narrow branch plays an essential role in the mechanism underlying the choice of route in ants. A mathematical model was developed to evaluate the importance of two mechanisms that could account for this result. The first is based on the difference in travel duration between the two paths. The second is based on the repulsive interactions between workers making head-on encounters. The model shows that travel duration per se is not sufficient to explain path choice. Rather, it is the interplay between trail following behaviour and repulsive interactions that allows ants to choose the path that minimizes their travel time. When choosing a path ants thus prefer to trade time against energy. Our results demonstrate that any environmental constraint that alters the dynamics of trail recruitment can lead to the emergence of adaptive foraging decisions without any explicit coding of information by the foragers at the individual level.     

Nomadic behavior of army ants in a desert-grassland habitat

Summary Nomadic behavior of the army ant Neivamyrmex nigrescens was studied in a desert-grassland habitat. Six colonies were followed through eight nomadic phases (94 nomadic days) while direction and distance of emigrations, growth of larvae, number of adults and larvae per colony were determined.In all colonies, the nomadic phase began when newly eclosed adults and small larvae were present, and ended when the larvae were fully grown. Average emigration distance was positively related to number of larvae in the colony. These findings support Schneirla's theory that brood stimulation is a proximate cause of the nomadic phase.Failures to emigrate were equally likely at all points in the nomadic phase, and there was no systematic increase in emigration distance as the phase progressed. These findings do not support Schneirla's version of brood-stimulative theory.Number of adults per colony was positively related to the directionality of the nomadic phase; however, both the direction and distance of emigrations varied unpredictably from one nomadic day to the next, in marked contrast to predictions from optimal foraging theory.Schneirla's theory is useful in predicting phase differences in colony behavior, but it does not account for characteristics such as frequency, direction, or distance of emigrations within the nomadic phase. These aspects of nomadic behavior are more closely related to characteristics of the habitat such as prey density and availability of nest sites. Army ant nomadism in this habitat may depart from the optimum because of high prey density, small colony size, or lack of nesting sites.     

Foaming in Pachycondyla: A new defense mechanism in ants

Summary When disturbed, two species of Malayan Pachycondyla release foam threads more than 10 cm in length or foam piles. The source of the proteinaceous foam is the enlarged venom gland, which is probably frothed up by air from the spiracles of the spiracular plates. The Dufour's gland normally producing hydrocarbons in stinging ants is atrophied. Therefore, absence of the Dufour's gland could be essential to the foaming ability, since the lipophilic hydrocarbons inhibit froth production in protein solutions. The release of foam is a mechanically acting defense mechanism, which is very effective against small mass-attacking ants. Pachycondyla species are also able to sting effectively.     

Variable sex ratio among colonies of ants

Summary Small colonies of ants often produce mostly male alates, while large colonies produce mostly female alates. I present a simple model consistent with this pattern in which males that compete for mates are related (Local Mate Competition). The model explains the observed trend even when relatedness among competing males is low, so that there is only a negligible effect on the predicted sex allocation ratio in the population. The reverse trend is expected when there is competition among related females for a limited resource, such as nest sites (Local Resource Competition); small broods are predicted to be mostly female and large broods are predicted to be mostly male.     

Conflicts,social economics and life history strategies in ants

Summary This paper presents a life history model for a perennial social insect colony. The model's purpose is to explore the evolutionary consequences, in terms of fitness of different parties within the colony, of alternative life history strategies. The model has been specifically developed for colonies of the slave-making ant, Harpagoxenus sublaevis, which has reproductive workers organized in dominance orders. It incorporates empirically obtained parameters, and uses computer algorithms based on numerical optimization to determine the optimum policy for a colony queen in allocating resources between workers, queens, and males. Variants of the model also consider alternative situations in which either (1) orphaned workers do not slave-raid, or (2) workers are sterile. The results correspond closely to data on colony growth and reproductive allocation obtained from the field. They suggest that a colony queen would suffer reduced fitness in the two theoretical cases as compared to the real situation. Reproduction by orphaned workers posthumously enhances the colony queen's fitness because a queen with sterile workers cannot produce enough extra sexuals in her lifetime to balance her loss in grandson production. The results also suggest that the division of labour between slave-raiding and nonraiding workers observed in H. sublaevis colonies can be explained as an outcome of worker-worker reproductive conflict: reproductively-inhibited subordinate workers can increase their inclusive fitness by slaveraiding for dominant, nonraiding egg-layers. These findings emphasize the evolutionary importance of the orphanage period and of intracolony conflict in monogynous social insect colonies.