Behavioral Ecology Advance Access originally published online on June 11, 2004
Behavioral Ecology 2004 15(5):857-863; doi:10.1093/beheco/arh098
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Male sperm expenditure under sperm competition risk and intensity in quacking frogs
Division of Botany and Zoology, The Australian National University, Canberra ACT 0200, Australia
Address correspondence to P. G. Byrne. E-mail: phillip.byrne{at}anu.edu.au.
Received 21 December 2003; revised 29 January 2004; accepted 2 February 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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In the frog Crinia georgiana, reproductive behavior comprises a "guarding tactic," in which males defend spawn sites and attract females by calling, and a "sneak tactic," in which males join spawning pairs. The aims of the present study were to (1) relate ejaculate expenditure by "guarding" and "sneak" males to their probability of mating with other males present (sperm-competition risk), and (2) determine if males adjust their ejaculate expenditure according to the number of males involved in a spawning (sperm-competition intensity). Theory predicts that because sneak males always mate with other males present, they will experience a higher sperm-competition risk and should release larger ejaculates relative to that of guarding males. However, as the proportion of sneaks in a population increases so does the risk of sperm competition to guarders, so expenditure by each tactic should move toward equality. Given that the incidence of sneak behavior is high in C. georgiana, guarders and sneaks were expected to experience similar risks of sperm competition and show similar investment in spermatogenesis. Comparison of testes size and ejaculate size showed no difference between tactics. Models of sperm-competition intensity predict that males should increase their ejaculate size when spawning in the presence of one other male but decrease their ejaculate size when spawning in the presence of multiple males. Here, males maintained a constant sperm number irrespective of whether a mating involved one, two, or three males. This result suggests that male C. georgiana do not facultatively adjust ejaculate investment in response to fluctuating intensities of sperm competition.
Key words: ejaculate expenditure, frogs, group spawning, sperm competition.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Within species, theoretical predictions about ejaculate expenditure and sperm competition depend on a distinction being made between sperm competition risk and sperm competition intensity. Sperm competition risk is the probability that a female mates with more than one male, and sperm competition intensity relates to the number of males competing when all matings are subject to sperm competition (Parker et al., 1996
). Based on the assumption that sperm compete in a situation analogous to a raffle and a male's success equates to the number of sperm ejaculated divided by the total number of competing sperm, the common prediction is that ejaculate size should always increase with risk (Parker et al., 1996). However, a range of ejaculate strategies is predicted depending on the male's mating role and the degree of information available to competitors (cf. Parker, 1998).
In situations in which raffles are fair but the information available to rivals differs such that males in one role have perfect information but males in the other have no information, unequal sperm expenditure is predicted. Parker (1990) considered two cases. In the first, all males pair with females but also perform extrapair copulations (EPC model). In the second, males compete for females and superior males guard territories, whereas inferior males use sneak tactics (sneak-guarder model). In both cases males with perfect information (sneaks and EPC males) are always exposed to sperm competition and are predicted to release larger ejaculates relative to guarding or paired males.
Crucial to the differential in investment between mating tactics is the proportion of sneaks in a population. If low, guarders are exposed to a low risk, and the differential should be great. However, as the proportion of sneaks increases so does the risk of sperm competition to guarders, and expenditure should move toward equality (Gage et al., 1995). In mammals and fish, investigations comparing testes size, ejaculate size, and gonosomatic index between mating tactics have claimed biological support for risk models (Gage et al., 1995; Stockley and Purvis, 1993; Taborsky, 1998). However, these findings may be flawed because the analysis used did not adequately control for the confounding effects of allometry. Recent investigations of gametic investment in dung beetles and sunfish have taken these problems into consideration (Neff et al., 2003; Simmons et al., 1999; Tomkins and Simmons, 2002). Controlled analysis of sperm investment patterns of males that use alternative mating tactics are now required for other groups.
Compared with risk models, the predictions of intensity models are more complex. Parker et al. (1996) modeled ejaculate expenditure for external fertilizers with sperm competition and concluded that males should decrease sperm numbers for matings involving any more than two males owing to decreasing returns per extra unit of sperm expenditure. This prediction is based on the assumption that fertilization is instantaneous and is consistent whether males (1) can accurately estimate the number of competitors, (2) can only assess the number of competitors relative to the average, or (3) can not estimate the number of competitors and respond according to the mean level of sperm competition (Parker et al., 1996).
More recently, Ball and Parker (1997) extended this model by considering contexts in which fertilization is continuous, which they claim to be biologically more realistic for external fertilizers. Two models were devised. The first looked at situations in which males cannot assess the number of competitors. Here, ejaculate expenditure was shown to vary in relation to the mean level of sperm competition. The second considered situations in which males have the ability to accurately count. The results confirmed the findings of Parker et al. (1996) in that expenditure should decrease with increasing intensity of competition but also showed that expenditure could decline in the absence of sperm competition (single-female single-male matings) when average fertility is low. Reanalysis of these models by Ball and Parker (1998) in relation to the amount of energy available per male for reproduction yielded comparable results.
Empirical evaluation of the "intensity model" has only been made in a few systems, and variable patterns of ejaculate expenditure have been detected. In red jungle fowl (Pizzari et al., 2003), as well as several species of fish (Candolin and Reynolds, 2002; Pilastro et al., 2002; Smith et al., 2003) and cricket (Schaus and Sakaluk, 2001; Simmons and Kvarnemo, 1997), males decrease sperm expenditure in response to increasing sperm competition intensity, responses that fit closely with the intensity model. In contrast, males in the butterfly Pieris rapae (Wedell and Cook, 1999) and in the field crickets Gryllodes sigillatus and Acheta domesticus (Gage and Barnard, 1996) increase the number of sperm they release in response to increasing sperm competition intensity. Males in the fish Etheostoma caeruleum (Fuller, 1998) and two species of gryllid cricket, Gryllus texensis and G. sigillatus (Schaus and Sakaluk, 2001), keep ejaculate expenditure constant regardless of the intensity of sperm competition. Given that empirical evaluation of the intensity model has only been made for a few systems and that the results are conflicting, continued evaluation is required across diverse taxa to determine its general applicability.
The frog Crinia georgiana offers a prime opportunity to investigate the effects of sperm competition risk and intensity upon ejaculate expenditure in an anuran amphibian. Approximately half of all matings involve two or more males, and these group spawns result in multiple paternity (Roberts et al., 1999). Within populations, males show extreme variation in body size; body mass is approximately bimodally distributed, with few males occurring between the modes (Smith and Roberts, 2003). Behavioral analysis has shown that mating tactics differ significantly with body size; males greater than 3 g typically guard potential spawning sites from other males and vocalize to attract females, whereas males less than 3 g predominantly adopt noncalling sneak roles and attain fertilizations by joining pairs once they are amplexed (sexual embrace of a female by a male; see Byrne, 2002b). Over a breeding season, approximately 80% of group spawns involve "guarders" and "sneaks" (Byrne, 2002b). Given this, all males should experience a high risk of sperm competition and show similar ejaculate investment (Gage et al., 1995). For matings in which sperm competition is intense (more than two males) ejaculatory responses are of particular interest because previous work has demonstrated that the proportion of a female's clutch that is fertilized declines as the number of amplectant males increases (Byrne and Roberts, 1999). One possible explanation for this is that sperm output per male is reduced in response to intense sperm competition, and sufficient sperm are not present to fertilize eggs. The aims of the present study were to test whether (1) males using different mating tactics show similar investment in ejaculate expenditure, and (2) whether males reduce ejaculate expenditure under conditions of intense sperm competition. The present study provides the first examination of how sperm competition influences sperm investment within a frog species.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Study species and collection location
Crinia georgiana is an abundant, widely distributed frog in southwestern Australia. Breeding is nocturnal and occurs in shallow water between mid autumn and late spring (Roberts et al., 1999
). In single-male matings, males amplex females dorsally. In multiple-male matings, males amplex females dorsally, ventrally, and dorsolaterally (for illustration, see Roberts et al., 1999). Oviposition consists of eggs being released in several discrete episodes. Behavioral observation, combined with water sampling between pairs before and after episodes of oviposition, indicates that ejaculation corresponds with egg release (Byrne, 2002b). Collections of breeding frogs were made from Kangaroo Gully, a natural breeding site located approximately 35 km east-southeast of Perth Western Australia, during the breeding seasons (June–August) of 1999 and 2000.
Comparison between the ejaculate expenditure of guarders and sneaks
Investment in ejaculate expenditure by guarders and sneaks was tested by comparing their ejaculate size and testes size. To obtain ejaculates, 27 sexually active males and females of variable body size were collected from the field and returned to the laboratory, where 27 matings were staged by placing pairs of males and females in transparent circular plastic containers (11 x 5 cm) that held a known volume of water. After oviposition and the release of females by males, frogs were removed and weighed (to the nearest 0.01 g), and water (containing ejaculates) was collected. The technique used to estimate the number of sperm released per male is outlined below. All matings were established in a constant temperature room maintained at 15°C. To estimate testes mass, 47 males of variable body size were collected on the first night of breeding to eliminate the chance of decline in testes volume due to sperm release. Males were weighed (to the nearest 0.01 g), killed via double pithing, and dissected, and testes were removed and weighed (to the nearest 0.0001 g). For analysis, mating tactics were differentiated based on body mass (more than 3 g = guard, less than 3 g = sneak). Body mass has been shown previously to be an accurate predictor of mating behavior in C. georgiana (Byrne, 2002b).
The relationships between log soma mass (body mass – testes mass) and log testes mass and between log soma mass and log sperm number were examined by using simple regression. Comparisons of testes size and ejaculate size between tactics were made by using ANCOVA. Mating tactic (guarding versus sneaking) were effects, log testes mass and log number of sperm were dependants, and log soma mass was a covariate. This is the most effective way to control for allometric relationships when testing for differences in investment between size classes (Tomkins and Simmons, 2002). However, it is important to note that ANCOVA is only a reliable measurement of the relative investment of males adopting different tactics when the relationship between the covariate (soma mass) and the response variable (testes size or sperm number) is isometric. This is examined within the ANCOVA from the interaction between mating tactic and the covariate (log soma mass; Tomkins and Simmons, 2002). Where interaction terms were not significant, they were omitted from ANCOVA models (Hendrix et al., 1982).
Effect of sperm competition intensity on ejaculate size
Ninety-six gravid females and 180 sexually active males were collected from the field and returned to the laboratory, where 96 matings were established by placing females in transparent circular plastic containers (11 x 5 cm), either with one, two, or three males. Thirty-four matings involved one female and one male, 40 matings involved one female and two males, and 22 matings involved one female and three males. Mating containers held a known volume of water, which was raised to accommodate matings with more frogs. After oviposition, frogs were removed and weighed (see Appendices A, B, and C), and water (containing ejaculates) was collected. The number of sperm released per male was then estimated (see below). All matings were established in a constant temperature room maintained at 15°C.
Procedure for estimating ejaculate size
For each mating, sperm solutions were agitated to suspend sperm, and using a micropipette, six aliquots, ranging in size from 1 to 5 ml, were deposited onto glass microscope slides and left to air dry. Aliquots were viewed under a binocular dissecting microscope, and images were digitized by using the image analysis program OPTIMAS 6.5. Aliquot areas were then traced and measured by using the program's area sampling function. After this, aliquots were viewed by using a Leica compound microscope and were once again digitized by using OPTIMAS. A sample square of known area, constructed by using the area creation tool, was then imposed upon the image, and the number of sperm within the sample square were counted. Sperm were included in the count if any portion occurred inside the sample boundary. Six replicate counts were made for each aliquot. Samples were taken systematically across the entire aliquot image to control for any edge effects. To calculate number of sperm within each aliquot, aliquot area was divided by the sample square area and multiplied by the mean number of sperm per sample. This value was then divided by aliquot volume and multiplied by the original volume of water in which the frogs mated. Finally, the average sperm number for six replicates was divided by the number of males involved in a mating to estimate the number of sperm ejaculated per male. The relationship between number of sperm ejaculated per male and the number of males involved in a mating was analyzed by using Kruskal-Wallis ANOVA because variances between mating categories were not equal (Levenes test F2,81 = 3.39, p <.05, JMP statistical package). Post hoc comparisons for Kruskal-Wallis ANOVA were made following the method of Zar (1984: 200).
Assessment of techniques for estimating ejaculate size
To test the accuracy of the technique for estimating the number of sperm within aliquots, total sperm numbers within 30 different aliquots were counted and compared with estimated numbers (see below) by using a paired t-test. The repeatability of aliquot area measures was also tested. Fifteen aliquots were measured six times each, and their areas were compared by using repeated-measures ANOVA. Within aliquots, estimated sperm numbers did not differ significantly from actual sperm numbers (t29 = –1.564, p =.13). For estimates of aliquot area, repeated-measures ANOVA showed there to be significant variance between measurements of different samples (F1,14 = 1621.51, p <.001) but not within (F1,75 = 1.762, p =.13). Therefore, the counting and measurement techniques were accepted as being reliable and repeatable.
To test whether the number of sperm released by males under laboratory conditions was representative of the number released under natural conditions, average ejaculate size from 27 single-male matings established in the laboratory was compared with the average ejaculate size from five naturally formed matings in the field by using an unpaired t test. Under field conditions, recently formed matings were placed in transparent circular plastic containers with a known volume of water. After oviposition and mating breakup, water containing sperm was transferred into plastic vials and returned to the laboratory, where number of sperm released was determined as for artificial matings (see above). No differences were detected, indicating ejaculate strategies were not altered under artificial mating conditions (t30 = 0.30, p =.76: mean ± SE sperm number for laboratory matings = 2.27 x 107 ± 5.8 x 106; mean ± SE sperm number for field matings = 1.85 x 107 ± 2.17 x 106).
Effect of male body mass and clutch size upon ejaculate size
Among external fertilizers ejaculate size may vary in response to factors other than sperm competition. Male body size may affect ejaculate size, if, for example, there is differential ejaculate expenditure by males adopting alternative, size-related mating tactics (Gage et al., 1995; Taborsky, 1998). Males may also adjust ejaculate size in relation to clutch size (Marconato and Shapiro, 1996). The effect of body size was tested in the comparison between the ejaculate expenditure of guarders and sneaks (see above). The effect of clutch size was tested for 27 single male matings by using regression. For the analysis female body mass after egg deposition was used to predict clutch size because egg counts for all matings were not available. Clutch size in C. georgiana is positively predicted by maternal mass (measured after egg deposition; F1,53 = 30.862, p <.001, r2 =.372, n = 54). Egg numbers were estimated from the regression equation y = 20.4275x (female body mass) + 40.8142.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Comparison between the ejaculate expenditure of guarders versus sneaks
A positive allometric relationship was evident between log soma mass and log testes mass; log testes mass increased with log soma mass with an allometric slope greater than one (b = 1.15 ± 0.19, F1,46 = 35.5, p <.001) (Figure 1). There was no significant relation between log sperm number released per male and log soma mass (b = 0.46 ± 1.14, F1,26 = 0.166, p =.68) (Figure 2).
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ANCOVA revealed that interactions between mating tactic (guarder or sneak) and log soma mass were not significant for either testes size or sperm number (testes size: F2,46 = 0.15, p =.69; sperm number: F2,26 = 0.49, p =.48). Therefore, it can be assumed that the relationships between soma mass and testes mass, and soma mass and sperm number, of the two alternative mating tactics (sneaks and guarders) are isometric (Tomkins and Simmons, 2002
). For the reduced models (interaction terms omitted) neither testes size (F1,46 = 0.599, p =.44, Table 1) nor number of sperm released (F1,26 = 2.893 p =.101) (Table 1) differed significantly between predicted mating tactics. This also demonstrates that body size does not affect ejaculate size.
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Effect of clutch size on ejaculate size
Regression analysis revealed that clutch size did not significantly affect number of sperm ejaculated (F1,26 = 1.12, p =.27, r2 =.048). Therefore, this variable was not controlled for in subsequent analysis.
Effect of sperm competition intensity on ejaculate size
Number of sperm released per male remained constant for matings that involved one, two, or three males (Kruskal-Wallis H = 2.228, p =.33). In all cases mean number of sperm released ranged from 1.80 x 107 to 2.05 x 107 (Figure 3).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Ejaculate expenditure in relation to mating tactic and sperm competition risk
In the Australian frog C. georgiana, calling (guarding) males and noncalling (sneak) males were found to invest equally in ejaculate expenditure irrespective of body size. This result was predicted because both tactics experience similar risks of sperm competition (Gage et al., 1995
). Sneak behavior is used opportunistically in response to the intensity of intrasexual competition, which varies with density of breeding males and operational sex ratio (OSR; Byrne, 2002b). When competition is low (low density, even OSR) most males call to attract females. Typically, however, density is high, the OSR is male biased, and males physically compete for call space within a chorus. Large males are superior, forcing small males into sneak tactics (Byrne, 2002b). Large calling males mate free from sperm competition when they attract females and the pair spawn alone. However, group spawns typically involve both guarders and sneaks, so males in both tactics frequently experience sperm competition (Byrne, 2002b). The disparity in risk between tactics is also low because small males can mate free from sperm competition if they intercept females and remain undetected by calling males or when they remain amplexed to females that break free from group spawns. Risk of sperm competition is further unified between tactics because all males resort to scramble competition when male density is extreme and OSR is heavily male biased (Byrne, 2002b). Because mating success under these conditions is not dictated by body size, all males experience an equal chance of sperm competition (Byrne, 2002b).
The way males in alternative mating roles respond to sperm competition has not been investigated previously in frogs. This is surprising because anuran amphibians offer substantial opportunity to rigorously investigate intraspecific predictions of game-theory models (Halliday, 1998). Comparative analysis of testes size in Japanese rhacophorids, African rhacophorids, and Australian myobatrachids suggest that sperm competition occurs frequently in frogs and selects for increased investment in spermatogenesis (Byrne et al., 2002; Jennions and Passmore, 1993; Kusano et al., 1991). Species in these groups provide a rich diversity of mating patterns, ranging from systems in which male-male competition is intense and sneak behavior is common to those in which opportunities for sneak copulations are limited (Byrne et al., 2002; Halliday, 1998). Based on the assumption that the proportion of sneaks in a population directly effects how males adopting different mating tactics invest in spermatogenesis (see Gage et al., 1995), it can be expected that frogs will exhibit a broad range of ejaculatory strategies. To date, most tests of risk models, in both internal and external fertilizers, have not controlled for allometry and consequently may have generated unreliable results (for discussion, see Simmons et al., 1999; Tomkins and Simmons, 2002). Therefore, comparison of testes size, ejaculate size, and gonosomatic index between mating tactics across frog species that are characterized by varying frequencies of sneak behavior would provide important tests of sperm competition theory.
Ejaculate expenditure in relation to sperm competition intensity
This study also investigated whether male C. georgiana adjust ejaculate expenditure according to the intensity of sperm competition. Current models of sperm competition predict males should reduce ejaculate expenditure as the intensity of sperm competition increases (Ball and Parker, 1997, 1998). This was not apparent here. The data suggest that male sperm output remained constant irrespective of the number of competitors. However, because the technique for estimating ejaculate size for matings involving more than two males did not directly measure sperm number for each frog, it is possible that some males increased ejaculate size whereas others reduced it, giving the false indication that all males kept a constant sperm output regardless of the intensity of sperm competition. To address this issue, ejaculates would need to be collected from individual males. In a study investigating paternity contributions in the group spawning frog Chiromantis xerempelina, Jennions and Passmore (1993) attached plastic sheaths to males in order to stop them transferring sperm. A similar technique was attempted as a means of collecting ejaculates in C. georgiana. However, it was ineffective because males would not mate under these conditions. Until direct, although noninvasive, techniques are devised for collecting anuran ejaculates, the possibility exists that individual male C. georgiana use a variety of sperm allocation patterns in response to varying intensities of sperm competition. However, given that theoretical models do not predict any circumstances in which this would be adaptive, combined with the fact that such patterns have not been reported previously for other groups, this scenario seems unlikely.
Why would male C. georgiana not reduce sperm output in response to increasing intensity of sperm competition? There may be several explanations. First, adjustment of ejaculate size may not be possible. In anurans the process of sperm release is poorly understood (Achi et al., 1996). Therefore, the ability of males to facultatively regulate ejaculate size remains highly speculative. In some fish, males can alter sperm output according to female body mass and clutch size (Marconato and Shapiro, 1996), possibly using musculature surrounding the sperm duct at the base of the caudal fin (Rasotto and Shapiro, 1998). Comparable data are not available for frogs.
An alternative explanation for maintenance of ejaculate size is that C. georgiana is not sperm limited. Testes size relative to body mass in C. georgiana is at least four times greater than in any other species in the genus (Byrne and Roberts, 1999). Behavioral observations have shown that males can mate many times both within and between nights. In one observation, a male mated with four separate females over a period of 3.5 h (Byrne, 2002b). Moreover, males do not voluntarily abandon matings, even when competition is extreme (Byrne, 2002b). This would be expected if there was strong selection for sperm economy (Fuller, 1998). Because breeding is not continuous but confined to episodes regulated by environmental conditions (Byrne, 2002a), males may often have the opportunity to replenish sperm stores between matings. A critical assumption of sperm competition models is a tradeoff between ejaculate size and number of matings (Ball and Parker, 1997, 1998). Evidence for sperm economy exists for fish. In an investigation of sperm allocation patterns in Thalassoma bifasciatum, Warner et al. (1995) demonstrated that more successful males release fewer sperm. C. georgiana may not experience such constraints.
Third, if males are unable to accurately count the number of competitors, then it will be impossible for them to effectively modify expenditure in relation to the level of sperm competition intensity. During spawn events the number of males involved can vary unpredictably owing to individuals joining pairs at various times after spawning commences as well as males physically displacing competitors during a spawning event (Byrne, 2002b). Considering this, it may be difficult for males to distinguish how many rivals are present at any given time. Under these conditions, the best strategy may be to optimize expenditure according to the average intensity of sperm competition. The outcome being that all males release approximately the same number of sperm irrespective of how many competitors are present (Ball and Parker, 1997).
Fourth, for adjustment to be effective, expenditure may have to be weighted not only to the number of amplectant males but also to other factors that critically affect the probability of ejaculate success: for example, proximity to eggs, temporal differences in ejaculate release, heterogeneity in fine-scale processes operating in the medium in which ejaculates are released, and interference from competitors. In C. georgiana, reduced fertilization success was reported as a cost of group spawning by Byrne and Roberts (1999). They proposed this might be the result of males reducing sperm output in response to intense sperm competition. However, the finding in the present study, that total number of sperm released is positively associated with the number of amplectant males, supports an alternative hypothesis that reduced fertilization success results from the physical obstruction of ejaculates by competitors (Byrne and Roberts, 1999). This high level of interference between competitors means that during group spawns, one or several males may have a reduced chance of fertilization success. Consequently, the actual level of sperm competition may not be as intense as competitors perceive it to be. If this is the case, males that reduce sperm output when multiple males are present would be at a selective disadvantage.
Finally, the possibility needs to be considered that for anuran amphibians factors that favor increased ejaculate expenditure, other than sperm competition, complicate ejaculate decisions. In a comparative study across frog species in the family ranidae, Emerson (1997) found a positive association between testes size and clutch size. However, in the present study there was no association between the clutch size of female C. georgiana and the ejaculate size of her mate. Concordantly, Byrne et al. (2002) found that across 181 species of Australian frogs variation in clutch size had no effect on testes size, although, the present study did reveal that oviposition habit (e.g., terrestrial, aquatic, foam nests) had a positive effect on testes size. Spawn environment may greatly influence sperm mortality or wastage and therefore ejaculate expenditure (Levitan, 1998). It is also conceivable that ova size may influence ejaculate expenditure. Large ova may increase the proportion of successful sperm/egg interactions and favor reduced sperm output, whereas small ova may have the reverse effect (Levitan, 1993). Benefits of ejaculate adjustment will only come if the combination of factors affecting ejaculate success can be accurately predicted. If they cannot, maintenance of ejaculate size across all intensities of sperm competition may be a more effective strategy.
In conclusion, the present study provides support for Gage et al.'s (1995) risk model of sperm competition by showing that investment in spermatogenesis by male quacking frogs was not significantly different between callers (guarders) and noncallers (sneaks) that experience similar risks of sperm competition. In addition, the present study provides the first evaluation of the intensity model of sperm competition (Ball and Parker, 1997, 1998; Parker et al., 1996) in a group-spawning frog. The data suggest that males do not facultatively adjust ejaculate investment in response to fluctuating intensities of sperm competition. Similar results have also been obtained for a group spawning fish (Fuller, 1998) and two species of gryllid cricket (Schaus and Sakaluk, 2001). If future investigations continue to find examples in which males show no ejaculatory response to varying intensities of sperm competition, then current models may need to consider how factors other than sperm competition intensity could influence the way sperm allocation strategies evolve.
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ACKNOWLEDGEMENTS |
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Many thanks to Leigh Simmons, Dale Roberts, Scott Keogh, and two anonymous reviewers for comments on the manuscript. Research was supported by a Gene Rodgerson Scholarship and the Australian Research Council. Research was conducted with permission of the Animal Ethics and Experimentation Committee (U.W.A.; approval no. 117/97).
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