Insect Science (2015) 0, 1–12, DOI 10.1111/1744-7917.12253
ORIGINAL ARTICLE
Is bigger better? Male body size affects wing-borne courtship signals and mating success in the olive fruit fly, Bactrocera oleae (Diptera: Tephritidae) Giovanni Benelli1 , Elisa Donati2 , Donato Romano1 , Giacomo Ragni2 , Gabriella Bonsignori2 , Cesare Stefanini2 and Angelo Canale1 1 Insect
Behaviour Group, Department of Agriculture,Food and Environment,University of Pisa, Pisa and
2 The
BioRobotics Institute,
Sant’Anna School of Advanced Studies, Pontedera, Italy
Abstract Variations in male body size are known to affect inter- and intrasexual selection outcomes in a wide range of animals. In mating systems involving sexual signaling before mating, body size often acts as a key factor affecting signal strength and mate choice. We evaluated the effect of male size on courtship displays and mating success of the olive fruit fly, Bactrocera oleae (Diptera: Tephritidae). Wing vibrations performed during successful and unsuccessful courtships by large and small males were recorded by high-speed videos and analyzed through frame-by-frame analysis. Mating success of large and small males was investigated. The effect of male–male competition on mating success was evaluated. Male body size affected both male courtship signals and mating outcomes. Successful males showed wing-borne signals with high frequencies and short interpulse intervals. Wing vibrations displayed by successful large males during copulation attempt had higher frequencies over smaller males and unsuccessful large males. In no-competition conditions, large males achieved higher mating success with respect to smaller ones. Allowing large and small males to compete for a female, large males achieve more mating success over smaller ones. Mate choice by females may be based on selection of the larger males, able to produce high-frequency wing vibrations. Such traits may be indicative of “good genes,” which under sexual selection could means good social-interaction genes, or a good competitive manipulator of conspecifics. Key words communication channels; courtship call; male–male competition; mate choice; sexual selection; social resources
Introduction Since the theory of sexual selection was introduced by Darwin (1871), it is generally recognized that the male–male competition for mates (i.e., intrasexual selection) and the active choice of individuals of one sex by
Correspondence: Giovanni Benelli, Insect Behaviour Group, Department of Agriculture, Food and Environment, University of Pisa, via del Borghetto 80, 56124 Pisa, Italy. Tel: +39 0502216141; fax: +39 0502216087; email: [email protected], [email protected]
individuals of the other (i.e., intersexual selection) are the two fundamental processes routing the evolution of sexual traits (Thornhill & Alcock, 1983; Rodriguero et al., 2002; Miller & Svensson, 2014; West-Eberhard, 2014). These processes may work at the same time, affecting variation in mating success across individuals and selecting particular traits. Investigations on which traits are related with mating success could provide important information about how sexual selection is working (Fiske et al., 1998). However, it is not simple to partition the effects of intra- and intersexual selection (Halliday, 1983). In several cases, the real weight of the female’s decision in producing variation of male mating success is not clearly 1
C 2015
Institute of Zoology, Chinese Academy of Sciences
2
G. Benelli et al.
elucidated and good evidence about female preferences for a given male phenotype is not common (Cotton et al., 2006). Among sexual traits, the role of male and female body size has been extensively investigated in a wide range of animals (Miller & Svensson, 2014), including insects. Strong correlations with different physiological and behavioral characters have been highlighted (Blanckenhorn, 2000). These characters include longevity (van den Assem et al., 1989), courtship (Webb et al., 1984; Brice˜no et al., 2002) fecundity (Krainacker et al., 1989), number of ovarioles (Fitt, 1990), quantity of ejaculates, spermatophore dimensions (Thornhill & Alcock, 1983), male mating success (Rodriguero et al., 2002), copula duration (Webb et al., 1984; Harano et al., 2012) and lifetime reproductive success (Partridge et al., 1987; Simmons, 1988). As in other taxa, in insects sexual selection generally favors larger body size in males. This seems to be due to the greater ability of larger males to influence female preferences (e.g., demonstrating greater vigor in courting and/or emitting higher quantities of pheromones over smaller ones), as well as to win male–male competition during courtship or when fighting to access limited resources (Thornhill & Alcock, 1983; Moradian & Walker, 2008; Blaul & Ruther, 2012). On the other hand, large males may incur additional costs and in some species sexual selection may favor small males (Blanckenhorn, 2000). This may be due to greater agility during courtship (McLachlan, 1987) as well as to the female’s preferences for small males (Steele & Partridge, 1988). Concerning Diptera, the role of male body size on courtship performances and mating success has been extensively investigated in the genus Drosophila (e.g., Partridge et al., 1987; Aspi & Hoikkala, 1995). Other research has been conducted on mosquitoes (e.g., Ma¨ıga et al., 2014), scatophagids (e.g., Gress et al., 2014), and tephritids (e.g., Brice˜no & Eberhard, 2002; Brice˜no et al., 2002; Cˆamara de Aquino & Joachim-Bravo, 2014). In tephritid flies, little is known on how body size affects male mating performances (see Benelli et al. 2014a for a recent review). However, it has been pointed out that some sexually selected traits are frequently good indicators of male fitness (Rodriguero et al., 2002; but see also Hunt et al., 2004). Knowledge of sexual selection processes may be useful for the successful implementation of the Sterile Insect Technique (SIT) against tephritids (Knipling, 1955), to select genotypes with high reproductive success, and to promote sexually selected male phenotypes through mass-rearing optimization (Burk & Calkins, 1983; Rodriguero et al., 2002; Benelli, 2014; Benelli et al., 2014a, 2014b).
The olive fruit fly, Bactrocera oleae (Rossi), is an invasive tephritid species that causes extensive damage to olive crops around the world, which special reference to the Mediterranean basin and North America. Over the last four decades, the control of B. oleae has been based mainly on the use of chemical insecticides, particularly the organophosphates (e.g., dimethoate and fenthion) formulated as food baits (i.e., insecticide mixed with an attractant) or cover sprays. In latest years, pyrethroids incorporated in food-baited traps have been introduced against the olive fruit fly in several countries as alternative tools for adult control. However, these chemicals may lead to the development of resistance and have negative effects on nontarget organisms (Daane & Johnson, 2010). Concerning biological control agents, several braconid species have been proposed, but to date no consistent results were obtained in the control of B. oleae populations (Daane et al., 2015). Previous attempts to use the SIT against this pest were not successful, and this may be linked to the inability to rear high-quality males in laboratory conditions. New improvements in rearing methods and additional understanding of the basic biology of the olive fruit fly may lead to a renewal of interest in using SIT against this important pest of olive crops (Estes et al., 2012). After emergence, both sexes of B. oleae complete gonad maturation in 5–8 d, thus becoming sexually mature (Canale et al., 2012). Olive fruit fly females are oligogamous and mate 1–3 times during their lifetime (Zouros & Krimbas, 1970) while males are polygamous and mate daily if receptive females are available (Zervas, 1982). At dusk, the olive fruit flies start rapid and whirling flights towards the host trees, forming noncompact swarms mainly composed of lekking males. Each B. oleae swarm usually establishes itself on the windward side of an olive plant. Within the lek, each male exhibits territoriality, defending a small area on a leaf (i.e., males are regularly spaced and they aggressively exclude conspecifics) and initiating courtship when a female lands in its territory (Benelli, 2014; Benelli et al., 2014a). The sequence of courtship events leading to copulation is divided into 3 main phases: (i) a mate-searching phase that culminates with the male’s arrestment near the female, (ii) a close-range phase, in which the male court the female through wing vibrations, and (iii) a final contact phase, with male copulation attempts (Benelli et al., 2012). Different physical and chemical stimuli affect the courtship and mating behavior of B. oleae. As regards to physical cues, when the male comes within close proximity of a female, he starts to court through fast wing vibrations. The male’s performances in wing vibration are able to affect the female’s decisions during courtship. Frequency and
C 2015 Institute of
Zoology, Chinese Academy of Sciences, 0, 1–12
Size and mating success in B. oleae
pulse duration of the male wing vibration are higher before a successful mating than before an unsuccessful one and when the male’s wings are removed the probability of successful mating is reduced (Benelli et al., 2012). The perception of female- and male-borne olfactory cues is crucial for B. oleae mating. Virgin females produce a multicomponent sex pheromone in their rectal glands (Baker et al., 1980; Canale et al., 2015). The most abundant component of this pheromone is 1,7-dioxaspiro[5.5]undecane (DSU) and it exhibits the highest biological activity toward males (Carpita et al., 2012). Young B. oleae males also produce DSU in the rectal glands (Benelli et al., 2013a). After having reached sexual maturity, they start to produce (Z)-9-tricosene, a compound unique to males, which is able to selectively attract the females (Carpita et al., 2012; Canale et al., 2013). Intra-sexual selection has a debated role in tephritid leks. The males form groups in which each individual defends a small territory performing courting rituals to attract the females (Witthier et al., 1994; H¨oglund & Alatalo, 1995; Segura et al., 2007; Benelli et al., 2014a, b; Cˆamara de Aquino & Joachim-Bravo, 2014). It remains unclear if the size of tephritid males can also affect male wing vibration performances and mate choice dynamics. Since the male’s wings generate the air-borne mechanic particle-displacement, which is perceived as a sound by the mate, we hypothesize that changes in wing’s area or mass may influence the parameters of courtship songs (Moradian & Walker, 2008). Moreover, large males could spend more energy performing wing vibrations, reaching higher frequencies and longer pulse durations that can affect positively the female mating decision. Do larger males perform better courtship displays over smaller ones? Are larger males able to achieve higher mating success over smaller competitors? In this research (i) the wing vibrations performed during courtship by large and small males were recorded by high-speed videos and analyzed through frame-by-frame analysis and (ii) the mating success of large and small males toward a virgin female was investigated. Also, (iii) allowing large and small males to compete for a virgin female, the effect of male– male competition for social resources (where resources are mates) on mating success was evaluated.
Materials and methods Insect rearing Insects used in this study were obtained from field-derived pupae (collected in a Tuscan olive-mill) during December 2013. Pupae were maintained in Pisa C 2015
Institute of Zoology, Chinese Academy of Sciences, 0, 1–12
3
laboratory under controlled conditions (22 ± 1 °C, 50% ± 5% R.H. and natural photoperiod) to wait for adult emergence. To obtain coeval virgin specimens, emergent flies were sexed, singly stored in clean glass vials (diameter 3 cm, length 7 cm) and fed on a dry diet (yeast and sugar at ratio 10 : 1) and water (Benelli et al., 2012; Canale & Benelli, 2012).
General observations and male body size measurements All experiments were conducted in laboratory conditions (22 ± 1 °C, 50% ± 5% relative humidity and 16 : 8 [L : D] photoperiod), in a room illuminated with daylight fluorescent neon tubes (Philips 30W/33). The light intensity in close proximity of the testing arena was approximately 1000 lux, estimated over the 300–1100 nm waveband using a LI-1800 spectroradiometer (LI-COR Inc., Lincoln, NE, USA), equipped with a remote cosine receptor. Diffused laboratory lighting was used to reduce possible reflection and phototaxis. Bioassays were all performed from December 15, 2013 to January 20, 2014 between 15.30 and 19.00 h, with 12–18 d old virgin flies. Each tested fly was immobilized (3 min at -10 °C) and its maximum head and thorax width were measured in dorsal view with a binocular microscope (Brice˜no & Eberhard, 2002; Brice˜no et al., 2002). Previous research showed that cooling did not affect the B. oleae behavior (Benelli et al., 2012). For the experiments, male flies of 2 size categories were used. Males with a thorax width >1.550 mm and
ORIGINAL ARTICLE
Is bigger better? Male body size affects wing-borne courtship signals and mating success in the olive fruit fly, Bactrocera oleae (Diptera: Tephritidae) Giovanni Benelli1 , Elisa Donati2 , Donato Romano1 , Giacomo Ragni2 , Gabriella Bonsignori2 , Cesare Stefanini2 and Angelo Canale1 1 Insect
Behaviour Group, Department of Agriculture,Food and Environment,University of Pisa, Pisa and
2 The
BioRobotics Institute,
Sant’Anna School of Advanced Studies, Pontedera, Italy
Abstract Variations in male body size are known to affect inter- and intrasexual selection outcomes in a wide range of animals. In mating systems involving sexual signaling before mating, body size often acts as a key factor affecting signal strength and mate choice. We evaluated the effect of male size on courtship displays and mating success of the olive fruit fly, Bactrocera oleae (Diptera: Tephritidae). Wing vibrations performed during successful and unsuccessful courtships by large and small males were recorded by high-speed videos and analyzed through frame-by-frame analysis. Mating success of large and small males was investigated. The effect of male–male competition on mating success was evaluated. Male body size affected both male courtship signals and mating outcomes. Successful males showed wing-borne signals with high frequencies and short interpulse intervals. Wing vibrations displayed by successful large males during copulation attempt had higher frequencies over smaller males and unsuccessful large males. In no-competition conditions, large males achieved higher mating success with respect to smaller ones. Allowing large and small males to compete for a female, large males achieve more mating success over smaller ones. Mate choice by females may be based on selection of the larger males, able to produce high-frequency wing vibrations. Such traits may be indicative of “good genes,” which under sexual selection could means good social-interaction genes, or a good competitive manipulator of conspecifics. Key words communication channels; courtship call; male–male competition; mate choice; sexual selection; social resources
Introduction Since the theory of sexual selection was introduced by Darwin (1871), it is generally recognized that the male–male competition for mates (i.e., intrasexual selection) and the active choice of individuals of one sex by
Correspondence: Giovanni Benelli, Insect Behaviour Group, Department of Agriculture, Food and Environment, University of Pisa, via del Borghetto 80, 56124 Pisa, Italy. Tel: +39 0502216141; fax: +39 0502216087; email: [email protected], [email protected]
individuals of the other (i.e., intersexual selection) are the two fundamental processes routing the evolution of sexual traits (Thornhill & Alcock, 1983; Rodriguero et al., 2002; Miller & Svensson, 2014; West-Eberhard, 2014). These processes may work at the same time, affecting variation in mating success across individuals and selecting particular traits. Investigations on which traits are related with mating success could provide important information about how sexual selection is working (Fiske et al., 1998). However, it is not simple to partition the effects of intra- and intersexual selection (Halliday, 1983). In several cases, the real weight of the female’s decision in producing variation of male mating success is not clearly 1
C 2015
Institute of Zoology, Chinese Academy of Sciences
2
G. Benelli et al.
elucidated and good evidence about female preferences for a given male phenotype is not common (Cotton et al., 2006). Among sexual traits, the role of male and female body size has been extensively investigated in a wide range of animals (Miller & Svensson, 2014), including insects. Strong correlations with different physiological and behavioral characters have been highlighted (Blanckenhorn, 2000). These characters include longevity (van den Assem et al., 1989), courtship (Webb et al., 1984; Brice˜no et al., 2002) fecundity (Krainacker et al., 1989), number of ovarioles (Fitt, 1990), quantity of ejaculates, spermatophore dimensions (Thornhill & Alcock, 1983), male mating success (Rodriguero et al., 2002), copula duration (Webb et al., 1984; Harano et al., 2012) and lifetime reproductive success (Partridge et al., 1987; Simmons, 1988). As in other taxa, in insects sexual selection generally favors larger body size in males. This seems to be due to the greater ability of larger males to influence female preferences (e.g., demonstrating greater vigor in courting and/or emitting higher quantities of pheromones over smaller ones), as well as to win male–male competition during courtship or when fighting to access limited resources (Thornhill & Alcock, 1983; Moradian & Walker, 2008; Blaul & Ruther, 2012). On the other hand, large males may incur additional costs and in some species sexual selection may favor small males (Blanckenhorn, 2000). This may be due to greater agility during courtship (McLachlan, 1987) as well as to the female’s preferences for small males (Steele & Partridge, 1988). Concerning Diptera, the role of male body size on courtship performances and mating success has been extensively investigated in the genus Drosophila (e.g., Partridge et al., 1987; Aspi & Hoikkala, 1995). Other research has been conducted on mosquitoes (e.g., Ma¨ıga et al., 2014), scatophagids (e.g., Gress et al., 2014), and tephritids (e.g., Brice˜no & Eberhard, 2002; Brice˜no et al., 2002; Cˆamara de Aquino & Joachim-Bravo, 2014). In tephritid flies, little is known on how body size affects male mating performances (see Benelli et al. 2014a for a recent review). However, it has been pointed out that some sexually selected traits are frequently good indicators of male fitness (Rodriguero et al., 2002; but see also Hunt et al., 2004). Knowledge of sexual selection processes may be useful for the successful implementation of the Sterile Insect Technique (SIT) against tephritids (Knipling, 1955), to select genotypes with high reproductive success, and to promote sexually selected male phenotypes through mass-rearing optimization (Burk & Calkins, 1983; Rodriguero et al., 2002; Benelli, 2014; Benelli et al., 2014a, 2014b).
The olive fruit fly, Bactrocera oleae (Rossi), is an invasive tephritid species that causes extensive damage to olive crops around the world, which special reference to the Mediterranean basin and North America. Over the last four decades, the control of B. oleae has been based mainly on the use of chemical insecticides, particularly the organophosphates (e.g., dimethoate and fenthion) formulated as food baits (i.e., insecticide mixed with an attractant) or cover sprays. In latest years, pyrethroids incorporated in food-baited traps have been introduced against the olive fruit fly in several countries as alternative tools for adult control. However, these chemicals may lead to the development of resistance and have negative effects on nontarget organisms (Daane & Johnson, 2010). Concerning biological control agents, several braconid species have been proposed, but to date no consistent results were obtained in the control of B. oleae populations (Daane et al., 2015). Previous attempts to use the SIT against this pest were not successful, and this may be linked to the inability to rear high-quality males in laboratory conditions. New improvements in rearing methods and additional understanding of the basic biology of the olive fruit fly may lead to a renewal of interest in using SIT against this important pest of olive crops (Estes et al., 2012). After emergence, both sexes of B. oleae complete gonad maturation in 5–8 d, thus becoming sexually mature (Canale et al., 2012). Olive fruit fly females are oligogamous and mate 1–3 times during their lifetime (Zouros & Krimbas, 1970) while males are polygamous and mate daily if receptive females are available (Zervas, 1982). At dusk, the olive fruit flies start rapid and whirling flights towards the host trees, forming noncompact swarms mainly composed of lekking males. Each B. oleae swarm usually establishes itself on the windward side of an olive plant. Within the lek, each male exhibits territoriality, defending a small area on a leaf (i.e., males are regularly spaced and they aggressively exclude conspecifics) and initiating courtship when a female lands in its territory (Benelli, 2014; Benelli et al., 2014a). The sequence of courtship events leading to copulation is divided into 3 main phases: (i) a mate-searching phase that culminates with the male’s arrestment near the female, (ii) a close-range phase, in which the male court the female through wing vibrations, and (iii) a final contact phase, with male copulation attempts (Benelli et al., 2012). Different physical and chemical stimuli affect the courtship and mating behavior of B. oleae. As regards to physical cues, when the male comes within close proximity of a female, he starts to court through fast wing vibrations. The male’s performances in wing vibration are able to affect the female’s decisions during courtship. Frequency and
C 2015 Institute of
Zoology, Chinese Academy of Sciences, 0, 1–12
Size and mating success in B. oleae
pulse duration of the male wing vibration are higher before a successful mating than before an unsuccessful one and when the male’s wings are removed the probability of successful mating is reduced (Benelli et al., 2012). The perception of female- and male-borne olfactory cues is crucial for B. oleae mating. Virgin females produce a multicomponent sex pheromone in their rectal glands (Baker et al., 1980; Canale et al., 2015). The most abundant component of this pheromone is 1,7-dioxaspiro[5.5]undecane (DSU) and it exhibits the highest biological activity toward males (Carpita et al., 2012). Young B. oleae males also produce DSU in the rectal glands (Benelli et al., 2013a). After having reached sexual maturity, they start to produce (Z)-9-tricosene, a compound unique to males, which is able to selectively attract the females (Carpita et al., 2012; Canale et al., 2013). Intra-sexual selection has a debated role in tephritid leks. The males form groups in which each individual defends a small territory performing courting rituals to attract the females (Witthier et al., 1994; H¨oglund & Alatalo, 1995; Segura et al., 2007; Benelli et al., 2014a, b; Cˆamara de Aquino & Joachim-Bravo, 2014). It remains unclear if the size of tephritid males can also affect male wing vibration performances and mate choice dynamics. Since the male’s wings generate the air-borne mechanic particle-displacement, which is perceived as a sound by the mate, we hypothesize that changes in wing’s area or mass may influence the parameters of courtship songs (Moradian & Walker, 2008). Moreover, large males could spend more energy performing wing vibrations, reaching higher frequencies and longer pulse durations that can affect positively the female mating decision. Do larger males perform better courtship displays over smaller ones? Are larger males able to achieve higher mating success over smaller competitors? In this research (i) the wing vibrations performed during courtship by large and small males were recorded by high-speed videos and analyzed through frame-by-frame analysis and (ii) the mating success of large and small males toward a virgin female was investigated. Also, (iii) allowing large and small males to compete for a virgin female, the effect of male– male competition for social resources (where resources are mates) on mating success was evaluated.
Materials and methods Insect rearing Insects used in this study were obtained from field-derived pupae (collected in a Tuscan olive-mill) during December 2013. Pupae were maintained in Pisa C 2015
Institute of Zoology, Chinese Academy of Sciences, 0, 1–12
3
laboratory under controlled conditions (22 ± 1 °C, 50% ± 5% R.H. and natural photoperiod) to wait for adult emergence. To obtain coeval virgin specimens, emergent flies were sexed, singly stored in clean glass vials (diameter 3 cm, length 7 cm) and fed on a dry diet (yeast and sugar at ratio 10 : 1) and water (Benelli et al., 2012; Canale & Benelli, 2012).
General observations and male body size measurements All experiments were conducted in laboratory conditions (22 ± 1 °C, 50% ± 5% relative humidity and 16 : 8 [L : D] photoperiod), in a room illuminated with daylight fluorescent neon tubes (Philips 30W/33). The light intensity in close proximity of the testing arena was approximately 1000 lux, estimated over the 300–1100 nm waveband using a LI-1800 spectroradiometer (LI-COR Inc., Lincoln, NE, USA), equipped with a remote cosine receptor. Diffused laboratory lighting was used to reduce possible reflection and phototaxis. Bioassays were all performed from December 15, 2013 to January 20, 2014 between 15.30 and 19.00 h, with 12–18 d old virgin flies. Each tested fly was immobilized (3 min at -10 °C) and its maximum head and thorax width were measured in dorsal view with a binocular microscope (Brice˜no & Eberhard, 2002; Brice˜no et al., 2002). Previous research showed that cooling did not affect the B. oleae behavior (Benelli et al., 2012). For the experiments, male flies of 2 size categories were used. Males with a thorax width >1.550 mm and
