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Keywords:

  • Cardiocondyla;
  • colony founding;
  • longevity;
  • queen number;
  • queen polymorphism;
  • trade-off reproduction–dispersal;
  • wing muscle polymorphism

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

The number of queens per colony is of fundamental importance in the life history of social insects. Multiple queening (polygyny), with dependent colony founding by budding, has repeatedly evolved from ancestral single queening (monogyny) and independent founding by solitary queens in waSPS, bees and ants. By contrast, the reversal to monogyny appears to be rare, as polygynous queens often lack morphological adaptations necessary for dispersal and independent colony founding. In the ant genus Cardiocondyla, monogynous species evolved from polygynous ancestors. Here, we show that queens of monogynous species found their colonies independently, albeit in an unusual way: they mate in the maternal nest, disperse on foot and forage during the founding phase. This reversal appears to be associated with the occurrence of a wing polymorphism, which reflects a trade-off between reproduction and dispersal. Moreover, queens of monogynous species live considerably longer than queens in related polygynous taxa, suggesting that queen life span is a plastic trait.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Cooperation among biological entities is a powerful force in evolution underlying major transitions, such as those from prokaryotes to eukaryotes, from single-celled organisms to multicellular organisms, and from solitary animals to animal societies (Maynard-Smith & Szathmáry, 1997). Reversions of the evolutionary trajectories may occasionally occur (Siddall et al., 1995; Wcislo & Danforth, 1997) but are comparatively rare and their causes and consequences are not well understood.

The evolution of polygyny (presence of multiple reproductive females per nest) from monogyny (single reproductive female per nest) in social insects has been likened to the origin of eusociality, in that in both cases individuals give up their reproductive autonomy and are forced to find a compromise with others about the partitioning of reproduction (Rosengren & Pamilo, 1983; Keller & Vargo, 1993; Keller, 1995). Queen number is believed to be a rather plastic trait (Ross & Carpenter, 1991). Most eusocial bee species are strictly monogynous, but polygyny has evolved convergently in waSPS and in many ants.

In ants in particular the switch from one to many queens per nest is often associated with dramatic changes in dispersal and colony-founding strategies and, connected to this, morphological alterations. Young queens of monogynous species normally disperse to mate and find their own societies independently either alone or together with other founding queens (pleometrosis). In contrast, queens of polygynous species mate in or near their maternal nests, and new colonies are initiated by colony fission or ‘budding’ with the help of workers (Keller, 1991, 1993; Ross & Carpenter, 1991). Accordingly, the evolution of multiple queening appears to be favoured whenever the success of solitary colony founding is low, as with habitat saturation or patchiness, resource limitation, high predation risk and high dispersal costs (Hölldobler & Wilson, 1977, 1990; Ross & Carpenter, 1991; Herbers, 1993; Bourke & Heinze, 1994).

Nondispersing, dependently founding queens do not need the large resources required for solitary colony founding and are therefore often much smaller and have fewer fat reserves than queens of monogynous species (Keller & Passera, 1989; Keller, 1991; Stille, 1996). Thus, queens of species with regular polygyny have often lost the capability of establishing new colonies independently (Keller, 1995). Furthermore, because queens from polygynous species produce sexuals faster and probably have a higher extrinsic mortality, they have a considerably shorter life span in perennial social insects than queens from monogynous species (Tsuji & Tsuji, 1996; Keller & Genoud, 1997).

All these adaptations impede the reversal from polygyny to monogyny, and such an evolutionary pathway has, to our knowledge, not yet been demonstrated in social insects, although the occasional association of monogyny with peculiar colony-founding tactics suggests that it might occur in a number of taxa with both monogynous and polygynous species. For example, several monogynous species of slave-making ants have presumably evolved from polygynous ancestors (Beibl et al., 2005). The young slave-maker queens found new colonies in a parasitic way by usurping colonies of other ant species and killing the resident queens (Buschinger, 1986, 1990; Hölldobler & Wilson, 1990; Bourke & Franks, 1991). In this case, the reversal to monogyny is adaptive, as the reproductive output of a queen is limited by the number of locally available slave workers (J. Heinze & S. Foitzik, unpublished).

A phylogenetic analysis suggests a similar reversal from ancestral polygyny to derived monogyny in the nonparasitic ant genus Cardiocondyla. Monogyny evolved at least once in a clade comprising several species from xeric habitats in Southern Europe and Central Asia (Heinze et al., 2005). In at least four species of this phylum, obligatory single queening is well supported (Marikovsky & Yakushkin, 1974; Tarbinsky, 1976; Heinze et al., 2002; Seifert, 2003; Schrempf et al., 2005a; J.C. Lenoir et al., in press; A. Schrempf, unpublished), and appears to be associated with seasonal production of sexuals.

Queens of polygynous Cardiocondyla usually found new colonies by budding (Stuart, 1990; Heinze & Delabie, 2005; A. Schrempf, unpublished) after mating with males in their maternal nests (Heinze & Hölldobler, 1993; Heinze et al., 2005). They are only a little larger than the workers (Seifert, 2003) and extremely short-lived (Schrempf et al., 2005b). Whereas only a single fertile queen is tolerated in colonies of monogynous Cardiocondyla, queens in polygynous species are mutually tolerant and workers do not attack surplus queens (Schrempf et al., 2005a). Queen–worker dimorphism is only a little more pronounced in the derived monogynous species (Seifert, 2003). The question therefore arises how female sexuals cope with problems resulting from the secondary switch to monogyny, and whether queen number in Cardiocondyla is indeed associated with other changes in life-history traits, in particular concerning the mode of colony founding. Field observations, according to which young queens of monogynous species leave their maternal nests after mating, disperse on foot and solitarily search for suitable nesting sites, suggest independent colony founding (Heinze et al., 2002).

In this study, we investigate colony-founding success and longevity of polygynous and secondarily monogynous Cardiocondyla species to determine how queen strategies have changed with the evolutionary reversal to monogyny. In addition, we examined the role of wing reduction, which appears to be common in monogynous Cardiocondyla but absent from polygynous taxa (Seifert, 2003). We show that female sexuals of monogynous Cardiocondyla nigra and Cardiocondyla batesii found their own nests solitarily after shedding their wings and dispersing on foot and during this period forage for food. Wing reduction appears to be associated with the replacement of wing muscles by fat, i.e. queens trade off dispersal capability with increased fecundity and higher survival rate during solitary founding. Finally, we document that the average reproductive life span of queens is considerably longer in monogynous than polygynous Cardiocondyla species.

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Study sites and ants

We investigated colony-founding success, queen anatomy and longevity of two monogynous species, Cardiocondyla batesii (Forel, 1894) and Cardiocondyla nigra (Forel, 1905), and one polygynous species, Cardiocondyla minutior (Forel, 1899). Although budding has already been reported from polygynous Cardiocondyla (Stuart, 1990; Heinze & Delabie, 2005), we included the latter species to be able to compare directly the outcome of founding experiments, queen anatomy and longevity between the three species. All queens of polygynous species eclose with well-developed, long wings (macropterous, ‘M-queens’). In contrast, female sexuals of monogynous species are either long winged or short winged (brachypterous, ‘B-queens’).

Female sexuals and wingless males of the monogynous species C. batesii and C. nigra mate inside the maternal nest in autumn. Young queens hibernate in the nest and shed their wings immediately before emigrating from the nest in spring to disperse on foot (Heinze et al., 2002). In contrast, female sexuals and both winged and wingless male ants of C. minutior are produced year-round.

We collected a total of 988 young queens of C. batesii from 52 colonies at four different localities near Granada, Spain, in April 2001, April 2004 and April 2005. Young queens from Padul, Víznar, Agrón and from 21 colonies from Sierra Elvira were exclusively long-winged M-queens. In contrast, 13 additional colonies from Sierra Elvira contained on average 25 % short-winged B-queens and 75 % queens which externally resembled M-queens but on closer inspection were found to lack flight muscles. They therefore represent flightless, macropterous queens and henceforth are referred to as ‘m-queens’.

All 621 young queens of C. nigra collected in March 2003 in 27 colonies from Panagia Evangelistria, Kalavasos and Alassa near Limassol (Southern Cyprus) were short-winged B-queens. All 210 long-winged M-queens of C. minutior eclosed in laboratory colonies, which were reared from nests collected at CEPLAC, Ilhéus, Brazil, in October 2002 (for an overview on the investigated species and queen morphs see Table 1). If not mentioned otherwise, experiments were conducted with winged young queens collected from their nests before dispersal.

Table 1.   Collecting sites, queen number and queen morph of the investigated species.
SpeciesCardiocondyla nigraCardiocondyla batesiiC. batesiiCardiocondyla minutior
  1. In correspondence with wing muscle development (see text), queen morphs of C. batesii are entitled throughout the manuscript as given below (nomenclature).

Collecting sitesCyprus: Panagia, Evangelistria, Kalavasos, AlassaSpain: Sierra ElviraSpain: Padul, Agron, Viznar, Sierra ElviraBrazil: Itabuna
Queen numberMonogynyMonogynyMonogynyPolygyny
Queen morph found in the coloniesShort-winged brachypterousShort-winged (25 %) and long-winged (75 %) brachypterous and flightless macropterousLong-winged macropterousLong-winged macropterous
Wing muscle development++
NomenclatureC. nigra (B)B-queens and m-queensM-queensC. minutior (M)

Founding experiments

We investigated whether queens of the different species and morphs are able to found solitarily (independent founding), together with other queens (pleometrotic founding), or only together with the help of workers (dependent founding). Founding experiments were conducted with single queens (C. nigra: 122 B-queens; C. batesii: 110 M-queens from Padul, 44 B-queens and 40 m-queens from Sierra Elvira; C. minutior: 110 M-queens), groups of four queens each (C. nigra: 24 groups with B-queens; C. batesii: 24 groups with M-queens and 8 groups with B-queens; C. minutior: 12 groups with M-queens), single queens assisted by three workers each (n = 12 for each species; C. batesii only M-queens) and, for C. minutior, 10 single queens assisted by 10 workers each. Some queens of C. nigra and several M-queens of C. batesii from Padul had already shed their wings and left the maternal nest at the time they were collected. M-queens from the different populations did not differ in ovarian status and fat content (see below) and the founding ability of M-queens from Padul might be representative also for M-queens from other populations. In an additional experiment, two young queens were placed together with approximately 30 workers from their maternal colonies into large plastic boxes, where they had the opportunity to separate into distant nest chambers (n = 3 for each species).

Experimental colonies were housed as described previously (Schrempf et al., 2005a) at rearing temperatures resembling natural conditions (C. nigra and C. batesii: 27–24 °C; 12/12 h; C. minutior: 30–25 °C; 12/12 h). Honey and pieces of cockroaches were offered three times per week in a separate nest chamber. The numbers of eggs, larvae, pupae and adults, as well as the location of the queens (e.g. nest chamber, feeding sites) were recorded twice per week. Colony foundation was considered successful once young workers eclosed.

Fat content, development of flight muscles and ovarian status

Fat content of young queens was determined by gravimetry (Peakin, 1972) immediately after transfer to the laboratory. After drying for 24 h at 65 °C and individual weighing to the nearest 0.1 μg, each queen was soaked in diethyl ether for 48 h, dried again for 24 h and reweighed to estimate the weight of extracted fat. Queens that disintegrated during this procedure were excluded from the analysis. Fat content could be determined in 10 B-queens from five C. nigra colonies, eight M-queens from four C. minutior colonies, and a total of 59 C. batesii queens (eight or nine queens each from different colonies from each of the three collecting sites with only M-queens, and 12 B-queens, 12 m-queens, and nine M-queens from Sierra Elvira).

Flight muscle development was investigated in semi-thin sections (0.5–1 μm, epoxy resin, stained with 1 % Toluidin blue) of the thorax of six young B-queens of C. nigra, five M-queens of C. minutior and 15 queens of C. batesii from different populations. Furthermore, we prepared sections of three young C. batesii M-queens from Padul, which had been collected in autumn 2003 before hibernation, and of four freshly eclosed M-queens from laboratory colonies originating from Padul. Flight muscle and thorax volume were measured in two representative individuals each using image analysis 2.1 (Soft Imaging Software GmbH, Münster, Germany).

Ovary development was investigated by dissecting young queens of C. batesii a few hours after collection in the field (10 B-queens, 10 m-queens and 30 M-queens from different populations). Number and length of ovarioles per ovary, number of developing eggs and presence or absence of yellow bodies (remnants of previously developed eggs) were recorded. Ovaries were categorized according to the presence of small egg cells only (I), maturing eggs of intermediate size (II) or mature eggs (III). For a comparison between queen morphs, the number of a queen's eggs was multiplied with a size factor (one, two and three according to the ovarian category; henceforth referred to as ovarian status).

Queen longevity and timing of sexual offspring production

To determine the longevity of queens, we set up small laboratory colonies with freshly inseminated young queens. To be able to compare the life span of queens from monogynous species from seasonal habitats with hibernation and polygynous, tropical Cardiocondyla, we subtracted the duration of the inactive winter period from the life span of queens from monogynous species. Furthermore, we recorded the time when the first sexuals were produced in the experimental colonies.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Founding experiments

Solitary queens of monogynous C. batesii and C. nigra were able to found colonies in a semi-claustral way, i.e. they foraged for food until the first workers eclosed. During the inspection of the colonies, at least 36 (29.5 %) of the queens of C. nigra and 49 (25.3 %) of C. batesii queens were observed at least once at the provided food. Short-winged B-queens of both C. batesii and C. nigra had a significantly higher colony-founding success than long-winged M-queens of C. batesii and C. minutior. The difference is caused by the higher success of short-winged B-queens, which were similarly successful in C. batesii and C. nigra and more successful than long-winged M-queens with developed flight muscles of C. batesii (Table 2). Flightless m-queens of C. batesii, i.e. long-winged queens without wing muscles, did not differ in colony-founding success from short-winged B-queens of C. batesii. Similarly, egg laying rates of isolated queens differed significantly among species and morphs. They were higher in B-queens and m-queens of C. batesii and B-queens of C. nigra than in M-queens of C. batesii and queens of C. minutior (Fig. 1). Furthermore, queens of C. nigra and B-queens and m-queens of C. batesii started egg laying earlier than M-queens of C. batesii (Kruskal–Wallis H-test, H4,180 = 133.54, P < 0.0001; Table 2), and accordingly, first workers eclosed earlier in colonies initiated by queens of C. nigra (46 days) and B-queens and m-queens of C. batesii (54 and 57 days) than in colonies founded by M-queens (72 days). Although queens of C. minutior started egg laying already after 3 days they never succeeded in raising brood solitarily and no workers eclosed.

Table 2.   Percentage of Cardiocondyla queens that were successful in founding a colony either in isolation, with other queens or assisted by workers, and the start of egg laying in colonies with single queens (mean ± standard deviation).
 Cardiocondyla nigraB-queensm-queensM-queensCardiocondyla minutior
  1. Identical letters indicate groups of queens that did not differ significantly (chi-squared tests and Mann–Whitney U-test respectively; P > 0.05).

Single queens23.7 % (29/122) a13.6 % (6/44) a10 % (4/40) a3.6 % (4/110) b0 % (0/110) c
Start of egg laying (days)6.9 ± 1.27 a6.05 ± 2.29 a6.00 ± 0.81 a18.7 ± 7.82 b3.04 ± 1.08 c
Four queens0 % (0/24)0 % (0/8)No data0 % (0/24)33.3 % (4/12)
One queen, three workers75 % (9/12)No dataNo data41.6 % (5/12)0 % (0/12)
One queen, 10 workersNo dataNo dataNo dataNo data20 % (2/10)
image

Figure 1.  Differences in egg laying rates of Cardiocondyla queens (median, quartiles and range). Significant differences are indicated by different letters. Kruskal–Wallis H-test, H4,175 = 70.24, P < 0.0001; Mann–Whitney U-test: B vs. m: U = 153, P = 0.137; B vs. M: U = 60.5, P < 0.0001; m vs. M: U = 83.0, P < 0.0004; Cardiocondyla nigra vs. B: U = 638, P = 0.521; C. nigra vs. all other groups: P < 0.02; Cardiocondyla minutior vs. M: U = 416, P = 0.235; C. minutior vs. all other groups: P < 0.0003.

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When accompanied by three workers, the proportion of successful foundresses was much higher than that of solitary queens of C. batesii and C. nigra (Table 2), but C. minutior queens succeeded only when assisted by 10 workers. Cooperative colony founding failed in both C. batesii and C. nigra, as queens became aggressive and fed on each others’ eggs. Queens even fought when they had the opportunity to separate in a spatial manner with a few workers from each other, and workers attacked queens until only one survived. In contrast, C. minutior queens tolerated each other and workers also did not exhibit any aggression. Four of 12 queen associations of C. minutior queens succeeded in raising young workers.

Fat content, development of flight muscles and ovarian status

The different queen types of C. batesii differed in dry weight, although with only marginal significance (anova: F5,59 = 2.23, P = 0.06). To eliminate a possible influence of queen weight we also analysed fat content data with an ancova, using dry weight as cofactor. Queens from mixed colonies (B-queens and m-queens) had significantly more fat than M-queens from the four collecting sites (ancova, F5,59 = 3.86, P < 0.01; post hoc Fisher LSD: B versus m: P = 0.53; B versus M: P < 0.01 for all four sites; m versus M: P < 0.01 for all four sites; comparison between different collecting sites: Mi versus Mj: P > 0.2 for all comparisons). Queens of C. nigra had a similar fat content as B-queens of C. batesii, whereas the fat content of C. minutior was even lower than in the least fat M-queens of C. batesii (see Fig. 2).

image

Figure 2.  Fat content of Cardiocondyla queens (LS mean and 95 % confidence interval). Significant differences are indicated by different letters. ancova, F4,77 = 36.36, P < 0.0001; post hoc Fisher LSD: B versus m: P = 0.56; m versus M: P < 0.005; B, m and M versus Cardiocondyla nigra: P < 0.0005; Cardiocondyla minutior versus all: P < 0.0001.

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Flight muscles of young, winged C. minutior queens were well developed (18 % muscle volume/thorax), completely replaced by fat in C. nigra queens and B-queens of C. batesii, and undeveloped or strongly reduced in m-queens (3.8 %). Flight muscles were well developed in four M-queens (12.8 %) and to some extent reduced in five others (9.36 %). Flight muscles were already reduced in M-queens collected before hibernation and also in freshly eclosed queens from the laboratory.

All dissected young winged queens had a filled spermatheca and 2 × 3 ovarioles without visible yellow bodies. The number of ovarioles corresponds to that of C. nigra and C. minutior. In C. batesii, the ovaries of all B-queens and of eight of 10 m-queens, but only of eight of 30 M-queens contained maturing eggs and over all, ovarian status was highest in B-queens, intermediate in m-queens and lowest in M-queens (see Fig. 3).

image

Figure 3.  Differences in ovarian status (egg number multiplied with a size factor) between morphs of Cardiocondyla batesii (median, quartiles and range). There is a significant difference between all morphs. Kruskal–Wallis H-test; H2,50 = 31.16; P < 0.0001; Mann–Whitney U-test: B vs. m: U = 8, P = 0.001; B vs. M: U = 1.5, P < 0.001; m vs. M: U = 57, P = 0.003.

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Queen longevity and time of sexual offspring production

It has previously been reported that queens of polygynous Cardiocondyla, such as Cardiocondyla obscurior (Schrempf et al., 2005b) and Cardiocondyla emeryi (J. Heinze, unpublished), are short lived compared with other ant queens. C. obscurior queens survived for an average of 26 weeks, with a maximum lifespan of 56 weeks (n = 47). In contrast, queens of C. batesii and C. nigra survived longer, even when corrected for inactivity during hibernation by subtracting 6 months for field collected queens and additional 4 months for each hibernation under artificial conditions in the laboratory. Queens of C. nigra survived on average 31.9 weeks (n = 26; maximum lifespan 66 weeks), and queens of C. batesii on average 56.1 weeks (n = 23, maximum 112 weeks; all three species: Kruskal–Wallis H-test: H2,96 = 27.23, P < 0.0001; Mann–Whitney U-tests: P < 0.05 for all comparisons between C. obscurior, C. nigra and C. batesii). Sexual offspring eclosed in the autumn of the first year in 13 of 26 C. nigra colonies and two of 23 C. batesii colonies. The other colonies reared sexuals in the autumn of the second year. In C. obscurior, first sexuals were reared approximately 6 weeks after the queens had mated.

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

With the evolutionary reversal from polygyny to monogyny, queens of the ant genus Cardiocondyla successfully ‘re-invented’ solitary colony founding, albeit in a rather unconventional way. Queens of monogynous Cardiocondyla resemble those of their polygynous relatives in that they mate in the maternal nest, usually shed their wings before emigrating, and disperse on foot to found solitarily. Although the founding success of queens was considerably higher when they were accompanied by workers than when they founded solitarily, worker aggression towards young queens and field observations of solitary dispersal show that budding is not a likely option for young queens in the field. Despite the low success rate of solitarily founding queens, which in the field is presumably even lower than in the laboratory, individual colonies of monogynous Cardiocondyla produce sufficiently high numbers of female sexuals so that a stable population size is maintained (e.g. Marikovsky & Yakushkin, 1974; Heinze et al., 2002; Schrempf et al., 2005a).

Many ant species with independent founding have queens with a large body size and a voluminous thorax, which allows rearing the first young with histolysed body tissues in complete seclusion (claustral founding). In contrast, queens of monogynous Cardiocondyla have remained only a little larger than the queens of their polygynous relatives (Seifert, 2003) and instead provision themselves and their brood by foraging (semi-claustral founding). Queens have indeed been observed carrying food items in the field (A. Schrempf, unpublished). Foraging appears to be complemented by the histolysis of body tissues, in particular in short-winged queens, whose unneeded wing muscles are replaced by fat.

For social insects in general our study documents that a simple reversal from regular polygyny to strict monogyny is not easy once queens have lost adaptations associated with solitary founding. Phylogenetic analyses document that polygyny is derived in most studied genera (e.g. Tetramorium, Schlick-Steiner et al., 2005; Lasius, Steiner et al., 2004; Solenopsis, Krieger & Ross, 2005; Shoemaker et al., 2006), and examples for the reversed evolutionary pathway are scarce. Other than Cardiocondyla, they are restricted as yet to a few socially parasitic ants (see above). The ant genus Cataglyphis might be another likely candidate, as the queens of monogynous Cataglyphis cursor found new colonies in an unusual way, i.e. on foot by budding (Lenoir et al., 1988). Unfortunately, the phylogeny of polygynous and monogynous Cataglyphis is not yet resolved. Simple transitions between polygyny and monogyny therefore appear to be confined to the large number of facultatively polygynous species, where queen number varies within species between colonies and populations. In these species, the morphology of queens from polygynous and monogynous colonies typically does not differ strongly, or if so, exhibits a genetically or environmentally mediated queen polymorphism (Buschinger & Heinze, 1992; Rüppell & Heinze, 1999).

Whereas numerous hypotheses have been forwarded for the evolution of polygyny, we can at present only speculate about the causes of the reversal to monogyny in Cardiocondyla. Polygynous Cardiocondyla species appear to occur predominantly in tropical and subtropical areas, whereas monogynous species live in Eurasian deserts, semi-deserts and steppes with a more stable, seasonal climate. This distribution is puzzling, as the latter habitats were thought to pose higher risks to dispersing and solitarily founding queens (e.g. Tinaut & Heinze, 1992; Tinaut & Ruano, 1992). The presence of rare, suitable patches within large, unsuitable areas has probably favoured female brachyptery (Seifert, 2003) and the two queen morphs might be associated with a dispersal polymorphism as in other ant species.

Although our study did not reveal a difference between the founding tactics of long-winged and short-winged queens of monogynous Cardiocondyla, short-winged queens were more successful in establishing an own colony than long-winged queens, because they had a significantly higher fat content, similar to that of other semi-claustrally founding ants (Keller & Passera, 1989). Moreover, short-winged queens start egg laying earlier, which is also reflected by a more progressive ovary development in spring. Queen polymorphism in C. batesii therefore nicely documents the trade-off between dispersal and reproductive success commonly found in solitary insects, where the large-winged morph is also significantly less fecund and lays eggs later than the flightless morph (Roff, 1984; Dixon & Howard, 1986; Ritchie et al., 1987). Although we observed winged female sexuals running, but never flying, in the field, and long-winged queens did not fly even in a wind tunnel (A. Schrempf, unpublished), winged queens might eventually be drifted farther away by strong winds than short-winged queens and thus could serve as colonizers of empty habitat patches. Then, the dimorphism could be stable, in spite of considerable costs for long-winged queens concerning colony-founding success.

As in other species (Tanaka, 1993; Mole & Zera, 1994; Zera & Denno, 1997), the reduction of wing muscles in short-winged Cardiocondyla queens does presumably not result from histolysis but from the inhibition of muscle growth. Wings are less costly than wing muscles (Roff, 1986), and the fitness differences between winged and wingless insects without wing muscles are therefore negligible (Zera & Mole, 1994; Zera & Denno, 1997). This explains why the founding success of long-winged C. batesii queens from mixed colonies does not differ from that of short-winged queens.

Finally, queens of monogynous Cardiocondyla have an increased life span in comparison with queens of their polygynous relatives, although they are still very short-lived compared with queens of monogynous ants with similar colony and body size, such as Temnothorax, which live three to four times longer (Keller & Genoud, 1997). Monogyny and polygyny in Cardiocondyla are associated with different habitats and a simple comparison of life spans between seasonally and continuously producing queens is therefore not possible. Our correction for the inactive hibernation period might have underestimated the true difference between the taxa. In any case, queen life span appears to be a plastic trait that evolves with changing life history. A similar point has previously been made concerning the life span of socially parasitic queens that kill the host queen and switch from iteroparity and long life to semelparity and short life (Heinze & Tsuji, 1995; Bekkevold & Boomsma, 2000). The prolongation of queen life span in monogynous Cardiocondyla is essential, as queens have to build up large workforce before they can rear sexuals. In contrast, queens of polygynous Cardiocondyla species are assisted by workers from the maternal nest ab ovo and produce sexual offspring almost immediately after mating.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Supported by Deutsche Forschungsgemeinschaft (He 1623 and Schr 1135/1-1) and Deutscher Akademischer Austauschdienst, DAAD (Acciones Integradas). We thank Dr A. Tinaut and C. Wanke for their support in the field, A. Schulz for information concerning C. nigra collecting sites and B. Lautenschläger for semi-thin sections and measurement of flight muscles.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References
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