The Two-Locus Model with Sex Differences in Recombination

The criteria for stability of the equilibrium with D=0 are obtained for the two locus model with multiplicative or symmetric fitnesses when the recombination values in males and females are different. It is shown that if r is defined to be equal to the average of the recombination values in males and females, then the criteria are exactly the same as in the standard two locus model.-The equilibrium values with D not equal0 are obtained for the symmetric fitness model. At this equilibrium, the absolute value of D is always greater (for the same average recombination value) if the recombination values in males and females differ than if they are equal.     

Soay rams target reproductive activity towards promiscuous females' optimal insemination period

Female promiscuity is thought to have resulted in the evolution of male behaviours that confer advantages in the sperm competition that ensues. In mammalian species, males can gain a post-copulatory advantage in this sperm 'raffle' by inseminating females at the optimal time relative to ovulation, leading to the prediction that males should preferentially associate and copulate with females at these times. To the best of our knowledge, we provide the first high-resolution test of this prediction using feral Soay sheep, which have a mating system characterized by male competition for access to highly promiscuous females. We find that competitive males time their mate guarding (and hence copulations) to occur close to the optimal insemination period (OIP), when females are also increasingly likely to 'cooperate' with copulation attempts. Subordinate males practice an alternative mating tactic, where they break the integrity of the consort pair and force copulations on females. The timing of these forced copulations is also targeted towards the OIP. We thus provide quantitative evidence that female promiscuity has resulted in the evolution of reproductive strategies in which males 'load' the sperm raffle by targeting their mating activity towards female OIPs, when the probability of sperm-competition success is at its greatest.     

Don'T throw bateman out with the bathwater!

Bateman identified two aspects of sexual selection. The first,called Bateman's principle, is that sexual selection favorsincreased promiscuity of males but not of females as a resultof differences in parental investment in gametes. The secondis that the variance in mate number of males is the fundamentalcause of a sex difference in fitness variance. We argue thatBateman's insight about the source of sexual selection is morefundamental than his speculation about patterns of parentalinvestment. We show that, when the sex ratio is 1:1, the averagefemale must be as promiscuous as the average male, because eachcopulation involves one male and one female. Because mean maleand female promiscuity are tied together in the same manneras mean male and female fitness, a sex difference in matingpropensity must be the result of either (1) a sex differencein the covariance between matings and number offspring, or (2)Fisherian run-away sexual selection, wherein female reluctanceto mate is a weak form of female choice. We show how femalepromiscuity can limit the evolution of male promiscuity, turningthe central argument of parental investment theory on its head.     

The strength of direct selection against female promiscuity is associated with rates of extrapair fertilizations in socially monogamous songbirds

A costs-benefits approach has frequently been used to understand the evolutionary origin and maintenance of promiscuity in animal populations. Recent meta-analyses suggest that direct costs to unfaithful females outweigh indirect benefits from infidelity in socially monogamous songbirds, suggesting that in this taxa, extrapair fertilization (EPF) evolved primarily as a self-interest male tactic. Here we present results of comparative analysis to show that standardized selection gradients acting against female infidelity (direct costs of promiscuity) explain variation in EPF rates at an interspecific level in passerines. This result confirms that costs to females resulting from reduced parental care from cheated males constrain promiscuity in this group. Our data indicate that females exert resistance over EPFs when the costs of infidelity are high and, conversely, that the rate of EPFs increases when selection on females to defend themselves against EPF attempts by males is weak and costs of infidelity are low.     

Mating promiscuity and reproductive tactics in female black and gold howler monkeys (Alouatta caraya) inhabiting an island on the Parana river,Argentina

In several primate species, females mate promiscuously and several adult males peacefully co‐reside in the same social group. We investigated female mating behavior in two neighboring multimale–multifemale groups of Alouatta caraya in northern Argentina (27°20′S–58°40′W). All adult individuals in each group were marked with identification anklets and ear tags, and followed for five consecutive full days per month during 20 consecutive months. We recorded 219 copulations for eight resident females in these two groups. Thirty‐two percent of matings involved extra‐group copulations and 68% were with resident males. During periods when females were likely to conceive and during periods when females were nonfertile (pregnancy and lactation), there were no significant differences in the average number of resident and nonresident males with which they copulated (G‐test: G_adj=0.1, df=3, P>0.05). In both of our study groups, adult males were tolerant of the mating activities between resident males and resident females, but acted aggressively and collectively (howling, border vigilance, and fighting) when extragroup males attempted to enter the group and mate with resident females. Given the frequency of extragroup matings, we examined the distance females traveled to engage in these copulations, time engaged in pre‐ and postcopulatory behavior, and the risk of injury during extragroup copulations. These costs were found to be relatively small. We suggest that female promiscuity is the prime driver or constraint on male reproductive opportunities in this species. Female promiscuity in A. caraya appears to represent a mixed mating strategy that may serve to increase opportunities for genetic diversity between a female's successive offspring as well as minimize the risk of infanticide by spreading paternity estimates across a larger number of adult males. Am. J. Primatol. 72:734–748, 2010. © 2010 Wiley‐Liss, Inc.     

Fixed and dilutable benefits: female choice for good genes or fertility

Benefits accruing to females who exercise mate choice have been defined to be either 'direct' or 'indirect'. We suggest an alternative distinction: benefits can be considered 'fixed', meaning they are on average equal to all females mating with the same male (e.g. good genes' benefits) or 'dilutable', meaning they are shared between females mating with the same male, so that the more mates a male has, the lower the average benefit to each (e.g. fertility benefits or many forms of direct benefit). Using a simple model, we show that this distinction has a major effect on the form of female preference. We predict that mating skew will be far greater in species where the benefits are fixed when compared with those where the benefits are dilutable.     

Male Reproductive Success in a Promiscuous Armoured Catfish Corydoras Aeneus (Callichthyidae)

Among a variety of fish mating systems, promiscuity with random-mating seems to be most prevalent. However, detailed studies of promiscuity have been rare due partly to the peculiar difficulty in examination of male mating and reproductive success in the random mating. Females of the armoured catfish Corydoras aeneus (no sexual dimorphism other than size of males > females) spawn 10–20 egg-clutches with multiple males at a time, but an entire egg clutch is inseminated by sperm of a single male. We studied mating system of this fish in aquarium. Males had neither mating territories nor monopolized females, never being aggressive against rival males. Evidence of female preference for certain male traits including size was not detected. Females mated a male in proportion to his relative courtship frequency among males. Courtship frequency was not related to male size, and male mating success was not different between small and large males. Clutch size and insemination rate were different neither between small and large males nor between frequently and less frequently courting males. Thus, the male reproductive success will not be related to the male size, but directly to courtship frequency, indicating the random mating in this fish. There seemed to be fecundity advantage with size in female, and the consequent sexual difference in energy allocation will be responsible to the sexual dimorphism. We also discuss the low male-GSI in this promiscuous fish in which sperm competition hardly occurred.     

Female Promiscuity Does Not Lead to Increased Fertility or Fecundity in an Arctiid Moth (<Emphasis Type="Italic">Utetheisa ornatrix</Emphasis>)

Prior work has demonstrated significant phenotypic benefits to female promiscuity in the arctiid moth Utetheisa ornatrix. We were interested in determining whether U. ornatrix females also derive genetic benefits from mating multiply. We specifically tested whether, by mating with several males, females are able to exploit postcopulatory mechanisms that decrease the risk of fertilization by incompatible sperm. We show evidence that U. ornatrix females are not taking multiple mates as fertilization insurance because: (1) females that mate once are as fertile as those that mate three times; and (2) females that take three different mates are no more fertile than those that mate three times with the same male.     

Male-derived cuticular hydrocarbons signal sperm competition intensity and affect ejaculate expenditure in crickets

Female sexual promiscuity can have significant effects on male mating decisions because it increases the intensity of competition between ejaculates for fertilization. Because sperm production is costly, males that can detect multiple matings by females and allocate sperm strategically will have an obvious fitness advantage. The presence of rival males is widely recognized as a cue used by males to assess sperm competition. However, for species in which males neither congregate around nor guard females, other more cryptic cues might be involved. Here, we demonstrate unprecedented levels of sperm competition assessment by males, which is mediated via the use of chemical cues. Using the cricket Teleogryllus oceanicus, we manipulated male perception of sperm competition by experimentally coating live unmated females with cuticular compounds extracted from males. We found that males adjusted their ejaculate allocation in response to these compounds: the viability of sperm contained within a male's ejaculate decreased as the number of male extracts applied to his virgin female partner was increased. We further show that males do not respond to the relative concentration of male compounds present on females, but rather to the number of distinct signature odours of individual males. Our results conform to sperm competition theory, and show for the first time, to our knowledge, that males can detect different intensities of sperm competition by using distinct chemical cues of individual males present on females.     

Extreme Female Promiscuity in a Non-Social Invertebrate Species

Background

While males usually benefit from as many matings as possible, females often evolve various methods of resistance to matings. The prevalent explanation for this is that the cost of additional matings exceeds the benefits of receiving sperm from a large number of males. Here we demonstrate, however, a strongly deviating pattern of polyandry.

Methodology/Principal Findings

We analysed paternity in the marine snail Littorina saxatilis by genotyping large clutches (53–79) of offspring from four females sampled in their natural habitats. We found evidence of extreme promiscuity with 15–23 males having sired the offspring of each female within the same mating period.

Conclusions/Significance

Such a high level of promiscuity has previously only been observed in a few species of social insects. We argue that genetic bet-hedging (as has been suggested earlier) is unlikely to explain such extreme polyandry. Instead we propose that these high levels are examples of convenience polyandry: females accept high numbers of matings if costs of refusing males are higher than costs of accepting superfluous matings.     

The cost of peripheral males in a brook trout mating system

A focus on the reproductive contributions of males displaying alternative life histories has neglected the role of size-dependent peripheral males in salmonine mating systems. We documented mating behaviour of brook trout Salvelinus fontinalis, including observations of spawning, over two breeding seasons to determine the mating costs of peripheral males to dominant males (kleptogamy) and females (egg cannibalism). For males and females, the mating costs of peripheral males were substantial because more than half (56%) of all observed brook trout spawnings involved peripheral males. Males that paired with large females experienced a greater incidence of kleptogamy due to increased numbers of peripheral males present. Large males face a conflict when mating in that they prefer to spawn with large females; however, these same females attract numerous males against which the dominant male cannot defend. From paternity studies, we estimated that males that had peripheral males participate in spawning may fertilize, on average, equal numbers of eggs compared to males spawning solely with a smaller female. Females that paired with relatively smaller males had significantly more eggs eaten by peripheral males than females that paired with relatively larger males. Latency to spawn by females increased when paired with a relatively small male, and resulted in females obtaining a larger spawning partner. The observed patterns of size-assortative mating, kleptogamy and cannibalism are discussed in relation to mate choice for this population of brook trout. Copyright 1999 The Association for the Study of Animal Behaviour.     

Sneaky Monkeys: An Audience Effect of Male Rhesus Macaques (Macaca mulatta) on Sexual Behavior

Males and females have different sexual interests and subsequently may show conflicting sexual strategies. While dominant males try to monopolize females, promiscuity benefits females and subordinate males. One way to escape monopolization by dominant males is to copulate in their absence. We tested this inhibitory effect of males on the sexual behavior of their group members in captive group‐living Rhesus macaques. Copulations between females and nonalpha males almost exclusively took place when the alpha male was out of sight. Furthermore, the inhibiting effect was not unique for the alpha male. An upcoming nonalpha male also inhibited copulations of its group members, and three other nonalpha males inhibited female copulation solicitations. Females adjusted their behavior to the presence of bystander males, as they initiated and accepted initiations more often in absence than in presence of bystander males. Although not significant, in males, a similar pattern was found. The observed reduction in mating behavior in presence of bystander males is in accordance with an “audience effect,” in which the behavior is modulated in relation to the presence or absence of third parties. This audience effect may serve as an important mechanism to reduce (aggressive) interruptions of subordinate male copulations.     

Multiple paternities increase genetic diversity of offspring in Brandt's voles

Mating system and philopatry influence the genetic structure of a social group in mammals. Brandt's vole (Lasiopodomys brandtii) lives in social groups year-round and has male biased dispersal, which makes the vole a model system for studies of genetic consequences of mating system and philopatry. This study aimed to test the hypotheses that: (1) multiple paternity (MP) would exist in Brandt's voles, enhance offspring genetic diversity and reduce genetic relatedness between littermates; (2) promiscuity would occur in this species in that males and females mate with multiple partners; and (3) plural breeders of a social group would be genetically related because of philopatry of female juveniles in Brandt's voles. Paternity analysis indicated that MP occurred in 11 (46%) of 24 social groups examined and that promiscuity existed in this species. Multiple paternity litters had twice the offspring genetic diversity and half the average within-litter genetic relatedness of single paternity litters. We also found plural breeding females in six social groups. Average pairwise genetic relatedness of plural breeders ranged from 0.41 to 0.72 in four social groups, suggesting first-order kinship. Future studies need to investigate effects of reproductive skew and MP on population genetic structure of Brandt's voles.     

Male nutritional history affects female fecundity in a male-dimorphic mite: Evidence for a nuptial gift?

In male-dimorphic species, males are often either armoured ‘majors’ that can monopolise access to females, or unarmoured and defenceless ‘minors’ that cannot. However, majors, unlike minors, have to spend energy to maintain their weaponry. Like-for-like, this could mean that minors have relatively more energy available to increase their reproductive output through e.g. sperm competition, or the transference of nutrients by means of a nuptial gift. Such a fitness advantage to minors could therefore contribute to explaining the coexistence of both morphs in single populations. We tested if food-deprived females of the male-dimorphic bulb mite Rhizoglyphus robini produced more eggs when mated to a minor or to a major male, and whether egg production depended on whether their mates were starved or fed prior to mating. We found no effect of male morph on female fecundity, but females did produce more eggs when mated to previously fed males. We also found that females increased in mass, but males decreased in mass over the course of the experiment. From these observations we infer that fed males are able to transfer nutrients, a nuptial gift, to their mate. This is the first observation to suggest nuptial gift transfer in mites. Though males of both morphs appeared able to produce nuptial gifts; other factors, like habitat complexity, should be considered to identify fitness benefits of minor over major males to understand why the two morphs coexist.     

Geographic parthenogenesis and the common tea‐tree stick insect of New Zealand

Worldwide, parthenogenetic reproduction has evolved many times in the stick insects (Phasmatidae). Many parthenogenetic stick insects show the distribution pattern known as geographic parthenogenesis, in that they occupy habitats that are at higher altitude or latitude compared with their sexual relatives. Although it is often assumed that, in the short term, parthenogenetic populations will have a reproductive advantage over sexual populations; this is not necessarily the case. We present data on the distribution and evolutionary relationships of sexual and asexual populations of the New Zealand stick insect, Clitarchus hookeri. Males are common in the northern half of the species’ range but rare or absent elsewhere, and we found that most C. hookeri from putative‐parthenogenetic populations share a common ancestor. Female stick insects from bisexual populations of Clitarchus hookeri are capable of parthenogenetic reproduction, but those insects from putative‐parthenogenetic populations produced few offspring via sexual reproduction when males were available. We found similar fertility (hatching success) in mated and virgin females. Mated females produce equal numbers of male and female offspring, with most hatching about 9–16 weeks after laying. In contrast, most eggs from unmated females took longer to hatch (21–23 weeks), and most offspring were female. It appears that all C. hookeri females are capable of parthenogenetic reproduction, and thus could benefit from the numerical advantage this yields. Nevertheless, our phylogeographic evidence shows that the majority of all‐female populations over a wide geographic area originate from a single loss of sexual reproduction.     

Overt and covert competition in a promiscuous mammal: the importance of weaponry and testes size to male reproductive success

Male contests for access to receptive females are thought to have selected for the larger male body size and conspicuous weaponry frequently observed in mammalian species. However, when females copulate with multiple males within an oestrus, male reproductive success is a function of both pre- and postcopulatory strategies. The relative importance of these overt and covert forms of sexual competition has rarely been assessed in wild populations. The Soay sheep mating system is characterized by male contests for mating opportunities and high female promiscuity. We find that greater horn length, body size and good condition each independently influence a male's ability to monopolize receptive females. For males with large horns at least, this behavioural success translates into greater siring success. Consistent with sperm-competition theory, we also find that larger testes are independently associated with both higher copulation rates and increased siring success. This advantage of larger testes emerges, and strengthens, as the number of oestrous females increases, as dominant males can no longer control access to them all. Our results thus provide direct quantitative evidence that male reproductive success in wild populations of mammals is dependent upon the relative magnitude of both overt contest competition and covert sperm competition.     

Selection on sperm size in response to promiscuity and variation in female sperm storage organs

Sperm cells are exceptionally morphologically diverse across taxa. However, morphology can be quite uniform within species, particularly for species where females copulate with many males per reproductive bout. Strong sexual selection in these promiscuous species is widely hypothesized to reduce intraspecific sperm variation. Conversely, we hypothesize that intraspecific sperm size variation may be maintained by high among-female variation in the size of sperm storage organs, assuming that paternity success improves when sperm are compatible in size with the sperm storage organ. We use individual-based simulations and an analytical model to evaluate how selection on sperm size depends on promiscuity level and variation in sperm storage organ size (hereafter, female preference variation). Simulations of high promiscuity (10 mates per female) showed stabilizing selection on sperm when female preference variation was low, and disruptive selection when female preference variation was high, consistent with the analytical model results. With low promiscuity (2–3 mates per female), selection on sperm was stabilizing for all levels of female preference variation in the simulations, contrasting with the analytical model. Promiscuity level, or mate sampling, thus has a strong impact on the selection resulting from female preferences. Furthermore, when promiscuity is low, disruptive selection on male traits will occur under much more limited circumstances (i.e. only with higher among-female variation) than many previous models suggest. Variation in female sperm storage organs likely has strong implications for intraspecific sperm variation in highly promiscuous species, but likely does not explain differences in intraspecific sperm variation for less promiscuous taxa.     

Reproductive Synchrony and Extra-pair Copulation in Birds

We present a model which explores the idea that females may reduce their risk of suffering a forced extra-pair copulation, by breeding synchronously with other females in the population. Information from three bird species with contrasting ecology and behaviour shows that synchrony does not automatically confer an advantage on females: synchrony is only advantageous to females if certain conditions pertain. We identify three main factors which influence female extra-pair copulation risk, these are: (i) whether or not males can identify fertile females, (ii) male and female ‘availability’ (e.g. in some seabirds copulation and extra-pair copulations occur only at the colony: females may be absent for long periods and hence are unavailable for extra-pair copulations), (iii) the timing of extra-pair copulations by males, relative to when their partners lay.     

Male mate choice influences female promiscuity in Soay sheep

In most animal species, males are predicted to compete for reproductive opportunities, while females are expected to choose between potential mates. However, when males' rate of reproduction is constrained, or females vary widely in 'quality', male mate choice is also predicted to occur. Such conditions exist in the promiscuous mating system of feral Soay sheep on St Kilda, Scotland, where a highly synchronized mating season, intense sperm competition and limitations on sperm production constrain males' potential reproductive rate, and females vary substantially in their ability to produce successful offspring. We show that, consistent with predictions, competitive rams focus their mating activity and siring success towards heavier females with higher inclusive fitness. To our knowledge, this is the first time that male mate choice has been identified and shown to lead to assortative patterns of parentage in a natural mammalian system, and occurs despite fierce male-male competition for mates. An additional consequence of assortative mating in this population is that lighter females experience a series of unstable consorts with less adept rams, and hence are mated by a greater number of males during their oestrus. We have thus also identified a novel male-driven mechanism that generates variation in female promiscuity, which suggests that the high levels of female promiscuity in this system are not part of an adaptive female tactic to intensify post-copulatory competition between males.     

Evolutionary significance of promiscuity in an aphidophagous ladybird, Propylea dissecta (Coleoptera: Coccinellidae)

The short- and long-term consequences of monogamy and promiscuity in an aphidophagous ladybird, Propylea dissecta (Mulsant) in terms of reproductive performance and offspring fitness were investigated. Promiscuous females were more fecund and laid more viable eggs than monogamous ones. Amongst promiscuous females, those mated with multiple males (freedom for mate choice) had significantly higher reproductive output than those mated daily with a new unmated male. The increased reproductive output of females mated with an unmated male each day in comparison to monogamous ones might possibly be a result of their increased ejaculate size. The further increase in reproductive output, when a female was allowed to choose a mate, may be because of repeated preferential mating with the fitter males. Results on long-term consequences of promiscuity revealed faster development and increased survival in offspring of promiscuous females at 25, 27 and 30 degrees C. The percent adult emergence was more strongly influenced than development by the mating regime. There were no statistically significant differences in survival of offspring of promiscuous females (mate choice) at the three temperatures, while the percent survival of offspring of monogamous females differed significantly. Offspring of promiscuous females (mate choice) were fittest in terms of development and survival, which indicates their probable better genetic constitution and subsequently that of chosen mates. Thus, promiscuity has both short- and long-term benefits and is advantageous to ladybirds. The presence of long-term benefits in terms of fitter offspring probably indicates the evolutionary rationale behind the prevalence of promiscuity in ladybirds.