Is the hook of muroid rodent's sperm related to sperm train formation?

Competition between spermatozoa of rival males to gain fertilizations has led to a wide array of modifications in sperm structure and function. Sperm cells of most muroid rodents have hook‐shaped extensions in the apical–ventral tip of the head, but the function of this structure is largely unknown. These ‘hooks’ may facilitate aggregation of spermatozoa in so‐called ‘trains’, as an adaptation to sperm competition, because sperm in trains may swim faster than free‐swimming cells. However, there is controversy regarding the role of the hook in train formation, and in relation to whether it is selected by sperm competition. We examined spermatozoa from muroid rodents with varying levels of sperm competition to assess whether (i) sperm aggregates are common in these taxa, (ii) presence of a hook relates to the formation of sperm aggregations, and (iii) formation of sperm aggregations is explained by sperm competition. Our analyses in 25 muroid species revealed that > 92% of spermatozoa swim individually in all species, with the exception of the wood mouse, Apodemus sylvaticus, which has ~50% spermatozoa swimming freely. Species with hooked spermatozoa had higher sperm competition levels and longer sperm than species whose sperm lack a hook. Neither the presence of hook nor sperm competition levels were related to the percentage of sperm in aggregations. Thus, (i) sperm aggregates in muroid rodents are an exceptional trait found only in a few species, (ii) evolution of the sperm hook is associated to sperm competition levels, but (iii) the hook is unlikely to be related to the formation of sperm aggregates. The evolutionary significance of the sperm head hook thus remains elusive, and future studies should examine potential roles of this pervasive structure in sperm's hydrodynamic efficiency and sperm–female tract interactions.     

Functional and morphological diversity of sperm in Drosophila

Unlike mammals, where the males produce huge quantities of tiny spermatozoa, insects, and Drosophila in particular, exhibit a wide range of reproductive strategies. Sperm gigantism in Drosophila deviates from the rules that normally govern anisogamy, i.e. differences in the size and quantity of male and female gametes. Sperm gigantism has driven anatomical, physiological and cytological adaptations that affect the correlated evolution of the male and female reproductive systems, and has led to the evolution of a new structure, the roller, located between the testis and the seminal vesicle, and to sperm coiling to form pellets. The diversification of sperm strategy is investigated in the light of sexual selection processes that occur in the female genital tract after copulation. These processes, which bias paternity, result from interactions either between spermatozoa from different males, or between the spermatozoa and the environment within the female reproductive tract. In Drosophila, increased sperm size does not confer any reproductive advantage on the male. The evolution of sperm gigantism does not seem to be attributable to competition between spermatozoa from different males, as has been shown to occur in some vertebrate species. Alternative mechanisms, such as interactions between spermatozoa and the female reproductive system, are therefore currently viewed as being more likely explanations. In particular, the impact of sperm size on female reproductive physiology is being investigated to find out whether having large spermatozoa increases the likelihood of male reproductive success. Correlated adaptations of the spermatozoa and female storage organs also seem to be a major factor in determining sperm success, and their role in male-female conflicts is discussed briefly.     

A cost for high levels of sperm competition in rodents: increased sperm DNA fragmentation

Sperm competition, a prevalent evolutionary process in which the spermatozoa of two or more males compete for the fertilization of the same ovum, leads to morphological and physiological adaptations, including increases in energetic metabolism that may serve to propel sperm faster but that may have negative effects on DNA integrity. Sperm DNA damage is associated with reduced rates of fertilization, embryo and fetal loss, offspring mortality, and mutations leading to genetic disease. We tested whether high levels of sperm competition affect sperm DNA integrity. We evaluated sperm DNA integrity in 18 species of rodents that differ in their levels of sperm competition using the sperm chromatin structure assay. DNA integrity was assessed upon sperm collection, in response to incubation under capacitating or non-capacitating conditions, and after exposure to physical and chemical stressors. Sperm DNA was very resistant to physical and chemical stressors, whereas incubation in non-capacitating and capacitating conditions resulted in only a small increase in sperm DNA damage. Importantly, levels of sperm competition were positively associated with sperm DNA fragmentation across rodent species. This is the first evidence showing that high levels of sperm competition lead to an important cost in the form of increased sperm DNA damage.     

Geometric Morphometrics of Rodent Sperm Head Shape

Mammalian spermatozoa, particularly those of rodent species, are extremely complex cells and differ greatly in form and dimensions. Thus, characterization of sperm size and, particularly, sperm shape represents a major challenge. No consensus exists on a method to objectively assess size and shape of spermatozoa. In this study we apply the principles of geometric morphometrics to analyze rodent sperm head morphology and compare them with two traditional morphometry methods, that is, measurements of linear dimensions and dimensions-derived parameters calculated using formulae employed in sperm morphometry assessments. Our results show that geometric morphometrics clearly identifies shape differences among rodent spermatozoa. It is also capable of discriminating between size and shape and to analyze these two variables separately. Thus, it provides an accurate method to assess sperm head shape. Furthermore, it can identify which sperm morphology traits differ between species, such as the protrusion or retraction of the base of the head, the orientation and relative position of the site of flagellum insertion, the degree of curvature of the hook, and other distinct anatomical features and appendices. We envisage that the use of geometric morphometrics may have a major impact on future studies focused on the characterization of sperm head formation, diversity of sperm head shape among species (and underlying evolutionary forces), the effects of reprotoxicants on changes in cell shape, and phenotyping of genetically-modified individuals.     

The ultrastructure of the spermatozoa of Boa constrictor occidentalis, with considerations on its mating system and sperm competition theories

Sperm ultrastructure has been described for several families of Squamata in which it has been considered a valuable character source for phylogenetic studies. However, sperm competition and mating systems have been demonstrated to influence variations in the sperm morphology and dynamics. The mating system of Boa constrictor occidentalis is likely to have a high degree of sperm competition. We investigated, for the first time, the ultrastructure of the spermatozoa of B. c. occidentalis and, thus, of the family Boidae. Active mating groups were captured from the field, and the spermatozoa of the males was collected by ejaculation and processed to obtain transmission electron micrographs and fluorescence micrographs. The spermatozoa are filiform and their morphology fits the general model described for snakes, and several synapomorphies belonging to the squamates can be identified in these cells. Nevertheless, the head is 25% longer and the midpiece presents a lower frequency of mitochondrial transformations than that of other snakes. We propose that this last trait, along with the extraordinary elongation of the midpiece and the system of multilaminar membranes covering this section (both synapomorphies of the snake spermatozoa), are adaptive responses to processes of sperm competition and sperm storage.     

The tailored sperm cell

Sperm are ubiquitous and yet unique. Genes involved in sexual reproduction are more divergent than most genes expressed in non-reproductive tissues. It has been argued that sperm have been altered during evolution more than any somatic cell. Profound variations are found at the level of morphology, motility, search strategy for the egg, and the underlying signalling mechanisms. Sperm evolutionary adaptation may have arisen from sperm competition (sperm from rival males compete within the female’s body to fertilize eggs), cryptic female choice (the female’s ability to choose among different stored sperm), social cues tuning sperm quality or from the site of fertilization (internal vs. external fertilization), to name a few. Unquestionably, sperm represent an invaluable source for the exploration of biological diversity at the level of signalling, motility, and evolution. Despite the richness in sperm variations, only a few model systems for signalling and motility have been studied in detail. Using fast kinetic techniques, electrophysiological recordings, and optogenetics, the molecular players and the sequence of signalling events of sperm from a few marine invertebrates, mammals, and fish are being elucidated. Furthermore, recent technological advances allow studying sperm motility with unprecedented precision; these studies provide new insights into flagellar motility and navigation in three dimensions (3D). The scope of this review is to highlight variations in motile sperm across species, and discuss the great promise that 3D imaging techniques offer into unravelling sperm mysteries.     

Sperm quality, secondary sexual characters and parasitism in roach (Rutilus rutilus L.)

According to sperm competition models, a male spawning in a disfavoured role should have spermatozoa with higher velocity but shorter longevity compared with a male spawning in a favoured role. Moreover, immunosuppressive androgens are needed to produce both secondary sexual characters and sperm cells. The 'sperm protection' hypothesis suggests that the immunosuppressive action of androgens has evolved to protect haploid spermatozoa, which are antigenic, from autoimmune attacks. Therefore, a male with high sexual ornamentation may be more susceptible to diseases but may possess better quality ejaculate than his less ornamented rival. We studied sexual ornamentation (breeding tubercles), ejaculation quality (sperm concentration, longevity and spermatozoal velocity) and intensity of parasitism in the cyprinid, Rutilus rutilus . Sperm longevity and spermatozoal velocity were positively correlated. Males having elaborated sexual ornamentation had longer-lived sperm and more Myxobolus mülleri parasites in the liver compared with males with low ornamentation. However, no difference was found between males with different degrees of ornamentation with respect to sperm concentration, spermatozoal velocity or other parasites. Since the highly ornamented males had higher sperm longevity, the present results are partly consistent with the predictions of the sperm competition models and the 'sperm protection' hypothesis.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 81 , 111–117.     

No evidence of trade‐offs in the evolution of sperm numbers and sperm size in mammals

Post‐copulatory sexual selection, in the form sperm competition, has influenced the evolution of several male reproductive traits. However, theory predicts that sperm competition would lead to trade‐offs between numbers and size of spermatozoa because increased costs per cell would result in a reduction of sperm number if both traits share the same energetic budget. Theoretical models have proposed that, in large animals, increased sperm size would have minimal fitness advantage compared with increased sperm numbers. Thus, sperm numbers would evolve more rapidly than sperm size under sperm competition pressure. We tested in mammals whether sperm competition maximizes sperm numbers and size, and whether there is a trade‐off between these traits. Our results showed that sperm competition maximizes sperm numbers in eutherian and metatherian mammals. There was no evidence of a trade‐off between sperm numbers and sperm size in any of the two mammalian clades as we did not observe any significant relationship between sperm numbers and sperm size once the effect of sperm competition was taken into account. Maximization of both numbers and size in mammals may occur because each trait is crucial at different stages in sperm's life; for example size‐determined sperm velocity is a key determinant of fertilization success. In addition, numbers and size may also be influenced by diverse energetic budgets required at different stages of sperm formation.     

BOTH MALE AND FEMALE NOVEL TRAITS PROMOTE THE CORRELATED EVOLUTION OF GENITALIA BETWEEN THE SEXES IN AN ARTHROPOD

The correlated evolution of genitalia between sexes has been demonstrated in many taxa. However, it remains unclear whether female rather than male genitalia can play a key role in the correlated evolution of male and female genitalia. We conducted an extensive cross‐population analysis of the divergence patterns of genital structures, weights of whole genital organs, and the bodies of both sexes, and male genital length in a group of xystodesmid millipedes showing diverse genital morphologies. We demonstrate that the correlated evolution of male and female genitalia toward exaggerated states has occurred in the millipedes, which have evolved novel traits in both males (forceps‐like gonopods) and females (retractable bellows). Enlargement and elongation of forceps‐like gonopods may be advantageous in sperm competition, whereas enlargement and elongation of the bellows may facilitate acceptance/rejection of insemination for ensuring the female's fitness. These male and female genital parts have affected the correlated evolution in the opposite sex, resulting in diversification and exaggeration of genital morphology. Our study suggests that evolutionary novel traits in not only males but also in females could play an important role in the correlated evolution of genitalia between the sexes.     

Postcopulatory Sexual Selection Results in Spermatozoa with More Uniform Head and Flagellum Sizes in Rodents

Interspecific comparative studies have shown that, in most taxa, postcopulatory sexual selection (PCSS) in the form of sperm competition drives the evolution of longer and faster swimming sperm. Work on passserine birds has revealed that PCSS also reduces variation in sperm size between males at the intraspecific level. However, the influence of PCSS upon intra-male sperm size diversity is poorly understood, since the few studies carried out to date in birds have yielded contradictory results. In mammals, PCSS increases sperm size but there is little information on the effects of this selective force on variations in sperm size and shape. Here, we test whether sperm competition associates with a reduction in the degree of variation of sperm dimensions in rodents. We found that as sperm competition levels increase males produce sperm that are more similar in both the size of the head and the size of the flagellum. On the other hand, whereas with increasing levels of sperm competition there is less variation in head length in relation to head width (ratio CV head length/CV head width), there is no relation between variation in head and flagellum sizes (ratio CV head length/CV flagellum length). Thus, it appears that, in addition to a selection for longer sperm, sperm competition may select more uniform sperm heads and flagella, which together may enhance swimming velocity. Overall, sperm competition seems to drive sperm components towards an optimum design that may affect sperm performance which, in turn, will be crucial for successful fertilization.     

Evolution of sperm structure and energetics in passerine birds

Spermatozoa exhibit considerable interspecific variability in size and shape. Our understanding of the adaptive significance of this diversity, however, remains limited. Determining how variation in sperm structure translates into variation in sperm performance will contribute to our understanding of the evolutionary diversification of sperm form. Here, using data from passerine birds, we test the hypothesis that longer sperm swim faster because they have more available energy. We found that sperm with longer midpieces have higher levels of intracellular adenosine triphosphate (ATP), but that greater energy reserves do not translate into faster-swimming sperm. Additionally, we found that interspecific variation in sperm ATP concentration is not associated with the level of sperm competition faced by males. Finally, using Bayesian methods, we compared the evolutionary trajectories of sperm morphology and ATP content, and show that both traits have undergone directional evolutionary change. However, in contrast to recent suggestions in other taxa, we show that changes in ATP are unlikely to have preceded changes in morphology in passerine sperm. These results suggest that variable selective pressures are likely to have driven the evolution of sperm traits in different taxa, and highlight fundamental biological differences between taxa with internal and external fertilization, as well as those with and without sperm storage.     

How the sperm lost its tail: the evolution of aflagellate sperm

The typical sperm is comprised of a head, midpiece and flagellum. Around this theme there is an enormous diversity of form--giant sperm, multi-flagellate sperm and also sperm that lack flagella entirely. Explaining this diversity in sperm morphology is a challenging question that evolutionary biologists have only recently engaged in. Nonetheless, one of the selective forces identified as being an important factor in the evolution of sperm form is sperm competition, which occurs when the sperm of two or more males compete to fertilize a female's ova. In species with a truly monandrous mating system, the absence of sperm competition means that the selection pressure on males to produce motile sperm may be relaxed. Potentially aflagellate sperm are less costly to produce, both in terms of energy and time. Thus, selection may therefore favour the loss of the sperm flagellum and any other motile mechanisms in monandrous taxa. A review of the literature revealed that 36 taxonomic groups, from red algae to fish, were found independently to have evolved aflagellate sperm. I review what is known about the mating systems of each of these taxa and their nearest sister taxa. A sister-group analysis using this information provided weak evidence suggesting that the evolution of aflagellate sperm could be linked to the removal of selective pressures generated by sperm competition.     

Female competition and its evolutionary consequences in mammals

Following Darwin's original insights regarding sexual selection, studies of intrasexual competition have mainly focused on male competition for mates; by contrast, female reproductive competition has received less attention. Here, we review evidence that female mammals compete for both resources and mates in order to secure reproductive benefits. We describe how females compete for resources such as food, nest sites, and protection by means of dominance relationships, territoriality and inter‐group aggression, and by inhibiting the reproduction of other females. We also describe evidence that female mammals compete for mates and consider the ultimate causes of such behaviour, including competition for access to resources provided by mates, sperm limitation and prevention of future resource competition. Our review reveals female competition to be a potentially widespread and significant evolutionary selection pressure among mammals, particularly competition for resources among social species for which most evidence is currently available. We report that female competition is associated with many diverse adaptations, from overtly aggressive behaviour, weaponry, and conspicuous sexual signals to subtle and often complex social behaviour involving olfactory signalling, alliance formation, altruism and spite, and even cases where individuals appear to inhibit their own reproduction. Overall, despite some obvious parallels with male phenotypic traits favoured under sexual selection, it appears that fundamental differences in the reproductive strategies of the sexes (ultimately related to parental investment) commonly lead to contrasting competitive goals and adaptations. Because female adaptations for intrasexual competition are often less conspicuous than those of males, they are generally more challenging to study. In particular, since females often employ competitive strategies that directly influence not only the number but also the quality (survival and reproductive success) of their own offspring, as well as the relative reproductive success of others, a multigenerational view ideally is required to quantify the full extent of variation in female fitness resulting from intrasexual competition. Nonetheless, current evidence indicates that the reproductive success of female mammals can also be highly variable over shorter time scales, with significant reproductive skew related to competitive ability. Whether we choose to describe the outcome of female reproductive competition (competition for mates, for mates controlling resources, or for resources per se) as sexual selection depends on how sexual selection is defined. Considering sexual selection strictly as resulting from differential mating or fertilisation success, the role of female competition for the sperm of preferred (or competitively successful) males appears particularly worthy of more detailed investigation. Broader definitions of sexual selection have recently been proposed to encompass the impact on reproduction of competition for resources other than mates. Although the merits of such definitions are a matter of ongoing debate, our review highlights that understanding the evolutionary causes and consequences of female reproductive competition indeed requires a broader perspective than has traditionally been assumed. We conclude that future research in this field offers much exciting potential to address new and fundamentally important questions relating to social and mating‐system evolution.     

Implications of diversity in sperm size and function for sperm competition and fertility

Sperm competition is now recognised as a potent selective force shaping many male reproductive traits. While the influence of sperm competition on sperm number is widely accepted, its effects upon sperm size remain controversial. It had been traditionally assumed that there is a trade-off between sperm number and sperm size, so that an increase in sperm number would result in a decrease in sperm size, under conditions of sperm competition. Contrary to this prediction, we proposed some time ago that sperm competition favours an increase in sperm size, because longer sperm swim faster and are more likely to win the race to fertilize ova. Comparative studies between species show that in many taxa such a relationship exists, but the consequences of an increase in sperm size may vary between taxa depending on the environment in which spermatozoa have to compete. We present new evidence showing that in mammals longer sperm swim at higher speeds. We also show that mean swimming speed is highly correlated with maximum swimming speed, so even if the fastest swimming sperm are more likely to fertilize, both measures are informative. When individuals of the same species are compared, ratios between the dimensions of different sperm components, as well as the shape of the head, seem better at explaining sperm swimming velocity. Finally, we show that mean and maximum sperm swimming speed determine male fertility. Other studies have shown that in competitive contexts, males with faster swimming sperm have higher fertilization success. We conclude that the available evidence supports our original hypothesis.     

EXPERIMENTAL EVOLUTION OF SPERM QUALITY VIA POSTCOPULATORY SEXUAL SELECTION IN HOUSE MICE

Individuals of many species copulate with multiple mates (polygamy). Multiple mating by females (polyandry) promotes sperm competition, which has broad implications for the evolution of the ejaculate. Multigenerational studies of polygamous insects have shown that the removal of sexual selection has profound fitness consequences for females, and can lead to an evolutionary divergence in ejaculate traits. However, the evolutionary implications of polygamous mating across successive generations have not before been demonstrated in a vertebrate. By manipulating the mating system we were able to reinstate postcopulatory sexual selection in a house mouse population that had a long history of enforced monogamy. Following eight generations of selection, we performed sperm quality assays on males from both the polygamous and monogamous selection lines. We applied a principal component analysis to summarize the variation among 12 correlated sperm traits, and found that males evolving under sperm competition had significantly larger scores on the first axis of variation, reflecting greater numbers of epididymal sperm and increased sperm motility, compared to males from lines under relaxed selection. Moreover, we found a correlated response in the size of litters born to females in lines subject to sperm competition. Thus, we present significant evidence that sperm competition has profound fitness consequences for both male and female house mice.     

Comparative studies of epididymal morphology and sperm distribution in dasyurid marsupials during the breeding season

The structural features of the epididymis and the number and distribution of spermatozoa along the duct, during the breeding season, were examined in two semelparous and three iteroparous dasyurid marsupials. Total numbers of epididymal spermatozoa were extremely low in all of these species when compared with epididymal sperm numbers in most other marsupials and eutherian mammals. Although semelparous dasyurids had significantly more epididymal spermatozoa than itcroparous species, very few spermatozoa were seen in the distal cauda epididymidis of any of the species examined. This coincided with distinct changes in duct shape and the surface area of the lumen in caudal regions which resulted in a reduced sperm storage capacity in the cauda epididymidis of these species. The data suggest that, like Antechinus stuartii (Taggart & Temple-Smith, 1990a), sperm content of the ejaculates in these species will be extremely low, and that sperm motility and/or transport in the female tract is highly efficient. The functional and evolutionary significance of the reproductive strategies of semelparous and iteroparous dasyurid marsupials is still obscure and further study is needed to determine if the length of sperm storage in the female and sperm competition for storage sites is related to sperm distribution in the male and mating activities. This study does, however, clearly indicate that large numbers of spermatozoa are not required to ensure successful fertilization in either semelparous or iteroparous members of the family Dasyuridae.     

POSTCOPULATORY SEXUAL SELECTION INCREASES ATP CONTENT IN RODENT SPERMATOZOA

Sperm competition often leads to increase in sperm numbers and sperm quality, and its effects on sperm function are now beginning to emerge. Rapid swimming speeds are crucial for mammalian spermatozoa, because they need to overcome physical barriers in the female tract, reach the ovum, and generate force to penetrate its vestments. Faster velocities associate with high sperm competition levels in many taxa and may be due to increases in sperm dimensions, but they may also relate to higher adenosine triphosphate (ATP) content. We examined if variation in sperm ATP levels relates to both sperm competition and sperm swimming speed in rodents. We found that sperm competition associates with variations in sperm ATP content and sperm‐size adjusted ATP concentrations, which suggests proportionally higher ATP content in response to sperm competition. Moreover, both measures were associated with sperm swimming velocities. Our findings thus support the idea that sperm competition may select for higher ATP content leading to faster sperm swimming velocity.     

Sperm competition and the evolution of gamete morphology in frogs

Despite detailed knowledge of the ultrastructure of spermatozoa, there is a paucity of information on the selective pressures that influence sperm form and function. Theoretical models for both internal and external fertilizers predict that sperm competition could favour the evolution of longer sperm. Empirical tests of the external-fertilization model have been restricted to just one group, the fishes, and these tests have proved equivocal. We investigated how sperm competition affects sperm morphology in externally fertilizing myobatrachid frogs. We also examined selection acting on egg size, and covariation between sperm and egg morphology. Species were ranked according to probability of group spawning and hence risk of sperm competition. Body size, testis size and oviposition environment may also influence gamete traits and were included in our analyses. After controlling for phylogenetic relationships between the species examined, we found that an increased risk of sperm competition was associated with increased sperm head and tail lengths. Path analysis showed that sperm competition had its greatest direct effect on sperm tail length, as might be expected under selection resulting from competitive fertilization. Sperm competition did not influence egg size. Oviposition location had a strong influence on egg size and a weak influence on sperm length, with terrestrial spawners having larger gametes than aquatic spawners. Our analysis revealed significant correlated evolution between egg morphology and sperm morphology. These data provide a conclusive demonstration that sperm competition selects for increased sperm length in frogs, and evidence for evolutionary covariance between aspects of male and female gamete morphology.     

Sperm competition games: sperm selection by females

We analyse a co-evolutionary sexual conflict game, in which males compete for fertilizations (sperm competition) and females operate sperm selection against unfavourable ejaculates (cryptic female choice). For simplicity, each female mates with two males per reproductive event, and the competing ejaculates are of two types, favourable (having high viability or success) or unfavourable (where progeny are less successful). Over evolutionary time, females can increase their level of sperm selection (measured as the proportion of unfavourable sperm eliminated) by paying a fecundity cost. Males can regulate sperm allocations depending on whether they will be favoured or disfavoured, but increasing sperm allocation reduces their mating rate. The resolution of this game depends on whether males are equal, or unequal. Males could be equal: each is favoured with probability, p, reflecting the proportion of females in the population that favour his ejaculate (the 'random-roles' model); different males are favoured by different sets of females. Alternatively, males could be unequal: given males are perceived consistently by all females as two distinct types, favoured and disfavoured, where p is now the frequency of the favoured male type in the population (the 'constant-types' model). In both cases, the evolutionarily stable strategy (ESS) is for females initially to increase sperm selection from zero as the viability of offspring from unfavourable ejaculates falls below that of favourable ejaculates. But in the random-roles model, sperm selection decreases again towards zero as the unfavourable ejaculates become disastrous (i.e. as their progeny viability decreases towards zero). This occurs because males avoid expenditure in unfavourable matings, to conserve sperm for matings in the favoured role where their offspring have high viability, thus allowing females to relax sperm selection. If sperm selection is costly to females, ESS sperm selection is high across a region of intermediate viabilities. If it is uncostly, there is no ESS in this region unless sperm limitation (i.e. some eggs fail to be fertilized because sperm numbers are too low) is included into the model. In the constant-types model, no relaxation of sperm selection occurs at very low viabilities of disfavoured male progeny. If sperm selection is sufficiently costly, ESS sperm selection increases as progeny viability decreases down towards zero; but if it is uncostly, there is no ESS at the lowest viabilities, and unlike the random-roles model, this cannot be stabilized by including sperm limitation. Sperm allocations in the ESS regions differ between the two models. With random roles, males always allocate more sperm in the favoured role. With constant types, the male type that is favoured allocates less sperm than the disfavoured type. These results suggests that empiricists studying cryptic female choice and sperm allocation patterns need to determine whether sperm selection is applied differently, or consistently, on given males by different females in the same population.     

BODY SIZE,SPERM COMPETITION,AND DETERMINANTS OF REPRODUCTIVE SUCCESS IN MALE SAVANNA BABOONS

One component of sexual selection is sperm competition. It has been reasoned that the intensity of sperm competition may be reflected in the relative testicular sizes of animals. Among males residing in multimale breeding systems, testicular size is relatively larger than among males residing in unimale mating systems. Information on whether differences in testicular size within a species can account for differences in male reproductive success is unavailable for natural populations of primates. A population of six troops of savanna baboons in Kenya was surveyed for morphometric analysis, and one of these troops was the subject of extensive behavioral observations afterwards. Testicular weights could not be obtained, but measurements of linear dimensions were transformed into volumetric estimates. Male weight accounted for 30% of the variance in testicular volume. Neither body size nor testicular volume was associated with differences in male reproductive activity. The outcome of fights over access to females could not be related to male body size, and ejaculatory patterns of males were independent of testicle size. Both sperm competition and aggressive competition intensified during the four-day optimum conception period, but fights over access to consort females were infrequent. Among savanna baboons, the probability of an ejaculation resulting in a conception is fairly low, which may account for the infrequency of injurious fights. Although testicle size influences sperm production, it does not influence either the timing of mating or the fertilizing capacity of spermatozoa, and both of these factors probably account for a substantial fraction of the variance in male baboon paternity. Sperm competition is an adjunct to agonistic competition as a mechanism affecting male baboon reproductive success. It is concluded that male reproductive success in baboons is affected more by social factors than by morphological traits associated with size.