Kinship structure and mating system in a solitary subterranean rodent, the silvery mole-rat

The African mole-rats (Bathyergidae, Rodentia) are subterranean rodents that have evolved a wide range of social organisations varying from solitary to eusocial. Due to some unusual features of their breeding and social systems, much attention has been focused on social species, with solitary species being virtually ignored. Here, we present the first genetic study on the mating system of any solitary mole-rats, the silvery mole-rat (Heliophobius argenteocinereus) - a species which is assumed to be monogamous. Microsatellite markers were used to analyse the mating system and the kinship structure in populations in southern Malawi. Isolation by distance between individuals was apparent in two studied populations, but not in the third, probably, as a result of barriers limiting dispersal in the latter population. Polygyny was found to be a mating system in this population, where a strongly female-biased adult sex ratio was present. In this case, large distances between the burrow systems of mating partners exclude the possibility of belowground searching for mates, suggesting that the males might seek females aboveground. Interestingly, among analysed litters from various localities, one multiple-sired litter was found. Therefore, the results suggest that the mating system in potentially monogamous solitary subterranean rodents may be much more variable than expected and can differ among populations.     

Polygyny and male parental care in Savannah sparrows: effects on female fitness

Summary To determine the effects of male mating status on female fitness, we compared the reproductive success, survival, and future fecundity of female Savannah sparrows (Passerculus sandwichensis) mated to monogamous vs. polygynous males in a 5-year study on Kent Island, New Brunswick, Canada. The proportion of males with more than one mate varied from 15 to 43% between years and sites. Polygynous and monogamous males fledged young of equal size in every year of the study. Females who shared paternal care with other females laid as many eggs per clutch and clutches per season as monogamously mated females. In most years polygynously mated females showed no delay in laying a second clutch, and they suffered no reduction in fecundity the following year. Recruitment of a female's offspring into the breeding population was generally independent of her mating status. Fitness costs of being mated to a polygynous male were only apparent in one year of the study, during which females mated to polygynous males had higher over-winter mortality than those mated to monogamous males. That same year, young raised by polygynous males were only one-third as likely to survive to reproductive maturity (as inferred by returns) as those raised by monogamous males. A male's mating status had no effect on his own survivorship. A male's mating status did not necessarily reflect his contributions to raising nestlings, which may partially explain why monogamously and polygynously mated females had equal fitness. At 35 nests the proportion of food deliveries brought by individual males varied from 0 to 75%; on average, males brought fewer than 30% of all food deliveries. Yet parental care by polygynous males was no less than that of monogamous males, at least at the nests of their primary females. Secondary females tended to receive less male assistance during the nestling stage, but their reproductive success was indistinguishable from that of primary females. Females feeding young without male assistance made as many food deliveries/h as did pairs in which males brought at least 30% of all food deliveries. Unassisted females did not suffer diminished fledging success or produce smaller fledglings. The benefits of polygyny for male Savannah sparrows are clear: polygynous males recruit more surviving offspring into the breeding population than monogamous males. The fitness of females, on the other hand, appears to be unaffected by whether their mate was monogamous or polygynous except in occasional years. Polygyny may be maintained in this population by the constraints of a female-biased sex ratio, the inability of females to predict a male's paternal care based on his morphology or behavior, the poor correlation between a male's mating status and his assistance at the nest, and inconsistent natural selection against mating with a polygynous male. Correspondence to: N.T. Wheelwright     

Breeding success of a brood parasite is associated with social mating status of its host

Reproductive success of brood parasites varies considerably both among and within host species, mainly due to differences in host egg-rejection rates and survival of parasitic chicks. Here, we investigated the breeding success of the cuckoo (Cuculus canorus) in one of its major hosts, the great reed warbler (Acrocephalus arundinaceus), with respect to host social mating status. In this passerine, polygynous males provide less parental care to their young per nest than monogamous males. Consequently, their less-assisted females may fledge lower numbers of nestlings than monogamous females. This may be especially true for secondary females, which often receive limited or no paternal help with young at all. Based on these findings, we expected higher cuckoo reproductive success in nests of socially monogamous than polygynous great reed warbler males. More specifically, we predicted lower fledging success of cuckoo young in nests of secondary than primary or monogamous females. In line with the prediction, we found higher cuckoo fledging success in nests of monogamous than polygynous males, monogamous nests being more than twice as successful as secondary nests. We detected, however, only a tendency to lower cuckoo success in primary compared to monogamous nests and no differences between primary and secondary nests. Moreover, neither parasitism nor host egg-rejection rates differed among the nests of different status. Our results show, for the first time, that the social mating status of a host may influence the overall reproductive success of a brood parasite and thus should be considered in further studies.     

Female mate choice in the Pied Flycatcher Ficedula hypoleuca

Summary We have examined male and territorial factors which might influence female mate choice in the Pied Flycatcher (Ficedula hypoleuca). Mating success of males was largely determined by the order of arrival on the breeding grounds. Females actively selected those males that had been longest in the area. This is likely to be due to territorial cues, early males having the best territories. About 15% of the males become polygynous, and these are the males that arrived earliest.Male age was relatively unimportant for breeding success, but because old males tend to arrive earlier than yearlings, most polygynous males were old. On average, old males are somewhat darker than young males, but male colour also varies within males of the same age. Polygynous and monogamous males did not differ significantly in colour, and female breeding success was not correlated with the colour of their mates. We show theoretically that it does not pay for females to select young males to avoid polygyny unless polygyny frequency or the number of reliable monogamous males almost double. Hence our hypothesis of polygyny-by-deceit in the Pied Flycatcher remains realistic. Because of male polyterritoriality, females are unaware of the males' status when mating.     

Paternal care in the European starling,Sturnus vulgaris: nestling provisioning

The extent to which male birds in polygynous species with biparental care assist in nestling feeding often varies considerably between nests of different mating status. Both how much polygynous males assist and how they divide their effort between nests may have a profound effect on the evolution of mating systems. In this study we investigated how males in the facultatively polygynous European starling Sturnus vulgaris invested in their different nests. The amount of male assistance affected the quality of the offspring. Polygynous males invested as much as monogamous males, but divided their effort asymmetrically between nests, predominantly feeding nestlings of first-mated (primary) females. Although females partly compensated for loss of male assistance, total feeding frequency was lower at primary females’ nests than at monogamous females nests. Secondary females received even less assistance with nestling rearing, and the extent to which males assisted decreased with the length of the interval between the hatching of the primary and secondary clutches. These results are contrasted with those from a Belgian populations of starlings with a much more protracted breeding season and thus greater opportunities for males to attract additional mates during the nestling rearing period. The results show that both the “defence of male parental investment model” and the “asynchronous settlement model” have explanatory power, but that their validity depends on the potential length of the breeding season. Received: 21 July 1995/Accepted after revision: 13 July 1996     

Inter-sexual conflicts over copulations in the European starling: evidence for the female mate-guarding hypothesis

Recent studies have suggested that conflicts of interests between the sexes may lead to variation in copulation behavior among pairs. We examined differences in the rate and timing of copulation solicitations and copulations among females of different mating status in the facultatively polygynous starling in an attempt to explain why females copulate repeatedly with their mate. All within-pair copulations were female-solicited indicating that females control copulations in the starling. Before egg laying, females solicited copulations at a high rate (usually more than 2 per hour). In contrast to most other species studied so far, females continued to solicit copulations throughout the egg laying period, and also solicited after egg laying (the latest solicitation occurred when the nestlings were 4 days old). Primary females solicited more copulations than monogamous females both during and after the fertile period. Many copulation solicitations of primary females occurred at the nestbox where their male was singing to obtain an extra female. Both primary and secondary females solicited more copulations after the egg-laying period than monogamous females. A large proportion of female copulation solicitations was refused by the male partner: female solicitation resulted in more male refusals in primary females than in monogamous females. Primary females were more likely to be chased aggressively when they solicited a copulation than monogamous females; most aggressive chases occurred when primary females flew towards their male to solicit copulation when he was singing at another nestbox. Overall, our results demonstrate that there is a conflict over copulation between males and females in polygynous pairs. The conflict presumably relates to the cost of sharing male parental investment: females use copulation solicitation behavior to interrupt their singing males apparently in an attempt to prevent them from becoming polygynous. We present the first empirical evidence that female songbirds use copulation solicitation behavior as a form of mate guarding, often in a non-reproductive context. We did not find a positive relationship between copulation rate during the fertile period and the amount of male parental care as is predicted by the paternity confidence hypothesis.     

Reproductive success of monogamous and polygynous pied flycatchers (Ficedula hypoleuca)

One of the most interesting aspects of the mating system of pied flycatchers is the regular occurrence of polygyny. Here we present data on the reproductive success of polygynously paired pied flycatcher males compared to monogamous males based on paternity analyses through DNA fingerprinting. Males paired with two females suffered a higher loss in reproductive output per female compared to monogamous males due to (1) a greater proportion of unhatched eggs in their broods, (2) greater nestling mortality and (3) a greater probability of being cuckolded. Nevertheless, the number of fledglings was significantly greater for polygynous males. Based on the number of nestlings that returned for subsequent breeding seasons, however, the reproductive success of monogamous and polygynous males did not differ significantly. These data raise the question as to why males attempt polygyny. Received: 16 August 1999 / Received in revised form: 15 March 2000 / Accepted: 18 March 2000     

Natal dispersal in two mice species with contrasting social systems

We compared the natal dispersal behaviour of two mice species under laboratory conditions. Natal dispersal is a movement of an animal from its birthplace to its breeding area. This behaviour is known to be influenced by the mating system. In polygamous species, males are more likely to disperse, while in most of the monogamous species, both sexes disperse. Our subjects, the house mouse (Mus musculus) and the mound-building mouse (Mus spicilegus) are two sympatric species of the genus Mus. Both are native in Hungary, but they differ in their habitat type mating system and overwintering strategy. The house mouse is a polygynous species and adapted to human environment, known for mature and reproduce early. On the contrary, the mound-building mice are monogamous, and they inhabit extensively used agricultural fields, where they spend the unfavourable winter period in nest chambers under mounds, which they construct from soil and plant material. Successful overwintering for this species demands delayed maturity and reduced dispersion during the winter. Our results showed that the natal dispersal of these two species differ; both sexes of the mound-building mice dispersed later than the house mice, where a difference between sexes also occurs; house mice males dispersed earlier than females. The mound-building mice showed no sexual dimorphism in this behaviour.     

Alternative reproductive tactics and male-dimorphism in the horned beetle Onthophagus acuminatus (Coleoptera: Scarabaeidae)

Adult dung beetles (Onthophagus acuminatus) exhibit continuous variation in body size resulting from differential nutritional conditions experienced during larval development. Males of this species have a pair of horns that protrude from the base of the head, and the lengths of these horns are bimodally distributed in natural populations. Males growing larger than a threshold body size develop long horns, and males that do not achieve this size grow only rudimentary horns or no horns at all. Previous studies of other horned beetle species have shown that horned and hornless males often have different types of reproductive behavior. Here I describe the mating behaviors of the two male morphs of O. acuminatus during encounters with females. Females excavate tunnels beneath dung, where they feed, mate and provision eggs. Large, horned males were found to guard entrances to tunnels containing females. These males fought with all other males that attempted to enter these tunnels. In contrast, small, hornless males encountered females by sneaking into tunnels guarded by other males. In many instances, this was accomplished by digging new tunnels that intercepted the guarded tunnels below ground. Side-tunneling behavior allowed sneaking males to enter tunnels beneath the guarding male, and mate with females undetected. Both overall body size and relative horn length significantly affected the outcome of fights over tunnel ownership. These results suggest that alternative reproductive tactics may favor divergence in male horn morphology, with long horns favored in males large enough to guard tunnels, and hornlessness favored in smaller males that adopt the “sneaking” behavioral alternative. Received: 12 October 1996 / Accepted after revision: 8 August 1997     

Female encounter rates and fighting costs of males are associated with lek size in Drosophila mycetophaga

Male costs and benefits associated with male display size in field populations of an Australian lekking Drosophila species were examined. Results suggested that male mating success increased with display size, since matings appeared to be more common in large displays, and since the probability of males encountering a female increased as displays contained more males. Female encounter probabilities did not increase once about 20 males or more were present on a display. Male size and fighting costs tended to increase with display size. The distribution of males among displays did not follow the ideal free distribution in the sense that each male did not have equal mating opportunity per unit time. Deviation from an ideal free distribution may have been due to female preference for mating in aggregations rather than with solitary males, since in a field experiment females were more willing for mating in an aggregation of five males than with solitary males. Received: 22 May 1997 / Accepted after revision: 1 November 1997     

The annual testosterone profile in cooperatively breeding superb fairy-wrens, Malurus cyaneus, reflects their extreme infidelity

Superb fairy-wrens are cooperatively breeding birds that combine stable, socially monogamous pair bonds and high levels of paternal care, with extreme levels of extra-pair mating and high levels of sexual competition. Our aim was to determine which testosterone correlates would prevail in such a life history that combines features that are conventionally associated with divergent hormone profiles. Unlike the situation in other species with monogamous pair bonds and high levels of paternal care, testosterone was elevated for a very long period of several months. During breeding there was a broad peak in testosterone followed by a gradual decline: this resembles the profile found in polygynous and promiscuous species. We found that three factors correlated with testosterone: development of the sexually selected nuptial plumage, social status and extra-group mating opportunities. Testosterone started increasing months prior to breeding, when the males that are later preferred as extra-group sires develop their nuptial plumage. Although these males did not have higher testosterone levels during breeding, they sustained high testosterone for much longer, and this might lend reliability to this sexual signal. Dominant males in groups had higher testosterone than pair-dwelling males and subordinate helpers. This was not due to differences in age, reproductive capability or mating opportunities, but was presumably associated with the assertion of dominance. In contrast to findings in other species, male testosterone level was not correlated with whether the resident female was fertile or had dependent nestlings. However, testosterone was strongly correlated with the total number of fertile females in the population, and hence with the opportunities for extra-group mating.     

Sexual conflict in the house sparrow: interference between polygynously mated females versus asymmetric male investment

Summary The reproductive success of female house sparrows mated with polygynous males depended to some extent on the aid received from their mates. Polygynous males fed nestlings at the same rate as monogamous males although polygynous males gave aid almost exclusively to one of their mates (the preferred). As a consequence, the number and quality of young raised by preferred females were similar to those of monogamous females, whereas nonaided females experienced a reduction of clutch size, hatching success, and fledgling quality. Males aided the harem female that hatched her clutch earliest (in 11 out of 12 cases). However, harem females that laid eggs earliest (i.e., the potential preferred) lost eggs and/or nestlings more frequently than harem females that delayed laying. This resulted from competing females' infanticide suited for redirecting males' aid. All polygynously mated females delayed the start of a new clutch and reproduced fewer times per season than monogamous females. Consequently, monogamy seems to be the optimum mating situation for females; whereas polygynous males raised more young per year than monogamous males, interference between harem females reduced their maximum expected success.     

Males in the shade: habitat use and sexual segregation in the Galápagos sea lion (Zalophus californianus wollebaeki)

In many sexually dimorphic species adult sexes tend to segregate socially, spatially, or in habitat use. Several hypotheses have been formulated regarding underlying mechanisms. We investigated terrestrial habitat use and sexual segregation in a tropical otariid, the Galápagos sea lion (Zalophus californianus wollebaeki), where most of the hypotheses can be ruled out a priori. Factors relating to thermoregulation and costs of locomotion were of prime importance for habitat use. Habitats directly adjacent to the sea, with simple structured flat surfaces, shade, and tide pools were most frequented, but sexes and age classes differed in their usage patterns. Sexual segregation, both spatial and by habitat was pronounced in the reproductive period (RP), but remained high during the nonreproductive period (NRP). A GLM model of habitat use showed that in both seasons adult males frequented habitat types that adult females and other age classes used much less. Males were most abundant in suboptimal inland habitats, which offered only shade for cooling. Females with newborns differed in habitat use from females with older offspring and lone females. Spatial and habitat segregation are explained most parsimoniously as by-products of social processes, primarily intrasexual competition and female avoidance of male harassment, linked to the polygynous mating system.Electronic Supplementary Material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Mating in bighorn sheep: frequent male reproduction via a high-risk “unconventional” tactic

Rocky Mountain bighorn rams use three distinct tactics in competition for mates. Two tactics (tending and blocking) feature defense and cooperative mating over a relatively prolonged consort period (up to 3 days). In the coursing tactic, subordinate rams fight dominants for temporary copulatory access (lasting seconds) to defended ewes. By combining population-wide genetic (microsatellite) exclusion of paternity, behavioral data and a model of bighorn reproductive competition, we estimated that coursing rams fathered 44% of 142 lambs assigned paternity in two natural populations. In one population, the probability of successful defense against coursing was lowest among rams that had many female consorts and held highest dominance rank. Even so, per capita annual male reproductive success was positively associated with social rank in both herds when measured in terms of fall conceptions. The proportion of coursing versus defending ram matings in each population (0.36 and 0.39) was similar to the corresponding fraction of lambs (0.43 and 0.47) fathered by coursing rams, suggesting that sperm competition approximated a fair lottery. Male traits important in gaining social status and obtaining cooperative consorts with ewes were different and potentially in conflict with those needed to defend against (and practise) coursing. Although the concussive weapons (horns) of rams are less dangerous than, for example, the piercing weapons of other bovids, injury from falls and horn blows during coursing brawls may cause death, handicap future mating competition or increase the risk of predation. Coursing is a rare example of an unconventional alternative mating tactic that is high-gain and high-risk. Received: 17 April 1996 / Accepted after revision: 15 March 1997     

Intrasexual selection in Mirza coquereli : evidence for scramble competition polygyny in a solitary primate

The primates of Madagascar (Lemuriformes) deviate from fundamental predictions of sexual selection theory in that polygynous species lack sexual dimorphism, have even adult sex ratios and often live in female-dominated societies. It has been hypothesized that intrasexual selection in these species is either reduced or primarily focused on traits related to scramble competition. The goal of this study was to examine these hypotheses by studying the mating system of a solitary nocturnal species, Mirzacoquereli. During a 4-year field study in western Madagascar, I captured and followed 88 individually marked animals. I found that adult males were significantly larger than females, providing the first evidence for sexual size dimorphism in lemurs. In addition, the adult sex ratio was biased in favour of females in 3 out of 4 years. There was no significant sex difference in canine size, however. Males showed pronounced seasonal variation in testis size with a 5-fold increase before and during the short annual mating season. During the mating season, males had more injuries than females and more than quadrupled their home ranges, overlapping with those of more than ten females, but also with about the same number of rivals. Only about one social interaction per 10 h of observation was recorded, but none of them were matings. Together, these results indicate that these solitary lemurs are clearly subject to intrasexual selection and that male-male competition is primarily, but not exclusively, of the scramble type. In addition, they suggest that the above-mentioned idiosyncracies may be limited to group-living lemurs, that social systems of solitary primates are more diverse than previously thought, and that the temporal distribution of receptive females is responsible for this particular male mating strategy. Received: 11 January 1997 / Accepted after revision: 18 April 1997     

Adult sex ratio affects divorce rate in the monogamous endoparasite Schistosoma mansoni

“Divorce” (mate switching) rate is known to vary largely both between and within socially monogamous species. Although the adult sex ratio can have an important influence on mating patterns, very few studies have investigated the influence of sex ratio on divorce rate in monogamous species, and even less so from an experimental point of view. In addition, most studies on the causes and consequences of divorce have been performed on vertebrate species, whereas data for invertebrate monogamous species remain scarce. Schistosoma mansoni is a monogamous endoparasite with a complex life cycle characterized by asexual reproduction in the intermediate host and sexual reproduction in the definitive host. In the wild, populations of S. mansoni inside their definitive hosts are characterized by a male-biased sex ratio. We studied the influence of experimentally varying the adult sex ratio on divorce rate in S mansoni, using controlled infections of hosts with clonal populations. The more male-biased the sex ratio was, the more the divorce rate increased, whereas no such effect was observed under a female-biased sex ratio. In this study and for the first time, we showed, by handling the sex ratio, that the divorce rate increases in adult male-biased sex ratio conditions in a monogamous species.     

Sex in the dark: determinants and consequences of mixed male mating tactics in<Emphasis Type="Italic"> Microcebus murinus</Emphasis>, a small solitary nocturnal primate

In mammals with solitary females, the potential for males to monopolize matings is relatively low, and scramble competition polygyny is presumed to be the predominant mating system. However, combinations of male traits and mating tactics within this type of polygyny have been described. The main aim of our study was to identify the relative importance of, and interactions among, potential determinants of contrasting male reproductive tactics, and to determine their consequences for male reproductive success in a small solitary nocturnal Malagasy primate, the gray mouse lemur (Microcebus murinus). We studied their mating behavior over three consecutive annual mating seasons. In addition, we determined the genetic relationships among more than 300 study animals to quantify the reproductive success of individual males. We found that, with a given relatively low overall monopolization potential, successful male mouse lemurs roamed extensively in search of mates, had superior finding ability and mated as early as possible. However, contest competition was important too, as temporary monopolization was also possible. Males exhibited different mating tactics, and heavier males had a higher reproductive success, although most litters had mixed paternities. Switching between tactics depended on short-term local variation in monopolization potential determined by a pronounced dynamic in fertilization probability, number of alternative mating opportunities, and the operational sex ratio. This study also revealed that the dynamics of these determinants, as well as the mutual interactions between them, necessitate a detailed knowledge of the mating behavior of a species to infer the impact of determinants of alternative mating tactics.Electronic Supplementary Material Supplementary material is available in the online version of this article at Communicated by S. AlbertsThis revised version was published online in August 2004 with corrections to Figure 2.     

Parental investment, adult sex ratios, and sexual selection in a socially monogamous seabird

Although most birds are monogamous, theory predicts that greater female parental investment and female-biased adult sex ratios will lower the polygyny threshold. This should result in polygynous mating, unless obligate biparental care or the spatial and temporal distribution of fertilizable females constrains a male’s ability to take advantage of a lowered polygyny threshold. Here we present data on the extent of male sexually dimorphic plumage, adult sex ratios and breeding season synchrony in three populations of a socially monogamous seabird, the brown booby Sula leucogaster. For one of these populations, San Pedro Mártir Island, we also present data on differences in male and female parental investment, mortality and probability of pairing. The extent of plumage dimorphism varied among populations. Sex ratios were female biased in all populations. On San Pedro Mártir Island, parental investment was female biased, females failed more often than males to find a mate, but there was no polygyny. We suggest that on San Pedro Mártir: (1) a period of obligate biparental care coupled with a relatively synchronous breeding season constrained the ability of males to take advantage of a high environmental polygamy potential and (2) the resulting socially monogamous mating system, in combination with the female-biased adult sex ratio, caused females to be limited by the availability of males despite their greater parental investment. Received: 18 November 1999 / Accepted: 24 January 2000     

Sexual size dimorphism in fallow deer (Dama dama): do larger, heavier males gain greater mating success?

Sexual size dimorphism may evolve as a result of both natural and sexual selection. In polygynous mammals, the main factor resulting in the evolution of large body size in males is the advantage conferred during competition for mates. In this study, we examined whether sexual selection acts on body size in mature fallow bucks (Dama dama) by examining how the following traits are inter-related: age, body (skeletal) size, body mass, prerut dominance rank, rut dominance rank and mating success. This is the first study to examine how all these factors are together related to the mating success of a large sexually dimorphic and polygynous mammal. We found that male mating success was directly related to body size, but not to body mass. However body mass was related to prerut dominance rank which was in turn strongly related to rut dominance rank, and thus there was an indirect relationship between mating success and body mass. Rut dominance rank was the variable most strongly related to mating success. Mating success among mature males was unrelated to age. We conclude that larger mature fallow bucks have advantages over other males when competing for matings, and sexual selection therefore continues to act on sexual size dimorphism in this species. Heavier fallow bucks also have advantages, but these are mediated through the dominance ranks attained by males before the rut.     

Genetic relatedness of worker nestmates in Myrmica ruginodis (Hymenoptera : Formicidae) populations

Summary Genetic relatedness and mating structure of the red ant, Myrmica ruginodis, were studied from a large data set in several natural populations, one of which was a substructured archipelago population. Within-colony relatedness of worker nestmates was measured as genotypic correlation and mating structure as deviation from random mating; both were calculated from the genotype frequency data. The average relatedness of worker nestmates was rather high across populations (0.43–0.73), and random mating was the rule within populations. The data were used to deduce the social structure of populations. With one possible exception, populations turned out to be weakly polygynous, implying that they consisted of the macrogyne form of the species. The exceptional population was relatively more polygynous and inbred, suggesting that it might include the microgyne form. This exceptional population lived in a habitat not noticeably different from that of the other populations, challenging previous suggestions of the habitat specialization of the two forms. The colonies were facultatively polygynous, and a comparison with earlier studies shows that polygyny is functional. Relatedness of worker nestmates varied slightly among populations, indicating that the social structure of Myrmica colonies is flexible.