DOTTORATO DI RICERCA IN
ETOLOGIA, ECOLOGIA ANIMALE E
ANTROPOLOGIA
(XXV CICLO)
The male phenotype in Polistes dominula:
sexual selection and host-parasite interactions
Tesi di
Federico Cappa
Coordinatore Prof. Alberto Ugolini
Tutor Prof. Laura Beani
UNIVERSITÀ DEGLI STUDI DI FIRENZE
DIPARTIMENTO DI BIOLOGIA EVOLUZIONISTICA L. PARDI
DOTTORATO DI RICERCA IN ETOLOGIA,
ECOLOGIA ANIMALE E ANTROPOLOGIA
(XXV CICLO, BIO 05, BIO 07)
The male phenotype in Polistes dominula:
sexual selection and host-parasite interactions
Tesi di
Federico Cappa
Coordinatore Prof. Alberto Ugolini
Tutor Prof. Laura Beani
(2013)
INDEX
1. SUMMARY... 3 2. CHAPTER 1: Introduction ... 5 3 CHAPTER 2: Visual over chemical cues in sex-recognition of the primivitely
eusocial wasp Polistes dominula ... 17 4. CHAPTER 3: Males do not like the working class: male choice according to
caste in a primitive wasp society... 34 5. CHAPTER 4: When a parasite breaks all the rules of a colony: morphology
and fate of wasps infected by a strepsipteran endoparasite ... ... 56
6. CHAPTER 5: Host sex and parasite manipulation: dramatic differences in the fate of female and male Polistes dominula wasps infected by the parasite Xenos vesparum ... 78 7. CHAPTER 6: Testing the haploid susceptibility hypothesis in two different species of social insects with different level of social organizatio ... 97
SUMMARY
In the female-centered hymenopteran societies males have always represented the dark side of the moon. Not taking any conspicuous part in the social life and activities of the colony, they have drawn little attention, if not for their lazy condition, which earn them the discreditable epithets of “intra-colony parasites” or “an example of absolute egocentrism”, compared to their mates and sisters, engrossed in ruling and maintaining their usually complex, yet sometimes ephemeral, empires. Males have often been regarded as being merely short-lived “small and simple mating machine” or “flying sperm containers”. Against this common view, this PhD thesis investigates the male phenotype of eusocial Hymenoptera, focusing on the effects of sexual selection and host-parasite interactions with the aim of demonstrating that eusocial Hymenoptera males are not just ‘simple and small mating machine’ unworthy of attention. On the contrary, they represent excellent opportunities to understand how the forces of kin selection and sexual selection combine in shaping the role of males and their phenotype in the Hymenoptera societies. By using a model species for sociobiology studies, the primitively eusocial wasp Polistes dominula, and an integrated approach involving behavioural, morphological and immunological studies, I start from the investigation of traditional subjects such recognition, so far neglected with respect to males, to the reproductive strategies and behavioural and physiological adaptations evolved by males to cope with their parasites and pathogens and to maximize their reproductive success.
Prior to this work, little was known regarding the dynamics of sex-recognition and male choice in social insects; therefore, in the first part of the thesis I address these topics. I show that P. dominula wasps are able to discriminate between individuals belonging to different sex and that visual cues rather than chemical ones play an important role in such recognition process. The results obtained in the context of mate choice represent the first evidence of male choice in a social insect, demonstrating that male P. dominula are not just “flying bags of sperm” eager to mate and competing to be chosen. Instead, they are able to distinguish among females with different reproductive potential and exert a mate choice towards those females that will provide, theoretically, the highest returns in terms of fitness. The second part of the thesis focuses on the impact of parasites and pathogens on the male phenotype of P. dominula with respect to female wasps. By comparing the effects of the specific strepsipteran
parasite X. vesparum on female and male wasps, I underline the dramatic differences in the impact of this parasite on hosts of different sex. Finally, I test the assumptions of the “haploid susceptibility hypothesis” using both the primitively eusocial wasp P. dominula and the highly eusocial honeybee A. mellifera. According to this hypothesis, males of eusocial Hymenoptera should suffer reduced disease resistance compared to females, because of the lack in genetic variability due to their haploid condition. The results are quite interesting since they show different patterns in the two studied species. A. mellifera males exhibited a lower immunocompetence than females as in other social Hymenoptera species previously examined and in agreement with what predicted by the theoretical hypothesis. Conversely, P. dominula male larvae showed a better antibacterial response with respect to females. The different results found in the two species are discussed in relation to the different life history of their males. Overall, by deepening the knowledge on the too often neglected subject of male phenotype in social Hymenoptera, this PhD thesis provide a different perspective for the study of insect societies compared to the female-centered point of view typical of studies focusing on eusocial Hymenoptera.
Chapter 1
Introduction
1. INTRODUCTION
1.1.Being a male in the Hymenoptera societies
When we think about Hymenoptera societies, regardless of their level of complexity or social organization, we usually think of female worlds. Most of the studies that over the decades have deeply investigated the different aspects of the social life of wasps, ants and bees, have always focused on the colony and the females who form the center of its living structure (Wilson 1971). Kin selection, queen-workers conflict over reproduction and caste determination, division of labour and task specialization, social immunity and recognition, all these issues have always been studied predominantly in females (Wilson 1965; Wilson 1971; Hamilton 1972; Oster & Wilson 1978; O’Donnell 1998; Queller & Strassmann 1998; Bourke & Ratnieks 1999; Mehdiabadi et al. 2003; Ratnieks et al. 2006; Cremer 2007; van Zweden & d’Ettorre 2010).
In the female-centered hymenopteran societies males have always represented the dark side of the moon. Not taking any conspicuous part in the social life and activities of the colony, they have drawn little attention, if not for their lazy condition, which earn them the discreditable epithets of “intra-colony parasites” or “an example of absolute egocentrism” (Wilson 1971), compared to their mates and sisters, engrossed in ruling and maintaining their usually complex, yet sometimes ephemeral, empires. Males have often been regarded as being merely short-lived “small and simple mating machine” or “flying sperm containers” (Tsuji 1996; Baer 2003). They are produced only for a brief period of time during a specific phase of the colony cycle (Boomsma et al. 2005), and, with few exceptions (Hamilton 1979; Heinze et al. 1998; O’Donnell 1999), their lifespan is very brief and after reaching sexual maturity they depart from their native colonies, never to return. They die after one or a few matings, realized during a short mating period, and persist as long-lived sperm stored by the queens with whom they mated (Wilson 1971; Michener 1974; Hölldobler & Wilson 1990; Boomsma et al. 2005). They are haploid, produce clonal sperm, and sire only female offspring (Simmons 2001; Baer 2003; Baer & Boomsma 2004; Boomsma et al. 2005). The brevity of their somatic survival together with the absence of any involvement in the social life of the colony can help explaining why eusocial Hymenoptera males represent a too often neglected subject compared to the well-studied female societies.
1.2. Male eusocial Hymenoptera as models for the study of sexual selection
Sexual selection has long held the interest of naturalists and biologists, due in large part to the exotic and bizarre adaptations it creates. How organisms compete for access to mates or to entice members of the opposite sex into mating opportunities are not only fascinating, but also some of the most dynamic and important processes in nature.
It has been argued that because of the constraints imposed by kin selection and haplodiploidy males of social Hymenoptera have few if any sexually selected traits (Boomsma et al. 2005; Boomsma 2007). Against this view Baer (2003) pointed out that social Hymenoptera males represent instead great opportunities to study sexual selection, and the lack of studies devoted to such topic is somewhat surprising given the fact that they offer several characteristics that make them interesting model organisms to test existing sexual selection theory in unconventional ways.
To understand how and why social Hymenoptera males might represent suitable models for the study of sexual selection it is necessary to move the focus of attention away from the colony-fortresses dominated by females. The role of males might be inconspicuous on nests, but their fate in terms of reproductive success will be decided in the field, their phenotype is the result of the combined forces of kin selection and sexual selection (Baer 2003; Boomsma et al. 2005; Boomsma 2007). In several species males compete fiercely to get access to females, evolving fighting weaponry as in the case of the ergatoid males of some ants species belonging to the genera Hypoponera and Cardiocondyla, where peaceful winged males are produced along aggressive wingless morphs which lost their wings in exchange for elongated mandibles that they use to kill their rivals (Kinomura & Yamauki 1987; Heinze & Hölldobler 1993; Yamauki et al. 1996; Heinze et al. 1998).Lethal fighting with the evolution of robust mandibles as a weapon for intra-sexual competition is also found in the primitively eusocial wasp Polistes gigas, where males fight to the death in order to achieve a mate (Lee & Starr 2007). In other species males do not compete aggressively for access to females other than by scramble competition (Beani & Turillazzi 1990; Paxton 2005), nonetheless, they engage in prolonged costly mate-searching behaviours and courtship displays and spend almost all their time and energies, if not their entire transitory existence, in trying to achieve a mate (Thornhill & Alcock 1983; Beani 1996; Boomsma et al. 2005). Males of the primitively eusocial Stenogastrinae wasps patrol particular hover-sites daily, where they perform scent-marking behaviours and display sex-dimorphic tergal stripes to each other as a socially costly badge of
status (Beani & Turillazzi 1999; Beani et al. 2002). Males of different species of Polistes wasps gather in great numbers in lek-like aggregations in proximity of prominent landmarks where they try to acquire and defend a territory against their rivals, waiting for weeks the arrival of reproductive females (Beani & Turillazzi 1988; Beani et al. 1992; Beani 1996). Male bumblebees patrol mating territories or other colonies for several weeks (Goulson, 2003) and they also also deposit mating plugs, which effectively prevent intromission by subsequent males and demotivate queens to continue sexual activity (Baer et al. 2000; Baer et al. 2001; Sauter et al. 2001; Brown et al. 2002). Similar mating plugs occur in fire ants (Mikheyev 2003) and most likely in fungus-growing ants (Baer & Boomsma 2004; Boomsma et al. 2005).
Finally, the effects of sexual selection on the male phenotype of eusocial Hymenoptera are not limited to the pre-mating phases. Although multiple mating by females of social Hymenoptera might seem a rare event (Strassmann 2001), multiple queen-mating has evolved secondarily in honeybees, vespine wasps, leafcutter ants, army ants, harvester ants, and a few minor taxa (Hughes et al. 2008; Boomsma 2009; den Boer et al. 2010; Kronauer et al. 2011). Thus, opportunities for postcopulatory sexual conflict have repeatedly emerged and may have induced convergent adaptive responses; in particular, sperm competition is expected to occur in lineages where queens receive sperm from multiple males. In facts, by comparing monandrous and polyandrous sister groups of ants and bees den Boer and colleagues (2010) showed that seminal fluid of polyandrous species has a more positive effect on the survival of a male’s own sperm than on other males’ sperm. A difference not observed in the monandrous species, suggesting that incapacitation of competing sperm may have independently evolved in both bees and ants (den Boer et al. 2010). All of these studies carried out so far on eusocial Hymenoptera males showed that, although their role may be marginal in the colonies ruled by their mates, they are targets of both pre-mating and post-copulatory selective pressures and that is why the represent excellent models to study the action of sexual selection.
1.3. Parasites, pathogens and haploid susceptibility
Parasites are ubiquitous and exert strong selective pressures on their host. They represent significant challenges to insect societies. The high density, interaction rate and relatedness of individuals within colonies are all predicted to make social insect colonies particularly vulnerable to parasites (Schmid-Hempel 1998, 2005; Cremer et al. 2007). Inside nests of social Hymenoptera, males
might represent a particularly vulnerable target of infection. Wasps, ants and bees are all characterized by a haplodiploid system of sex determination where males are haploid and therefore lack allelic variation at the individual level. Genetic variation at the individual level may confer fitness advantages, particularly when codominant alleles at resistance loci contribute to the defence against pathogens (Ewald 1994; Brown 1997; Baer & Schmid-Hempel 2003). According to the “haploid susceptibility hypothesis” (O’Donnell & Beshers 2004), because of such lack in genetic variability, males in haplodiploid Hymenoptera may suffer reduced disease resistance. Furthermore, susceptible males may act as parasite vehicles, carrying infections into their social groups. Haploid susceptibility should therefore favour the evolution of adaptations to diminish the risks imposed by susceptible males (O’Donnell & Beshers 2004). So far, few empirical studies tested the ‘haploid susceptibility hypothesis’ in social hymenoptera using species with different level of social organization and different experimental approaches (Vainio et al. 2004; Baer et al. 2005; Baer & Schmid-Hempel 2006; Ruiz-González & Brown 2006; Gillespie 2010; Laughton et al. 2011; Huth-Schwarz et al. 2012). With only a single exception (Ruiz-González & Brown 2006), all of these studies confirmed the assumptions of the hypothesis finding a higher susceptibility in males compared to females. In all these cases, however, it is not always clear if the higher susceptibility showed by males might be a direct consequence of their haploid condition or the result of a “live hard, die young” overall evolutionary strategy (Zuk 2009) in which males channel their resources towards mating behaviour and reproduction rather than towards immunity and survivorship.
A higher susceptibility of the male sex with regard to parasites and pathogens may be an ultimate consequence of the different reproductive strategies favoured by selection in the two sexes (Bateman 1948; Rolff 2002; Zuck 2009 According to the Bateman’s principle, in facts, mating rate is a major determinant for fitness in males, whereas longevity is more important in females (Bateman 1948; Rolff 2002). If immune function is linked to differences in life history, we might expect males of species with different reproductive strategies to invest relatively more or less in immune function according to their necessities. Once more, eusocial Hymenoptera males, with their wide range of reproductive strategies, represent great opportunities to study sexual selection with respect to host-parasite interactions.
1.4. Aim of the thesis
This PhD thesis takes into examination the male phenotype of eusocial Hymenoptera, focusing on the effects of sexual selection and host-parasite interactions with the aim of demonstrating that eusocial Hymenoptera males are not just ‘simple and small mating machine’ unworthy of attention. On the contrary, they represent excellent opportunities to understand how the forces of kin selection and sexual selection combine in shaping the role of males and their phenotype in the Hymenoptera societies.
By using a model species for sociobiology studies, the primitively eusocial wasp Polistes dominula, I start from the investigation of traditional subjects such recognition, so far neglected with respect to males, to the reproductive strategies and behavioural and physiological adaptations evolved by males to cope with their parasites and pathogens and to maximize their reproductive success.
In the first part of my thesis I address the problem of recognition and sexual selection, investigating the topic of sex-recognition and the importance of visual and chemical cues in such process, and the existence of some degree of male mate choice in P. dominula. In the last decades, in facts, an ever-growing body of evidence on the study of male mate choice has undermined the sexual selection stereotypes of female “choosiness” versus male “undiscriminating eagerness” to mate. Male mate choice and sex-role reversal represent a new and expanding field of enquiry, and have been demonstrated in a soaring number of both vertebrates and invertebrates species, while receiving very little attention in social insects. Nonetheless, given some characteristic traits of male eusocial Hymenoptera as an ephemeral lifespan, a limited not refillable set of sperm, the constraint of being physically allowed to mate only once or a few times in a lifetime and, especially, the fact that they encounter females belonging to different castes which dramatically differ in their reproductive potential, the evolution of male mate choice in such species becomes plausible. Thus, by means of simple behavioural assays, I studied whether P. dominula males are able to discriminate among females with different reproductive potential and if they exert a differential mate choice according to female reproductive quality. The second part of my thesis focuses on the impact of parasites and pathogens on the male phenotype of P. dominula with respect to females. I describe the effects of a specific parasite, the strepsipteran parasitic castrator Xenos vesparum, on the phenotype of both female and male wasps highlighting the differences between the two sexes of the host in response to the parasitic infection. By comparing the behaviour and the morpho-physiology of
parasitized female and male wasps with respect of healthy individuals I underline the changes induced by manipulation of the parasite on its hosts of different sex.
Finally, I use both the primitively eusocial wasp P. dominula and the highly eusocial honeybee Apis mellifera to test the ‘haploid susceptibility hypothesis’ (O’Donnell & Beshers 2004), which predicts a higher vulnerability of haploid male Hymenoptera compared to diploid females.
Using a protocol of immune challenge with the pathogen Escherichia coli, I investigate the immunocompetence of male and female larvae of the two species. I compare the two species because they represent different levels of social evolution and their males show considerable differences in their life histories. Therefore, the immune system of these males might reflect such differences since the capacity for immune defence represent an important male life history trait that affects individual survival and mating success.
Overall, by deepening the knowledge on the too often neglected subject of male phenotype in social Hymenoptera, the main goal of this PhD thesis is to provide a different perspective for the study of insect societies compared to the female-centered point of view typical of studies focusing on eusocial Hymenoptera.
REFERENCES
Baer, B. 2003. Bumblebees as model organisms to study male sexual selection
in social insects. Behavioral Ecology & Sociobiology, 54: 521-533.
Baer, B. & Boomsma, J. J. 2004. Male reproductive investment and queen
mating frequency in fungus growing ants. Behavioral Ecology, 15: 426-432.
Baer, B. & Schmid-Hempel, P. 1999. Experimental variation in polyandry
affects parasite loads and fitness in a bumble bee. Nature, 397: 151-154.
Baer, B. & Schmid-Hempel, P. 2003. Bumblebee workers from different sire
groups vary in susceptibility to parasite infection. Ecology Letters, 6: 106-110.
Baer, B. & Schimd-Hempel, P. 2006. Phenotypic variation in male and worker
encapsulation response in the bumblebee Bombus terrestris. Ecological Entomology, 31: 591-596.
Baer, B., Maile, R., Schmid-Hempel, P., Morgan, E. D. & Jones, G.R. 2000.
Chemistry of a mating plug in bumblebees. Journal of Chemical Ecology, 26: 1869-1875.
Baer, B., Morgan, E. D. & Schmid-Hempel, P. 2001. A non-specific fatty
acid within the bumblebee mating plug prevents females from remating. Proceedings of the National Academy of Sciences USA, 98: 3926-3928.
Baer, B., Krug, A., Boomsma, J. J. & Hughes, W. O. H. 2005. Examination
of the immune responses of males and workers of the leaf-cutting ant Acromyrmex echinatior and the effect of infection. Insectes Sociaux, 52: 298-303.
Bateman, A. J. 1948. Intra-sexual selection in Drosophila. Heredity, 2:
349-368.
Beani L. 1996. Lek-like courtship in paper wasps: ‘‘a prolonged, delicate, and
troublesome affair.’’ In: The natural history and evolution of paper-wasps (Ed. By S. Turillazzi & M. J. West-Eberhard), pp. 113-125. Oxford, UK: Oxford University Press.
Beani, L. & Turillazzi, S. 1988. Alternative mating tactics in males of Polistes
dominulus (Hymenoptera: Vespidae). Behavioral Ecology & Sociobiology, 22: 257-264.
Beani, L. & Turillazzi, S. 1990. Male swarms at landmarks and scramble
competition polygyny in Polistes gallicus (Hymenoptera, Vespidae). Journal of Insect Behaviour, 3: 545-556.
Beani, L. & Turillazzi, S. 1999. Stripes display in hover-wasps (Vespidae:
Stenogastrinae): a socially costly status badge. Animal Behaviour, 57: 1233-1239.
Beani, L., Cervo, R., Lorenzi, C. M. & Turillazzi, S. 1992. Landmark-based
mating systems in four Polistes species (Hymenoptera: Vespidae). Journal of the Kansas Entomological Society, 65: 211-217.
Beani, L., Sledge, M. F., Maiani, S., Boscaro, F., Landi, M., Fortunato, A. & Turillazzi, S. 2002. Behavioral and chemical analyses of
scent-marking in the lek system of a hover-wasp (Vespidae, Stenogastrinae). Insectes Sociaux, 49: 275-281.
den Boer, S. P. A., Baer, B. & Boomsma, J. J. 2010. Seminal fluid mediates
ejaculate competition in social insects. Science, 327: 1506-1509.
Boomsma, J. J. 2007. Kin selection versus sexual selection: why the ends do
not meet. Current Biology, 17: R673-R683.
Boomsma, J. J. 2009. Lifetime monogamy and the evolution of eusociality.
Philosophical Transactions of the Royal Society B, 364: 3191-3207.
Boomsma, J.J., Baer, B. & Heinze, J. 2005. The evolution of male traits in
Bourke, A. F. G. & Ratnieks, F. L. W. 1999. Kin conflict over caste
determination in social Hymenoptera. Behavioral Ecology & Sociobiology, 46: 287-297.
Brown, J. L. 1997. A theory of mate choice based on heterozygosity.
Behavioral Ecology, 8: 60-65.
Brown, M. J. F., Baer, B., Schmid-Hempel, R. & Schmid-Hempel, P. 2002.
Dynamics of multiple mating in the bumblebee Bombus hypnorum. Insectes Sociaux, 49: 315-319.
Cremer S., Armitage S. A. O. & Schmid-Hempel P. 2007. Social immunity.
Current Biology, 17: 693-702.
Ewald, P.W. 1994. Evolution of infectious disease. Oxford: Oxford University
Press.
Gillespie, S. 2010. Factors affecting parasite prevalence among wild
bumblebees. Ecological Entomology, 35: 737-747.
Goulson, D. 2003. Bumblebees: their Behaviour and Ecology. Oxford
University Press, Oxford.
Hamilton, W. D. 1972. Altruism and related phenomena, mainly in the social
insects. Annual Review of Ecology & Systematics, 3: 193-232.
Hamilton, W. D. 1979. Wingless and fighting males in fig wasps and other
insects. In: Sexual selection and reproductive competition in insects, eds. Blum, M.S., & Blum, N. A., New York: Academic.
Heinze, J. & Hölldobler, B. 1993. Fighting for a harem of queens: Physiology
of reproduction in Cardiocondyla male ants. Proceedings of the National Academy of Sciences USA, 90: 8412-8414.
Heinze, J., Hölldobler, B. & Yamauchi K. 1998. Male competition in
Cardiocondyla ants. Behavioral Ecology & Sociobiology, 42: 239-246.
Hölldobler, B. & Wilson, E. O. 1990. The Ants. Cambridge, MA: Harvard
University Press.
Hughes, W. O. H., Oldroyd, B. P., Beekman, M. & Ratneiks, F. L. W. 2008.
Ancestral monogamy shows kin selection is key to the evolution of eusociality. Science, 320: 1213-1216.
Huth-Schwarz, A., Settele, J., Moritz, R. F. A. & Kraus, F. B. 2012. Factors
influencing Nosema bombi infections in natural populations of Bombus terrestris (Hymenoptera: Apidae). Journal of Invertebrate Pathology 110: 48-53.
Kinomura, K. & Yamauchi, K. 1987. Fighting and mating behaviors of
dimorphic males in the ant Cardiocondyla wroughtoni. Journal of Ethology, 5: 75-81.
Kronauer, D. J. C., O’Donnell, S., Boomsma, J. J. & Pierce, N. E. 2011.
Strict monandry in the ponerine army ant genus Simopelta suggests that colony size and complexity drive mating system evolution in social insects. Molecular Ecology, 20: 420-428.
Laughton, A. M., Boots, M. & Siva-Jothy, M. T. 2011. The ontogeny of
immunity in the honey bee, Apis mellifera L. following an immune challenge. Journal of Insect Physiology, 57: 1023-1032.
Lee, J. X. Q. & Starr, C. K. 2007. Violent combat among Polistes gigas males
(Hymenoptera: Vespidae). Sociobiology, 50: 337-342.
Mehdiabadi, N. J., Reeve, H. K. & Mueller, U. G. 2003. Queens versus
workers: sex-ratio conflict in eusocial Hymenoptera. Trend in Ecology & Evolution, 18: 88-93.
Michener, C. D. 1974. The social behavior of the bees. Cambridge, MA:
Harvard University Press.
Mikheyev, A. S. 2003. Evidence for mating plugs in the fire ant Solenopsis
invicta. Insectes Sociaux, 50: 401-402.
O'Donnell, S. 1998. Reproductive caste determination in eusocial wasps
(Hymenoptera: Vespidae). Annual Review of Entomology, 43, 323-346.
O’Donnell, S. 1999. The function of male dominance in the eusocial wasp,
Mischocyttarus mastigophorus (Hymenoptera: Vespidae). Ethology, 105: 273-282.
O’Donnell, S. & Beshers, S. N. 2004. The role of male disease susceptibility in
the evolution of haplodiploid insect societies. Proceedings of the Royal Society B, 271: 979-983.
Oster, G. & Wilson, E. O. 1978. Caste and ecology in the social insects.
Princeton, NJ: Princeton Univesity Press.
Paxton, R. J. 2005. Male mating behaviour and mating systems of bees: an
overview. Apidologie, 36: 145-156.
Queller, D. C. & Strassmann ,J. E. 1998. Kin selection and social insects.
Bioscience, 48: 165-175.
Ratnieks, F. L. W., Foster, K. R. & Wenseleers, T. 2006. Conflict resolution
in insect societies. Annual Review of Entomology, 51: 581-608.
Rolff, J. 2002. Bateman's principle and immunity. Proceedings of the Royal
Society B, 269: 867-872.
Ruiz-González, M. X. & Brown, M. J. F. 2006. Males vs workers: testing the
assumptions of the haploid susceptibility hypothesis in bumblebees. Behavioral Ecology & Sociobiology, 60: 501-509.
Sauter, A., Brown, M. J. F. , Baer, B. & Schmid-Hempel, P. 2001. Males of
social insects can prevent queens from multiple mating. Proceedings of the Royal Society B, 268: 1449-1454.
Schmid-Hempel, P. 1998. Parasites in Social Insects. Princeton, NJ: Princeton
University Press.
Schmid-Hempel, P., 2005. Evolutionary ecology of insect immune defenses.
Annual Review of Entomology, 50: 529-551.
Simmons, L.W. 2001. Sperm competition and its evolutionary consequences in
the insects. Princeton, NJ: Princeton University Press.
Strassmann, J. E. 2001. The rarity of multiple mating by females in the social
Hymenoptera. Insectes Sociaux, 48: 1-13.
Stuart, R. J., Francoeur, A. & Loiselle, R. 1987. Lethal fighting among
dimorphic males of the ant, Cardiocondyla wroughtonii. Naturwissenschaften, 74: 548-549.
Thornhill, R. & Alcock, J. 1983. The evolution of insect mating systems.
Cambridge, MA: Harvard University Press.
Vainio, L., Hakkarainen, H., Rantala, M. J. & Sorvari, J. 2004. Individual
variation in immune function in the ant Formica exsecta; effects of the nest, body size and sex. Evolutionary Ecology, 18: 75-84.
Wilson, E. O. 1965. Chemical communication in the social insects. Science.
149: 1064-1071.
Wilson, E. O. 1971. The Insect Societies. Cambridge, MA: Harvard University
Press.
Yamauchi, K., Kimura, Y., Corbara, B., Kinomura, K. & Tsuji, K. 1996.
Dimorphic ergatoid males and their reproductive behaviour in the ponerine ant Hypoponera bondroiti. Insectes Sociaux, 43: 119-130.
Zuk, M. 2009. The sicker sex. PLoS Pathogens, 5: e1000267.
van Zweden, J. S. & d’Ettorre, P.
2009. Genetic, individual, and groupfacilitation of disease resistance in insect societies. Annual Review of Entomology, 54: 405-423.
Chapter 2
Visual over chemical cues in
sex-recognition of the primitively eusocial wasp
Polistes dominula
The results of this study have been
submitted for publication
Visual over chemical cues in sex-recognition of the primitively
eusocial wasp Polistes dominula.
Federico Cappa1, Laura Beani1, Felicia Menicucci1& Rita Cervo1
1Università degli Studi di Firenze, Dipartimento di Biologia
Evoluzionistica “Leo Pardi”, Via Romana 17, 50125, Firenze, Italy. e-mail:david.baracchi@gmail.com
ABSTRACT
Recognition is an essential component of interactions in many social systems, allowing the maintainance of the complex, often hierarchical structure fundamental for the survival of the society Over the years a vast number of studies have used the wasps of the genus Polistes as a model to test the presence of different types of recognition, from nestmate to individual recognition. However, the existence of some mechanisms for sex-recognition represents a basic topic that has received only marginal attention. Here, we used lure presentation experiments to address the issue of sex-recognition in Polistes dominula, investigating the relevance of the different visual and chemical cues potentially involved in such process. Our results showed that P. dominula wasps are able to discriminate between individual of different sex and that visual cues rather than chemical ones are responsible for this discrimination. Further trials with visually-altered lures showed a higher aggressiveness towards male lures bearing black mandibles, a typical female trait. The lower level of aggression elicited by visually unaltered male lures in every test may indicate the completely yellow face of males as a signal of appeasement and harmless subordinate condition.
1. INTRODUCTION
Social contexts are based on more or less flexible webs of interactions among different individuals and, to preserve the structure of the society and the cooperation at its basis, the presence of effective recognition systems, together with the ability to recognize members of their group, prove essential for social organisms. Colonies of social insects represent some of the highest peaks reached by evolution in the expression of sociality (Wilson 1971). Their level of complexity is so high that they have been compared to superorganisms (Reeve & Hölldobler 2007; Hölldobler & Wilson 2009; Queller & Strassmann 2009) in which every part, consisting of each single individual, must recognize its role and the ones of the others in order to maintain the hierarchical structure fundamental for the survival of the society and to defend it from outer threats. A recognition system based on the use of chemical cues was believed to be the more efficient inside social insect colonies because of the characteristics of the chemical channel of communication (Wilson 1965; Wyatt 2003). In an underground nest made of dark tunnels or inside a hollow tree the message carried by specific odours might be more easily perceived, spread more quickly or be more persistent according to necessity (Wilson 1965; Wyatt 2003). Nonetheless, not all social insects’ species nest in dark places, and other systems to convey information using different sensory channels are expected to evolve every time they might improve the pre-existing communication system. In the last decade a considerable number of studies have showed the function of visual cues, in addition to the well-known chemical ones, in the recognition process of primitively eusocial wasps (Zanetti et al. 2001; Tibbetts 2002; Tibbetts & Dale 2004; Cervo et al. 2008; Tannure-Nascimento et al., 2008; Ortolani et al. 2010; Sheehan & Tibbetts, 2011; Baracchi et al. 2012). Many issues related to this topic have been investigated, especially using the model genus Polistes, demonstrating the importance of visual cues and visual signals in a number of recognition processes characteristic of these species. Visual traits play a role in different contexts, conveying information about the status, the identity and the quality of an individual (Tibbetts 2002; Tibbetts & Dale 2004; Tibbetts 2006; Tibbetts & Curtis 2007; Tibbetts & Dale 2007; Tibbetts & Lindsay 2008; Sheehan & Tibbetts 2010). Visual signals can be used to discriminate a nestmate from an intruder (Tibbetts 2002, Baracchi et al. 2012), to recognize its hierarchical rank inside the colony or its fighting ability (Tibbetts & Dale 2004; Tibbetts & Shorter 2009; Tibbetts et al. 2010), and they are also exploited by social parasites to usurp congeneric hosts’ colonies
(Ortolani et al. 2010). Over the years an ever-growing body of research focused in particular on the wasp Polistes dominula (formerly P. dominulus, for more information concerning the name change see Castro & Dvořák 2009) studying the importance of visual signals, especially facial patterns of black spots sometimes present on the clypeus of female wasps, across different populations of the species both in the USA, where these wasps are invasive (Cervo et al. 2000; Tibbetts & Dale 2004; Tibbetts & Lindsay 2008, Sheehan & Tibbetts 2010; Tibbetts et al. 2010), and in its European native range (Cervo et al. 2008; Zanette & Field 2009; Green & Field 2011; Tibbetts et al. 2011). The results obtained from these studies, albeit sometimes controversial (Cervo et al. 2008; Zanette & Field 2009), showed a geographic variation in the role of visual cues in the system of communication of P. dominula. Despite the still fruitfully debated topic, most of the research on visual signals in the context of recognition in primitively eusocial wasps has focused almost exclusively on female wasps. Rare exceptions to this trend are represented by two studies on sexually dimorphic visual cues, consisting of male wasps’ abdominal stripes for the Stenogastrinae Parischnogaster mellyi and abdominal spots in P. dominula males, in the context of sexual selection and mate choice (Beani & Turillazzi 1999; Izzo & Tibbetts 2012). The lack of attention towards male eusocial Hymenoptera represents a common trait of sociobiology studies on this group of insects, probably because males have always been regarded as “simple and small mating machine” (Tsuji 1996) of scarce interest if compared to the complexity of the female-dominated hymenopteran societies (Wilson 1971; Boomsma 2007). As a consequence, subjects of study, such as recognition, deeply investigated in female wasps have been largely neglected in males. Here, we test the relevance of visual versus chemical cues in the sex-recognition of P. dominula to verify the ability of female wasps to recognize individual belonging to different sexes and to assess the differential importance of these two sensory channels in the recognition process.
2. MATERIALS AND METHODS
2.1. Colony collection and maintenance
In June and July 2010, 2011 and 2012, we collected a total of 48 colonies of P. dominula from three different populations in the surroundings of Florence, Italy (6 colonies collected in July 2010 for the first population used for lures’ preparation, 20 colonies from the second population collected in June 2011 tested in our first experiment on visual and chemical cues, and 22 colonies from the third population collected in June 2012 tested in our second experiment on the relevance of different visual cues). We collected post-emergence colonies with both foundresses and workers on the nest, the 6 colonies which provided the individuals used for lures’ preparation were collected at the end of July 2010, when individual of both sexes were likely to eclose from each colony. All the collected colonies were transferred to the laboratory where they were maintained in glass boxes (15x15x15 cm) under natural light and temperature conditions with water, sugar and maggot larvae ad libitum until they were used for experiments. The nests were tested at the beginning of July 2011 and 2012.
2.2 General experimental procedure
The procedure for all the experiments consisted of the simultaneous presentation of two objects (two freeze-killed wasps, two freeze-killed pentane-washed wasps and their pentane extracts on square pieces of filter paper for the first set of experiments on visual and chemical cues; two pentane-washed heads for the second set of experiments on the relevance of different visual cues) with different characteristics or treatments. We used a stick 30 cm long with a fork at one end; the two objects were mounted on the fork, 1.5 cm apart, and were randomly placed on the left or right. The fork device was slowly introduced into the colony box while the wasps were on the nest and held 1 cm from the nest comb for 1 min after the first interaction between the wasps and the presented stimulus; each presentation was video recorded. There was an interval of at least one day between successive tests on the same colony. Each year, all the colonies were tested in a similar stage of development (measured as number of cells, larvae and pupae) to ensure equal motivation in nest defence. All the experiments were carried out during the same daytime interval (from 1100 to 1500 hours), on sunny days to ensure a similar activity rate of tested wasps.The videos were watched blindly by one observer to avoid observer bias in timing the bites during the presentation time. Bites are the more evident and more
quantifiable aggressive behaviour performed by wasps during this kind of experiment (Ortolani et al. 2010).
2.3. First experiment: visual and chemical cues
2.3.1. Male versus female visual and chemical cues
The most notable difference in visual appearance between males and females of P. dominula consists of the almost completely yellow face of males with respect to black areas on the mandibles, the frons and, sometimes, the clypeus of females (Fig. 1a, b). The high variability in black facial pattern of the female clypeus, together with the fact that the most common facial pattern of Italian population is represented by females with a totally yellow clypeus (Cervo et al. 2008) prompted us to use lures consisting of males and females individuals with a yellow clypeus in our first set of experiment testing the relevance of visual versus chemical cues.
In July 2011, we recorded the wasps’ reaction to the individuals of different sex by simultaneously presenting to each of 20 P. dominula colonies an unmanipulated male lure and an unmanipulated female lure. The lures consisted of 20 freeze-killed P. dominula males and 20 freeze-killed P. dominula females. Each nest was presented with a couple of lures of different sex, belonging to different colonies and of comparable head size. All the wasps used for the preparation of the lures were frozen at the age of 10 days post-eclosion to allow all the wasps to develop their full cuticular hydrocarbon (CHC) profile. The freeze-killed individuals used as lures came from the six colonies collected in 2010. The lures belonged to a different population with respect to the tested colonies to assure no relatedness among lures and tested wasps.
2.3.2. Male versus female chemical cues
To test the wasps’ reaction to the mere chemical cues, two square pieces of filter paper with the pentane-extract of either a male or a female wasp were simultaneously presented to each P. dominula colony (N = 20). For the CHC extraction method and extract reapplication on lures, we used a procedure similar to the one reported by Dani et al. (1996) for the same species. To prepare the CHC extracts, we washed each of 20 P. dominula males and 20 P. dominula females in 250ml of pentane for 15 min. All the extracts were kept separately in a freezer. All the individuals used for CHC extraction came from the six colonies also used for the visual and chemical cues experiment. A similar number of males and females from each nest were used. Before the experiment, all the extracts were dried at room temperature and resuspended in 100 l of pentane. Lures were obtained by placing the resuspended extract of each individual on a square piece of clean filter paper (rinsed with the solvent) of approximately 0.5 cm side. The lures were never presented more than once. 2.3.3. Male versus female visual cues
To evaluate the wasps’ response to the visual cues alone, we simultaneously presented 20 nests with two P. dominula lures of both sexes deprived of their odour. To obtain the lures for presentations, we washed 20 freeze-killed P. dominula males and 20 freeze-killed P. dominula female specimens in pentane for a sufficient time to remove all the cuticular hydrocarbons (24 h as reported by Dani et al. 1996 for the same species). We then placed two washed, similar-sized P. dominula lures of different sex on the tips of the fork device for presentation to the nests. Both the lures had a completely yellow clypeus. Each pair of lures was presented only once.
2.4. Second experiment: manipulation of visual cues
The results obtained from our first set of experiments carried out in 2011 gave evidence of a preponderancy of visual over chemical cues in the process of sex-recognition showing a higher level of aggressive responses of the tested wasps towards females (see RESULTS below). To determine which visual traits of the female’s face elicited the wasps’ more aggressive response, we artificially modified the typical visual appearance of male lures and performed a second set of experiments to test the relevance of single visual traits characteristic of the female wasps. Specifically, we altered the colour of the mandibles and the frons of a male’s head, both areas showing a black coloration in females, while completely yellow in males. The second set of visual experiments was carried out by simultaneously presenting two heads (altered versus unaltered male head) placed on the tips of the fork device to different P. dominula colonies. The heads used for the experiments had been previously washed in pentane for 24 h to remove the cuticular hydrocarbons. To avoid any bias in wasps’ reaction due to the enamel used for manipulation, the visually altered lure received the black enamel on the area of interest (mandibles or frons), while the visually unaltered lure (control) received a similar amount of enamel on the black area over the antennae.
2.4.1. Manipulated mandibles versus control
We chose male head pairs with equal width and altered one head per pair, changing its mandibles’ colour. We covered the yellow surface of the mandibles with black paint (TESTORS modelling paints from Enamel) (Fig. 1c). As a control, unaltered male heads were painted with black paint on the already black region of the head. We used a total of 44 male’s heads (22 altered vs 22 control) to obtain 22 pairs of lures. Each pair was presented only once to the nests (N = 22).
2.4.2. Manipulated frons versus control
We repeated the visually altered versus control experiment by manipulating the males’ frons instead of the mandibles. Similarly to the previous experiment, the yellow area of the frons of the male’s head (N = 22) was covered with black
paint (Fig. 1d). Controls were once more represented by visually unaltered male heads (N = 22) painted with black paint on the already black region of the head. Each pair of lures was was presented only once to the nests (N = 22).
Figure 1. Examples of lures used in visual presentation experiments. (a, b) unmanipulated heads of (a) male and (b) female P. dominula. (c, d) altered male heads with (c) black-painted mandibles and (d ) black-painted frons.
2.5. Statistical analyses
Since the data had a non-normal distribution, we analysed them with the nonparametric Wilcoxon test. All analyses were performed with SPSS 16.0 (SPSS Inc., Chicago, IL, U.S.A.).
3.1. First experiment: visual and chemical cues
3.1.1. Male versus female visual and chemical cues
Wasps were able to discriminate between male and female lures. They bit significantly more times female lures rather than male ones (Wilcoxon test: Z = -2.558, N = 20, P = 0.011) (Fig. 2).
Figure 2.Number of bites received by male and female lures in the presence of both visual and chemical cues (N = 20). Box plots show the median, 25-75% percentiles and range.
3.1.2. Male versus female chemical cues
Wasps did not discriminate between the epicuticular extract of male, biting the two paper lures at a comparable rate (Wilcoxon test, Z = -0.584, N = 20, P = 0.559) (Fig. 3a).
Wasps did not discriminate between the epicuticular extract of male, biting the two paper lures at a comparable rate (Wilcoxon test, Z = -0.584, N = 20, P = 0.559) (Fig. 3b).
Figure 3: Number of bites received by (a) filter paper lures with the pentane-extracts of males and females (N = 20) and (b) male and female lures with only visual cues (N = 20).
3.2. Second experiment: manipulation of visual cues
Altered mandibles’ lures received a higher number of bites compared to controls (Wilcoxon test, Z = -1.999, N = 22, P = 0.046) (Fig. 4a). The difference in the aggressive response for the altered frons lures versus controls was not statistically significant (Wilcoxon test, Z = -0.487, N = 22, P = 0.626) (Fig. 4b).
Figure 4: Number of bites received by (a) mandibles-altered male lures versus unaltered controls (N = 22) and (b )frons-altered male lures versus unaltered controls (N = 22). Box plots show the median, 25-75% percentiles and range. 4. DISCUSSION AND CONCLUSIONS
The results of our first set of experiment showed that P. dominula wasps are able to discriminate between female and male individuals and the cues responsible for this discrimination were the visual ones rather than the chemical ones. In facts, in both the experiments were the visual cues were present, the female lures received a significantly higher aggressive response. Therefore, the wasps clearly perceived some differences between the presented lures and such differences must be related to the different visual appearance of males and females. However, albeit not involved in the discrimination process, chemical cues appear to have some role in driving the rate of aggression, since the number of bites received by the presented lures was higher in the first experiment where both types of cues were present with respect to the experiment testing only the visual components (Fig. 2, 3b). The role of chemical cues in the recognition system of Polistes wasps has already been largely demonstrated in a number of studies (Gamboa et al. 1986; Espelie et al. 1994; Dani et al. 2001; Cini et al. 2009; Bruschini et al. 2010; Bruschini et al. 2011). Nonetheless, the use of chemical signals in the communication processes between male and female Polistes wasps is still a largely unexplored field (Bruschini et al. 2010); the few studies carried out so far focused predominantly on what happens outside a colony, in the context of sexual selection and mate choice. For example, both males and gynes (i. e. future queens) of P. fuscatus use their CHC profile to discriminate nestmates of the opposite sex in order to avoid inbreeding (Ryan & Gamboa 1986), while territorial males of P. dominula rub their abdomen and legs over the substrate of the defended perches at leks and the glands associated with the rubbing behaviour may be sources of pheromones (Beani & Calloni 1991). However, chemical cues did not appear to play a relevant part in the discrimination of the sexes in P. dominula (Fig. 3a). The results of our first set of experiments showed that sexually dimorphic visual traits could be instead responsible for the recognition of males and females and they might be at the basis of the different behaviour observed on nests towards individuals of the two sexes. The behavioural interactions between males and females on nests represent another poorly investigated issue in the Polistes societies. It has been demonstrated that females of P. canadensis showed no aggressive behaviour towards males coming from foreign colonies when they are tethered in proximity of the nest guarded by females, which even allowed the male intruders to crawl onto the nest and remain there ignored by them (Polak 1992). Conversely, the same females vigorously attacked foreign conspecifics females once tethered nearby their nest (Polak 1992). Also in P. dominula the behaviour of males and females on a nest is quite different; while females perform all the different activities of provisioning and maintenaince of the colony according to their rank, males are not involved in any social task inside the colony, spending most of their time inactive on the back of the nest (pers. obs.), in a position of subordination with respect to the other sex. Females
can also prevent males’access to food by forcefully confining them inside the comb’s cells (Starks & Poe 1997). Given the fact that the black facial pattern of the clypeus represent a badge of status in this species (Tibbetts & Dale 2004; Ortolani et al. 2010) and according to our findings, the completely yellow face of P. dominula males may represent a sort of signal of appeasement evolved by males to show their subordinate position in order to avoid continuous harassment by females. In this case we would be in the presence of a sort of “inversed badge of status” evolved to minimize social costs advertising not the male’s quality but rather its harmless subordinate condition. The higher level of aggressiveness showed by our wasps towards the female lures may due to the fact that a conspecific foreign female might also represent a greater threat to the colony if compared to a foreign male. Alien females may try to usurp or raid the nest, taking over the colony and killing the brood (Starks 2001). On the contrary, males are a minor threat for a colony, representing only a cost in terms of consumption of resources (Wilson 1971).
With the second set of our visual experiments we assessed the relevance of different visual traits characteristic of a female’s face by altering the male’s appearance. Previous studies on visual cues focused predominantly of the facial pattern of the clypeus, the area that shows the highest level of variability in a female wasp’s face (Tibbetts & Dale 2004; Tibbetts & Lindsay 2008; Green & Field 2010; Tibbetts et al. 2010). In our case, however, the conditions were a little different because males have a completely yellow face with no variable traits, and even the females chosen as lures had a yellow clypeus. As a consequence, the differences in the levels of aggression showed in the experiments with the visual cues had to be related to the presence or the absence of one or more specific traits rather then to their variability.
The results obtained from our experiment with visually-altered lures showed that male lures with black-painted mandibles elicited a higher level of aggression (Fig. 4a), while a black-painted frons did not (Fig. 4b). The mandibles are indeed the primary weapon used in fights among Polistes wasps, which strongly bite each other during aggressive contests. Therefore, the black colour of the mandibles, a threatening signal exploited also by the social parasite P. sulcifer exploits this trait to frighten its P. dominula hosts (Ortolani et al. 2010), might be a relevant visual feature for the wasps to discriminate between a potentially aggressive female from a harmless male in P. dominula. ACKNOWLEDGEMENTS
The authors thank Dr. David Baracchi and Dr. Iacopo Petrocelli for their helpful suggestions and their assistance during the realization of this study. The research was founded by the University of Florence.
Baracchi, D., Petrocelli, I., Cusseau, G., Pizzocaro, L., Teseo, S., & Turillazzi, S. 2012. Facial markings in the hover wasps: quality signals
and familiar recognition cues in two species of Stenogastrinae. Animal Behaviour, in press.
Beani, L., & Calloni, C. 1991. Leg tegumental glands and male rubbing
behavior at leks in Polistes dominulus (Hymenoptera: Vespidae). Journal of insect behaviour, 4: 449-462.
Beani, L., & Turillazzi, S. 1999. Stripes display in hover-wasps (Vespidae:
Stenogastrinae): a socially costly status badge. Animal Behaviour, 57: 1233-1239.
Boomsma, J. J. 2007. Kin selection versus sexual selection: why the ends do
not meet. Current Biology, 17, R673–R683.
Bruschini, C., Cervo, R. and Turillazzi, S. 2010. Pheromones in social wasps.
In: Vitamins and Hormones (ed. G. Litwack), pp. 447-492. Burlington: Academic Press.
Bruschini, C., Cervo, R., Cini, A., Pieraccini, G., Pontieri, L., Signorotti, L., & Turillazzi, S. 2011. Cuticular hydrocarbons rather than peptides are
responsible for nestmate recognition in Polistes dominulus. Chemical senses, 36: 715-723.
Castro, L. & Dvořák, L. 2009. New and noteworthy records of vespid wasps
(Hymenoptera: Vespidae) from the palaearctic region (II). Boletín Sociedad Entomológica Aragonesa, 44: 295-304.
Cervo, R., Zacchi, F., & Turillazzi, S. 2000. Polistes dominulus
(Hymenoptera, Vespidae) invading North America: some hypotheses for its rapid spread. Insectes Sociaux, 47: 155-157
Cervo, R., Dapporto, L., Beani, L., Strassmann, J. E. & Turillazzi, S. 2008.
On status badges and quality signals in the paper wasp Polistes dominulus: body size, facial colour patterns and hierarchical rank. Proceedings of the Royal Society B, 275: 1189-1196.
Cini, A., Gioli, L., and Cervo, R. 2009. A quantitative threshold for nest-mate
recognition in a paper social wasp. Biology Letters, 5: 459-461.
Dani, F. R., Fratini, S. & Turillazzi, S. 1996. Behavioural evidence for the
involvement of Dufour’s gland secretion in nestmate recognition in the social wasp Polistes dominulus (Hymenoptera: Vespidae). Behavioral Ecology and Sociobiology, 38: 311-319.
Dani, F. R., Jones, G. R., Destri, S., Spencer, S. H., Turillazzi, S. 2001.
Deciphering the recognition signature within the cuticular chemical profile of paper wasps. Animal Behaviour, 62: 165-171.
Detrain, C., & Deneubourg, J. L. 2006. Self-organized structures in a
superorganism: do ants “behave” like molecules? Physics of Life Reviews, 3: 162-187.
Espelie, K. E., Gamboa, G. J., Grudzien, T. A., & Bura, E. A. 1994.
Cuticular hydrocarbons of the paper wasp, Polistes fuscatus: A search for recognition pheromones. Journal of Chemical Ecology, 20: 1677-1687.
Gamboa, G. J., Reeve, H. K., & Pfennig, D. W. 1986. The evolution and
ontogeny of nestmate recognition in social wasps. Annual Review of Entomology, 31: 431-454.
Green, J. P., & Field, J. 2011. Interpopulation variation in status signalling in
the paper wasp Polistes dominulus. Animal Behaviour, 81: 205–209.
Hölldobler, B., & E. O. Wilson. 2009. The Superorganism: The Beauty,
Elegance, And Strangeness of Insect Societies. New York: W. W. Norton.
Izzo, A. S. & Tibbetts, E. A. 2012. Spotting the top male: sexually selected
signals in male Polistes dominulus wasps. Animal Behaviour, 83: 839-845.
Ortolani, I., Zechini, L., Turillazzi, S., & Cervo, R. 2010. Recognition of a
paper wasp social parasite by its host: evidence for a visual signal reducing host aggressiveness. Animal Behaviour, 80: 683-688.
Polak, M. 1992. Distribution of virgin females influences mate-searching
behavior of male Polistes canadensis (L.) (Hymenoptera: Vespidae). Journal of Insect Behavior, 5: 531-535.
Queller D. C., & Strassmann J. E. 2009. Beyond society: the evolution of
organismality. Philosophical Transactions of the Royal Society B., 364: 3143-3155.
Reeve, H.K. & Hölldobler, B. 2007. The emergence of a superorganism
through intergroup competition. Proceedings of the National Academy of Sciences USA., 104: 9736-9740.
Ryan, R. E. & Gamboa, G. J. 1986. Nestmate recognition between males and
gynes of the social wasp Polistes fuscatus (Hymenoptera: Vespidae). Annals of the Entomological Society of America, 79: 572-575.
Sheehan, M. J., & Tibbetts, E. A. 2010. Selection for individual recognition
and the evolution of polymorphic identity signals in Polistes paper wasps. Journal of Evolutionary Biology, 23: 570-577
Sheehan, M. J., & Tibbetts, E. A. 2011. Specialized face learning is associated
with individual recognition in paper wasps. Science, 334: 1272-1275.
Starks, P. 2001. Alternative reproductive tactics in the paper wasp Polistes
dominulus with specific focus on the sit-and-wait tactic. Annales Zoologici Fennici, 38: 189-199.
Starks, P. and Poe, E. 1997. Male stuffing in wasp societies. Nature, 389: 450. Tannure-Nascimento, I. C., Nascimento, F. S., & Zucchi, R. 2008. The look
of royalty: visual and odour signals of reproductive status in a paper wasp. Proceedings of the Royal Society B, 275: 2555-2561
Tibbetts, E. A. 2002. Visual signals of individual identity in the wasp Polistes
fuscatus. Proceedings of the Royal Society B., 269: 1423-1428.
Tibbetts, E. A. 2006. Badges of status in worker and gyne Polistes dominulus
wasps. Annales Zoologici Fennici, 43: 575-582.
Tibbetts, E. A., & Curtis, T. 2007. Rearing conditions influence quality
signals but not individual identity signals in Polistes wasps. Behavioral Ecology, 18: 602-607.
Tibbetts, E. A., & Dale, J. 2004. A socially enforced signal of quality in a
paper wasp. Nature, 432: 218-222.
Tibbetts, E. A., & Dale, J. 2007. Individual recognition: it is good to be
different. Trends in Ecology & Evolution, 22: 529-537.
Tibbetts, E. A. & Lindsay, R. 2008. Visual signals of status and rival
assessment in Polistes dominulus paper wasps. Biology Letters, 4: 237-239.
Tibbetts, E. A. & Shorter, J. R. 2009. How do fighting ability and nest value
influence usurpation contests in Polistes wasps? Behavioral Ecology and Sociobiology, 63: 1377-1385.
Tibbetts, E. A., Mettler, A. & Levy, S. 2010. Mutual assessment via visual
status signals in Polistes dominulus wasps. Biology Letters, 6: 10-13.
Tibbetts, E. A., Skaldina, O., Zhao, V., Toth, A. L., Skaldin, M., Beani, L., & Dale, J. 2011. Geographic variation in the status signals of Polistes
dominulus paper wasps. PloS ONE, 6 (12): e28173.
Wilson, E. O. 1965. Chemical communication in the social insects. Science.
149: 1064-1071.
Wilson, E. O. 1971. The Insect Societies. Cambridge, MA: Harvard University
Wyatt T. D. 2003. Pheromones and animal behaviour. Cambridge, UK:
Cambridge University Press.
Zanette, L. & Field, J. 2009. Cues, concessions, and inheritance: dominance
hierarchies in the paper wasp Polistes dominulus. Behavioral Ecology, 773-780.
Zanetti, P., Dani, F. R., Destri, S., Fanelli, D., Massolo, A., Moneti, G., Pieraccini, G. & Turillazzi, S. 2001. Nestmate recognition in
Parischnogaster striatula (Hymenoptera Stenogastrinae), visual and olfactory recognition cues. Journal of Insect Physiology, 47: 1013-1020.
Chapter 3
Males do not like the working class: male
choice according to caste in a primitive
wasp society
The results of this study have been
submitted for publication
Males do not like the working class: male choice according to
caste in a primitive wasp society
Federico Cappa1, Claudia Bruschini1 Rita Cervo1, Stefano Turillazzi1,2 & Laura Beani1
1Università degli Studi di Firenze, Dipartimento di Biologia
Evoluzionistica “Leo Pardi”, Via Romana 17, 50125, Firenze, Italy. e-mail: federico.cappa@unifi.it
2Centro Interdipartimentale di Spettrometria di Massa dell’Università di
Firenze, Viale G. Pieraccini, 5000 Firenze, Italy.
ABSTRACT
Despite being demonstrated in a soaring number of species, male mate choice in social contexts has received little attention. It has been argued that kin selection and sexual selection usually take different evolutionary routes and as a consequence the ‘ant’ and the ‘peacock’ rarely meet. In the female-dominated hymenopteran societies males have often been regarded as ‘flying sperm containers’ spending all their time and energies in trying to achieve a mate. Here, we report the first evidence of male mate choice in an insect society using as a model the primitively eusocial wasp Polistes dominula. By presenting males with females of different reproductive potential and recording the males’ behaviour and number of sexual interactions we showed that males exert a mate choice based on female caste, strongly preferring reproductive gynes with respect to workers regardless of female age, body size or health condition. The results obtained show that in our species caste is more important that actual fecundity in determining the reproductive value of a female. Our study offers a proof that social Hymenoptera males are not always ‘small mating machine’ eager to mate and that sometimes the ‘ant’ and the ‘peacock’ actually meet. Keywords: male choice, sexual selection, social wasps, caste.
1. INTRODUCTION
In the last decades an ever-growing body of evidence has undermined the sexual selection stereotypes of female “choosiness” versus male “undiscriminating eagerness” to mate crystallized in the Bateman’s principle (Bateman 1948; Knight 2002). Due to their higher reproductive investment, ultimately ascribable to the ancestral asymmetry in the energetic cost of egg production with respect to sperm, females usually appear to be the choosier sex (Andersson 1994; Andersson & Simmons 2006, Clutton-Brock 2009; Hosken & House 2011). Sometimes, however, even costs faced by males can increase both in terms of energetic investment and risk of choosing a mate that does not guarantee a high profit in terms of fitness (Dewsbery 1982; Wong & Jennions 2003). Male mate choice is particularly predicted to evolve in systems where male mating effort or parental investment is high (Burley 1977; Gwynne 1981; Gwynne 1991; Jones & Hunter 1993), or, alternatively, in response to a large variation in female quality, especially when ‘mate availability’ exceeds male ‘capacity to mate’, (Kvarnemo & Simmons 1999; Bonduriansky 2001; Edward & Chapman 2011). Despite being demonstrated in a soaring number of both vertebrates and invertebrates species, male mate choice in social insects has never received much attention. In the female-centered hymenopteran societies males have often been regarded as merely short-lived “flying sperm containers” or “simple and small mating machines” (Tsuji 1996; Baer 2003). Produced only for a brief period of time during a specific phase of the colony cycle, with rare exceptions (Yamauchi et al. 1996; Heinze et al. 1998), after reaching sexual maturity, they depart from their native colonies, never to return. Their fate in terms of reproductive success will be decided in the field, their phenotype is the result of the combined forces of kin selection and sexual selection (Boomsma et al. 2005; Boomsma 2007). Although males do not compete aggressively for access to females other than by scramble competition, they engage in prolonged costly mate-searching behaviours and courtship displays and spend almost all their time and energies, if not their entire transitory existence, in trying to achieve a mate (Thornhill & Alcock 1983; Beani 1996; Boomsma et al. 2005). Looking for male choosiness in systems based on high levels of male intrasexual competition may seem apparently controversial because we do not expect the competitive sex to be also choosy. Nevertheless, given some characteristic traits of male eusocial hymenoptera as an ephemeral lifespan, a limited not refillable set of sperm, and, often, the constraint of being physically allowed to mate only once or a few times in a lifetime (Boomsma et al. 2005), the evolution of some degree of male mate choice becomes more plausible. The last remark gains further credit if we take into account the fact that eusocial hymenopteran males usually encounter females belonging to different castes, which dramatically differ in their reproductive potential (Wheeler 1986; O’Donnell 1998). Hence, without the ability of discriminating reproductive
