When females mate with multiple males, sperm competition plays an important role in fertilization.
While humans are generally viewed as monogamous, this reproductive strategy is actually a rarity. In most sexually reproducing species, females mate with multiple males. Because of this, sperm competition becomes an important aspect of reproduction for males. Even when pre-copulatory male choice by females appears to determine which males will sire offspring, resulting paternity is often different from what is expected. Additionally, traits that these "sexy" males possess can make them vulnerable to predators or decrease their immune function, shortening their life span and possible mating opportunities. This provides an avenue for males to instead invest resources elsewhere, like in sneak copulations, or their sperm. Sneaky males and promiscuous females alter the proportion of offspring sired by a "favorable" male, and often sperm competition plays a large role in this. Some traits are correlated with sperm quality, but there are a surprising number of factors that can affect how many offspring a male will sire. This board explores the amazing evolutionary adaptations by males in the race to produce as many offsprign as they possibly can.
Abstract: Traits that influence the interactions between males and females can evolve very rapidly through sexual selection or sexually antagonistic coevolution. Rapid change in the fertilization systems of independent populations can give rise to reproductive incompatibilities between populations, and may contribute to speciation. Here I provide evidence for cryptic reproductive divergence among three sibling species of Drosophila that leads to a form of postmating isolation. When a female mates with both a conspecific and a heterospecific male, the conspecific sperm fertilize the vast majority of the eggs, regardless of the order of the matings. Heterospecific sperm fertilize fewer eggs after these double matings than after single matings. Experiments using spermless males show that the seminal fluid of the conspecific male is largely responsible for this conspecific sperm precedence. Moreover, when two males of the same species mate sequentially with a female from a different species, a highly variable pattern of sperm precedence replaces the second-male sperm precedence that is consistently found within species. These results indicate that females mediate sperm competition, and that second-male sperm precedence is not an automatic consequence of the mechanics of sperm storage.
Pub.: 14 Aug '97, Pinned: 07 Jun '17
Abstract: Paternity is often determined by competition between the ejaculates of different males. Males can also use particular behaviours or structures to manipulate how females use sperm. However, the ability of females to bias sperm utilization in favour of preferred males independently of male manipulation has not been demonstrated. Females are predicted to respond differentially to the sperm of different males when the reproductive interests of the sexes differ and when females are coerced into copulating. Here we show that in female feral fowl most copulations are coerced, and that females consistently bias sperm retention in favour of the preferred male phenotype. Females prefer to copulate with dominant males, but when sexually coerced by subordinate males, they manipulate the behaviour of dominant males to reduce the likelihood of insemination. If this fails, females differentially eject ejaculates according to male status in the absence of any male manipulation and preferentially retain the sperm of dominant males.
Pub.: 24 Jun '00, Pinned: 07 Jun '17
Abstract: Evolutionary ecologists are attempting to explain how parents make behavioural decisions about how much care to provide to their young. Theory predicts that when genetic relatedness to young is decreased by cuckoldry, for example, parents should reduce their care in favour of alternative broods that provide greater reproductive success. Experimental manipulation of perceived paternity has been used to test the theory, but such studies have generated mixed results. Some manipulations can fail to alter a parent's perceived paternity, whereas others may directly affect parental behaviour when, for instance, the manipulation involves capturing the parent. No study has demonstrated parental care adjustment in a manner uncomplicated by experimental design or life history correlates. Here I test the theory using the fact that nest-tending parental male bluegill sunfish (Lepomis macrochirus) can assess their paternity using both the visual presence of parasitic cuckolder males during spawning, and olfactory cues released by newly hatched eggs. By manipulating both types of cues I show that parental males dynamically adjust their parental care, favouring broods that are apparently most closely related. These results confirm the importance of genetic relatedness in parental care decision-making.
Pub.: 18 Apr '03, Pinned: 07 Jun '17
Abstract: Most birds are monogamous, but recent studies have shown that extra-pair copulations (EPCs) occur frequently despite a range of paternity guards, including mate-guarding and frequent copulation. Although EPCs are known to result in extra-pair paternity, there are no previous quantitative estimates of the success of EPCs in fertilizing eggs. We present here estimates of the likelihood of success of extra-pair copulations in a monogamous passerine, the zebra finch Poephila guttata. We show that (1) EPCs occurring under semi-natural conditions in captivity result in extra-pair paternity, (2) sperm from the last male to mate has precedence over previous matings: a single EPC occurring last is disproportionately successful in fertilizing eggs, but EPCs followed by further pair copulations have a low probability of success. These results have important implications for sexual selection theory.
Pub.: 07 Jul '88, Pinned: 07 Jun '17
Abstract: Following a copulation, males in many species of vertebrates (particularly birds) and invertebrates remain near the inseminated female and repel other suitors with displays or force. Guarding males must delay resumption of competitive mate searching, but they may insure their paternity by reducing possibilities for secondary matings and sperm competition. Among mammals, post-copulatory mate guarding has been reported in rodents, mongooses, ungulates and primates, including humans, but the effects of such behaviour on male reproductive success have not been determined genetically. I report here that mate guarding by male Idaho ground squirrels (Spermophilus brunneus) enhances a male's probability of paternity. Furthermore, an analysis based on game theory shows that mate guarding is an evolutionarily stable strategy for male S. brunneus, but not male Belding's ground squirrels (S. beldingi), which resume searching once copulation is completed.
Pub.: 30 Mar '89, Pinned: 07 Jun '17
Abstract: Sperm design and function are important determinants of male reproductive success and are expected to be under strong selection. The way that spermatozoa phenotypes evolve is poorly understood, because there have been few studies of the quantitative genetics of sperm. Here we show, in the zebra finch Taeniopygia guttata, an extraordinary degree of inter-male variation in sperm design that is independent of sperm swimming velocity. A quantitative genetics study using data from over 900 zebra finches in a complex breeding experiment showed that sperm head, mid-piece and flagellum length are heritable, that negative genetic correlations exist between sperm traits, and that significant indirect (maternal) genetic effects exist. Selection on the zebra finch sperm phenotype may be low because sperm competition is infrequent in this species, and this, in combination with negative genetic correlations and maternal genetic effects, may account for the variation in sperm phenotype between males. These results have important implications for the evolution of sperm in other taxa.
Pub.: 18 Mar '05, Pinned: 07 Jun '17
Abstract: Females commonly mate with more than one male, and polyandry has been shown to increase reproductive success in many species. Insemination by multiple males shifts the arena for sexual selection from the external environment to the female reproductive tract, where sperm competition or female choice of sperm could bias fertilization against sperm from genetically inferior or genetically incompatible males. Evidence that polyandry can be a strategy for avoiding incompatibility comes from studies showing that inbreeding cost is reduced in some egg-laying species by postcopulatory mechanisms that favour fertilization by sperm from unrelated males. In viviparous (live-bearing) species, inbreeding not only reduces offspring genetic quality but might also disrupt feto-maternal interactions that are crucial for normal embryonic development. Here we show that polyandry in viviparous pseudoscorpions reduces inbreeding cost not through paternity-biasing mechanisms favouring outbred offspring, but rather because outbred embryos exert a rescuing effect on inbred half-siblings in mixed-paternity broods. The benefits of polyandry may thus be more complex for live-bearing females than for females that lay eggs.
Pub.: 13 Jan '06, Pinned: 07 Jun '17
Abstract: Females often mate with several males before producing offspring. Field studies of vertebrates suggest, and laboratory experiments on invertebrates confirm, that even when males provide no material benefits, polyandry can enhance offspring survival. This enhancement is widely attributed to genetic benefits that arise whenever paternity is biased towards males that sire more viable offspring. Field studies suggest that post-mating sexual selection biases fertilization towards genetically more compatible males and one controlled experiment has shown that, when females mate with close kin, polyandry reduces the relative number of inbred offspring. Another potential genetic benefit of polyandry is that it increases offspring survival because males with more competitive ejaculates sire more viable offspring. Surprisingly, however, there is no unequivocal evidence for this process. Here, by experimentally assigning mates to females, we show that polyandry greatly increases offspring survival in the Australian marsupial Antechinus stuartii. DNA profiling shows that males that gain high paternity under sperm competition sire offspring that are more viable. This beneficial effect occurs in both the laboratory and the wild. Crucially, there are no confounding non-genetic maternal effects that could arise if polyandry increases female investment in a particular reproductive event because A. stuartii is effectively semelparous. Our results therefore show that polyandry improves female lifetime fitness in nature. The threefold increase in offspring survival is not negated by a decline in maternal lifespan and is too large to be offset by an equivalent decline in the reproductive performance of surviving offspring.
Pub.: 03 Nov '06, Pinned: 07 Jun '17
Abstract: Among the extraordinary adaptations driven by sperm competition is the cooperative behaviour of spermatozoa. By forming cooperative groups, sperm can increase their swimming velocity and thereby gain an advantage in intermale sperm competition. Accordingly, selection should favour cooperation of the most closely related sperm to maximize fitness. Here we show that sperm of deer mice (genus Peromyscus) form motile aggregations, then we use this system to test predictions of sperm cooperation. We find that sperm aggregate more often with conspecific than heterospecific sperm, suggesting that individual sperm can discriminate on the basis of genetic relatedness. Next, we provide evidence that the cooperative behaviour of closely related sperm is driven by sperm competition. In a monogamous species lacking sperm competition, Peromyscus polionotus, sperm indiscriminately group with unrelated conspecific sperm. In contrast, in the highly promiscuous deer mouse, Peromyscus maniculatus, sperm are significantly more likely to aggregate with those obtained from the same male than with sperm from an unrelated conspecific donor. Even when we test sperm from sibling males, we continue to see preferential aggregations of related sperm in P. maniculatus. These results suggest that sperm from promiscuous deer mice discriminate among relatives and thereby cooperate with the most closely related sperm, an adaptation likely to have been driven by sperm competition.
Pub.: 22 Jan '10, Pinned: 07 Jun '17
Abstract: Sperm competition theory predicts that males should use cues indicating the risk and intensity of sperm competition to tailor their sperm investment accordingly. Rival males are an important source of social information regarding sperm competition risk. However, revealing such information may not be in the rival males' interest. Here we use a theoretical approach based on informed and uninformed games to investigate when information transfer about sperm competition risk to competitors is beneficial for a male, and when it is not. The results show that signalling to potential future mates that a female has already mated is beneficial when the signalling male has a sperm competition disadvantage, whereas it is unfavourable when the signaller has an advantage. The reason for this counterintuitive result is that the rival males' optimal response is to reduce sperm investment when the signaller has a disadvantage and conversely, to increase investment when the signaller has an advantage. Furthermore, we analysed scenarios where males use alternative reproductive tactics. In this situation, signalling the awareness of sperm competition risk rarely pays; instead, it is beneficial to maintain an information advantage. Thus, it may be beneficial for bourgeois males to accept cuckoldry instead of revealing their sperm competition awareness to reproductive parasites. These results provide new insight in the evolution of communication between rivals in the context of sperm competition. This article is protected by copyright. All rights reserved.
Pub.: 05 Apr '17, Pinned: 07 Jun '17
Abstract: Sperm competition is pervasive and fundamental to determining a male's overall fitness. Sperm traits and seminal fluid proteins (Sfps) are key factors. However, studies of sperm competition may often exclude females that fail to remate during a defined period. Hence, the resulting datasets contain fewer data from the potentially fittest males that have most success in preventing female remating. It is also important to consider a male's reproductive success before entering sperm competition, which is a major contributor to fitness. The exclusion of these data can both hinder our understanding of the complete fitness landscapes of competing males and lessen our ability to assess the contribution of different determinants of reproductive success to male fitness. We addressed this here, using the Drosophila melanogaster model system, by (i) capturing a comprehensive range of intermating intervals that define the fitness of interacting wild type males, and (ii) analysing outcomes of sperm competition using selection analyses. We conducted additional tests using males lacking the sex peptide (SP) ejaculate component versus genetically matched (SP(+) ) controls. This allowed us to assess the comprehensive fitness effects of this important Sfp on sperm competition. The results showed a signature of positive, linear selection in wild type and SP(+) control males on the length of the intermating interval and on male sperm competition defense. However, the fitness surface for males lacking SP was distinct, with local fitness peaks depending on contrasting combinations of remating intervals and offspring numbers. The results suggest that there are alternative routes to success in sperm competition and provide an explanation for the maintenance of variation in sperm competition traits. This article is protected by copyright. All rights reserved.
Pub.: 10 Apr '17, Pinned: 07 Jun '17
Abstract: Males among many species, including humans, evaluate cues of sperm competition risk and adjust accordingly their sperm competition tactics. The number of potential sexual rivals can serve as an index of sperm competition risk. Therefore, men may adjust their in-pair copulatory interest in accordance with the presence of sexual rivals. Using self-reports from 45 married men, we test the hypotheses that the time a man's wife spends with other men-either male friends or male coworkers-will positively predict a man's copulatory interest in his wife (Hypothesis 1) and his anger (Hypothesis 2), upset (Hypothesis 3), and frustration (Hypothesis 4) in response to his wife's sexual rejection. The results show that the time wives spend with male friends (but not male coworkers) predicts their husbands' anger, upset, and frustration in response to sexual rejection, providing support for Hypotheses 2-4. Discussion highlights novel contributions of the current research and provides a potential explanation for the discrepant findings regarding male friends versus male coworkers.
Pub.: 20 Apr '17, Pinned: 07 Jun '17
Abstract: The mosquito-parasitic nematode, Strelkovimermis spiculatus (Mermithidae: Nematoda) emerges from hosts and aggregates to form mating clusters characterized by intense male-male competition for females. Successful males deposit an adhesive copulatory plug over the female vulva after insemination. In choice experiments, males strongly preferred virgin females, whereas plugged females were ignored. Males made no attempt to displace copulatory plugs deposited by previous males. Plugged females repelled males without the need for physical contact. The observed chemical repellency was independent of females, since excised plugs alone showed the same negative male response. Removal of the plug shortly after mating reduced fecundity by 90%, presumably due to spermatids leaking from the vulva. The plug as a nutritional gift hypothesis was rejected because there was no post-mating reduction in plug size that would have indicated absorption. Despite being a male adaption to sperm competition, we conclude that the copulatory plug serves the fitness needs of both males and females in multiple roles that include paternity preservation, reduced male harassment, improved male search for mates, increased fecundity, and sperm retention.
Pub.: 24 Apr '17, Pinned: 07 Jun '17