A N N A L E S Z O O L O G I C I (Warszawa), 2006, 56(4): 667-673 DO PERMANENTLY MIXED COLONIES OF WOOD ANTS (HYMENOPTERA: FORMICIDAE) REALLY EXIST? WOJCIECH CZECHOWSKI and ALEXANDER RADCHENKO Laboratory of Social and Myrmecophilous Insects, Museum and Institute of Zoology, Polish Academy of Sciences, Wilcza 64, 00-679 Warsaw, Poland; e-mails: [email protected], [email protected] Abstract.— We describe the composition of two colonies of wood ants (FM-1 and FM-2) from southern Finland, identified on the basis of morphological investigations of workers (for FM-1, also of alate gynes and males) as mixed colonies comprising individuals with phenotypes typical of Formica aquilonia Yarr., F. polyctena Först. and F. rufa L. The prevailing species (phenotypes) were F. polyctena in FM-1, and F. rufa in FM-2. Colony FM-1 was observed every year in the period 1996–2006, almost from the moment it was formed. A first tentative investigation in 1999 revealed that it was already a mixed one and was probably also polygynous. Systematic follow-up investigations from 2002 to 2006 demonstrated relative stability of the proportions of individual species (phenotypes). A possible origin of this permanently mixed colony is postulated and discussed. ± Key words.— Ants, Formicidae, Formica rufa-group, Formica polyctena, Formica aquilonia, Formica rufa, mixed colonies, polygyny, morphology, phenotypes. INTRODUCTION able intraspecific variability of individuals often make it difficult, and sometimes impossible, to determine the Palaearctic wood ants, i.e. the species of the sub- species affiliation of a given colony. Such difficulties genus Formica s. str., and especially the members of are especially spectacular in certain geographic re- the Formica rufa-group (sensu Dlussky 1967), have gions, where they usually concern particular pairs (or been the object of intensive studies since the very ear- triplets) of species, e.g. F. rufa L. and F. polyctena ly days of myrmecology and have been discussed and Först. in Central Europe and southern Sweden, F. aqui- described in numerous papers. Even so, they still pose lonia Yarr., F. lugubris Zett. and F. paralugubris a taxonomic problem, and often puzzle field research- Seifert in the Alps, F. polyctena and F. aquilonia in ers. Vepsäläinen and Pisarski (1981) called the F. rufa Finland (Gösswald et al. 1965, Rosengren 1977, group “a taxonomic enfant terrible” among wood ants Douwes 1979, Pamilo et al. 1979, 1992, Vepsäläinen and likened the state of taxonomy of these species to and Pisarski 1981, Maeder et al. 2005). It occurs that “chaos before order.” workers from the same nest bear intermediate species Diagnosis of the F. rufa-group is based on the hairi- characters or that individuals within a colony seem to ness and, to a lesser degree, on the microsculpture of represent different species. Moreover, it happens that various parts of the body, mainly in workers (e.g. Yar- the appearance of workers within a colony changes in row 1955, Dlussky 1967, Dlussky and Pisarski 1971, time, and nest samples from different years may be dif- Kutter 1977, Collingwood 1979, Douwes 1981, Seifert ferently identified (Pamilo and Vepsäläinen 1977, Vep- 1996, Czechowski et al. 2002, Radchenko et al. 2004). säläinen and Pisarski 1981, Czechowski 1996). However, the generally high morphological similarity Recent investigations, both morphological and among species within the group together with consider- molecular, have thrown some light on this problem. On 668 W. CZECHOWSKI and A. RADCHENKO the one hand temporary social parasitism, i.e. taking F. polyctena, artificially established in 1987 (see Cze- over colonies (queenless or queenright ones) by hete- chowski 1990); at the time of colonization, the ants rospecific gynes (see Pisarski and Czechowski 1994, were identified as F. aquilonia, and they sponta- Czechowski 1993a, b, 1996), has been shown to occur neously transformed into morphologically typical within the subgenus Formica s. str., one result being F. polyctena within the period of study. There had the establishment of temporarily mixed colonies. On also been one natural F. lugubris colony on the island the other hand the possibility of interspecific hybridiza- before, but it died out in the early nineties. The two tion within the F. rufa-group has been evidenced (e.g. existing colonies of F. polyctena produced their own Seifert 1991, 1999, Czechowski 1993c, 1996, Goropash- numerous reproductives (both gynes and males). naya et al. 2004, Gyllenstrand et al. 2004, Seifert and Colony FM-1 was noticed for the first time as an Goropashnaya 2004, Sorvari 2006). Nevertheless, not almost incipient colony (however, already with no For- all cases encountered in the field can be conveniently mica fusca L. workers) in 1996. It was situated be- explained in terms of one of the above phenomena. Fre- tween the territories of the two already existing F. po- quent polygyny in wood ant colonies, which is partly lyctena colonies, outside the searching range of both. due to geographical factors (e.g. Pisarski 1981, Mabelis In 1999, the colony moved from a forest onto a sparse- 1986, Mabelis and Soesbergen 1989, Pamilo et al. 1992, ly overgrown open rocky outcrop, and later its size Rosengren et al. 1993; see also Czechowski 1996), and began to increase dramatically. In the meantime, colo- multiple mating (polyandry) of wood ant gynes ny FM-1 had moved 2–3 times before finally (in 2000) (Yamauchi et al. 1994, Boomsma and Sundström 1998) settling in the former nest site of F. lugubris, while further compound the problem. still remaining monodomous. In 2004, its mound had Anyway, colonies are found here and there which reached ca. 1 m in diameter and 45 cm in height (com- evidently appear to be (at least potentially) perma- pared to 40 and 20 cm, respectively, in 2000) and main- nently comprised of different species. The present tained these dimensions until 2006 (Fig. 1). The territo- paper offers circumstantial proof of the existence of ry of FM-1 enlarged considerably, pushing aside the such mixed wood ant societies, drawing this conclusion territories of both F. polyctena colonies. from long-term observations of the morphological com- J. Sorvari (personal comm.) was the first to notice, position of a particular colony. in 1999, that FM-1 consisted of workers that seemed to represent different species. Then the mixed nature of the colony was confirmed when nest samples taken in STUDY AREA, MATERIAL AND METHODS the years 2002, 2004, 2005 and 2006 were morphologi- cally analysed. In the years 2002–2005 the samples The study was conducted in the vicinity of Tvär- were collected in July, when only workers were avail- minne in the region of Hanko Peninsula in southern able. The nest sample of 2006 was taken in mid-June, Finland, where annual Polish-Finnish myrmecological and it included workers as well as alate sexuals, gynes investigations have been carried out since the late and males, which were having their nuptial flights at 1970’s. This region is well-known to Finnish and visit- that time. ing foreign myrmecologists (K. Vepsäläinen, P. Pamilo, Another ‘mixed’ colony, FM-2, is also considered in W. Fortelius, J. Sorvari, C. Collingwood, E. O. Wilson, this paper. It was situated on the mainland within B. Pisarski, W. Czechowski, and others) for unusually a complex of sand dunes being overgrown with pine frequent and often inexplicable inter- and intracolonial forest close to Tvärminne village. There were several variability of the local wood ants of the F. rufa group, typical mono- or oligodomous F. rufa colonies in the represented there by F. rufa, F. polyctena, F. aqui- immediate vicinity (see Czechowski and Markó 2005). lonia and F. lugubris (see Czechowski 1996). F. polyctena also occurred there, with its nearest The main object of the present study was colony colonies at a distance of ca. 250 m. FM-2 was followed FM-1 on the island of Vikaskär, a 2-hectare rocky up at that location from 2002 to 2004. In 2005, the colo- island within the Tvärminne archipelago, two-third ny disappeared, leaving an intact nest. The colony was forested, situated 50 m off the mainland (Långholmen not too big (mound diameter ca. 50 cm, height 25 cm; peninsula) and 60 m from the big (10 ha) island of Fig. 2), but very active, so most probably it moved Joskär, both densely populated by wood ants. The somewhere else. A worker nest sample of FM-2 was mainland supports all Finnish species of the F. rufa- taken once in 2004. group. Joskär, at the time of the present investigations, Nest samples (including worker samples containing was populated by a polydomous F. polyctena colony; at least 100 individuals per sample; Table 1) were tak- earlier, in the seventies and eighties, there were also en from the mound surface. Individuals were subse- single nests of F. lugubris and F. aquilonia (Pisarski quently classified into morphological groups corre- and Vepsäläinen 1989 and unpubl. data). The island sponding to different species based on commonly ac- of Vikaskär itself had two monodomous colonies of cepted key criteria, especially the presence, numbers DO PERMANENTLY MIXED COLONIES OF WOOD ANTS REALLY EXIST? 669 Figure 1. Nest of colony FM-1 in 2004 (photo W. Czechowski). Figure 2. Nest of colony FM-2 in 2004 (photo W. Czechowski). and appearance of standing hairs on the occipital mar- second group was considerably differentiated, and this gin of the head, the ventral surface of the head and the differentiation represented a continuous (with no hia- pilosity of the dorsal part of the alitrunk (see e.g. tus) transition from the typical phenotype of F. poly- Dlussky 1967, Dlussky and Pisarski 1971, Collingwood ctena to that typical of F.