Nestmate discrimination in the queenless ponerine ant Diacamma sp. from Japan

We examined the nestmate discrimination ability of Diacamma sp., an ant that reproduces by colony budding. We also tested for a relationship between internest distance and hostility. Hostility toward non‐nestmates was significantly stronger than that toward nestmates, suggesting that Diacamma sp. discriminates between nestmates and non‐nestmates. There was no significant correlation between internest hostility and internest distance, which indicates the absence of a “dear enemy” phenomenon in this species.     

Male sexual attractiveness and aggressiveness in rodents having different mating systems

Male aggressiveness can affect male reproductive success both directly by increasing competitiveness and indirectly through female preference. Assuming that significance of male aggressiveness in species having different mating systems can be different, we studied how male aggressiveness relates to sexual attractiveness in polygynous rodents, the water vole (Arvicola terrestris) and the house mouse (Mus musculus), and in a monogamous species, the steppe lemming (Lagurus lagurus). Our analysis revealed that the relation between odor attractiveness and aggressiveness is nonlinear. In polygynous species, males are more aggressive, so females opt for aggressive, albeit not too aggressive, males. In the monogamous steppe lemming, males show low level of intermale aggressiveness, and the most attractive are slightly aggressive males who have greater reproductive potential.     

Intercolony aggression within and among local populations of the invasive ant, Myrmica rubra (Hymenoptera: Formicidae), in coastal Maine

Myrmica rubra L. was introduced into New England in the early 20th century, and at present, has a patchy distribution in parts of northeastern North America, including records from 31 communities in Maine. M. rubra is highly polygynous, and colonies reproduce vegetatively, forming dense local populations where conditions are favorable. Using mobile nests and baited arenas in a series of field aggression bioassays, we tested patterns of internest tolerance within and among local populations on Mt. Desert Island, ME. We found that foragers originating from fragments of the same colony or from neighboring nests retained a high level of intraspecific tolerance over several months, whereas significant intercolony aggression among workers was present between colonies within the same local patch separated by approximately 10 m. Within populations, aggression score values were found to increase linearly with internest distance within a site. Aggression was highest between colonies from spatially different populations on the island and was higher still when nests were assayed against colonies at an off-island site 70 km away in Castine, ME. These data strongly suggest a multicolonial organization within and among local populations of M. rubra in parts of its introduced range. These findings contradict the loss of intraspecific aggression and unicolonial social structure over large geographic areas that have previously been observed in other invasive ant species, particularly Linepithema humile Mayr.     

Does cooperation mean kinship between spatially discrete ant nests?

Eusociality is one of the most complex forms of social organization, characterized by cooperative and reproductive units termed colonies. Altruistic behavior of workers within colonies is explained by inclusive fitness, with indirect fitness benefits accrued by helping kin. Members of a social insect colony are expected to be more closely related to one another than they are to other conspecifics. In many social insects, the colony can extend to multiple socially connected but spatially separate nests (polydomy). Social connections, such as trails between nests, promote cooperation and resource exchange, and we predict that workers from socially connected nests will have higher internest relatedness than those from socially unconnected, and noncooperating, nests. We measure social connections, resource exchange, and internest genetic relatedness in the polydomous wood ant Formica lugubris to test whether (1) socially connected but spatially separate nests cooperate, and (2) high internest relatedness is the underlying driver of this cooperation. Our results show that socially connected nests exhibit movement of workers and resources, which suggests they do cooperate, whereas unconnected nests do not. However, we find no difference in internest genetic relatedness between socially connected and unconnected nest pairs, both show high kinship. Our results suggest that neighboring pairs of connected nests show a social and cooperative distinction, but no genetic distinction. We hypothesize that the loss of a social connection may initiate ecological divergence within colonies. Genetic divergence between neighboring nests may build up only later, as a consequence rather than a cause of colony separation.     

Multiple queens in founding colonies of the neotropical ant Pachycondyla striata Smith (Formicidae: Ponerinae)

In social insects, the typical mode of colony foundation occurs when a single queen is inseminated by a male and establishes a new colony, although we can find interspecific and intraspecific variations in queen number and queen-mating frequencies in a single colony. This study aimed to verify the queen number in Pachycondyla striata (Smith) colonies and to evaluate the level of aggressiveness among workers. We collected 14 colonies of P. striata. The behaviors of individuals from five multiple-queen colonies maintained in laboratory were studied by the method of scan sampling. In order to evaluate aggressiveness, dyadic encounters among heterocolonial and homocolonial workers were performed. The results showed that colonies of P. striata can have two or more mated queens (polygynous colonies) besides to monogynous ones (colony containing one queen). Because in polygynous colonies the number of workers was relatively low, such colonies could represent colonies in the foundation phase that characterize a pleometrosis state. In fact, ovarian development analysis from queens showed that the number of queens in the colonies seemed to be unstable. Despite a few cases of oophagy (egg cannibalism), social hierarchy among queens is unclear in comparison to other Pachycondyla species. In addition, aggressiveness increased with distance among nests. Nearby colonies (less than 1 m apart) showed a low level of aggressiveness, suggesting the presence of polydomy, that is, a unique colony can occupy multiple nests. Polygyny associated to polydomy in founding colonies may confer benefits on growth and dispersion of colonies in the studied environments.     

Yearly variation in polygyny in the termite <Emphasis Type="Italic">Macrotermes michaelseni</Emphasis> (Sjöstedt)

Summary Macrotermes michaelseni, an abundant termite species in semi-arid ecosystems of eastern Africa, exhibits considerable fluctuations in the proportion of polygynous colonies across years (between 5 and 50%). We found a negative correlation between rainfall and the proportion of polygynous colonies with a time lag of two years. A decrease in the percentage of polygynous colonies was particularly obvious in 1999 and 2000, two years after an El Niño-event. Rainfall and subsequent primary production seem to influence the degree of polygyny. Variation in the proportion of polygynous colonies could be due to several mutually non-exclusive factors, including better survival of polygynous colonies during years with low resource availability and increased aggression between nestmate queens with decreasing resource availability. Evaluating several lines of circumstantial evidence we suggest that the degree of polygyny depends on ecological factors.Received 11 July 2003; revised 14 October and 3 February 2004; accepted 9 March 2004.     

Encounter-induced hostility to neighbors in the ant Pristomyrmex pungens

Field observations have demonstrated that internest hostilityis negatively correlated with the distance between nests inthe Japanese queenless ant, Pristomyrmex pungens. This runscounter to the "dear enemy" phenomenon. This result led us tohypothesize the existence of encounter-induced hostility inP. pungens. We created "neighbor(s)" and "stranger(s)" by anexperimental method and tested the ant's ability to discriminatethe neighbors from strangers. The results indicated that theant could distinguish the neighbors from the strangers and displayedsignificantly stronger hostility toward the neighbors. The recognitionof neighbors was quickly established after a few encountersand was maintained for at least 10 days after the most recentencounter. Nest mates without direct encounter experience withneighbors did not show hostile behavior toward neighbors. Thesefindings suggest that information about neighbors (probablycolony odor) is not transmitted to nest mates within the colonies.Aggressiveness toward the neighbors appears to be caused andmaintained on the basis of individual memories from direct contact.It may be adaptive for a species that changes nest sites frequentlyto discriminate strangers from neighbors and selectively attackthe latter.     

Population genetics of the socially polymorphic ant <Emphasis Type="Italic">Formica podzolica</Emphasis>

Summary. We used microsatellite markers to analyze the hierarchical genetic structure of the North American mound building ant, Formica podzolica. About one-third of all colonies were headed by a single queen (monogynous) whose effective mating frequency was close to one (nestmate worker relatedness r = 0.70), while the remaining colonies were polygynous, with low average nestmate relatedness (r = 0.16). The low worker relatedness found in most polygynous colonies furthermore suggested that the numbers of queens in polygynous colonies of this ant are usually high. Contrary to what has been described from other ants with a queen number dichotomy, we did not find an effect of social form variation on the partitioning of genetic variation above the level of the colony. We found no significant differentiation between the sympatric social forms of F. podzolica, nor did differentiation among populations appear to be affected by colony social organization. These unexpected patterns of genetic structure may have resulted from differences either in the spatial distribution of the social forms or in their social flexibility.Received 12 January 2004; revised 23 February 2004; accepted 10 March 2004.     

Queen–worker caste ratio depends on colony size in the pharaoh ant (<Emphasis Type="Italic">Monomorium pharaonis</Emphasis>)

The success of an ant colony depends on the simultaneous presence of reproducing queens and non-reproducing workers in a ratio that will maximize colony growth and reproduction. Despite its presumably crucial role, queen–worker caste ratios (the ratio of adult queens to workers) and the factors affecting this variable remain scarcely studied. Maintaining polygynous pharaoh ant (Monomorium pharaonis) colonies in the laboratory has provided us with the opportunity to experimentally manipulate colony size, one of the key factors that can be expected to affect colony level queen–worker caste ratios and body size of eclosing workers, gynes and males. We found that smaller colonies produced more new queens relative to workers, and that these queens and workers both tended to be larger. However, colony size had no effect on the size of males or on the sex ratio of the individuals reared. Furthermore, for the first time in a social insect, we confirmed the general life history prediction by Smith and Fretwell (Am Nat 108:499–506, 1974) that offspring number varies more than offspring size. Our findings document a high level of plasticity in energy allocation toward female castes and suggest that polygynous species with budding colonies may adaptively adjust caste ratios to ensure rapid growth.     

Polygyny reduces rather than increases nestmate discrimination cue diversity in Formica exsecta ants

Although the majority of social insect colonies are headed by a single queen, some species possess nests that contain numerous reproductive queens (polygyny), a trait that is particularly widespread amongst the ants. Polygyny is often associated with a lack of conspecific inter-nest aggression between workers. This is hypothesised to result from increased nestmate cue diversity within nests, since polygynous nests are more genetically diverse than monogynous nests. Alternatively, it may reflect the common origin of polygynous nests that form polydomous networks. We exploit the recent discovery that the nestmate discrimination system in the ant Formica exsecta is based on cuticular hydrocarbons to investigate cue (Z9-alkenes) diversity in several monogynous and polygynous populations. Contrary to previous predictions, in all polygynous populations, the variation between nests in the Z9-alkene profiles was reduced relative to that found in monogynous populations. However, nest-specific Z9-alkene profiles with little variation amongst nestmate workers were still maintained irrespective of nest type or population. This suggests a very effective gestalt mechanism that homogenises the chemical discrimination cues, despite genetic diversity within colonies. Although the reduction in variation between nests was associated with reduced worker aggression on the population level, it cannot totally explain the weak aggression associated with polygynous populations.     

Colony genetic structure in the Australian jumper ant <Emphasis Type="Italic">Myrmecia pilosula</Emphasis>

Eusocial insects vary significantly in colony queen number and mating frequency, resulting in a wide range of social structures. Detailed studies of colony genetic structure are essential to elucidate how various factors affect the relatedness and the sociogenetic organization of colonies. In this study, we investigated the colony structure of the Australian jumper ant Myrmecia pilosula using polymorphic microsatellite markers. Nestmate queens within polygynous colonies, and queens and their mates, were generally unrelated. The number of queens per colony ranged from 1 to 4. Queens were estimated to mate with 1–9 inferred and 1.0–11.4 effective mates. This is the first time that the rare co-occurrence of polygyny and high polyandry has been found in the M. pilosula species group. Significant maternity and paternity skews were detected at the population level. We also found an isolation-by-distance pattern, and together with the occurrence of polygynous polydomy, this suggests the occurrence of dependent colony foundation in M. pilosula; however, independent colony foundation may co-occur since queens of this species have fully developed wings and can fly. There is no support for the predicted negative association between polygyny and polyandry in ants.     

COMPLEX COLONY STRUCTURE IN SOCIAL INSECTS: I. ECOLOGICAL DETERMINANTS AND GENETIC CONSEQUENCES

For social insect species, intraspecific variation in colony social structure provides an opportunity to relate the evolution of social behavior to ecological factors. The species Myrmica punctiventris is a cavity-dwelling forest ant that exhibits very different colony structures in two populations in the northeastern United States. Combined data from seasonal censuses, allozyme electrophoresis, and worker hostility tests showed that a population of M. punctiventris in Vermont was strictly monogynous and seasonally polydomous. The same procedures showed that a population of M. punctiventris in New York was facultatively polygynous and predominantly monodomous. Genetic relatedness among colony-mates was not different from Hamilton's expected values in the Vermont population and was consistent with little exchange of ants between colonies and single-mating of queens. In contrast, relatedness was lower in New York, and examination of nest-mate genotypes revealed exchange of ants between colonies, high rates of colony loss and replacement of queens, or multiple-mating of queens. The genetic structure of the Vermont population was consistent with no inbreeding, but in New York, the population genetic structure reflected microgeographic subdivision and inbreeding. Previous study of the ant communities at these sites implicates nest-site limitation in New York as a primary constraint on social structure.     

Nest Defence of Nesting Chinstrap Penguins (Pygoscelis antarctica) against Intruders

We tested some predictions of parental investment theory by studying the aggressive behaviour of colonial nesting chinstrap penguins (Pygoscelis antarctica) against human intruders into their nesting territories. We tested for differences in the aggressive behaviour of penguins according to offspring age (eggs vs. chicks), offspring number, nest location in the colonies (central vs. peripheral) and sex. Offspring age was the main factor influencing nest defence, although nest location and sex were also important. Chicks were defended more strongly than eggs, in accordance with changes in the reproductive value of offspring, and this increase in aggressiveness was not related to revisitation of the same individuals. The level of aggression of penguins breeding in central sites was higher than that of peripheral birds, a difference that could be due to the lower residual reproductive value of central-nesting, probably older, birds. The stronger aggressiveness of males could be due to a combination of factors related to sexual selection and life-history traits. Offspring number did not affect the level of nest defence.     

Social structure varies with elevation in an Alpine ant

Insect societies vary greatly in social organization, yet the relative roles of ecological and genetic factors in driving this variation remain poorly understood. Identifying how social structure varies along environmental gradients can provide insights into the ecological conditions favouring alternative social organizations. Here, we investigate how queen number variation is distributed along elevation gradients within a socially polymorphic ant, the Alpine silver ant Formica selysi. We sampled low‐ and high‐elevation populations in multiple Alpine valleys. We show that populations belonging to different drainage basins are genetically differentiated. In contrast, there is little genetic divergence between low‐ and high‐elevation populations within the same drainage basin. Thus, elevation gradients in each of the drainage basins represent independent contrasts. Whatever the elevation, all well‐sampled populations are socially polymorphic, containing both monogynous (= one queen) and polygynous (= multiple queen) colonies. However, the proportion of monogynous colonies per population increases at higher elevation, while the effective number of queens in polygynous colonies decreases, and this pattern is replicated in each drainage basin. The increased prevalence of colonies with a single queen at high elevation is correlated with summer and winter average temperature, but not with precipitation. The colder, unpredictable and patchy environment encountered at higher elevations may favour larger queens with the ability to disperse and establish incipient monogynous colonies independently, while the stable and continuous habitat in the lowlands may favour large, fast‐growing polygynous colonies. By highlighting differences in the environmental conditions favouring monogynous or polygynous colonies, this study sheds light on the ecological factors influencing the distribution and maintenance of social polymorphism.     

Nestmate recognition in a neotropical polygynous wasp

In social insects, nestmate recognition systems can be dynamic and modulated in response to various kinds of genetic and environmental cues. For example, multiple-queen colonies can possess weak recognition abilities relative to single-queen colonies, due to broader exposure to heritable and environmentally derived nestmate recognition cues.We conducted field experiments to examine nestmate recognition ability in a neotropical polygynous wasp, Polybia paulista. Despite the fact that the effective queen number in P. paulista is the highest ever recorded in polygynous wasps, this species exhibits a well functioning nestmate recognition system, which allows colony entry only to nestmate individuals. Similar to other social Hymenoptera, young wasps express colony specific chemical signatures within several days after emergence. This is the first study to show that the polygynous epiponine wasp is able to distinguish nestmates from non-nestmates. Received 23 May 2006; revised 6 October 2006; accepted 23 October 2006.     

Low intraspecific aggressiveness in two obligate plant-ant species

Little is known about the aggressiveness of plant-ants typically living in isolated trees nor about how that aggressiveness varies based on this isolation. Here, we examine intra- and interspecific aggressiveness between workers of two Allomerus species associated with two different myrmecophytes. In both cases, the level of intraspecific aggressiveness is very low whatever the distance separating the tested nests, while interspecific conflicts are always violent. Similar patterns of aggressiveness have been reported in various ant species, but the strictly arboreal life of Allomerus ants associated with the isolation of their adult colonies highlight different ecological conditions that might explain the lack of aggressiveness between conspecifics. Received 18 December 2007; revised 12 March 2008; accepted 13 March 2008.     

Polygynous great reed warblers Acrocephalus arundinaceus suffer more cuckoo Cuculus canorus parasitism than monogamous pairs

There is increasing evidence that hosts within a population may not be parasitized by common cuckoos Cuculus canorus with equal probability. Such non‐randomness has been documented, for example, for host nest sites and host quality. In this study we demonstrate association between successful cuckoo parasitism and host social mating system. We found that nests of socially polygynous great reed warblers Acrocephalus arundinaceus were more often successfully parasitized than the nests of their monogamous counterparts. We hypothesize that lack of parental assistance provided by polygynous males to their mates during egg laying period and higher nest activity in their territories could contribute to this discrepancy. These data imply that social mating system should be taken into account in future studies of brood parasitism.     

Egg Cannibalism in a Facultative Polygynous Ant: Conflict for Reproduction or Strategy to Survive?

Policing, i.e. all behaviours that prevent a nestmate from reproducing, is currently observed in social insects. It is presumed to have evolved to regulate potential conflicts generated by genetic asymmetries or to enhance colony efficiency by avoiding surplus reproductives and brood. In the ant, Ectatomma tuberculatum, individual queen fecundity was similar in monogynous and polygynous colonies issued from a Mexican population. Egg cannibalism, however, occurred in the polygynous colonies. The stealing and destruction of reproductive queen‐laid eggs involved only nestmate queens, even if they were highly related. No queen appeared to monopolize reproduction in the polygynous colonies. But, the observed value of relatedness among workers differed from the expected value, suggesting an unequal sharing of reproduction between queens. We discussed whether the cannibalism of queen‐laid eggs in E. tuberculatum results from a competition for reproduction among queens or if this phenomenon is related to constraints on nutritional resources.     

Absence of Within‐Colony Kin Discrimination in a Multiple‐Queen Ant,Leptothorax acervorum

Inclusive fitness theory predicts that, other things equal, individuals within social groups should direct altruistic behaviour towards their most highly related group‐mates to maximise indirect fitness benefits. In the social insects, most previous studies have shown that within‐colony kin discrimination (nepotism) is absent or weak. However, the number of studies that have investigated within‐colony kin discrimination at the level of individual behaviour remains relatively small. We tested for within‐colony kin discrimination in the facultatively multiple‐queen (polygynous) ant, Leptothorax acervorum. Specifically, we tested whether workers within polygynous colonies treated queens differently as a function of their relatedness to them. Colonies containing two egg‐laying queens were filmed to measure the rate at which individually marked workers antennated and groomed or fed each queen. Relatedness between individual queens and workers was calculated from their genotypes at four microsatellite loci. The results showed there were no differences in the rates at which workers antennated or groomed/fed their more related queen and their less related queen. Workers interacted preferentially with their potential mother queen with respect to grooming/feeding but not with respect to antennation. However, because of high queen turnover, the frequency of adult workers with their potential mother queen still present within the colony was relatively low. Overall, therefore, we found no evidence for within‐colony kin discrimination in the context of the average worker's treatment of queens in polygynous L. acervorum colonies.     

The Effect of Queen Number on Nestmate Discrimination in the Facultatively Polygynous Ant Pseudomyrmex pallidus (Hymenoptera: Formicidae)

We present evidence indicating that the level of nestmate discrimination in the facultatively polygynous ant Pseudomyrmex pallidus varies predictably with the number of queens in the colony. P. pallidus workers were introduced into observation arenas in either nestmate or non-nestmate pairs. During the 5-min period immediately following the first contact between test ants, all interactions and relative distances were recorded. Aggression between non-nestmates was negatively correlated with the number of queens in the colony and distance between nestmates was positively correlated. These results are consistent with predictions of Reeve's (1989) optimal acceptance threshold model.