Male sperm reserves and copulation frequency in birds

Summary Male passerine birds store sperm in a cloacal protuberance during the breeding season. We consider three hypotheses to explain interspecific variation in relative cloacal protuberance size. The copulation efficiency hypothesis states that a relatively large cloacal protuberance facilitates sperm transfer and predicts more rapid copulation in species with larger protuberances. There is insufficient data to test this idea rigorously, but we found no evidence for such an effect. The spermatozoa size hypothesis is a non-functional hypothesis which states that cloacal protuberance size is merely a consequence of sperm size, and predicts that species with larger spermatozoa have relatively larger cloacal protuberances. Sperm length was positively correlated with protuberance size, providing support for this hypothesis, although it seems unlikely that variation in sperm size alone is sufficient to account for variation in protuberance size because the numbers of sperm stored in the cloacal protuberance were also positively correlated with its size. The sperm competition hypothesis states that the cloacal protuberance is a sperm store and predicts that when sperm competition is intense, as measured by male copulation frequency and or mating system, males will have relatively large protuberances and testes mass. The sperm competition hypothesis was supported: in a comparative study in which we controlled for phylogenetic effects, relative cloacal protuberance size was significantly and positively correlated with copulation frequency. Across all mating systems protuberance size was also positively correlated with the mass of seminal glomera tissue, the number of sperm stored in the seminal glomera, and with relative testes size. These results suggest that where sperm competition is intense, a large cloacal protuberance is required to maintain a large sperm reserve for a high copulation rate.Correspondence to: T.R. Birkhead     

Mating system and alternative male mating tactics in the grass goby Zosterisessor ophiocephalus (Teleostei: Gobiidae)

 Peculiar fertilization dynamics, with males releasing sperm in mucous trails lasting several hours, characterize some demersal spawning fish. The mating system was investigated in a natural population of one of these species: the grass goby Zosterisessor ophiocephalus (Pallas, 1814), a large coastal goby inhabiting seagrass meadows in shallow brackish water. Adult males ranged in size from 7.4 to 23 cm total length, but only larger ones were observed to dig and defend a burrow, where they performed parental care on eggs laid by one to several females. Field observations together with analyses of age, sperm production, trail sperm content and sperm competition tests indicated the occurrence of alternative male mating tactics, likely the expression of an ontogenetic gradient. Larger males are older than smaller ones, and while the former are territorial, the latter “sneak” territorial male spawns. The ejaculate characteristics indicate that grass goby males have functionally polymorphic spawns: in fact sperm trails of larger males last longer and release fewer sperm than those of smaller males. Sperm production over several days is more constant in larger than in smaller males, but the total number of sperm released is higher in the latter. The influence of seminal fluid in the functional intraspecific variability in sperm release in this species is discussed. Received: 30 December 1999 / Accepted: 31 July 2000     

The relative roles of predation and sperm competition on the duration of the post-copulatory association between the sexes in the blue crab, Callinectes sapidus

In many species, post-copulatory mate guarding prevents other males from mating with the guarded female. In crabs, males stay with their mates to protect the female from predators because, in some species, mating occurs when she is soft and vulnerable after molting. I tested the relative roles of sperm competition and predation on the duration of the post-copulatory association in the blue crab, Callinectes sapidus. Unpaired females suffered greater predation mortality than paired females and males stayed with the female longer in the presence of predators than in their absence, suggesting that the post-copulatory association protects females during their vulnerable period. However, the association may also occur in blue crabs because of sperm competition since spermathecal contents of females in the field indicate that 12.4% mated twice. Females experimentally mated with two males contained both males ejaculates and each ejaculate had access to the unfertilized eggs, suggesting that the size of a male's ejaculate influences his fertilization rate in a multiply-mated female. Males stayed longest in response to a high risk of sperm competition. Longer post-copulatory associations allowed the first male's ejaculate to harden into a type of sperm plug, which limited the size of a second inseminator's ejaculate in a non-virgin female as compared with a virgin. Males passed larger ejaculates in the presence of rivals and when previous ejaculates were in the female spermathecae, another response to sperm competition. Larger ejaculates may need longer post-copulatory associations before a more effective sperm plug forms. Large males stayed with the female longer, which is consistent with their ability to pass larger ejaculates than small males and suggests that there may be costs to minimizing the duration of the post-copulatory association. In the field, associations last long enough to protect the female during her vulnerable phase and may ensure that the guarding male fertilizes the most eggs in the female, even if she remates. Thus, the post-copulatory association protects female blue crabs from additional inseminators as well as from predators. Received: 23 January 1996 / Accepted after revision: 9 November 1996     

Sperm removal and ejaculate size correlate with chelae asymmetry in a freshwater crayfish species

Asymmetry in traits of sexual relevance may impair copulation behaviour and sexual performance of males, ultimately resulting in a fitness cost. Freshwater crayfish males use chelae, a sexually selected trait, to secure and position the female prior to and during mating. Thus, a relatively large chelae asymmetry, resulting from accidental loss and regeneration of one cheliped after autotomy, could have great consequences for male sexual behaviour. We studied copulatory behaviour and sperm expenditure of males paired to a mated female in Austropotamobius italicus, a freshwater crayfish species where both male and female mate multiply and where last-mating males are able to actively remove previously deposited sperm. We aimed at assessing whether male sperm removal and expenditure varied according to sperm allocated by first-mating males, and according to copulation behaviour and phenotypic traits (carapace length, chelae length and relative chelae asymmetry) of second-mating males. Second-mating males did not adjust their ejaculate size in relation to first-mating male ejaculate, nor to the first-mating male’s sperm removed. Moreover, the amount of sperm removed by second-mating males increased with increasing first-mating males ejaculate size, and first-mating male sperm remaining after removal did not correlate with the original first-mating male ejaculate size. Interestingly, the amount of sperm removed by second-mating males decreased with increasing relative chelae asymmetry, while increasing with male body size. However, second-mating (but not first-mating) asymmetric-clawed males produced larger ejaculates than symmetric-clawed ones. Importantly, the proportion of second-mating male sperm remaining after the two matings did not vary with relative chelae asymmetry nor with body size of second-mating males. Thus, small, asymmetric-clawed crayfish males appear to adopt sperm allocation tactics that allow them to fully compensate for their inferior sperm removal ability.     

Determinants of ejaculate size in a grasshopper (Chorthippus parallelus)

Models for the evolution of ejaculate expenditure predict that ejaculate size (ES) depends on male body condition, on female fecundity, or on the risk of sperm competition. These ideas were tested in the grasshopper Chorthippus parallelus. ES ranged over four orders of magnitude, around a mean of 442,000 sperm. ES was independent of male body size and female fecundity. None of the male condition parameters - age, degree of parasitism, macropterism score (a measurement of competitive energy allocation to other sources), and the mass-femur length residuals - explained variation in ES. As expected in species where females are cyclically non-receptive after mating, male-male encounters before copulation were a more reliable indicator of the risk of sperm competition than the number of males present during copulation. In particular, the summative number of male-male encounters rather than the male-male encounter frequency was a significant predictor of ES variation (24.9%).     

Sperm economy and limitation in spiny lobsters

Sperm limitation, when female fertilisation success is constrained by the supply of sperm, is generally perceived to be an uncommon feature of reproduction in species which directly transfer gametes during copulation. Male size, previous copulations, and the balance of expected reproductive return and future mating opportunity may, however, limit the amount of sperm males transfer to females. We used laboratory experiments where mate size could be manipulated and its consequences on spermatophore size and clutch size determined, to show that in two genera of spiny lobsters (Crustacea: Palinuridae) male reproductive output limits the size of clutches brooded by females. In Panulirus argus from the Florida Keys, we show that while male size affects spermatophore area, males also vary the amount of ejaculate positively with female size. Furthermore, the area of the spermatophore has a greater influence than female size on subsequent clutch weight. In Jasus edwardsii from New Zealand, female size, male size and mate order all affect clutch weight. In both species, clutches fertilised by small males in the laboratory are significantly smaller than clutches fertilised by large males. These results suggest that to ensure they receive sufficient sperm, females should either mate several times prior to oviposition, mate as early as possible in the reproductive season, or choose large, preferably unmated males as partners and thus compete with other females for preferred males. Sperm-limited female fecundity has the potential to limit the egg production of fished populations where large males are typically rare. Received: 18 May 1998 / Received in revised form: 20 November 1998 / Accepted: 30 November 1998     

Ideal dominance distributions: a test using red-winged blackbirds (Agelaius phoeniceus)

Summary The ideal dominance distribution model predicts that competition between individuals of a species for territories will result in socially dominant individuals acquiring territories in higher quality habitat than their subordinates. Although the dispersion and relative reproductive success of male red-winged blackbirds (Agelaius phoeniceus) across habitats in eastern Ontario appears to conform to the ideal dominance distribution model, data from a study of three captive groups comprised of males from both high (marsh) and low (upland field) quality habitats failed to support the prediction that males from marsh habitat are dominant to those from upland habitat. Contrary to the prediction males from uplands were generally dominant to males from marshes. We found a significant positive correlation between dominance and both increased epaulet size and increased body size. Controlling for these positive effects, upland males remained generally dominant to marsh males. Measurements of independent samples of males from both habitats indicated that the overall distribution of males does not conform to ideal dominance. We suggest that the strong between-year territory fidelity shown by male red-winged blackbirds and chance events when they initially acquire territories may contribute to this lack of conformity.     

Mechanisms enabling sperm economy in blenniid fishes

Sperm competition, cost of spermatogenesis and spawning frequency are known to influence ejaculate expenditure. Accordingly, males, particularly those with high reproductive costs, are expected to have evolved mechanisms enabling them to prudently allocate sperm, such as the fractioning of ejaculate expenditure or the semi-cystic type of spermatogenesis, hypothesised to favour the production of small ejaculates. In this study, we investigate sperm competition risk, ejaculate size and mode of ejaculate release in seven polygynous blenniid fish where males provide sole paternal care of eggs. In addition, we estimated the relative size of the two parts composing the male gonad, the strictly testicular (testicular lobules or testis) and the glandular (testicular gland), as the development of the latter is indicative of the level of semi-cystic spermatogenesis. In all the examined species, eggs were laid one by one, and the sperm expenditure at mating, evaluated as the total number of sperm released per mating, was parcelled out in several successive ejaculations, allowing males to adjust the release of sperm to the duration of egg deposition. In accordance with sperm competition theory, species experiencing higher sperm competition risk allocated more in sperm, both considering ejaculate size and ejaculate expenditure per mating. An increase in sperm expenditure was paralleled by the development of the testis at the expense of the testicular gland. Smaller species, whose males do not face sperm competition risk and fecundity is low, produced smaller ejaculates and exhibited a more developed testicular gland, supporting the hypothesis that a semi-cystic type of spermatogenesis is a mechanism allowing sperm economy.     

Male genital traits and mating interval affect male fertilization success in the water strider Gerris lacustris

Parker's seminal work brought attention to the possibility of postmating sexual selection by non-random fertilization success. Mechanisms for these processes are still only partly understood and there is clearly a need for more studies of intraspecific variation in sperm precedence. Here, we report results from an experimental study of the variation in fertilization success between males of the water strider Gerris lacustris. Genital morphology, male body size, and copulation duration were examined as possible correlates of paternity. The significance of guarding duration was also analysed. Only male genital morphology was correlated to fertilization success. This is one of the first studies showing a relationship between male genital traits and fertilization success, supporting the view that sexual selection may be responsible for the rapid and divergent evolution of genital structures in animals with internal fertilization. The fertilization success of last males varied considerably after double matings with a short mating interval (10 min). Last-male priority ranged from 0 to 100% and usually one of the males involved fertilized almost all the eggs. After double matings with a short mating interval, the proportion of eggs fertilized by the last male averaged 0.68 and was greater than 0.5. In contrast, the average fertilization success was biased towards the first male when the matings were more spread out over time (24 h). These results do not support earlier suggestions of a widespread last-male sperm priority in water striders. Received: 28 July 1998 / Received in revised form: 15 March 1999 / Accepted: 28 March 1999     

Sperm competition and ejaculate investment in red squirrels (<Emphasis Type="Italic">Tamiasciurus hudsonicus</Emphasis>)

Sperm competition is widespread in mammals and occurs when ejaculates from two or more males compete within the female’s reproductive tract to fertilize the ova. Enlarged testes are associated with sperm competition because they produce sperm, but the accessory glands produce fluids and proteins that are also important for fertilization success. Sperm morphology can also have consequences for fertilization success because of its influence on sperm motility. Red squirrels engage in multiple mating, and thus sperm competition is likely. Here, we assess levels of multiple paternity in a natural population of red squirrels, test the prediction that testis size is correlated with size of the accessory glands and sperm morphometry, and test the prediction that ejaculate investment is condition-dependent. Five of six litters (83%) showed evidence of multiple paternity, indicating that sperm competition is likely to have occurred. Testis size was correlated with the size of all three accessory glands (prostate, seminal vesicle, epididymides), and there was a generally positive relationship between the size of the accessory glands and sperm length. Sperm morphology showed significant variation in size and shape among individual male squirrels. There was no evidence of condition dependence of testis size or the size of the accessory glands, but sperm midpiece length was negatively related to body condition. Further work should include determining the fitness consequence of variation in sperm morphometry, testis size, and accessory gland size, and determining the effects of variation in ejaculate investment on sperm motility.     

Behavioral aspects of sperm competition in cuttlefish, Sepia officinalis (Sepioidea: Cephalopoda)

Sexual selection studies in cephalopods indicate that sperm competition is a central feature of their mating systems, yet this has not been studied experimentally in any detail. In 1998 we staged 20 matings of the cuttlefish, Sepia officinalis L., in the laboratory. Males rapidly initiated mating in the “head-to-head” position, with no apparent courtship. Mating lasted an average of 10 min (range 7 to 14 min). For the first 6 min (on average 63% of the mating duration), the male flushed strong jets of water directly at the female's buccal membrane, which sometimes resulted in the expulsion of parts of spermatangia placed there in recent matings. Then, in a single discrete movement that lasted an average of only 14 s, the male's modified fourth left arm – the hectocotylus – wrapped around a single large bundle of spermatophores (ca. 150 to 300) and transferred them to the female's buccal membrane. For the remainder of the mating (average 3 min, range 1.5 to 5.0 min), the hectocotylus repeatedly broke the spermatophores open, and manipulated them, so that sperm were released and many spermatangia were attached along the ventral buccal membrane, near the paired seminal receptacles. Approximately 140 spermatangia were attached in rows 3 to 5 deep around the ventral buccal membrane in a single mating; the rest were usually discarded during mating. Histology revealed that each of the seminal receptacles consists of a series of sperm storage bulbs connected by a central duct, which leads to a single pore at the surface of the buccal membrane. Baseline data on sperm motility were obtained, but the mechanism by which sperm enter the seminal receptacle remains unknown. Females seemed to initiate termination of mating, then males guarded their mates temporarily. These results, combined with other recent laboratory experiments, provide evidence that sperm competition may be a major feature of the mating system of S. officinalis. Received: 4 January 1999 / Accepted: 18 May 1999     

Breeding synchrony and extra-pair mating in red-winged blackbirds

Using data from a 6-year paternity study of red-winged blackbirds, I tested the hypotheses that increased nesting synchrony should either promote extra-pair mating by increasing the advantage of extra-pair mating to females, or decrease extra-pair mating by constraining males from seeking extra-pair copulations. Contrary to these hypotheses, the occurrence of extra-pair paternity did not vary with nesting synchrony over the breeding season, or vary with the number of synchronous nests within territories or within marshes, or with nesting order on territories. However, for nearly all nests with extra-pair young, there were fewer females synchronous with that nest on the cuckolder's territory than on the territory of the cuckolded male. This “advantage” of a synchrony difference was less pronounced for older males that cuckolded younger males, particularly when the two males were not neighbors. Collectively, these results suggest that breeding synchrony affects extra-pair mating by affecting mate guarding, but that breeding synchrony alone can not be used to predict which females are more likely to engage in extra-pair mating, nor with which extra-pair males they will mate. Understanding why extra-pair mating by older males is less affected by breeding synchrony may explain much about both the proximate and ultimate causes of extra-pair mating in red-winged blackbirds. Received: 7 June 1996 / Accepted after revision: 25 November 1996     

Sperm allocation in relation to male traits, female size, and copulation behaviour in freshwater crayfish species

Sperm competition is a well-recognised agent in the evolution of sperm and ejaculate structure, as well as variation in female quality. Models of the evolution of ejaculate expenditure predict that male body condition, female fecundity and the risk and intensity of sperm competition may be the ultimate factors shaping optimal ejaculate size. We investigated sperm allocation in Austropotamobius italicus, a freshwater crayfish exhibiting a coercive mating system and external fertilisation, in relation to male and female traits and copulation behaviour under laboratory conditions. We found that mating males were sensitive to female size and produced larger ejaculates when mating with larger females, which were more fecund in terms of number of eggs produced. We found no evidence for female egg production being sperm-limited, as the number of eggs was not dependent on male sperm expenditure. Copulation duration and number of ejaculations reliably predicted the amount of sperm transferred, and both these behavioural measures positively covaried with female body size. These results indicate that male freshwater crayfish can modulate their sperm expenditure in accordance with cues that indicate female fecundity. In addition, a novel finding that emerged from this study is the decrease in sperm expenditure with male body size, which may either suggest that large, old male crayfish are better able than small males to economise sperm at a given mating to perform multiple matings during a reproductive season, or that they experience senescence of their reproductive performance.     

No cost of male mating experience on female reproductive success in the almond moth, <Emphasis Type="BoldItalic">Cadra cautella</Emphasis> (Lepidoptera; Pyralidae)

Male copulation experience may have a profound impact on female reproductive success if male reproductive investment declines over consecutive copulations and if females are unlikely to re-mate. Male reproductive investment is particularly interesting in lepidopterans because males produce dimorphic sperm: a fertilizing (eupyrene) and a non-fertilising (apyrene) sperm. In two experiments, we explored the lifetime reproductive investment of male almond moths, Cadra cautella (also known as Ephestia cautella) and examined its influence on female reproductive success. In the almond moth, females re-mate infrequently and males transfer sperm in a spermatophore. Attached to the spermatophore is a large chitinous process, the function of which is unknown. One group of males were permitted consecutive copulations with virgin females and the amount of sperm and size of the spermatophore transferred were compared for all females. We found that the number of both eupyrene and apyrene sperm per ejaculate decreased with his increased mating frequency, while the size of the spermatophore process decreased dramatically after the male’s first copulation. In a second experiment, we allowed males to mate with females throughout their lives and then compared female fecundity and fertilisation success. We found no obvious decrease in female fecundity and fertilisation success with increased male copulation experience, despite the likely reduction in male gametic investment. We discuss potential explanations for the development of this enlarged and elaborate first spermatophore of male almond moths given that it confers no clear fitness advantage to females.     

Patterns of sperm use in the scorpionfly Panorpa germanica L. (Mecoptera: Panorpidae)

Sperm competition can be a powerful selective force in the evolution of reproductive strategies and mating systems. In studies on sperm competition, patterns of sperm use are typically reported as the mean species value of P ₂, determined as the proportion of offspring sired by the second male to copulate with a doubly mated female. However, the within-species variance in P ₂ has mostly been ignored, although taking this variance into account may be crucial for understanding the underlying mechanisms of sperm competition. Paternity analysis among the offspring of doubly mated females of Panorpa germanica (Mecoptera, Panorpidae) revealed a relationship between relative copulation durations of both males and the proportion of offspring each male will sire. This correlation between proportional copulation durations and paternity suggests mixing of sperm from different males inside the female’s spermatheca. Yet, sperm mixing appears to be incomplete, as paternity was overall slightly shifted towards the second male on average fathering a higher proportion of the offspring than its relative copulation duration would predict in case of complete sperm mixing. For individual males, however, the outcome of sperm competition is rather unpredictable as the intraspecific variance in P ₂ was found to be very high, irrespective of copulation durations. Possible causes of the observed variance in P ₂ and the partial last male sperm precedence are discussed.     

Elucidating the function of ejaculate expulsion and consumption after copulation by female Euxesta bilimeki

Postcopulatory processes can influence male reproductive success in several animal species. Females can use different mechanisms to bias male paternity after copulation. One of such mechanisms consists in expelling all or part of the ejaculate after copulation. Euxesta bilimeki is an Ulidiid fly whose females not only frequently expel ejaculates after mating but also consume the ejaculate after expulsion. In order to understand the significance of these behaviors we examined video recordings of courtship, copulatory, and postcopulatory behaviours. The presence of sperm in female storage organs was confirmed after mating with males of different sizes and was correlated with duration of courtship, copulation, and the period from the end of copulation to ejaculate expulsion. The effect of ejaculate consumption on female fitness (fecundity and longevity) was compared among females held under different dietary treatments: a rich diet consisting of protein, sugar and water, an intermediate diet composed of sugar and water, a poor diet of only water and females that were completely deprived of food and water. All of the observed females expelled ejaculates after mating. The probability of storing sperm in the two spermathecae and the ventral receptacle was correlated with interactions between the duration of all behaviours examined and male size. Except for starved females, who lived longer when allowed to consume ejaculates, ejaculate consumption had no effect on fitness. Results suggest that females can bias sperm storage according to male mating effort, while the consumed ejaculate had some nutritional value only evident when females were completely starved.     

Song repertoires and density assessment in red-winged blackbirds: further tests of the Beau Geste hypothesis

Summary The Beau Geste hypothesis proposes that song repertoires are advantageous in territory defense because nonterritorial males, who are prospecting for territories, use the number of song types they hear to assess the density of territorial males, and then avoid densely-settled areas. A territorial male can then inflate the apparent density of singers on his territory by singing several distinct song type. This hypothesis assumes (1) a positive correlation between the density of song types and the density of territorial males, and (2) a negative correlation between the rate of trespassing and the densities of both song types and territorial males. We studied the behavior of male red-winged blackbirds (Agelaius phoeniceus) and found support for the first, but not the second, assumption. Results showing a positive correlation between density of song types and density of territorial males show that the proposed method of density assessment is feasible in this species. However, we could find no support for the assumption that nonterritorial male red-winged blackbirds avoid densely settled areas. In contrast to the assumed negative correlation, rate of trespass per territory was not consistently correlated with male density, and trespass rate per unit area was positively correlated with male density. Further, these relationships were retained when we controlled statistically for the effects of territory quality. We conclude that prospecting male red-winged blackbirds to not attempt to avoid densely settled areas, and that, although they do avoid territories defended by many song types, they do not use song type density to assess the density of territorial males. It is thus unlikely that the Beau Geste hypothesis adequately accounts for the evolution of song repertoires in the red-winged blackbird.     

Use of the song repertoire in intersexual and intrasexual contexts by male red-winged blackbirds

Summary Live and stuffed male and female red-winged blackbirds (Agelaius phoeniceus) were presented to territorial males in order to study the repertoire organization of males during intersexual and intrasexual encounters. Territorial male red-winged blackbirds switched song types more frequently and sang more song types in response to presentation of a female stimulus than during control periods, but decreased their switching frequency when a male stimulus was presented. Switching frequency in response to the female stimulus was three times that in response to the male stimulus. These results support the hypothesis that song repertoires of male red-winged blackbirds evolved primarily in response to intersexual rather than intrasexual selection. They also suggest that male red-winged blackbirds have been selected to de-emphasize their song repertoires during encounters with conspecific males as a result of some as yet unidentified cost. Offprint requests to: W.A. Searcy     

Sperm transfer,displacement and precedence in Ischnura graellsii (Odonata : Coenagrionidae)

Summary Copulation in Ischnura graellsii may be divided into three stages, according to the movements and position of the male's abdomen. We measured sperm volumes in males and females interrupted at different phases of copulation in laboratory-reared and field specimens. The results showed that males remove sperm from the female during stage I, and do not transfer sperm until stage Il of the copulation. In the field females interrupted during stage I of copulation had less sperm than postcopula females, and the volume of sperm in laboratory females mated once or twice was similar. These results suggest that males can remove most of the sperm during stage I of copulation. Preparations of in-copula specimens showed the horns of the penis (used to remove sperm) inside the bursa copulatoox and the spermatheca. Therefore males can remove sperm from both organs, in contrast to the other Ischnura species so far studied, where males can empty only the bursa. The length of these horns is positively correlated with male body length, and there are significant differences in length between the left and right horns of individual males. This suggests great variability in the male's ability to remove sperm. On the other hand, ejaculate volume is positively related to male and female size, and negatively to male age. Males are likely to be able to detect the presence of sperm in females: if the effect of population density and time of start of copulation are taken into account, copulations are longer with mated than with virgin females. Using genetic markers, sperm precedence was studied by rearing the female offspring of 6 females mated with two males of different genotype. In 5 out of 6 crosses, the second male fertilized all the eggs laid by the female in her first clutch. On average, the following clutches were progressively more fertilized by the first male, but there were striking differences between crosses. These differences are probably due to the variability in the amount of sperm transferred and/or removed. Offprint requests to: A. Cordero     

Mass and production rate of ejaculates in relation to monandry/polyandry in butterflies

Summary The mating system maintained in a species has a strong effect on the degree of sperm competition, and certainty of paternity should accordingly influence the optimal sperm content, nutrient content, and mass of the ejaculate. We investigated how ejaculate mass relates to the degree of polyandry in 20 species of butterflies belonging to the families Pieridae and Satyridae. We found that the degree of polyandry has a substantial effect on the reproductive performance of males. The allometric line between ejaculate mass and male body mass has a higher elevation in the pierids compared to the satyrids. The mean number of matings performed by the pierid species is also higher compared to the mean of the satyrids. Thus, the relative ejaculate mass is larger in the family in which polyandry is more pronounced. A within family effect of degree of polyandry on relative ejaculate mass was also detected in the pierids. Since males of polyandrous species on average mate more often than males of monandrous species, they should be expected to have a higher capacity for producing many ejaculates. We investigated how this capacity was influenced by the degree of polyandry, by allowing males of seven different species (Danaus plexippus, Lasiommata megera, Papilio machaon, Pararge aegeria, Pieris napi, Pieris rapae, and Polygonia c-album) to mate twice, with different time intervals between matings. The results showed that not only is the mass of the ejaculate greater in more polyandrous species, but also the rate at which males are able to produce sperm and accessory substances is greater. Hence our data indicate that sperm competition is important for explaining variation in ejaculate mass in butterflies.