Releasing small ejaculates slowly increases per‐gamete fertilization success in an external fertilizer: Galeolaria caespitosa (Polychaeta: Serpulidae)

The idea that male reproductive strategies evolve primarily in response to sperm competition is almost axiomatic in evolutionary biology. However, externally fertilizing species, especially broadcast spawners, represent a large and taxonomically diverse group that have long challenged predictions from sperm competition theory—broadcast spawning males often release sperm slowly, with weak resource‐dependent allocation to ejaculates despite massive investment in gonads. One possible explanation for these counter‐intuitive patterns is that male broadcast spawners experience strong natural selection from the external environment during sperm dispersal. Using a manipulative experiment, we examine how male reproductive success in the absence of sperm competition varies with ejaculate size and rate of sperm release, in the broadcast spawning marine invertebrate Galeolaria caespitosa (Polychaeta: Serpulidae). We find that the benefits of Fast or Slow sperm release depend strongly on ejaculate size, but also that the per‐gamete fertilization rate decreases precipitously with ejaculate size. Overall, these results suggest that, if males can facultatively adjust ejaculate size, they should slowly release small amounts of sperm. Recent theory for broadcast spawners predicts that sperm competition can also select for Slow release rates. Taken together, our results and theory suggest that selection often favours Slow ejaculate release rates whether males experience sperm competition or not.     

Sexual selection and sperm quantity: meta‐analyses of strategic ejaculation

Multiple mating or group spawning leads to post‐copulatory sexual selection, which generally favours ejaculates that are more competitive under sperm competition. In four meta‐analyses we quantify the evidence that sperm competition (SC) favours greater sperm number using data from studies of strategic ejaculation. Differential investment into each ejaculate emerges at the individual level if males exhibit phenotypic plasticity in ejaculate properties in response to the likely risk and/or intensity of sperm competition after a given mating. Over the last twenty years, a series of theoretical models have been developed that predict how ejaculate size will be strategically adjusted in relation to: (a) the number of immediate rival males, with a distinction made between 0 versus 1 rival (‘risk’ of SC) and 1 versus several rivals (‘intensity’ of SC); (b) female mating status (virgin or previously mated); and (c) female phenotypic quality (e.g. female size or condition). Some well‐known studies have reported large adjustments in ejaculate size depending on the relevant social context and this has led to widespread acceptance of the claim that strategic sperm allocation occurs in response to each of these factors. It is necessary, however, to test each claim separately because it is easy to overlook studies with weak or negative findings. Compiling information on the variation in outcomes among species is potentially informative about the relevance of these assumptions in different taxa or mating systems. We found strong evidence that, on average, males transfer larger ejaculates to higher quality females. The effect of female mating status was less straightforward and depended on how ejaculate size was measured (i.e. use of proxy or direct measure). There is strong evidence that ejaculate size increased when males were exposed to a single rival, which is often described as a response to the risk of SC. There is, however, no evidence for the general prediction that ejaculate size decreases as the number of rivals increases from one to several males (i.e. in response to a higher intensity of SC which lowers the rate of return per sperm released). Our results highlight how meta‐analysis can reveal unintentional biases in narrative literature reviews. We note that several assumptions of theoretical models can alter an outcome's predicted direction in a given species (e.g. the effect of female mating status depends on whether there is first‐ or last‐male sperm priority). Many studies do not provide this background information and fail to make strong a priori predictions about the expected response of ejaculate size to manipulation of the mating context. Researchers should be explicit about which model they are testing to ensure that future meta‐analyses can better partition studies into different categories, or control for continuous moderator variables.     

Male primer endocrine responses to preovulatory female cyprinids under natural conditions in Sweden

This study investigated two related aspects of male-female reproductive interactions in the family Cyprinidae: (1) whether ovulating female rudd Scardinius erythrophthalmus (subfamily Leuciscinae) induce endocrine and gonadal priming responses in conspecific males, a phenomenon which has been described only in species from the subfamily Cyprininae such as goldfish, Carassius auratus, crucian carp Carassius carassius and common carp, Cyprinus carpio and (2) whether the stimuli mediating these responses are species-specific. Field studies of three sympatric European cyprinids, two leuciscins (S. erythrophthalmus and white bream Blicca bjoerkna) and one cyprinin (C. carassius), were conducted on fishes captured in Sweden in the spawning season and held in net pens under natural conditions. As previously reported in C. carassius, male S. erythrophthalmus increased milt (sperm and seminal fluid) volume and plasma concentrations of the sperm maturation hormone 4-pregnen-17,20β-diol-3-one (17,20β-P) when they were held with female S. erythrophthalmus induced to ovulate by injection of Ovaprim (GnRH analogue plus dopamine antagonist). Male S. erythrophthalmus had larger milt volumes than male C. carassius prior to and following exposure to ovulatory conspecifics, but exhibited a smaller proportional milt increase in response to stimulation, suggesting species differences in sperm allocation at spawning. The presence of female S. erythrophthalmus and B. bjoerkna did not affect milt volumes of C. carassius under two experimental conditions: (1) ovulating S. erythrophthalmus and B. bjoerkna did not increase the milt volumes of C. carassius and (2) S. erythrophthalmus and B. bjoerkna did not interfere with the milt volume increase induced in male C. carassius by ovulating conspecifics. These results suggest that, as in C. auratus, C. carassius and C. carpio (subfamily Cyprininae), female S. erythrophthalmus (subfamily Leuciscinae) release a preovulatory pheromone that exerts priming effects on male hormones and sperm allocation. The findings also indicate that C. carassius discriminate between the reproductive odours of conspecifics and heterospecifics.     

Male age and strain affect ejaculate quality in the Mexican fruit fly

Aging in all organisms is inevitable. Male age can have profound effects on mating success and female reproduction, yet relatively little is known on the effects of male age on different components of the ejaculate. Furthermore, in mass‐reared insects used for the Sterile Insect Technique, there are often behavioral differences between mass‐reared and wild males, while differences in the ejaculate have been less studied. The ejaculate in insects is composed mainly of sperm and accessory gland proteins. Here, we studied how male age and strain affected (i) protein quantity of testes and accessory glands, (ii) the biological activity of accessory gland products injected into females, (iii) sperm viability, and (iv) sperm quantity stored by females in wild and mass‐reared Anastrepha ludens (Diptera: Tephritidae). We found lower protein content in testes of old wild males and lower sperm viability in females mated with old wild males. Females stored more sperm when mated to young wild males than with young mass‐reared males. Accessory gland injections of old or young males did not inhibit female remating. Knowledge of how male age affects different ejaculate components will aid our understanding on investment of the ejaculate and possible postcopulatory consequences on female behavior.     

Ejaculate traits in relation to male body size in the eastern mosquitofish Gambusia holbrooki

In the present study, the correlation between sperm number, sperm quality (speed, viability, longevity and length), sperm bundles quality (size and dissolving rate) and male body size has been tested in the eastern mosquitofish Gambusia holbrooki a poecilid species characterized by coercive mating tactics where males do not possess obvious ornaments, and the body size is the key determinant of pre‐copulatory male mating success. The results do not tally with theoretical predictions. Indeed, no correlation between male body size and either sperm or sperm‐bundle traits has been found, evidencing the lack of the theoretically expected trade‐off between the investment in characters involved in mate acquisition and the investment in ejaculate quality. An explanation for the observed pattern comes from the extremely dynamic mating system of G. holbrooki, characterized by variable size‐related male mating success and strong post‐copulatory selective pressure, with all males facing a similar high level of sperm competition. In this situation, a higher investment in growth and maintenance at the expense of ejaculate quality is not expected. These results underscore the necessity to comprehend detailed information on species’ reproductive biology and reproductive environment to understand both the evolution of ejaculate characteristics and possible deviations from theoretical predictions.     

Ejaculate investment and attractiveness in the stalk‐eyed fly,Diasemopsis meigenii

The phenotype‐linked fertility hypothesis proposes that male fertility is advertised via phenotypic signals, explaining female preference for highly sexually ornamented males. An alternative view is that highly attractive males constrain their ejaculate allocation per mating so as to participate in a greater number of matings. Males are also expected to bias their ejaculate allocation to the most fecund females. We test these hypotheses in the African stalk‐eyed fly, Diasemopsis meigenii. We ask how male ejaculate allocation strategy is influenced by male eyespan and female size. Despite large eyespan males having larger internal reproductive organs, we found no association between male eyespan and spermatophore size or sperm number, lending no support to the phenotype‐linked fertility hypothesis. However, males mated for longer and transferred more sperm to large females. As female size was positively correlated with fecundity, this suggests that males gain a selective advantage by investing more in large females. Given these findings, we consider how female mate preference for large male eyespan can be adaptive despite the lack of obvious direct benefits.     

Out‐of‐season production of 17,20β‐dihydroxypregn‐4‐en‐3‐one in the roach Rutilus rutilus

In this study, although the highest production of two physiologically significant progestins in teleosts [17,20β‐dihydroxypregn‐4‐en‐3‐one (17,20β‐P) and 17,20β,21‐trihydroxypregn‐4‐en‐3‐one (17,20β,21‐P)] was observed in the period just prior to spawning in both male and female roach Rutilus rutilus, there was also a substantial production (mean levels of 5–10 ng ml^?1 in blood; and a rate of release of 5–20 ng fish^?1 h^?1 into the water) in males and females in the late summer and early autumn (at least 7 months prior to spawning). During this period, the ovaries were increasing rapidly in size and histological sections were dominated by oocytes in the secondary growth phase [i.e. incorporation of vitellogenin (VTG)]. At the same time, the testes were also increasing rapidly in size and histological sections were dominated by cysts containing mainly spermatogonia type B. Measurements were also made of 11‐ketotestosterone (11‐KT) in males and 17β‐oestradiol and VTG in females. The 3 months with the highest production of 11‐KT coincided with the period that spermatozoa were present in the testes. In females, the first sign of a rise in 17β‐oestradiol concentrations coincided with the time of the first appearance of yolk globules in the oocytes (in August). The role of the progestins during the late summer and autumn has not been established.     

Synthesis of 17,20β,21‐trihydroxypregn‐4‐en‐3‐one by ovaries of reproductively mature Atlantic cod Gadus morhua

Atlantic cod Gadus morhua ovaries were incubated in vitro with tritiated 17‐hydroxypregn‐4‐ene‐3,20‐dione (17‐P) to determine whether 17,20β‐dihydroxypregn‐4‐en‐3‐one (17,20β‐P) or 17,20β, 21‐trihydroxypregn‐4‐en‐3‐one (17,20β,21‐P), or both, were more likely to be the steroid responsible for inducing oocyte final maturation (i.e. resumption of meiosis). Only 17,20β,21‐P was produced, in addition to 11‐deoxycortisol (17,21‐P), which is intermediate between 17‐P and 17,20β,21‐P. Also, the 5β‐reduced forms of 17‐P, 17,21‐P and 17,20β,21‐P were all found. Some sulphation of 21‐hydroxylated steroids was demonstrated. The ability of female G. morhua to make 17,20β,21‐P but not 17,20β‐P was confirmed by radioimmunoassay of plasma samples from spawning fish. Although small amounts of 17,20β‐P immunoreactivity were detected in a few plasma samples, this was shown, by thin‐layer chromatography, to be mostly due to cross‐reaction with other unidentified compounds. The evidence strongly suggests that 17,20β,21‐P is more likely than 17,20β‐P to be the maturation‐inducing steroid in G. morhua.     

SPERM COMPETITIVE ABILITY EVOLVES IN RESPONSE TO EXPERIMENTAL ALTERATION OF OPERATIONAL SEX RATIO

In naturally polygamous organisms such as Drosophila, sperm competitive ability is one of the most important components of male fitness and is expected to evolve in response to varying degrees of male–male competition. Several studies have documented the existence of ample genetic variation in sperm competitive ability of males. However, many experimental evolution studies have found sperm competitive ability to be unresponsive to selection. Even direct selection for increased sperm competitive ability has failed to yield any measurable changes. Here we report the evolution of sperm competitive ability (sperm defense‐P1, offense‐P2) in a set of replicate populations of Drosophila melanogaster subjected to altered levels of male–male competition (generated by varying the operational sex ratio) for 55–60 generations. Males from populations with female‐biased operational sex ratio evolved reduced P1 and P2, without any measurable change in the male reproductive behavior. Males in the male‐biased regime evolved increased P1, but there was no significant change in P2. Increase in P1 was associated with an increase in copulation duration, possibly indicating greater ejaculate investment by these males. This study is one of the few to provide empirical evidence for the evolution of sperm competitive ability of males under different levels of male–male competition.     

Repeated matings and sperm depletion in the freshwater crayfish Austropotamobius italicus

1. In sexually reproducing organisms, the energetic costs of spermatogenesis can be considerable, and can limit the reproductive potential of the males. In species where males mate more than once during the reproductive season, the costs of sperm production are generally predicted to result in a decrease of ejaculate size and quality with successive fertilizations. 2. In this study we examined the variation in ejaculate size among successive fertilizations in a long‐lived freshwater crayfish species, Austropotamobius italicus. 3. Sexually active adult males of various sizes were allowed to mate repeatedly with different females on consecutive days. Trials for a given male ended when he copulated but did not release any sperm or refused to mate. 4. Males fertilized between 0 and 4 females, and most (42.5%) fertilized a single female. The overall number of females fertilized by a given male decreased with increasing male body size. Ejaculate size decreased markedly with consecutive fertilizations in a similar fashion among both large and small males, while simultaneously increasing with female body size. The total ejaculate size over successive fertilizations decreased with increasing male size. 5. Our study indicates that either sperm production or release involves non‐trivial costs in freshwater crayfish, and suggests that large/old males may face greater difficulties in gamete release than small/young ones, as shown by the lower number of females fertilized by large compared with small males, which may reflect the ongoing senescence of their reproductive performance.     

Female sperm storage mediates post‐copulatory costs and benefits of ejaculate anticipatory plasticity in the guppy

Males of many species evolved the capability of adjusting their ejaculate phenotype in response to social cues to match the expected mating conditions. When females store sperm for a prolonged time, the expected fitness return of plastic adjustments of ejaculate phenotype may depend on the interval between mating and fertilization. Although prolonged female sperm storage (FSS) increases the opportunity for sperm competition, as a consequence of the longer temporal overlap of ejaculates from several males, it may also create variable selective forces on ejaculate phenotype, for example by exposing trade‐offs between sperm velocity and sperm survival. We evaluated the relationship between the plasticity of ejaculate quality and FSS in the guppy, Poecilia reticulata, a polyandrous live‐bearing fish in which females store sperm for several months and where stored sperm contribute significantly to a male's lifelong reproductive success. In this species, males respond to the perception of future mating opportunities by increasing the quantity (number) and quality (swimming velocity) of ready‐to‐use sperm (an anticipatory response called ‘sperm priming’). Here we investigated (a) the effect of sperm priming on in vitro sperm viability at stripping and its temporal decline (as an estimate of sperm survival), and (b) the in vivo competitive fertilization success in relation to female sperm storage using artificial insemination. As expected, sperm‐primed males produced more numerous and faster sperm, but with a reduced in vitro sperm viability at stripping and after 4 hr, compared with their counterparts. Artificial insemination revealed that the small (nonsignificant) advantage of primed sperm when fertilization immediately follows insemination is reversed when eggs are fertilized by female‐stored sperm, weeks after insemination. By suggesting a plastic trade‐off between sperm velocity and viability, these results demonstrate that prolonged female sperm storage generates divergent selection pressures on ejaculate phenotype.     

Condition‐dependent ejaculate production affects male mating behavior in the common bedbug Cimex lectularius

Food availability in the environment is often low and variable, constraining organisms in their resource allocation to different life‐history traits. For example, variation in food availability is likely to induce condition‐dependent investment in reproduction. Further, diet has been shown to affect ejaculate size, composition and quality. How these effects translate into male reproductive success or change male mating behavior is still largely unknown. Here, we concentrated on the effect of meal size on ejaculate production, male reproductive success and mating behavior in the common bedbug Cimex lectularius. We analyzed the production of sperm and seminal fluid within three different feeding regimes in six different populations. Males receiving large meals produced significantly more sperm and seminal fluid than males receiving small meals or no meals at all. While such condition‐dependent ejaculate production did not affect the number of offspring produced after a single mating, food‐restricted males could perform significantly fewer matings than fully fed males. Therefore, in a multiple mating context food‐restricted males paid a fitness cost and might have to adjust their mating strategy according to the ejaculate available to them. Our results indicate that meal size has no direct effect on ejaculate quality, but food availability forces a condition‐dependent mating rate on males. Environmental variation translating into variation in male reproductive traits reveals that natural selection can interact with sexual selection and shape reproductive traits. As males can modulate their ejaculate size depending on the mating situation, future studies are needed to elucidate whether environmental variation affecting the amount of ejaculate available might induce different mating strategies.     

Do male secondary sexual characters signal ejaculate quality? A meta‐analysis

There are two reasons why researchers are interested in the phenotypic relationship between the expression of male secondary sexual characters (SSCs) and ‘ejaculate quality’ (defined as sperm/ejaculate traits that are widely assumed to increase female fertility and/or sperm competitiveness). First, if the relationship is positive then females could gain a direct benefit by choosing more attractive males for fertility assurance reasons (‘the phenotype‐linked fertility’ hypothesis). Second, there is much interest in the direction of the correlation between traits favoured by pre‐copulatory sexual selection (i.e. affecting mating success) and those favoured by post‐copulatory sexual selection (i.e. increasing sperm competitiveness). If the relationship is negative this could lead to the two forms of selection counteracting each other. Theory predicts that the direction of the relationship could be either positive or negative depending on the underlying genetic variance and covariance in each trait, the extent of variation among males in condition (resources available to allocate to reproductive traits), and variation among males in the cost or rate of mating. We conducted a meta‐analysis to determine the average relationship between the expression of behavioural and morphological male secondary sexual characters and four assays of ejaculate quality (sperm number, viability, swimming speed and size). Regardless of how the data were partitioned the mean relationship was consistently positive, but always statistically non‐significant. The only exception was that secondary sexual character expression was weakly but significantly positively correlated with sperm viability (r = 0.07, P < 0.05). There was no significant difference in the strength or direction of the relationship between behavioural and morphological SSCs, nor among relationships using the four ejaculate quality assays. The implications of our findings are discussed.     

Determinants of male ejaculate investment in the cotton bollworm Helicoverpa armigera: mating history,female body size and male age

Many studies demonstrate that ejaculate size may be influenced by male condition, female quality and the risk or intensity of sperm competition. In the present study, the effect of male and female conditions, male mating history and female mating status on ejaculate sperm numbers in the polyandrous moth Helicoverpa armigera is examined. A large variation in ejaculate size is found and, although female body size and male age influence ejaculate size, female age and copula duration do not. Both male and female mating histories have significant effects on ejaculate sperm numbers. Males reduce ejaculate expenditure in successive matings but deliver significantly more apyrene and eupyrene sperm to nonvirgin than to virgin females.     

Differential sperm expenditure reveals a possible role for post‐copulatory sexual selection in a lekking moth

Male reproductive success in the lesser wax moth Achroia grisella is strongly determined by pre‐copulatory mate choice, during which females choose among males aggregated in small leks based on the attractiveness of ultrasonic songs. Nothing is known about the potential of post‐copulatory mechanisms to affect male reproductive success. However, there is evidence that females at least occasionally remate with a second male and that males are unable to produce ejaculates quickly after a previous copulation. Here we investigated the effects of mating history on ejaculate size and demonstrate that the number of transferred sperm significantly decreased from first (i.e., virgin) to second (i.e., nonvirgin) copulation within individual males. For males of identical age, the number of sperm transferred was higher in virgin than in nonvirgin copulations, too, demonstrating that mating history, is responsible for the decrease in sperm numbers transferred and not the concomitant age difference. Furthermore, the number of transferred sperm was significantly repeatable within males. The demonstrated variation in ejaculate size both between subsequent copulations as well as among individuals suggests that there is allocation of a possibly limited amount of sperm. Because female fecundity is not limited by sperm availability in this system, post‐copulatory mechanisms, in particular sperm competition, may play a previously underappreciated role in the lesser wax moth mating system.     

Methyl eugenol effects on Bactrocera dorsalis male total body protein,reproductive organs and ejaculate

Methyl eugenol (ME) and inclusion of protein into the adult diet increase the mating competitiveness of the Oriental fruit fly, Bactrocera dorsalis (Hendel). Exposing males to ME or protein is a promising post‐teneral treatment for males being released in the sterile insect technique (SIT). However, the effect of this post‐teneral treatment on male reproductive organs or the male ejaculate is unknown. During mating, males transfer sperm and accessory gland products (AGPs) to females and these compounds are reported to modulate female sexual inhibition. We studied the impact of male exposure to ME and a yeast hydrolysate (YH) diet on the protein reserves of males, male reproductive organ size, and the male ejaculate through sperm and AGPs. We show that males exposed to ME regardless of access to YH accumulated a greater amount of whole body protein. Males fed on YH also had increased total body protein and had bigger reproductive organs than YH‐deprived males, but no apparent effect of ME exposure was observed on reproductive organ size. Females stored less sperm when mated with males fed on YH and ME compared to males not fed on ME. YH and ME had no effect on male AGPs. Females injected with AGPs of males fed on YH and exposed to ME were just as likely to mate as females injected with AGPs of non‐treated males. However, females injected with AGPs of males exposed to ME mated faster than females injected with AGPs of non‐exposed males. We conclude that while exposure to ME increases male copulatory success and protein reserves in the male body, there seem to be some potential trade‐offs such as lower sperm stored by females. We discuss our results in terms of pre‐release protocols that may be used for B. dorsalis in SIT application.     

Male body size and condition affects sperm number and production rates in mosquitofish,Gambusia holbrooki

Sperm number is an important predictor of paternity when there is sperm competition. Sperm number is often measured as maximum sperm reserves, but in species where mating is frequent, males will often be replenishing their reserves. Thus, variation in how quickly males can produce sperm is likely to be important in determining male success in sperm competition. Despite this, little is known about how male size, body condition or diet affects sperm production rates. We counted sperm number in large and small Gambusia holbrooki (eastern mosquitofish) after 3 weeks on either a high or low food diet. Sperm number was significantly higher in both larger males and in well‐fed males. We then stripped ejaculates again either 1, 2, 3, 4 or 5 days later to investigate subsequent sperm production. The rate of sperm replenishment was influenced by an interaction between size and diet. Large, well‐fed males had consistently high levels of sperm available over the 5 days (i.e. rapid replenishment), whereas small poorly fed males showed consistently low levels of sperm availability over the 5 days (i.e. slow replenishment). In contrast, large, poorly fed and small, well‐fed males increased their sperm numbers over the first 3 days (i.e. intermediate replenishment). Our study highlights that when mating is frequent and sperm competition is high, size and condition dependence of maximal sperm number and of sperm production rate might both contribute to variation in male reproductive success.     

Spawning behavior of Arctic charr (Salvelinus alpinus): Spawning synchrony,vibrational communication,and mate guarding

A mismatch in synchrony between male and female gamete release in external fertilizers can result in reduced or failed fertilization, sperm competition, and reduced paternity. In Arctic charr (Salvelinus alpinus), males can adopt either a guard or sneak tactic resulting in both pre‐ and postcopulatory competition between males with alternative reproduction tactics. Here, spawning behavior of free‐living Arctic charr was video‐recorded, and their reproductive behavior was analyzed. From evaluating 157 spawning events, we observed that females mainly spawned with a guarding male and that the female and the guarding male synchronized timing of gamete release under sperm competition. Although sneakers spawned with higher synchrony than the guarding male in single‐male spawning events, the average sneaker released his milt less synchronized with the female than the guarding male under sperm competition. Approximately 50% of the recorded spawning events occurred under sperm competition, where each event included an average of 2.7 males. Additionally, sneakers were more exposed to sperm competition than guarding males. An influx of males, in close proximity to the female, occurred during the behavioral sequences leading up to egg release, but this influx seemed not dependent on egg release, suggesting that something else than gonadal product attracts sneaker males to the spawning female. Just before and during the actual release of gametes, the spawning couple vibrates their bodies in close contact and it seems likely that this vibrational communication between the spawning couple, which results in a larger amplitude sound wave than seen under regular courting, reveals time of gamete release to sneaker males. Thus, vibrational communication may enable synchrony between the guarding male and the female, and this might be traded against the cost of higher detectability from surrounding sneaker males, eavesdropping in close proximity.     

Differing behavioral and endocrinological effects of two female sex pheromones on male goldfish

Ovulatory female goldfish sequentially release at least two sex pheromones: 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (17,20 beta P) and a mixture of F prostaglandins (PGFs). This study sought to determine whether these pheromones have different endocrinological and behavioral actions and whether the PGF pheromone, which is released by spawning females, is responsible for increasing the gonadotropin (GtH) and milt (sperm and seminal fluid) levels of spawning males. Grouped and isolated males were exposed to combinations of these pheromones, food odor, and spawning and nonspawning females. 17,20 beta P stimulated GtH increases in both grouped and isolated males but had only minor effects on behavior; because its principal function appears to be physiological it may be considered a "primer" pheromone. In contrast, exposue to the PGFs elicited large increases in sexual behavior but increased GtH only when fish were exposed as groups; this pheromone's principal action appears to be behavioral and it should be considered a "releaser" pheromone. Although males had increased GtH and milt levels after 1 hr of spawning, males allowed to interact with nonspawning females also had elevated GtH; thus, behavioral interactions appear capable of elevating GtH in the absence of either pheromone. The existence of an independent behavioral mechanism which stimulates GtH was supported by the fact that males exposed to 17,20 beta P while spawning had GtH levels much greater than males exposed to only one of these stimuli.     

Male sperm expenditure under sperm competition risk and intensity in quacking frogs

In the frog Crinia georgiana, reproductive behavior comprisesa "guarding tactic," in which males defend spawn sites and attractfemales by calling, and a "sneak tactic," in which males joinspawning pairs. The aims of the present study were to (1) relateejaculate expenditure by "guarding" and "sneak" males to theirprobability of mating with other males present (sperm-competitionrisk), and (2) determine if males adjust their ejaculate expenditureaccording to the number of males involved in a spawning (sperm-competitionintensity). Theory predicts that because sneak males alwaysmate with other males present, they will experience a highersperm-competition risk and should release larger ejaculatesrelative to that of guarding males. However, as the proportionof sneaks in a population increases so does the risk of spermcompetition to guarders, so expenditure by each tactic shouldmove toward equality. Given that the incidence of sneak behavioris high in C. georgiana, guarders and sneaks were expected toexperience similar risks of sperm competition and show similarinvestment in spermatogenesis. Comparison of testes size andejaculate size showed no difference between tactics. Modelsof sperm-competition intensity predict that males should increasetheir ejaculate size when spawning in the presence of one othermale but decrease their ejaculate size when spawning in thepresence of multiple males. Here, males maintained a constantsperm number irrespective of whether a mating involved one,two, or three males. This result suggests that male C. georgianado not facultatively adjust ejaculate investment in responseto fluctuating intensities of sperm competition.