Behavioral Ecology Advance Access originally published online on May 5, 2006
Behavioral Ecology 2006 17(4):628-632; doi:10.1093/beheco/ark007
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Resource-dependent nuptial feeding in Panorpa vulgaris: an honest signal for male quality
Institut für Evolutionsbiologie und Ökologie, Rheinische Friedrich-Wilhelms-Universität Bonn, An der Immenburg 1, 53121 Bonn, Germany
Address correspondence to S. Engels. E-mail: sengels{at}evolution.uni-bonn.de.
Received 11 October 2005; revised 21 March 2006; accepted 1 April 2006.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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In mating systems that are characterized by resource-dependent male behavior like nuptial feeding, food limitation obviously plays a major role in male performance. In Panorpa vulgaris (Mecoptera: Panorpidae), the ability to produce nuptial gifts has major fitness consequences as the number of gifts determines copulation duration, which then determines the number of eggs fertilized by a given male. In the present study, we are able to show that males of P. vulgaris were limited in their production of salivary secretions. The number of saliva secretions males were able to produce declined in successive matings. Moreover, males of nutritionally high status produced more gifts than those of nutritionally low status. The proximate factor determining male saliva secretion was the development of the salivary gland, which in turn depended on the amount of food a male could access. The degree of male mating effort corresponded to the size of the salivary gland, yet while absolute investment increased with gland size, the relative investment decreased. Mating costs for males thus depend on nutritional status.
Key words: female and male choice, indicator theory, Mecoptera, nuptial feeding, sexual selection.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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The basic assumption in modern behavioral ecology is that animals should behave in a way maximizing their overall fitness. However, mating interests are often significantly different between the sexes (Parker 1979
; Andersson 1994; Parker and Partridge 1998; Arnqvist et al. 2000; Gavrilets et al. 2001; Stutt and Siva-Jothy 2001; Arnqvist and Rowe 2002; Chapman et al. 2003). Generally, in polyandrous species, the challenge for males is to transfer more sperm to more females than other males, thereby siring more offspring than their competitors. The way to succeed, however, can be very different depending on the species and, moreover, on intraspecific strategies. In some species, intrasexual competition between males over mates may play a major role, for example, fighting, mate guarding, or some other way of monopolizing access to several females (Andersson 1994; Krebs and Davies 1996). A more cryptic struggle for fertilizations that arises from polyandry is sperm competition (Parker 1970), which is very common and widespread among various animal species (Simmons 2001). But females are far from passive in reproduction. There are numerous examples where females are the ones in control by choosing those males that seem to fit their claims best (Andersson 1994; Sakaluk and Eggert 1996; Sauer et al. 1998). A very conspicuous form of female choice can be observed when females choose males on the basis of secondary sexual ornaments. In general, such ornaments can be viewed as a form of male advertising in order to get access to females. A special case of sexual advertising is nuptial feeding. In mating systems characterized by nuptial feeding, males have to offer gifts in order to obtain copulations, which can for instance be prey items, body parts, or glandular secretions (Vahed 1998). These nuptial gifts can be associated with considerable costs for males. If this is the case, males should not tend to waste but rather allocate their resources strategically by adjusting the degree of mating effort to female quality (Sauer 1996, 2002; Engqvist and Sauer 2001, 2002).
In the present study, we were concerned with the mating system of the scorpionfly Panorpa vulgaris (Mecoptera: Panorpidae). Males and females of this species are polygamous, resulting in a high level of sperm competition (Sauer et al. 1990, 1997, 1998, 1999; Sindern 1996). Males can adopt 2 alternative resource-dependent mating tactics in order to obtain copulations: they can offer females 1) a dead arthropod or 2) variable numbers of salivary masses (Sauer et al. 1998). Providing females with saliva secretions is the more profitable tactic because it results in longer copulation durations and therefore is preferentially applied by males (Sauer et al. 1998). Females adjust mating duration to the number of salivary masses they receive (Sindern 1996; Sauer et al. 1998). Because sperm transfer rate is continuous during copulation and fertilization of eggs follows the fair raffle principle (Parker et al. 1990), males have an interest in maximizing mating duration (Sauer et al. 1990, 1997, 1999).
Previous studies indicated that the males' capability of producing salivary masses depends on their nutritional status (Bockwinkel and Sauer 1994; Sauer et al. 1997, 1998). The aim of the present study was to examine whether and in which way salivary gland development in male P. vulgaris is influenced by the amount of food available during adulthood. Because saliva secretion is essential for males to obtain long copulations, the amount of saliva males are capable of investing in matings is the most prominent factor determining male reproductive success. To get further information on the function of salivary masses as a Zahavian quality indicator (Zahavi 1975) in the scorpionfly P. vulgaris (Sauer 1996; Sauer et al. 1998), we investigated if nuptial gift production implies differential marginal costs for males of various qualities. Furthermore, we looked for evidence of cryptic male choice and under which circumstances males may invest their mating resources strategically according to female quality.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Breeding
Animals used in the experiments were offspring of the second annual generation of 2002 and of the first generation of 2003 of P. vulgaris. Adults were collected at a field site near Freiburg i. Br., southwest Germany, and taken to Bonn for breeding. They were held in pairs in plastic boxes (10 x 10 x 7 cm) containing moist tissue paper, ad libitum food, and a small petri dish filled with moist peat for oviposition. F1 larvae of the spring generation of 2003 were reared at an 18:6 h light:dark cycle at 18 °C on moist tissue paper with ad libitum food at a maximum density of 20 larvae per petri dish (12-cm diameter). Third-instar larvae were transferred into soil-filled, open-bottomed plastic cylinders (40-cm diameter) placed outdoors in the ground, where they finished their diapause-free development. F1 larvae of the summer generation of 2002 were reared under the same conditions, but at a 12:12 h light:dark cycle. Moreover, after the second larval molting, they were kept singly in small petri dishes. After 20 days, they were transferred into plastic tubes filled with moist peat and entered diapause in October 2002.
Nutritional status and mating success of males (F1 offspring of the first generation of 2003)
Experimental treatment and recorded parameters
Hatching adults were kept singly in plastic tubes (8 x 3.5 cm) containing moist tissue paper. Males were assigned to either a high- or a low-nutrient treatment. They received food on the day of emergence (=day 0) and 5 days after emergence (=day 5). Well-nourished males were fed with 2 segments of Tenebrio molitor larvae, whereas poorly nourished males received only one segment per feeding. Females were provided with 2 segments every fourth day.
On day 9, males were separated into 3 groups and were mated 1, 2, and 3 times, respectively. Only virgin females were used for the copulations. Males did not receive any food in between the matings. Because males were derived from different food regimes (see above), we had a total of 6 different groups:
- i. Observed during a first mating: 1) well nourished (N = 25) and 2) poorly nourished (N = 9).
- ii. Observed during a second mating: 1) well nourished (N = 15) and 2) poorly nourished (N = 10).
- iii. Observed during a third mating: 1) well nourished (N = 7) and 2) poorly nourished (N = 9).
- ii. Observed during a second mating: 1) well nourished (N = 15) and 2) poorly nourished (N = 10).
Males assigned to the second and third group that failed at initiating a second or third copulation, respectively, were not included in the general linear model analysis. Before starting the mating trials, the body weights of all males were measured with a balance of Sartorius (model: BP 110S). During matings, we counted the number of salivary masses produced and determined copulation duration. Therefore, we were able to measure the effect of different nutrition and the number of previous matings on these 2 variables.
Nutritional status and male investment (F1 offspring of the second generation of 2002)
Experimental treatment and recorded parameters
Hatching animals were kept singly in plastic tubes (8 x 3.5 cm) containing moist tissue paper. The body weights of males were measured immediately after emergence (Sartorius balance, model: BP 110S). Males were separated into 3 groups exposed to different food availability. Males of one group received food on days 0 (=hatching date), 4, 8, and 12 (well-nourished males); the second group of males on days 0, 7, and 14 (medium-nourished males); and the last group of males on days 0 and 10 (poorly nourished males). Each feeding consisted of one segment of T. molitor larvae. Females were provided with 2 segments on days 0 and 8.
A total of 151 males were included in these treatments. On day 15, the body weights of all males and females were measured using a Sartorius balance (model: BP 110S). At this point, the salivary glands were dissected of approximately half (N = 76) of the males. The remaining 75 males were mated to females once. The duration of copulation and number of salivary masses were recorded. Additionally, a subsample of 51 salivary masses was collected for weighing. After copulation, the salivary glands of the males were dissected.
Measuring weights of salivary masses
During 51 copulations, we removed the second salivary mass that was produced. This was done using small tweezers and was generally possible without disturbing the copulating pair. Thus, we collected a total of 51 salivary masses. The secretions were transferred into Eppendorf reaction tubes and were dried at 20 °C until weight constancy. Dry weights of secretions (individual weight of salivary mass [IWSM]) were measured with a precision balance of Sartorius (model: 2004 MP).
Dissection and weighing of salivary glands
On day 15, males were anesthetized using CO2 and subsequently were killed by drowning them in 70% ethanol. After approximately 5 min, they were transferred into a water-filled preparation dish and placed under a binocular magnifier (Leica [Wetzlar, Germany] Wild M3B). Bodies were laterally opened by cutting the integument with a small dissection scissor starting at the end of the abdomen. Afterward, the insects were put back into 70% ethanol where they remained for at least 24 h. This was necessary to harden the secretion inside the salivary gland. The salivary gland is a very soft tissue, which is almost impossible to dissect properly without this treatment. After dissection, the glands were placed on a piece of aluminum foil inside a small petri dish (5-cm diameter) and were dried at 30 °C until weight constancy. The salivary gland dry weight was measured using a precision balance of Sartorius (model: 2004 MP).
Calculated values
We calculated male investment as the amount of saliva secretion males invested in a copulation: 
salivary masses x IWSM (see above). Accordingly, for a sample size of 51 males, we estimated salivary gland dry weight before copulation as follows: gland dry weight after copulation + salivary masses x IWSM.
Statistics
Statistical analyses were performed using SPSS 11.0. All tests were 2-tailed, and the level of significance was determined as 
= 0.05 for all cases. Mean values are given as mean ± standard error. All data analyzed by parametric tests were explored graphically to ensure that they did not differ from the assumptions of parametric statistics.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Nutritional status and male mating success
Male body weight determined immediately before the first copulation correlated with the number of salivary masses a male provided to the female (Spearman rank correlation—first copulation: N = 84, rs = 0.405, P < 0.001; second copulation: N = 44, rs = 0.650, P < 0.001; third copulation: N = 16, rs = 0.877, P < 0.001) as well as with the duration of copulation in all 3 copulations (Spearman rank correlation—first copulation: N = 84, rs = 0.420, P < 0.001; second copulation: N = 44, rs = 0.618, P < 0.001; third copulation: N = 16, rs = 0.715, P = 0.002). This relationship becomes stronger in successive copulations, showing that the importance of body weight increases with mating frequency. Therefore, male body weight could be a measure of the amount of available saliva. Different food availability resulted in males of significantly different body mass (well-nourished males: 46.18 ± 0.84 mg; poorly nourished males: 34.36 ± 0.92 mg; ANOVA: F1,82 = 88.14, P < 0.001). Accordingly, well-nourished males produced significantly more salivary secretions than poorly nourished males in 3 consecutive copulations (Figure 1; 2-way ANOVA: F1,69 = 32.55, P < 0.001). Additionally, the number of salivary masses decreased from the first to the third mating (Figure 1; 2-way ANOVA: F2,69 = 12.70, P < 0.001). The interaction between the nutrition regime and the number of copulation was not significant (Figure 1; 2-way ANOVA: F2,69 = 2.28, P = 0.11). Also, the duration of copulation was affected by food availability with well-nourished males achieving longer copulations than poorly nourished males (well-nourished males: first copulation, 249.16 ± 13.36 min; second copulation, 229.20 ± 15.89 min; third copulation, 164.86 ± 25.99 min; poorly nourished males: first copulation, 212.56 ± 22.36 min; second copulation, 88.20 ± 28.04 min; third copulation: 54.67 ± 13.19 min; 2-way ANOVA: F1,69 = 32.48, P < 0.001). Additionally, copulation duration decreased in consecutive copulations (2-way ANOVA: F2,69 = 17.48, P < 0.001) accompanied by an increasing difference between the differently nourished groups (2-way ANOVA—nutritional regime x number of copulation: F2,69 = 4.04, P = 0.02).
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Nutritional regime also altered the probability of mating. Well-nourished males were more likely to obtain matings than poorly nourished males (first copulation—G-test: well nourished, 49/50, poorly nourished, 36/50, G1 = 15.44, P < 0.001; second copulation—G-test: well nourished, 15/16, poorly nourished, 10/15, G1 = 3.89, P = 0.049; third copulation—G-test: well nourished, 7/8, poorly nourished, 9/11, G1 = 0.115, P = 0.735). The lack of significance between the high- and low-nutrition groups in the third mating trial is probably due to the small sample size and not due to the absence of the effect.
As already demonstrated, salivary mass production of males from both nourishment groups declined in successive matings. However, in well-nourished males, the number of salivary masses produced during matings decreased, but all individuals (with only one exception during the second copulation) were able to produce at least one salivary mass. In contrast, in poorly nourished males, the number of salivary masses produced declined, whereas the percentage of males not providing any salivary masses increased dramatically with increasing mating frequency (first copulation, 2.86%; second copulation, 42.86%; third copulation, 66.67%). These results demonstrate a strong selective pressure on male P. vulgaris to acquire nutritional resources.
Nutritional status and male investment
There was a positive correlation between male body weight and the dry weight of the salivary gland (Figure 2; Spearman rank correlation: rs = 0.659, N = 76, P < 0.001). Moreover, we found significant differences in salivary gland weights between males of nutritionally high, medium, and low status (males of nutritionally high status, 2.70 ± 0.16 mg; males of nutritionally medium status, 1.84 ± 0.12 mg; males of nutritionally low status, 1.13 ± 0.13 mg; ANOVA: F2,73 = 27.41, P < 0.001). We measured the dry weight of a salivary mass and the salivary gland of 51 males after copulation. The mean dry weight of a salivary mass was 0.15 ± 0.01 mg (N = 51), and the calculated mean dry weight of the salivary gland prior to copulation was 2.34 ± 0.15 mg (N = 51). Male investment in a copulation (=salivary masses x IWSM) correlated with the estimated salivary gland weight before copulation (Figure 3; Spearman rank correlation: rs = 0.507, N = 51, P < 0.001). Thus, with increasing salivary gland size, males produce more salivary masses. But the slope of the regression line is smaller than unity (slope b = 0.294 ± 0.071; ANOVA: F1,98 = 99.163, P < 0.001) and the intercept larger than zero (intercept = 0.452 ± 0.182), indicating that relative male investment decreases with increasing salivary gland size and hence with increasing male condition.
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No relation exists between female quality (body weight) at copulation and the number of salivary masses produced by males of nutritionally high and medium status (Spearman rank correlation—males of nutritionally high status: rs = –0.12, N = 30, P = 0.528; males of nutritionally medium status: rs = 0.027, N = 27, P = 0.894). Although not statistically significant, such a relationship seems to exist for males of nutritionally low status (Figure 4; Spearman rank correlation: rs = 0.431, N = 18, P = 0.074).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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The present study demonstrates experimentally that salivary gland development and saliva secretion in male P. vulgaris are resource-dependent traits heavily influenced by the availability of food. The degree of mating effort measured as the number of salivary masses offered per copulation decreased in successive matings, if the males were not allowed to feed in between. Males experiencing high food availability produced more salivary masses than those experiencing low food availability. Furthermore, whereas all well-nourished males (=high body mass) were capable of producing salivary masses in any of 3 successive copulations, the proportion of nonproducers within poorly nourished males (=low body mass) increased dramatically with mating frequency. This was due to a correlation between male body weight and salivary gland size, with salivary gland size being a measure of the availability of saliva. Higher food availability also resulted in a higher probability for males to obtain matings.
Because sperm transfer is continuous during copulation (Sauer et al. 1997), the number of sperm transferred is controlled by copulation duration, which in turn depends on the number of salivary masses a male is capable to invest (Sauer et al. 1998). This has again been demonstrated in this study. Because fertilization of eggs in the scorpionfly P. vulgaris follows the fair raffle principle (Parker et al. 1990; Sauer et al. 1990, 1998; Sindern 1996), saliva secretion can be referred to as mating effort (Sauer et al. 1998; Sauer 2002).
It has previously been suggested that nuptial gift production in P. vulgaris is dependent on food availability (Bockwinkel and Sauer 1994; Sauer et al. 1997, 1998). However, this is the first experimental approach showing the direct interaction between food availability, salivary gland development, and saliva allocation in P. vulgaris. Nutritional condition and also "male quality" have been shown to influence nuptial gift size in other insect species as well (Simmons et al. 1999; Jia et al. 2000). Our results strengthen the importance of traits like foraging ability and fighting prowess as basic requirements for a successful participation in the game of reproduction. Whereas copulation duration can be viewed as the most prominent proximate fitness determinant (Sauer et al. 1997, 1998, 1999), food acquisition is fundamental for the development of males' salivary glands. As the availability of saliva determines males' lifetime copulation duration, foraging ability plays an important role for males in terms of achieving mating durations long enough to be competitive in sperm competition. Therefore, foraging ability strongly influences males' lifetime reproductive success. Previous studies have stressed the importance of male fighting prowess for reproductive success and could show that offspring of superior fighters were able to win more fights over food, and sons of good fighters were more successful in obtaining matings when compared with offspring of inferior males (Thornhill and Sauer 1992). So far, the importance of fighting prowess in connection with gaining resources in P. vulgaris may have been overestimated to some extent. Even a good fighter has nothing to win when there is nothing to fight for. Due to the temporally and spatially unpredictable distribution of food, the capability of detecting food items should be at least of equal if not of higher relevance in order to develop or replenish the salivary glands.
Males of P. vulgaris are forced to adjust their mating effort to the size of their salivary glands, but relative investment decreases with increasing salivary gland size and hence with male condition. Previous studies (Thornhill and Sauer 1992; Sauer 1996; Sauer et al. 1998) could show that nuptial gift production in P. vulgaris fulfils the main predictions of the indicator models of sexual selection (Zahavi 1975; Andersson 1982, 1986, 1994; Maynard Smith 1991; Kokko et al. 2003). However, the interpretation of Sauer (1996) and Sauer et al. (1998) are based on rather indirect evidence concerning the assumption that indicators must be honest signals that are more costly for low- than for high-quality individuals. The present study is the first one to present direct evidence for relative male investment to decrease with male quality in P. vulgaris. Concluding from this, the marginal cost of advertising (Maynard Smith 1991) is higher for males in poor condition. These results support the idea of the nuptial gift's role being a Zahavian quality indicator serving as a basis for cryptic female choice (Sauer et al. 1998). The existence of female choice based on nuptial gift production in P. vulgaris has been demonstrated several times (Sindern 1996; Sauer et al. 1998) and cannot exclusively be explained by direct benefits for the females. The very strict termination of copulation by females if receiving no further salivary masses supports the idea of female choice for a certain amount of sperm transferred by a given male (Sakaluk and Eggert 1996; Simmons et al. 1999). If it were only for the direct benefits gained through the gifts, one would expect a much greater variation in female behavior concerning the termination of copulations because the costs of ongoing matings without additional gifts would be minimal. However, by adjusting mating duration to the number of saliva secretions provided by the male, females can allocate paternity of their different mates according to their ecological (=genetic) quality (see above, Sauer 1996; Sauer et al. 1998).
Sindern et al. (1994, 1995) and Sauer (1996) demonstrated the existence of male choice in P. vulgaris. In the present study, there was a nonsignificant tendency for male choice to be exhibited only by poorly nourished males. As already emphasized, the marginal costs of display are less for males of high quality so that, in general, mating costs are relatively higher for poorly nourished than for well-nourished males. Therefore, it would be reasonable if males of low nutritional status were choosy while males of high quality exhibited no discrimination among females concerning their fecundity. However, because the evidence from the present study is quite weak and not significant, from these data, we cannot ultimately decide about the existence of cryptic male choice in P. vulgaris.
The present study points out that salivary gland size of male P. vulgaris, determined by the ability to gain food, is crucial for male reproductive success. Our results in connection with results of previous studies (Sauer 1996; Sauer et al. 1998) support the view that nuptial gifts in P. vulgaris represent honest signals for male genetic quality in the sense of Zahavi's (1975) indicator models of sexual selection.
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ACKNOWLEDGEMENTS |
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We thank Leif Engqvist, Andreas Vermeulen, and Dagmar Kock for valuable comments and discussions and 2 anonymous referees whose reviews helped to improve an earlier version of the manuscript. This work was supported by the Deutsche Forschungsgemeinschaft [SA 259/7—1 to 4].
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