Acta Zoologica Lituanica, 2007, Volumen 17, Numerus 2 87 ISSN 1392-1657 COMPARISON OF MATING OF TEN EUMENINAE WASP SPECIES WITH A BRIEF REVIEW OF SEXUAL SELECTION THEORIES: A FRAMEWORK FOR FUTURE RESEARCH Anna BUDRIENË, Eduardas BUDRYS Institute of Ecology of Vilnius University, Akademijos 2, LT-08412 Vilnius-21, Lithuania. E-mail: [email protected], [email protected] Abstract. This paper represents an initial attempt to examine mating phases of ten wasp species of the genera Ancistrocerus, Discoelius and Symmorphus, posing the questions as to what species traits are potentially involved in interactions between sexes, and what forms of sexual selection impel the evolu- tion of these traits. The studied species significantly differ in the sex ratio and the relative body size of sexes, their willingness to mount and copulate, male displays (mode and frequency of antennation and position while copulating), female displays (intensity and frequency of rejecting behaviour), the pres- ence of the males copulatory and postcopulatory courtship. Results of the comparative study of mating behaviour are discussed in parallel with the review of literature on sexual selection and sexual conflict, defining the framework for future research. Single locus complementary sex determination (sl-CSD), regular inbreeding and female uniparental care in combination with predominant monandry are the fea- tures of these wasps that allow to use them as a model for studies into understanding of the evolutionary maintenance of monandry. Key words: courtship, female rejecting behaviour, monandry, sexual conflict, sexual selection, sl-CSD INTRODUCTION Most discussions on sexual selection mechanisms have focused on polyandrous species. Their reproductive The lifetime reproductive success of animals depends success may be affected by pre-mating events, result- largely on male-female behavioural interactions that re- ing in male and female choice, as well as post-mating sult in choosing a high-quality partner. Most animal males events, including male-male competition in the form including insects are more active competitors for their of sperm competition (Parker 1970; Danielsson 1998) mates than females (e.g. Andersson 1994; Andersson & and cryptic female choice (Eberhard 1996). The latter Iwasa 1996), and their mating efforts are usually higher two forms of postcopulatory sexual selection are as- than parental efforts (Clutton-Brock & Vincent 1991; sumed to be powerful evolutionary forces working both Clutton-Brock & Parker 1992; Arnold & Duvall 1994). at molecular and population levels (Birkhead & This difference in reproductive interests between males Pizzari 2002; Neff & Pitcher 2005; Snook 2005). Posi- and females provides a potential for sexually antagonis- tive consequences of female promiscuity have been tic selection that may result in the antagonistic coevolu- generally divided into direct effects on female fecun- tion of sexes (Parker 1979, 2006; Alexander et al. 1997; dity and lifespan, e.g. acquisition of nutritious acces- Chapman et al. 2003; Arnqvist & Rowe 2005). The lat- sory gland substances (Gwynne 1984; Arnqvist & ter is thought to shape mating behaviour like the pre- Nilsson 2000; Møller & Jennions 2001), as well as in- copulatory struggle between males and females in direct (genetic) effects on offspring fitness, e.g. by aquatic isopods (Jormalainen & Merilaita 1995; selecting compatible or good genes (Zeh & Zeh 1997; Jormalainen 1998), dung flies (Parker 1979) and water Møller 1998; Jennions & Petrie 2000; Birkhead & Piz- striders (Arnqvist 1992; Rowe 1994; Rowe et al. 1994; zari 2002; Tomkins et al. 2004; Neff & Pitcher 2005; Lauer et al. 1996; Arnqvist 1997; Watson et al. 1998; Simmons 2005). According to theoretical models, the Arnqvist & Rowe 2002). Sexually antagonistic selec- preferred male must provide genes that increase net tion is predicted to influence biological diversity not only offspring fitness as compared to the genes provided by affecting speciation rates (Parker & Partridge 1998; by less desirable males (Kokko et al. 2003; Mead & Arnqvist et al. 2000; Gavrilets 2000; Gavrilets et Arnold 2004). However, the prevalence of indirect ben- al. 2001; Martin & Hosken 2004), but also by altering efits is controversial because its genetic and physiologi- the likelihood of extinction (Kokko & Brooks 2003). cal background is poorly understood. 88 Budrienë A., Budrys E. It is suggested that females of most Hymenoptera spe- the less investing sex, OSR increases the competition cies mate only once during their lifetime (Alcock 1978; for mates among individuals of the more abundant sex Boomsma & Ratnieks 1996; Schmid-Hempel & and choosiness among those of the less abundant sex Schmid-Hempel 2000; Baer & Schmid-Hempel 2001; (Trivers 1972; Emlen & Oring 1977). In solitary wasps, Strassmann 2001; Baer 2003; Boomsma et al. 2005). the female choosiness is possible because females of Nonetheless, social Hymenoptera are better known many species can reject suitors as they can physically than the solitary ones in terms of the incidence of dominate smaller males (ONeill 2001; Budrienë & polyandry in some genera (Crozier & Fjerding- Budrys 2004). stad 2001; Paxton 2005), thus increasing the genetic As the mating behaviour of solitary wasps, including diversity of a colony (Crozier & Page 1985; Palmer & Eumeninae, is difficult to observe in nature, earlier Oldroyd 2000; Tarpy & Page 2001; for alternative hy- knowledge is mainly based on anecdotal reports, de- potheses see Brown & Schmid-Hempel 2003; Kraus et tailed investigations being rare (ONeill 2001). Only a al. 2004; Schlüns et al. 2005). Therefore, the ques- few studies have compared the mating behaviour of tion is why females do not copulate more than once, Eumeninae (Cowan 1986; Budrienë 2001), even less if polyandry brings some fitness benefits. In the evo- attention being given to the mode of sexual selection to lution of mating systems, costs of searching for and which this behaviour may be subject. This paper pro- discriminating among males (costs of being too vides new data on the mating behaviour of six species choosy) and costs of mating too often (costs of be- of Eumeninae and additional information on four pre- ing not choosy enough) are important (Kokko et viously studied species. al. 2003). Indeed, mating at very high rates is costly The first objective of this study was to shed light on to insect females in general (Thornhill & Alcock 1983; the basic features of the mating systems of Eumeninae Arnqvist & Nilsson 2000), including the social Hy- wasps, i.e. (1) their choosiness while selecting a mate, menoptera. Among the latter, monandry is favoured (2) the behavioural elements that are involved in mate due to a narrow time window for mating, lack of choice, and (3) their traits related to sexual selection material gain from males, lack of paternal care and such as sexual size dimorphism (SSD) and the sex apparent lack of any post-copulatory sperm discrimi- ratio. The second objective was to review the latest nation mechanisms (Strassmann 2001). achievements in sexual selection theories and to as- Solitary Eumeninae are important because, as a sister sess the potential of Eumeninae as a model group for group of the social subfamilies Stenogastrinae, Polisti- the future research and testing the hypotheses. nae and Vespinae of the family Vespidae (Carpen- ter 1981; Brothers 1999; Carpenter & Wheeler 1999), they possess behavioural features which may have ex- MATERIAL AND METHODS planatory implications for the evolutionary trajectories of sociality in insects (Kurzenko 1980; Itino 1986; Mating was observed in captivity, using wasps reared Cowan 1991; Chapman & Stewart 1996; Hunt 1999; from trap-nests. Also, occasional field observations of Wcislo 2000; Hunt & Amdam 2005). Solitary wasp mating were used for comparison. females gain little from additional matings, thus tend to The reed bundle trap-nests were exposed in four lo- be monandrous (ONeill 2001). They are often recep- calities of Lithuania, Varnupys (55°24'N 25°17'E), Bilðiai tive on emergence, therefore, the ability of males to (55°08'N 25°16'E), Kaunas city (54°54'N 23°54'E) and select sites of waiting for emerging virgin females is Papiðkiai (55°56'N 24°16'E) in 19912003 (details in crucial for their reproductive success (Alcock 1978; Budrienë 2003). Insects obtained before the year 1997 Batra 1978; Seidelmann 1999). Protandry (males emer- were used for the estimation of sex weight and sex ge several days earlier than females) is a common fea- ratio only. Wasps reared in 19972003 were used for ture of the species, whose males seek freshly enclosed observations of mating. females, and represents an adaptive aspect of the males We studied mating behaviour of ten Eumeninae spe- life-history strategies (Evans 1966; ONeill 2001). The cies: Ancistrocerus antilope (Panzer), A. gazella (Pan- intensity of competition among males measured by their zer), A. nigricornis (Curtis), A. trifasciatus (Müller), operational sex ratio (OSR) is defined as the ratio of Discoelius dufourii (Lepeletier), D. zonalis (Panzer), the number of sexually active males in a population to Symmorphus allobrogus (Saussure), S. crassicornis the number of receptive females (Emlen & Oring 1977). (Panzer), S. gracilis (Brulle) and S. murarius (L.). Of It is presumed to be much higher than the population these species, mating of A. gazella, A. nigricornis, sex ratio even in species with a small degree of A. trifasciatus, S. crassicornis, S. gracilis and D. du-