Sexual selection and alternative mating behaviours generate demographic stochasticity in small populations.

Recent theory predicts that environmental variation and small population size facilitate the coexistence of alternative phenotypes despite unequal mean fitness. However, traditional studies of reproductive strategies often assume that the stability of alternative mating behaviours relies on equal male fitness. We present results from field observations and experimental manipulations of thermal resources on territories demonstrating the coexistence of alternative reproductive behaviours with unequal fitness. The side-blotched lizard Uta stansburiana exhibits two alternative strategies for territoriality: "usurp" and "defend". Paternity analysis revealed significantly greater mean fitness for "usurpers" than "defenders" in our study of natural variation. Moreover, variance in fitness was significantly higher for usurpers on both experimental and natural plots, implying that "usurp" is a risky strategy with potentially large pay-offs or none at all. We show theoretically that significantly higher variance in usurper fitness can allow for coexistence with defenders despite higher mean fitness of usurpers. This coexistence is facilitated by small population size. Our results have general implications for the evolution of alternative strategies and the maintenance of genetic diversity in small populations.     

Emergence of Polymorphic Mating Strategies in Robot Colonies

Polymorphism has fascinated evolutionary biologists since the time of Darwin. Biologists have observed discrete alternative mating strategies in many different species. In this study, we demonstrate that polymorphic mating strategies can emerge in a colony of hermaphrodite robots. We used a survival and reproduction task where the robots maintained their energy levels by capturing energy sources and physically exchanged genotypes for the reproduction of offspring. The reproductive success was dependent on the individuals'' energy levels, which created a natural trade-off between the time invested in maintaining a high energy level and the time invested in attracting mating partners. We performed experiments in environments with different density of energy sources and observed a variety in the mating behavior when a robot could see both an energy source and a potential mating partner. The individuals could be classified into two phenotypes: 1) forager, who always chooses to capture energy sources, and 2) tracker, who keeps track of potential mating partners if its energy level is above a threshold. In four out of the seven highest fitness populations in different environments, we found subpopulations with distinct differences in genotype and in behavioral phenotype. We analyzed the fitnesses of the foragers and the trackers by sampling them from each subpopulation and mixing with different ratios in a population. The fitness curves for the two subpopulations crossed at about 25% of foragers in the population, showing the evolutionary stability of the polymorphism. In one of those polymorphic populations, the trackers were further split into two subpopulations: (strong trackers) and (weak trackers). Our analyses show that the population consisting of three phenotypes also constituted several stable polymorphic evolutionarily stable states. To our knowledge, our study is the first to demonstrate the emergence of polymorphic evolutionarily stable strategies within a robot evolution framework.     

Alternative Mating Tactics and Evolutionarily Stable Strategies

Difficulties with applying Evolutionarily. Stable Strategy (ESS)methodology and terminology to alternative mating behaviors(in which some males in local populations adopt strikingly different,often non-competitive, behavioral patterns) are reviewed. Definitionsfor "tactic" (behavioral phenotype) and "strategy" (evolvedset of rules for expressing tactics) are given. Inconsistentand incorrect applications of "mixed," "pure," and "conditional"ESSs are discussed. Cases of condition-dependent alternative mating tactics arereviewed. Because most alternative behaviors are condition dependent,neither their population-wide nor individual fitness contributionsare expected to equal the fitness contributions of "primary"tactics. Individuals should, however, switch tactics at "equalfitness points." A particular conditional tactic will persistwhen its maintenance cost (genetic or physiological) is lessthan its fitness contribution. In only exceptional cases arethe fitness contributions of tactics expected to be equal: 1)genetic polymorphisms, 2) stochastic "mixed" ESSs, 3) frequency-dependentchoice and, 4) arbitrary assessment. Although alternative tacticsmay occur in cases of genetic polymorphism or genetic equipotence,most mating tactics probably occur when continuous heritablevariation in underlying conditional strategy exists. Selectionfor genetically influenced "roles" (genetic background) mayalso uncover apparent heritability.     

Male reproductive success in a promiscuous mammal: behavioural estimates compared with genetic paternity

Molecular techniques have enabled behavioural ecologists to reassess mating systems from a genetic perspective. Studies of paternity frequently reveal that mating behaviour does not always reflect parentage, and may bring to light alternative mating tactics. Here we present a comparison of behavioural and genetic measures of male reproductive success in a mammalian mating system in which both sexes are highly promiscuous. Rather than having a stable harem social structure, Soay rams (Ovis aries) on the island of Hirta, St Kilda, UK usually consort with individual oestrous ewes sequentially. Not all matings occur between consort pairs, however, and ewes have been seen to mate with up to 10 different rams on the same day. Using locus-specific polymorphism at five protein and 10 microsatellite DNA loci, we determined paternity for 236 lambs born into three cohorts, and compared paternity with estimates of mating success derived from more than one census of rutting behaviour. The correlation between the number of ewes with which each ram was witnessed in consort and the number of paternities assigned was positive and statistically significant, and rams that were observed in consort with a ewe were 18 times more likely to have sired her offspring than other candidate rams. However, most lambs (73%) were not sired by a ram seen in consort with the oestrous mother. Many juveniles, yearlings and some adult rams were rarely seen in consort with ewes, yet were assigned a significant number of paternities. These results suggest that mating tactics differ between age groups, and that alternative mating strategies among adults that do not involve forming consorts with many females also confer mating success. For these reasons, census-based observations of consort associations between individuals cannot be used to accurately estimate individual male reproductive success in this population.     

Population dynamics and harvesting of semelparous species with phenotypic and genotypic variability in reproductive age

We study the evolution of polymorphic life histories in anadromous semelparous salmon and the effects of harvesting. We derive dynamic phenotypic and genetic ESS models for describing the evolutionary dynamics. We show in our deterministic analysis that polymorphisms are not possible in a panmictic random mating population. Instead, genetic or behavioral polymorphisms may be observed in populations with assortative mating systems. Positive assortative mating may be supported and generated by behavioral and phenotypic traits like male mate choice, spawning ground selection by phenotype, or within-river homing-migration-distance by size. In the case of an evolutionarily stable dimorphism, the ESS is characterized by a reproductive ideal free distribution such that at an equilibrium the individuals are indifferent from the fitness point of view between the two life histories of early and late reproduction. Different strategy models - that is, phenotypic and genetic ESS models - yield identical behavioral predictions and, consequently, genetics does not seem to play an important role in the present model. An evolutionary response to increased fishing mortality is obvious and may have resource management implications. High sea fishing mortalities drive the populations toward early spawning. Thus it is possible that unselective harvesting at sea may eliminate, depending on the biological system, behavioral polymorphisms or genetic heterozygozity and drive the population to a monomorphic one. If within-river homing migration distances depend on the size of fish, unselective harvesting at sea, or selective harvesting of spawning runs in rivers, may reduce local population sizes on spawning grounds high up rivers. Finally, harvesting in a population may cause a switch in a dominant life-history strategy in a population so that anticipated sustainable yields cannot be realized in practice.     

Litter quality and inflammatory response are dependent on mating strategy in a reptile

Maintenance of health and the production of offspring are competing processes that can result in trade-offs. As vertebrates invest substantial resources in their immune system, it is crucial to understand the interactions between immunity and reproductive strategies. In the lizard Zootoca vivipara, females have condition- and context-dependent mating strategies. We predicted that, if the risk of infection is higher for polyandrous females, then polyandrous females should invest more in immune system while monandrous females should invest more in reproduction. In order to test our prediction, we captured 62 gravid females of known age in a natural population; we kept them until parturition to access to their offspring. Then, using microsatellite marker-based paternity analyses within litters, we determine the mating strategy of females (monandrous or polyandrous). Females were also challenged with PHA to estimate their inflammatory response. Our results show that polyandrous females have a higher PHA response than the monandrous females, and that monandrous females produce more males and more juveniles of better body condition than polyandrous females. The relationship between mating behaviour and immune function may have consequences for females and may shape the evolution of mating systems.     

Individualized mating system estimation using genomic data

The estimation of outcrossing rates in hermaphroditic species has been a major focus in the evolutionary study of reproductive strategies, and is also essential for plant breeding and conservation. Surprisingly, genomics has thus far minimally influenced outcrossing rate studies. In this article, we generalize a Bayesian inference method (BORICE) to accommodate genomic data from multiple subpopulations of a species. As an empirical demonstration, BORICE is applied to 115 maternal families of Mimulus guttatus. The analysis shows that low‐level whole genome sequencing of parents and offspring is sufficient for individualized mating system estimation: 208 offspring (88.5%) were definitively called as outcrossed, 23 (9.8%) as selfed. After mating system parameters are established (each offspring as outcrossed or selfed and the inbreeding level of maternal plants), BORICE outputs posterior genotype probabilities for each SNP genomewide. Individual SNP calls are often burdened with considerable uncertainty and distilling information from closely linked sites (within genomic windows) can be a useful strategy. For the Mimulus data, principal components based on window statistics were sufficient to diagnose inversion polymorphisms and estimate their effects on spatial structure, phenotypic and fitness measures. More generally, mating system estimation with BORICE can set the stage for population and quantitative genomic analyses, particularly researchers collect phenotypic or fitness data from maternal individuals.     

A molecular genetic examination of the mating system of pumpkinseed sunfish reveals high pay-offs for specialized sneakers

Intrasexual variation in reproductive behaviour and morphology are common in nature. Often, such variation appears to result from conditional strategies in which some individuals (e.g. younger males or those in poor condition) adopt a low pay-off phenotype as a 'best of a bad job'. Alternatively, reproductive polymorphisms can be maintained by balancing selection, with male phenotypes having equal fitnesses at equilibrium, but examples from nature are rare. Many species of sunfish (genus Lepomis) are thought to have alternative male reproductive behaviours, but most empirical work has focused on the bluegill sunfish and the mating systems of other sunfish remain poorly understood. We studied a population of pumpkinseed sunfish (Lepomis gibbosus) in upstate New York. Field observations confirm the existence of two male reproductive strategies: 'parentals' were relatively old and large males that maintained nests, and 'sneakers' were relatively young and small males that fertilize eggs by darting into nests of parentals during spawning. The sneaker and parental male strategies appear to be distinct life-history trajectories. Sneaker males represented 39% of the males observed spawning, and sneakers intruded on 43% of all mating attempts. Microsatellite analyses revealed that sneaker males fertilized an average of 15% of the eggs within a nest. This level of paternity by sneaker males appears to be higher than seen in most other fishes, and preliminary analyses suggest that the two male reproductive strategies are maintained as a balanced polymorphism.     

Relative fitness of alternative male reproductive tactics in a mammal varies between years

1. In many species, males can use different behavioural tactics to achieve fertilization, so-called alternative reproductive tactics (ARTs). Few field studies have measured fitness consequences of ARTs under varying environmental conditions. 2. Here, we describe fitness consequences of three phenotypically plastic ARTs in the African striped mouse (Rhabdomys pumilio) and show that relative fitness of ARTs differs between years. Each year represents a different generation. 3. For the generation living under high population density, tactics differed in relative fitness in accordance with the theory of conditional strategies, with highly successful territorial breeding males having 10 times higher success than solitary roaming males and 102 times higher success than adult natally philopatric males. 4. For the generation living under intermediate population density, the territorial breeding and roaming tactics yielded similar fitness, which would be in agreement with the theory of mixed strategies. No philopatric males occurred. 5. For the generation living under low population density, roaming was the only tactic used and some roamers had very high fitness. 6. The main prediction of status-dependent selection for conditional strategies is a correlation between fitness and status, often measured as body mass, but we did not find this correlation within tactics when more than one tactic was expressed in the population. 7. Female distribution seems to have an effect on which reproductive tactics male chose: female defence polygyny when females are clumped (interference competition), but a searching tactic when females are dispersed (scramble competition). In contrast to predictions arising from theory on scramble competition, male body mass was important in determining fitness only in the year when females were dispersed, but not in other years. 8. Our results indicate that the differentiation between conditional and mixed strategies is not an absolute one. In many other species, environmental conditions might fluctuate temporally and spatially so that the normally suboptimal tactic yields similar fitness to the (usually) dominant tactic or that only a single tactic prevails. 9. We suggest the term single strategy, independent of current fitness consequences, for systems where tactics are not genetically determined, in contrast to genetically determined alternative strategies.     

Effects of Social Structure on the Behaviour and Performance of Alternative Reproductive Phenotypes in Male Rock Shrimp, Rhynchocinetes typus

Males that adopt alternative mating tactics within a conditional strategy often undergo costly morphological changes when switching to the next phenotype during ontogeny. Whether costs of changing to a subsequent reproductive phenotype are outweighed by a higher mating probability may depend on the frequencies of different phenotypes in a group of competitors. Benefits and costs associated with different phenotype frequencies depend on interactions within and between alternative phenotypes, but the underlying behavioural mechanisms have rarely been studied. Herein, we used the rock shrimp Rhynchocinetes typus as a model: ontogenetic male stages of this species differ in morphological and behavioural traits that indicate alternative reproductive phenotypes. The small, subordinate, male stage (typus) develops via several intermediate stages (intermedius) to the dominant male stage (robustus): in competitive interactions the typus males usually employ the sneaking tactic, while the robustus males invariably employ the monopolizing fighter tactic. In laboratory experiments, we manipulated phenotype frequencies to examine whether there are frequency‐dependent effects on searching behaviour, aggressiveness and mating probability. With increasing frequency of robustus males, the rate of aggressive interactions among them increased. Furthermore, robustus males increased walking velocity when more than one robustus male was present. In contrast, typus males did not adjust their searching or aggressive behaviour. The increase of aggressive interactions among robustus males provided more opportunities for typus males to seize a temporarily unguarded female. While typus males exploit fights among robustus males that produce mating opportunities for them, robustus males benefit from typus males, which reveal the presence of receptive females. We suggest that each phenotype benefits from the presence of the other phenotype and suffers costly interference among individuals of the same phenotype. Whether frequency‐dependent effects on the mating probability of subordinates also affect their ontogenetic switchpoint should be examined in future studies.     

Criminal offending as part of an alternative reproductive strategy: investigating evolutionary hypotheses using Swedish total population data

Criminality is highly costly to victims and their relatives, but often also to offenders. From an evolutionary viewpoint, criminal behavior may persist despite adverse consequences by providing offenders with fitness benefits as part of a successful alternative mating strategy. Specifically, criminal behavior may have evolved as a reproductive strategy based on low parental investment reflected in low commitment in reproductive relationships. We linked data from nationwide total population registers in Sweden to test if criminality is associated with reproductive success. Further, we used several different measures related to monogamy to determine the relation between criminal behavior and alternative mating tactics. Convicted criminal offenders had more children than individuals never convicted of a criminal offense. Criminal offenders also had more reproductive partners, were less often married, more likely to get remarried if ever married, and had more often contracted a sexually transmitted disease than non-offenders. Importantly, the increased reproductive success of criminals was explained by a fertility increase from having children with several different partners. We conclude that criminality appears to be adaptive in a contemporary industrialized country, and that this association can be explained by antisocial behavior being part of an adaptive alternative reproductive strategy.     

To self or not to self? Absence of mate choice despite costly outcrossing in the fungus Podospora anserina

Fungi have a large potential for flexibility in their mode of sexual reproduction, resulting in mating systems ranging from haploid selfing to outcrossing. However, we know little about which mating strategies are used in nature, and why, even in well-studied model organisms. Here, we explored the fitness consequences of alternative mating strategies in the ascomycete fungus Podospora anserina. We measured and compared fitness proxies of nine genotypes in either diploid selfing or outcrossing events, over two generations, and with or without environmental stress. We showed that fitness was consistently lower in outcrossing events, irrespective of the environment. The cost of outcrossing was partly attributed to non-self recognition genes with pleiotropic effects on fertility. We then predicted that when presented with options to either self or outcross, individuals would perform mate choice in favour of the reproductive strategy that yields higher fitness. Contrary to our prediction, individuals did not seem to avoid outcrossing when a choice was offered, in spite of the fitness cost incurred. Our results suggest that, although functionally diploid, P. anserina does not benefit from outcrossing in most cases. We outline different explanations for the apparent lack of mate choice in face of high fitness costs associated with outcrossing, including a new perspective on the pleiotropic effect of non-self recognition genes.     

Frequency-dependent reproductive success in female common lizards: a real-life hawk–dove–bully game?

Alternative strategies are characterised by context-dependent fitness payoffs, which means that their fitness depends on the frequency and the nature of their interactions with one or more strategies. The analysis of the variation of the fitness of each strategy in different social environments can elucidate the evolutionary dynamics played by the strategies. In the common lizard, three female colour types (yellow, orange and mixed) are associated with alternative reaction norms in reproduction and social behaviour that signal alternative strategies. To clarify the nature of colour-specific interactions and their influence on female fitness, we analysed the response of female reproductive success to an experimental manipulation of colour frequencies in natural populations. We found that juvenile body condition at birth for all colour types was negatively affected by the local frequency of yellow females. In addition, we found that mixed females had higher clutch hatching success in the populations where orange females were frequent. These results prove that female reproduction is sensitive to the social environment, and are consistent with a scenario of a hawk–dove–bully game, in which yellow females are aggressive hawks, orange females non-aggressive doves, and mixed females have a context-dependent bully strategy. In this system, the plastic bully strategy would confer a reproductive advantage to putative heterozygotes in some social environments, which could allow the maintenance of the system through context-dependent overdominance effects.     

Queen-controlled sex ratios and worker reproduction in the bumble bee Bombus hypnorum,as revealed by microsatellites

Social insect colonies provide model systems for the examination of conflicts among parties with different genetic interests. As such, they have provided the best tests of inclusive fitness theory. However, much remains unknown about in which party's favour such conflicts are resolved, partly as a result of the only recent advent of the molecular tools needed to examine the outcome of these conflicts. Two key conflicts in social insect colonies are over control of the reproductive sex ratio and the production of male offspring. Most studies have examined only one of these conflicts but in reality they occur in tandem and may influence each other. Using microsatellite analyses, the outcome of conflict over sex ratios and male production was examined in the bumble bee, Bombus hypnorum. The genotypes were determined for mother queens, their mates and males for each of 10 colonies. In contrast to other reports of mating frequency in this species, all of the queens were singly mated. The population sex ratio was consistent with queen control, suggesting that queens are winning this conflict. In contrast, workers produced over 20% of all males in queen-right colonies, suggesting that they are more effective in competing over male-production. Combining these results with previous work, it is suggested that worker reproduction is a labile trait that may well impose only small costs on queen fitness.     

Evolution of life‐history traits and mating strategy in males: a case study on two populations of a Drosophila parasitoid

Abiotic and biotic factors affect life‐history traits and lead populations to exhibit different behavioural strategies. Due to the direct link between their behaviour and fitness, parasitoid females have often been used to test the theories explaining these differences. In male parasitoids, however, such investigations are vastly understudied, although their mating strategy directly determines their fitness. In this study, we compared the pattern of life history traits and the mating strategy of males in two populations of the Drosophila parasitoid Asobara tabida, exposed to different biotic and abiotic conditions, with the major difference being that one of them was recently exposed to strong competition with the dominant competitor Leptopilina boulardi after recent climate change, the other population being settled in a location where L. boulardi has not been recorded. The results showed that individuals of both populations have a different reproductive strategy: in one population, females produced a more female‐biased sex ratio, while males accumulated more lipids during their larval development, invested more energy in reproduction and decreased their locomotor activity, suggesting a higher proportion of matings on their emergence patch, all of these factors being possibly linked to the new competition pressure. In both populations, mating without sperm transfer may persist for several days after males become sperm‐depleted, and may be more frequent than mating with sperm transfer over their whole lifespan. This point is discussed from an evolutionary point of view.     

The mechanisms of morph determination in the amphipod Jassa: implications for the evolution of alternative male phenotypes

The proximal basis for and the maintenance of alternative male reproductive strategies and tactics are generally not understood in most species, despite the occurrence of male polymorphism across many taxa. In the marine amphipod Jassa marmorata, males differ in morphology as well as behaviour. This dimorphism corresponds to two contrasting reproductive strategies: small sneaker males or 'minors', and large fighter males or 'majors'. This study uses quantitative genetic analyses in conjunction with experimental manipulations to assess the relative importance of genetic versus environmental factors in the determination and maintenance of these alternative mating strategies. Heritability analyses indicated the reproductive phenotypes do not reflect genetic differences between dimorphic males. By contrast, morph determination was significantly affected by diet quality. Majors essentially only developed on high-protein diets. Field studies also identified a strong correlation between seasonal shifts in the relative proportions of morphs and changes in food (i.e. phytoplankton) quantity and composition, corroborating that diet cues the switch between alternative reproductive tactics. Moreover, the comparison of major and minor growth trajectories identified a heterochronic shift in maturation times between morphs, indicating that ecological selective pressures, rather than just sexual selection, may be involved in the maintenance of this conditional strategy.     

Signaling of Reproductive Status in Captive Female Golden-Headed Lion Tamarins (Leontopithecus chrysomelas)

In species in which paternal care has an important impact on the offspring's fitness, concealment of reproductive status has been proposed as a strategy employed by females to prevent males from practicing desertion and polygamy, which would then lead to monogamous or polyandrous mating systems or both. We investigated whether the female's reproductive status is being concealed in golden-headed lion tamarins, which exhibit extensive paternal care and a mainly monogamous/polyandrous mating system. We used a combination of behavioral observations and endocrine data to determine female reproductive status and to examine changes in sociosexual behaviors over the ovarian cycle and between conceptive and nonconceptive cycles. Females clearly signaled their reproductive status by way of proceptive sexual presenting. Males showed increased frequencies of anogenital sniffing and mounting during the fertile period, indicating that they detected changes in olfactory and behavioral cues emitted by females, and they adjusted their mounting behavior accordingly. Males and females also remained in closer proximity before and during the fertile period, which suggests the existence of mate guarding. We discuss a possible function of behavioral advertisement of reproductive status in shaping the mating system in Leontopithecus chrysomelas.     

Further Mathematical Modelling of Mating Sex Ratios & Male Strategies with Special Relevance to Human Life History

Influential models of male reproductive strategies have often ignored the importance of mate guarding, focusing instead on trade-offs between fitness gained through care for dependants in a pair bond versus fitness from continued competition for additional mates. Here we follow suggestions that mate guarding is a distinct alternative strategy that plays a crucial role, with special relevance to the evolution of our own lineage. Human pair bonding may have evolved in concert with the evolution of our grandmothering life history, which entails a shift to male-biased sex ratios in the fertile ages. As that sex ratio becomes more male biased, payoffs for mate-guarding increase due to partner scarcity. We present an ordinary differential equation model of mutually exclusive strategies (dependant care, multiple mating, and mate guarding), calculate steady-state frequencies and perform bifurcation analysis on parameters of care and guarding efficiency. Mate guarding triumphs over alternate strategies when populations are male biased, and guarding is fully efficient. When guarding does not ensure complete certainty of paternity, and multiple maters are able to gain some paternity from guarders, multiple mating can coexist with guarding. At female-biased sex ratios, multiple mating takes over, unless the benefit of care to the number of surviving offspring produced by the mates of carers is large.     

Male reproductive strategies in new world primates

Patterns of three variables of reproductive strategies in male New World primates are examined: (i) how males obtain access to potential mates; (ii) how males obtain actual mating opportunities; and (iii) how males affect infant survival and female reproductive success. Male opportunities to associate with females, whether by remaining in their natal groups, dispersing and forming new groups, or dispersing and taking over or joining established groups, are strongly influenced by local population densities and correlate with female reproductive rates and the extent of female reproductive seasonality and synchrony. Differences in male mating success are affected by female accessibility, whether male-male and male-female relationships are hierarchical or egalitarian, and whether female reproduction is seasonally restricted. Patterns of male behavior toward infants, characterized as active assistance, overt interference, or benign tolerance, appear to co-vary with differences in the degree to which males can affect female reproductive rates. These qualitative analyses suggest that the reproductive strategies of male New World primates can be classified along a continuum ranging from conservative to daring depending on whether female reproductive rates are relatively slow or fast and whether reproduction is strongly or weakly linked to seasonal ecological variables. Males adopt the conservative strategy of staying in their natal groups, forfeiting exclusive mating opportunities, and treating infants with tolerance when female reproduction is constrained by ecological factors. Conversely, males adopt the more daring strategy of dispersing and competing when potential payoffs through their ability to affect female reproduction are high.     

Insights into population ecology and sexual selection in snakes through the application of DNA-based genetic markers

Hypervariable genetic markers have revolutionized studies of kinship, behavioral ecology, and population biology in vertebrate groups such as birds, but their use in snakes remains limited. To illustrate the value of such markers in snakes, we review studies that have used microsatellite DNA loci to analyze local population differentiation and parentage in snakes. Four ecologically distinct species of snakes all show evidence for differentiation at small spatial scales (2-15 km), but with substantial differences among species. This result highlights how genetic analysis can reveal hidden aspects of the natural history of difficult-to-observe taxa, and it raises important questions about the ecological factors that may contribute to restricted gene flow. A 3-year study of genetic parentage in marked populations of the northern water snake showed that (1) participation in mating aggregations was a poor predictor of genetic-based measures of reproductive success; (2) multiple paternity was high, yet there was no detectable fitness advantage to multiple mating by females; and (3) the opportunity for selection was far higher in males than in females due to a larger variance in male reproductive success, and yet this resulted in no detectable selection on morphological variation in males. Thus genetic markers have provided accurate measures of individual reproductive success in this species, an important step toward resolving the adaptive significance of key features including multiple paternity and reversed sexual size dimorphism. Overall these studies illustrate how genetic analyses of snakes provide previously unobtainable information of long-standing interest to behavioral ecologists.