Multiple paternity in reptiles: patterns and processes

The evolution of female promiscuity poses an intriguing problem as benefits of mating with multiple males often have to arise via indirect, genetic, effects. Studies on birds have documented that multiple paternity is common in natural populations but strong evidence for selection via female benefits is lacking. In an attempt to evaluate the evidence more broadly, we review studies of multiple paternity in natural populations of all major groups of nonavian reptiles. Multiple paternity has been documented in all species investigated so far and commonly exists in over 50% of clutches, with particularly high levels in snakes and lizards. Marine turtles and lizards with prolonged pair-bonding have relatively low levels of multiple paternity but levels are nevertheless higher than in many vertebrates with parental care. There is no evidence that high levels of polyandry are driven by direct benefits to females and the evidence that multiple paternity arises from indirect genetic benefits is weak. Instead, we argue that the most parsimonious explanation for patterns of multiple paternity is that it represents the combined effect of mate-encounter frequency and conflict over mating rates between males and females driven by large male benefits and relatively small female costs, with only weak selection via indirect benefits. A crucial step for researchers is to move from correlative approaches to experimental tests of assumptions and predictions of theory under natural settings, using a combination of molecular techniques and behavioural observations.     

Social structure influences extra-pair paternity in socially monogamous mammals

Using the genetic estimates of paternity available for 22 species of socially monogamous mammals, we investigated the impact of the social structure and of the type of pair bonding on the interspecific variations of extra-pair paternity rates. To this purpose, we classified species in three categories of social structure—solitary, pair or family-living species—and in two categories of pair bonding—intermittent or continuous. We show that interspecific variations of extra-pair paternity rates are better explained by the social structure than by the type of pair bonding. Species with intermittent and continuous pair bonding present similar rates of extra-pair paternity, while solitary and family-living species present higher extra-pair paternity rates than pair-living species. This can be explained by both higher male–male competition and higher female mate choice opportunities in solitary and family-living species than in pair-living species.     

The frequency of multiple paternity suggests that sperm competition is common in house mice (Mus domesticus)

Sexual selection is an important force driving the evolution of morphological and genetic traits. To determine the importance of male-male, postcopulatory sexual selection in natural populations of house mice, we estimated the frequency of multiple paternity, defined as the frequency with which a pregnant female carried a litter fertilized by more than one male. By genotyping eight microsatellite markers from 1095 mice, we found evidence of multiple paternity from 33 of 143. Evidence for multiple paternity was especially strong for 29 of these litters. Multiple paternity was significantly more common in higher-density vs. lower-density populations. Any estimate of multiple paternity will be an underestimate of the frequency of multiple mating, defined as the frequency with which a female mates with more than a single male during a single oestrus cycle. We used computer simulations to estimate the frequency of multiple mating, incorporating observed reductions in heterozygosity and levels of allele sharing among mother and father. These simulations indicated that multiple mating is common, occurring in at least 20% of all oestrus cycles. The exact estimate depends on the competitive skew among males, a parameter for which we currently have no data from natural populations. This study suggests that sperm competition is an important aspect of postcopulatory sexual selection in house mice.     

Mate discrimination in Littorina littorea (L.) and L. saxatilis (Olivi) (Mollusca:Prosobranchia)

The ability of males of Littorina littorea and L. saxatilis to discriminate between mates of different sex, species and size was examined. In partner choice experiments males of L. littorea had the possibility to initiate a copulation with either a female or a male. The males did not show a preference for either sex. There was therefore no evidence that they could determine the sex of a conspecific prior to copulation. The duration of intrasexual copulation was considerably shorter than for intersexual copulation, both in the field and in laboratory experiments. For the two species, intersexual copulations were far more frequent than intrasexual ones. This can partly be explained by the difference in copulation time.Few interspecific copulating pairs were found on the shore. This may reflect a low interspecific encounter rate rather than a mechanism of species recognition. On all of these occasions, however, the active male was of L. saxatilis. It is argued that selection against precopulatory species and sex recognition is a more likely explanation than an hypothesis that states that the required mutations for precopulatory mate identification has not yet occurred. L. littorea males copulated longer with large than with small females. Copulation time was short with parasitized females, which are sterile or of low fecundity. The allocation of mating effort by males is discussed.     

Effects of age and repeated mating on male sperm supply and paternity in a parasitoid wasp

Post-copulatory paternity biases after female multiple mating are major constraints on both male and female reproductive systems. The outcome of paternity in certain situations is only controlled directly by male sperm stock. This was tested experimentally in the parasitoid wasp Anisopteromalus calandrae (Howard) (Hymenoptera: Pteromalidae), in which sperm stocks are small (several hundred) and the fertilizing efficiency of stored sperm is high (the ratio of sperm stored/fertilized eggs is about 0.75). Sperm in seminal vesicles and paternity of males of different status (virgin young, virgin old, or young previously mated) were measured after female single and double mating. The amount of sperm in the seminal vesicle differed according to male status (increasing from previously mated males to old males), but there was no difference in sperm stored by females after a single mating. In double mating experiments with two males of different status, paternity increased linearly with the relative amount of sperm in seminal vesicles. Paternity distribution conforms to 'a fair raffle' of sperm from both donors following complete mixing of sperm prior to fertilization. Thus, in a female multiple mating context, male fitness depends principally on their sperm stock, which in turn depends on life history parameters, such as age and previous mating.     

Multiple paternity in wild-caught Drosophila mojavensis

Female remating frequency and sperm allocation patterns can strongly influence levels of sperm competition and reproductive success in natural populations. In the laboratory, Drosophila mojavensis males transfer very few sperm per copulation and females remate often, suggesting multiple paternity should be common in nature. Here, we examine female sperm loads, incidence of multiple paternity, and sperm utilization by genotyping progeny from 20 wild-caught females at four highly polymorphic microsatellite loci. Based on indirect paternity analyses of 814 flies, we found evidence for high levels of multiple paternity coupled with relatively low reproductive output, consistent with the high levels of female remating predicted in this sperm-limited species. Overall, we found little evidence for last -- male sperm precedence though some temporal variation in sperm utilization was observed, consistent with laboratory findings.     

Polyandry and paternity skew in natural and experimental populations of Drosophila serrata

Many species engage in polyandry, resulting in the potential for sexual selection to continue post-copulation through sperm competition and/or cryptic female choice. The relative importance of pre- vs. post-copulatory processes remains unknown for most species despite this information being fundamental for understanding the evolutionary consequences of sexual selection. The Australian fruit fly Drosophila serrata has become a prominent model system for studying precopulatory sexual selection, such as mating preferences and their influence on the evolution of sexually selected traits. Here, we investigated polyandry and the potential for post-copulatory sexual selection in this species using indirect paternity analysis. We genotyped 21 wild-caught and 19 laboratory-reared mothers and their offspring (a total of 787 flies) at six microsatellite loci and found extensive polyandry, with all broods surveyed having at least two sires. Female remating rates were higher than in other Drosophila surveyed to date and no significant differences were found between laboratory and field populations. Additionally, we found evidence for biased sperm usage in several broods of D. serrata . Paternity skew occurred more frequently in broods from the field population than the laboratory one, suggesting differences between the two environments in the level of post-copulatory sexual selection. Our data suggest that D. serrata represents a promising system for studying the interaction between pre- and post-copulatory sexual selection in driving the evolution of sexually selected phenotypes.     

High prevalence of multiple paternity in the invasive crayfish species,Procambarus clarkii

Reproductive strategy is a central feature of the ecology of invasive species as it determines the potential for population increase and range expansion. The red swamp crayfish, Procambarus clarkii, has invaded many countries and caused serious problems in freshwater ecosystems. However, little is known about the effects of environmental conditions on crayfish paternity and offspring traits in the wild. We studied these reproductive characteristics of P. clarkii in wild populations from two different habitats (ponds and ditches) in three locations with different environmental conditions in China. Genotyping of 1,436 offspring and 30 mothers of 30 broods was conducted by using four microsatellites. An analysis of genotyping results revealed that gravid females were the exclusive mother of the progeny they tended. Twenty-nine of 30 mothers had mated with multiple (2-4) males, each of which contributed differently to the number of offspring in a brood. The average number of fathers per brood and the number of offspring per brood were similar (P > 0.05) among six sampling sites, indicating that in P. clarkii multiple paternity and offspring number per brood are independent of environmental conditions studied. Indirect benefits from increasing the genetic diversity of broods, male and sperm competition, and cryptic female choice are a possible explanation for the high level multiple paternity and different contribution of fathers to offspring in this species.     

Determinants of Mating Frequency in the Spiny Orbweaving Spider,Micrathena gracilis (Araneae: Araneidae)

Male Micrathena gracilis require two copulations, separated by a dismount, in order to inseminate both reproductive tracts of the female. We examined several factors that might influence a male's copulatory success. Web structure influenced male courtship and dismount tactics, but not copulatory frequency. The presence of another male reduced the likelihood of a given male copulating with both tracts, a limitation mediated by sexual responsiveness of the female. Mating status of the female did influence copulatory frequency; males were less likely to copulate a second time with nonvirgin females. In summary, males modify mating activities to reduce predation by females, to reduce intermale competition, and to avoid expending gametes when there is little chance of fertilization. Females influence males by predatory activities, mediated through web structure, and enhancing sperm competition among males.     

Multiyear multiple paternity and mate fidelity in the American alligator, Alligator mississippiensis

We examined multiple paternity during eight breeding events within a 10-year period (1995–2005) for a total of 114 wild American alligator nests in Rockefeller Wildlife Refuge in south-west Louisiana. Our goals included examining (i) within population variation in multiple paternity among years, (ii) variation in multiple paternity in individual females and (iii) the potential for mate fidelity. To accomplish this, in the current study, eggs were sampled from 92 nests over 6 years and analysed along with 22 nests from a previous 2-year study. Genotypes at five microsatellite loci were generated for 1802 alligator hatchlings. Multiple paternity was found in 51% of clutches and paternal contributions to these clutches were highly skewed. Rates of multiple paternity varied widely among years and were consistently higher in the current study than previously reported for the same population. Larger females have larger clutches, but are not more likely to have multiply sired nests. However, small females are unlikely to have clutches with more than two sires. For 10 females, nests from multiple years were examined. Seven (70%) of these females exhibited long-term mate fidelity, with one female mating with the same male in 1997, 2002 and 2005. Five females exhibiting partial mate fidelity (71%) had at least one multiple paternity nest and thus mated with the same male, but not exclusively. These patterns of mate fidelity suggest a potential role for mate choice in alligators.     

Variation in multiple paternity in natural populations of a free-spawning marine invertebrate

For free-spawning marine invertebrates, fertilization processes control the genetic diversity of offspring. Each egg can potentially be fertilized by a sperm from a different male, and hence genetic diversity within a brood varies with levels of multiple paternity. Yet, few studies have characterized the frequency of multiple paternity in natural spawns. We analysed patterns of multiple paternity in two populations of the colonial ascidian Botryllus schlosseri using microsatellites. Because previous studies have shown that at moderate to high population densities, competition among male-phase B. schlosseri colonies results in the nearest male dominating the paternity of a brood, we specifically tested the effect of population density on patterns of paternity. Paternity was estimated using three multilocus indices: minimum number of fathers, counts of sperm haplotypes, and effective paternity (K(E)). Multiple paternity was evident in more than 92% of the broods analysed, but highly variable, with a few broods displaying unequal contributions of different males. We found no effect of population density on multiple paternity, suggesting that other factors may control paternity levels. Indirect benefits from increasing the genetic diversity of broods are a possible explanation for the high level of multiple paternity in this species.     

The male's dilemma: Increased offspring production is more paternity to steal

Summary Large potential effects of male care on the number of offspring females successfully raise are not sufficient to select for caring males because of the pervasive importance of mating competition. Males face a version of the social dilemma, in which increased production increases the pay-off for theft. Models of the allocation of male effort partitioned between caring for babies and competing for paternity show that the optimal allocation to care is very low under a wide range of conditions. Like sex allocation where the alternatives are male versus female function or sons versus daughters, the pay-offs to one alternative are always strongly frequency dependent. Because that alternative (male function, sons, male mating effort) pays so well when rare, it cannot remain rare under most conditions. Here we consider the consequences of partitioning mating effort into mate guarding and all other forms of mating conflict. If a male gets all his partner's conceptions while guarding, gaining them at a constant rate, there are two possible regions of stability. The evolutionarily stable strategy (ESS) depends on a parameter scaling the decisiveness of (non-guarding) mating conflict. When marginal returns from conflict decrease with scale, almost all effort goes into guarding. When marginal returns increase, the ESS devotes all effort to mating. Even when the potential effect of care is large, male equilibrium strategies allocate little effort to it. We also report the results of computer simulations showing that care increases if gains from guarding saturate quickly, so that a male is assured of the paternity of most of his partner's offspring with little guarding, and consequently the pool of unguarded conceptions open to competion shrinks sharply. But even when the male's dilemma is very much reduced, it still substantially limits the allocation to care. The results of both computer simulations and mathematical analysis converge with other lines of evidence that mating has much stronger effects than parenting in shaping male strategies.     

Multiple paternity in an aggregate breeding amphibian: the effect of reproductive skew on estimates of male reproductive success

Aggregate, or explosive, breeding is widespread among vertebrates and likely increases the probability of multiple paternity. We assessed paternity in seven field-collected clutches of the explosively breeding spotted salamander (Ambystoma maculatum) using 10 microsatellite loci to determine the frequency of multiple paternity and the number of males contributing to a female's clutch. Using the Minimum Method of allele counts, multiple paternity was evident in 70% of these egg masses. Simple allele counts underestimate the number of contributing males because this method cannot distinguish multiple fathers with common or similar alleles. Therefore, we used computer simulations to estimate from the offspring genotypes the most likely number of contributing fathers given the distributions of allele frequencies in this population. We determined that two to eight males may contribute to A. maculatum clutches; therefore, multiple paternity is a common strategy in this aggregate breeding species. In aggregate mating systems competition for mates can be intense, thus differential reproductive success (reproductive skew) among males contributing to a female's clutch could be a probable outcome. We use our data to evaluate the potential effect of reproductive skew on estimates of the number of contributing males. We simulated varying scenarios of differential male reproductive success, ranging from equal contribution to high reproductive skew among contributing sires in multiply sired clutches. Our data suggest that even intermediate levels of reproductive skew decrease confidence substantially in estimates of the number of contributing sires when parental genotypes are unknown.     

Female aggression predicts mode of paternity acquisition in a social lizard

Individual differences in behaviour are ubiquitous in nature. Despite the likely role of selection in maintaining these differences, there are few demonstrations of their fitness consequences in wild populations and, consequently, the mechanisms that link behavioural variation to variation in fitness are poorly understood. Specifically, the consequences of consistent individual differences in behaviour for the evolution of social and mating strategies have rarely been considered. We examined the functional links between variation in female aggression and her social and mating strategies in a wild population of the social lizard Egernia whitii. We show that female Egernia exhibit temporally consistent aggressive phenotypes, which are unrelated to body size, territory size or social density. A female''s aggressive phenotype, however, has strong links to her mode of paternity acquisition (within- versus extra-pair paternity), with more aggressive females having more offspring sired by extra-pair males than less aggressive females. We discuss the potential mechanisms by which female aggression could underpin mating strategies, such as the pursuit/acceptance of extra-pair copulations. We propose that a deeper understanding of the evolution and maintenance of social and mating systems may result from an explicit focus on individual-level female behavioural phenotypes and their relationship with key reproductive strategies.     

Inter-population variation in multiple paternity and reproductive skew in the guppy

We use microsatellite loci to detail the multiple paternity patterns in broods from 10 wild populations of the guppy ( Poecilia reticulata ) found in Northern Trinidad. The populations span two major drainages comprising the Caroni and the Oropouche, and include sites that are characterized by either high or low predation. Across the populations the frequency of multiple paternity is high with 95% (range: 70%–100%) of broods having multiple sires. Broods have an average of 3.5 sires (range: 1–9) and a mixed-model analysis suggests that broods from high predation sites have marginally more sires than do those from low predation sites, but this is true only in the Oropouche drainage. There is no difference in sire number between predation sites in the Caroni drainage. Brood size, but not female body length, is correlated with the number of sires and the correlation cannot be attributed solely to the stochastic process associated with sperm competition and a 'fair raffle'. Within broods there is significant skew in reproductive success among males, which may reflect variation in sperm competitiveness or female choice. There is, however, no difference in the skew among populations from different predation regimes or drainages. Finally, high predation populations were characterized by increased genetic variability at the microsatellite loci, suggesting a larger effective population size. We discuss explanations for the high degree of multiple paternity but the general lack of any major differences among broods from ecologically different populations.     

Copulation behaviour and paternity in shy albatrosses (Thalassarche cauta)

We investigated the mating system of shy albatrosses Thalassarche cauta by combining behavioural observations during the pre-laying period with genetic paternity analysis. Genetic data on the mating systems of several procellariiform seabirds have recently become available, but data on the reproductive behaviour of these species are rarely obtained. Our main aims were to describe the copulatory behaviour of this species and identify how males achieve within-pair and extra-pair paternity (EPP). Most copulations occurred on the nest, were unforced and were within-pair. Females controlled the success of copulations and were observed soliciting extra-pair matings. Within-pair and extra-pair copulations were behaviourally similar. A low frequency (7–10%, n =29 chicks) of EPP was detected despite male use of frequent copulation as a paternity guard. The pre-laying foraging exodus of female shy albatrosses differed from that in other albatrosses: it was relatively short in length, lasting c . 2 days, and within-pair copulations occurred after the female's return 2 days before laying. This may reflect the close proximity of feeding grounds to the breeding colony.     

Multiple mating, paternity, and body size in a simultaneous hermaphrodite, Aplysia californica

Sperm displacement and sperm competition prove difficult tomeasure, but are crucial elements in predicting sex allocationstrategies of sperm-storing hermaphrodites. Body size is predictedto affect sex allocation so that within a population, largeanimals invest a greater proportion of resources in female functionthan do small animals. These mating strategies depend on spermdisplacement abilities and lead to similar levels of paternityacross body sizes despite differences in resource level. Thepresent study investigated mating patterns, multiple paternity,and sperm competition in a field population of a simultaneouslyhermaphroditic sea slug, Aplysia californica (California seahare). Animals mating in the female role were larger than themean for the population, indirectly supporting theoretical predictionsfor increased investment in female function with body size.However, contrary to predictions, animals mating in the malerole were not different in size from the population mean orthe animals they inseminated. Individual tagging revealed thatsea slugs are capable of moving across distances that allowfor the sampling of many potential mates, and that they materepeatedly in both sexual roles. Microsatellite paternity analysisdemonstrated that multiple mating in the field leads to multiplepaternity, and last-sperm donors achieve high levels of paternity.There was no effect of body size on paternity. Further paternitystudies are needed to reveal the mechanisms of sperm precedencepatterns in A. californica.     

Copulation behaviour in mammals: evidence that sperm competition is widespread

We review information on copulation behaviour and sperm competition in mammals using data primarily from the literature. Female mammals of many species regularly copulate with more than one male during each oestrous period. Such multi-male copulations are reported more often in social, compared with solitary species. In addition, the mates of males of polygynous species experience multi-male copulations as often as the mates of males or monogamous species. Male mammals attempt to increase their certainty of paternity through a number of reproductive tactics. Copulation frequency is higher in specks with multi-male copulations compared with other species. Consortships, which can br regarded as a form of mate guarding, are often absent from species in which more than one male copulates with each female during her oestrous period. Most solitary burrow-living small mammal species copulate in the open, whereas social species often copulate inside their burrows. Insurance copulations may occur in many species since high copulation rates occur in four circumstances: (i) when mates are reunited, (ii) when a new male takes over a female, (iii) when a strange male steals a copulation, and (iv) when an audience of males is present.