ARCTIC VOL. 46, NO. 4 (DECEMBER 1993) P. 354-358

Life Histories of Subarctic JÜRGEN HEINZE’

(Received 29 June 1992; accepted in revised form 5 October 1992)

ABSTRACT. belonging to seven genera occur in habitats near the tree line in the Northern Hemisphere.An analysis of colony founding strategies suggests that in addition to physiological cold resistance, behavioral and sociometric adaptations might be important for survival and propagation of ants in subarctic biomes. Key words: polygyny, budding, ants, Formicidae, &SUMÉ. Des esphes de fourmis appartenant B sept genres vivent dans des habitats aux environs de la limite d’arbres. Une analyse des strat6giesde fondation des soci6t6s suggbre que, en plus d’une r6sistance physiologique au froid, des adaptations sociomktriques et comportementales pourraient être importantes pour la survie et la propagation dans les biomes subarctiques. Mots cl6s : polygynie, bouturage, fourmis, Formicidae, Leptothorax

INTRODUCTION tree line in tundra habitats (e.g., Gregg, 1972). The ants ranging farthest north are taxa of Leptothorax (sstr.): a The ant fauna of boreal and alpine biomes consists of a colony of Leptothorax “muscorum” was collected 80 km monotonously small number of species and thus has been north of the tree line on Richards Island, N.W.T. (Brown, given little attentionby myrmecologists (Gregg, 1972; 1955); Arnoldi (1968, 1969) lists L. acervorum as the only Francoeur, 1983; Nielsen, 1987). Nevertheless, ants locally species occurring inRussian tundra; andat least two occur in high densities even near and beyond the tree line, Leptothorax species are common in tundra habitatsin Quebec suggestingthat they play an important role in subarctic (Francoeur, 1983; pers. obs.). Of the other species, at least ecosystems, both as prey and predators (Nielsen, 1987). Camponotus herculeanus most probably does not advance Borealants have to cope withharsh environmental far into treelessareas, as it requires woodas nesting material. conditions, such as extremely low temperatures and long Ants are completely absent from Greenland and Iceland winters, but little is known about how they are adapted to today, but fossil Camponotus and Formica were found in life in thecold. Ants hibernate in a dense clusterworkers, of the Plio-Pleistocene Kap K$benhavn Formation in northern- queens, and in some species also larvae, and only few workersmost Greenland (Bocher, 1989). are found solitarily near the nest entrance and in other parts of the colony (Eidmann, 1943). Nest cavitiesof Leptothorax, LIFE HISTORIES OF BOREAL ANTS: LEPTOTHORAX subgenus Leptothorax (s.str.), may be partially filled with ice crystals, and even under protective snowcover With atotal length of less than 5 mm, Leptothorax (sstr.) temperatures in the nest mayfall below -20°C (Berman et are by far the smallest boreal ants and also havethe smallest al., 1982). HibernatingLeptothorax acervorumfrom Siberia colonies. Typical nests contain from a few dozento hundreds are known to survive temperatures of -40°Cby accumulating of workers (Fig. 1) andare built in cavitiesin decaying twigs, anti-freeze polyols (Leyrikh, 1989). moss, or bark, or under stones. Recently it was suggested that in addition to physiological A comparative analysis of colony and population structures adaptations, changed life history strategies might facilitate of leptothoracine ants from variousbiomes suggests a the propagation of boreal ants (Heinze, 1991). correlation between life in northern habitats and optional In the following, records of ants from areas near the tree dependent colony founding strategies. The majority of the line and dataon their colony structures and colony founding approximately 300 Leptothorax species live in temperate strategies are summarized. Possible interrelations between or subtropic biomes, such as the Mediterranean, the Near life history traits and the occurrence in areas with long and East, or the southern United States.In most southern species cold winters are examined. (subgenera Myrafant, , Macromischa, etc.) nests are obligatorily monogynous, i.e., they contain only RECORDS OF ANTS NEAR THE TREE LINE a single inseminated egg-laying queen. New colonies are foundedindependently by young queens, which isolate According to the sparse data scattered in the literature themselves in small cavities directly after matingin midsummer and from personal observation, species of seven genera of (Table 2; see also Buschinger, 1968,1974; Plateaux, 1970). ants live near the tree line in northern and America In contrast, in colonies ofall species belongingto the boreal (Table l), three belongingto the subfamily Formicidae subgenus Leptothorax (sstr.) and in many boreal and alpine (Camponotus, Formica, Lusius) and four to the L. (Myrafant) several inseminated queens may be present (,Halpagoxenus, Leptothorax, Myrmica). All (e.g., Buschinger, 1968; Alloway et al., 1982; Heinze and species are common throughout boreal coniferous forests, Buschinger, 1988; Stille et al., 1991). Solitaryfounding but only few reports document their occurrence beyond the queenshave rarely been found. Direct observations and

‘Theodor-Boveri-Institutfiir Biowissenschaften, Zoologie II, Am Hubland, D-97074 Würzburg, F.R. Germany @The Arctic Institute of X

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X 356 / J. HEINZE and openly aggressive interactions in early spring, resulting mating in spring or early summer (Holldobler and Wilson, in the formation of rank orders. In each nest onlythe highest 1990). In Quebec, founding queens of Camponotus may ranking queen began to lay eggs (Heinze and Smith, 1990; therefore be found already in earlyJune (pers. obs.), whereas Heinze and Lipski, 1990; Heinze and Ortius, 1991). in other species mating and colony founding takes place only In both polygynous and functionally monogynous in late July and August. Leptothorax (s.str.), spontaneous fractioning of colonies (budding) and queen emigration were observed in the ADAPTIVE VALUE OF DEPENDENT COLONY FOUNDING laboratory. In Leptothorax sp.A and L. cf. canadensis from Quebec and New England andL. in acervomm from northern Generally, natural selection is thought to favor colony Norway and Alaska some inseminated queensleft their nests founding by solitary ant queens and the monopolization of directly after hibernation. Queens movedinto empty nesting reproduction by single queens(e.g., Holldobler andWilson, sites and were occasionally joined by workers from their 1990). In southern ant faunas, a large percentage of species maternal nests. The presence of very small colonies of are strictly monogynousand new nests are initiated by L. sp.A, L. gredleri, and L. acervorum in early spring dispersing queens immediately after mating in midsummer. suggests that budding occurs regularly also in the field. Alternative strategies - queen adoption, polygyny, and budding - appear to be adaptations to special ecological conditions, such as unstable nest sites, habitat patchiness, SOCIAL ORGANIZATION OF OTHER BOREAL ANTS or resource limitation (e.g., Holldobler and Wilson, 1977, Though data on colony and populationstructures of boreal 1990; Bolton, 1986). ants are rare, queen adoption and budding appear to be In six of seven boreal ant genera, independent colony common strategies also among other boreal genera (Tablefounding 1). is at least partly replaced by dependent strategies: Many Myrmica and Formica species are facultatively queen adoption after mating and budding after hibernation polygynous, and colony budding was reported from both or parasitic invasion of host species colonies. genera (e.g., Rosengren and Pamilo, 1983; Czechowski, Asyet it is not clear whether the surprisingly high 1984; Uchmariski and+tal, 1982). Queen emigration is likely frequency of dependent founding among boreal species is in Myrmica (e.g., Elmes, 1982). Though most species of accidental or causally connected with life the in cold. Several Lasius (s.str.) are strictly monogynous, facultative polygyny species of Lasius, Formica, Leptothorax,and Myrmica from was reported from Lasius flavus (Waloff, 1957). temperate habitats are also facultatively polygynous, and A rather high percentage of boreal antsare social parasites Formicalugubris is mainlymonogynous in Finland but - i.e., they depend on the help of other ant species at least polygynous in Central Europe (Pamilo et al. , 1992). Hence, during parts of their life cycles. Queens of temporary it appears unlikely that dispersal strategies changed in all parasites, e.g., species belonging to the Formica rufa and these generaas an adaptation to lifein boreal biomes. Rather, Formica exsecta groups, invade nests of other Formica already existing life history traits might have favored species, where they somehow eliminatethe host queen. Host facultatively polygynous species during the colonization of workers take care of the parasite’s brood but are eventually habitats near the tree line. Colony fractioning might be replaced by the parasite queen’s own workers. New nests facilitated in boreal biomes by the comparatively simple may also arise through colony budding and nest splitting(e.g., structuring of ecosystems, in which potential nestsites have Holldobler and Wilson, 1990). sublaevis and little diversity but occur in high densities. Furthermore, H. canademis are slave makers. Queensenter a Leptothorax resource limitation and the shortness of summers in taiga (s.str.) nest and kill or expel all adult residents. Workers and tundra might have favored species that produce few that eclose from the conquered Leptothorax pupae nurse the queens with a “safe” colony founding strategy rather than Harpagoxenus brood. Harpagoxenus workers are inefficient numerous dispersing queens with a high rate of founding in brood care and do not forage; thus the parasites failure. permanently depend on their hosts. New host workers are In addition to budding, boreal ants might benefitalso from obtained in slave raids, during which Harpagoxenus workers another aspect of their changed life histories. Some pillage neighboring Leptothorax colonies (e.g., Holldobler Leptothorax (s.str.) queens emigrate from their nests directly and Wilson, 1990). Formicoxenus are guest antsin the nests after hibernation and probably attempt solitary founding in of Myrmica or Formica. Though they beg food from their spring rather than budding. Similarly, due to the unusual hosts, they are typically capable of rearing their brood rearing time of winged sexualsCamponotus, in new colonies themselves. New colonies may be founded by budding within are founded by solitary queens in spring. Due to the delay a host nest (Lenoir et al., 1992). of colony founding from late summer and fall to spring, Camponotus herculeanusappears to be the only boreal ant solitary hibernation is avoided and the period during which species in which new nests are founded independently by ayoung queen can rear her first brood is extended. In solitary queens. Mature colonies are typically monogynous. preliminary laboratory studies with Leptothorax cf. Queens are intolerant of each other, but occasionally two canadensis, solitarily hibernating ant workers and ants whose or more queens may coexist in different parts of the same winter clusters were experimentally scattered suffered colony (“oligogyny” ; Holldobler, 1962).In contrast to other distinctly increased mortality (Heinze, unpubl.data). Herbers ants, male and female sexuals of Camponotus are reared in (1986) reported that polygynous colonies of Leptothorax late summer and hibernate in their maternal nests before (Myrafant) longispinosus survived winter better than SUBARCTIC ANTS / 357 monogynous nests. A positive effect group of size on winter CZECHOWSKI, W. 1984. Colony fission and intraspecific contests in mortality, probably causedby a higher resistanceof clustered Myrmicu hevinodis NYL. (,Formicidae). Annales Zoologici 38(4):99-199. ants against both desiccation (Sigal and Arlian, 1982; but EIDMANN, H. 1943.: Die ijberwinterung der Ameisen. Zeitschrift fir see Copp, 1983) and excessive moisture, might thus favor Morphologie und Okologie der Tiere 39(2):217-275. hibernation of young queens in established nests. ELMES, G.W. 1982. The phenology of five species of Myrmicu (Hym. Formicidae)from South Dorset, England. Insectes Sociaux Further studies on hibernation strategies and colony and 29(4):548-559. population structures of boreal Leptothorux (s.str.)are ESPADALER, X., PLATEAUX, L., and CASEVI’FZ-WEULERSSE, J. planned to substantiate these ideas. Data on the social 1984. Leptothorax melus, asp., deCorse. NotesBcologiques et organization of sub-Antarctic ants, which belong to other biologiques [Hymenoptera, Formicidae].Revue franpise d’Entomologie 6(3):123-132. subfamilies (Ponerinae: Acunthoponeru; Dolichoderinae: FRANCOEUR, A. 1983. The ant fauna near the tree-line in northern Dorymyrmex, Tupinoma) and thus have adapted convergently Quebec (Formicidae, Hymenoptera). Nordicana 47: 177-180. to cold climates, will be of special interest. GREGG, R.E. 1972. The northward distribution of ants in North America. Canadian Entomologist 104: 1073-1091. HEINZE, J. 1991. Koloniegriindung in der Kiilte: Strategien von Ameisen- ACKNOWLEDGEMENTS weibchen in borealenHabitaten. Verhandlungen der Deutschen Zoologischen Gesellschaft 84:308. Studies on colony and population structures nearcticof ants were HEINZE, J., andBUSCHINGER, A. 1988. Polygynyand functional supported bythe DFG Leibniz-Award to B. Holldobler (Wiirzburg) monogyny in Leptothorax ants (Hymenoptera: Formicidae). Psyche and a DFG grant (He1623/2-1). Thanks are due to B. Holldobler 95(3-4):309-325. for comments on the manuscript and to C. Peeters, Wurzburg, for HEINZE, J., and LIPSKI, N. 1990. Fighting and usurpation in colonies of the palaearctic ant . Naturwissenschaften the French abstract. This presented talk during paper was as a the 77:493-495. 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