Interspecific mate choice by late-courting male western grebes

In mixed populations of western and Clark's grebes, advertisingcalls by males and females play a critical role in mate choiceand reproductive isolation. We conducted field playback experimentsthat tested whether courting western grebe males became lesschoosy in their responses to female Clark's grebe calls as themating season progressed and mating opportunities diminished.Late-courting western grebe males were much more likely to answerand approach advertising calls of Dark's grebe females thanwere males courting earlier in the season. This change in responsivenessoccurred as the operational sex ratio index of the populationapproached 3:1 male calls per female call. These and field censusdata support the hypothesis that late-season hybridization betweenthese two closely related species may be a result not of speciesmisidentification, but of active and adaptive mate choice byindividuals with limited alternatives.     

Environmental change mediates mate choice for an extended phenotype,but not for mate quality

Sexual cues, including extended phenotypes, are expected to be reliable indicators of male genetic quality and/or provide information on parental quality. However, the reliability of these cues may be dependent on stability of the environment, with heterogeneity affecting how selection acts on such traits. Here, we test how environmental change mediates mate choice for multiple sexual traits, including an extended phenotype–‐the structure of male‐built nests – in stickleback fish. First, we manipulated the dissolved oxygen (DO) content of water to create high or low DO environments in which male fish built nests. Then we recorded the mate choice of females encountering these males (and their nests), under either the same or reversed DO conditions. Males in high DO environments built more compact nests than those in low DO conditions and males adjusted their nest structure in response to changing conditions. Female mate choice for extended phenotype (male nests) was environmentally dependent (females chose more compact nests in high DO conditions), while female choice for male phenotype was not (females chose large, vigorous males regardless of DO level). Examining mate choice in this dynamic context suggests that females evaluate the reliability of multiple sexual cues, taking into account environmental heterogeneity.     

Precision in sex allocation is influenced by mate choice in Drosophila melanogaster

Theory predicts that a 1 : 1 sex ratio is favoured in the absence of countervailing selection pressures. In an experiment with Drosophila melanogaster, we found significantly greater variation in the offspring sex ratios of freely mated flies than would be expected by the binomial distribution. In a surprise result, control flies given no mate choice exhibited significant under-dispersal in their sex ratio variation, possibly from sperm limitation. Both treatments, however, produced populations with a 1 : 1 sex ratio. This supports the hypothesis that sexually antagonistic selection for reproductive success in sons, and fecundity in daughters, may overcome selection for an equal sex ratio. Such precision in sex allocation may allow for the maintenance of genetic variation underlying trade-offs between male and female reproductive success.     

Female mate choice across mating stages and between sequential mates in flour beetles

Few studies have examined how female premating choice correlates with the outcome of copulatory and post-copulatory processes. It has been shown that polyandrous Tribolium castaneum females discriminate among males before mating based on olfactory cues, and also exert cryptic choice during mating through several mechanisms. This study tested whether a male's relative attractiveness predicted his insemination success during copulation. Bioassays with male olfactory cues were used to rank two males as more and less attractive to females; each female was then mated to either her more attractive male followed by less attractive male, or vice versa. Dissections immediately after second copulations revealed a significantly higher percent of successful inseminations for females that remated with more attractive males compared with those that remated with less attractive males. These results indicate that cryptic female choice during copulation reinforces precopulatory female choice in T. castaneum, and suggest that females could use cryptic choice to trade up to more attractive males, possibly gaining better phenotypic or genetic quality of sires.     

The evolution of male mate choice in insects: a synthesis of ideas and evidence

Mate choice by males has been recognized at least since Darwin's time, but its phylogenetic distribution and effect on the evolution of female phenotypes remain poorly known. Moreover, the relative importance of factors thought to underlie the evolution of male mate choice (especially parental investment and mate quality variance) is still unresolved. Here I synthesize the empirical evidence and theory pertaining to the evolution of male mate choice and sex role reversal in insects, and examine the potential for male mating preferences to generate sexual selection on female phenotypes. Although male mate choice has received relatively little empirical study, the available evidence suggests that it is widespread among insects (and other animals). In addition to 'precopulatory' male mate choice, some insects exhibit 'cryptic' male mate choice, varying the amount of resources allocated to mating on the basis of female mate quality. As predicted by theory, the most commonly observed male mating preferences are those that tend to maximize a male's expected fertilization success from each mating. Such preferences tend to favour female phenotypes associated with high fecundity or reduced sperm competition intensity. Among insect species there is wide variation in mechanisms used by males to assess female mate quality, some of which (e.g. probing, antennating or repeatedly mounting the female) may be difficult to distinguish from copulatory courtship. According to theory, selection for male choosiness is an increasing function of mate quality variance and those reproductive costs that reduce, with each mating, the number of subsequent matings that a male can perform ('mating investment') Conversely, choosiness is constrained by the costs of mate search and assessment, in combination with the accuracy of assessment of potential mates and of the distribution of mate qualities. Stronger selection for male choosiness may also be expected in systems where female fitness increases with each copulation than in systems where female fitness peaks at a small number of matings. This theoretical framework is consistent with most of the empirical evidence. Furthermore, a variety of observed male mating preferences have the potential to exert sexual selection on female phenotypes. However, because male insects typically choose females based on phenotypic indicators of fecundity such as body size, and these are usually amenable to direct visual or tactile assessment, male mate choice often tends to reinforce stronger vectors of fecundity or viability selection, and seldom results in the evolution of female display traits. Research on orthopterans has shown that complete sex role reversal (i.e. males choosy, females competitive) can occur when male parental investment limits female fecundity and reduces the potential rate of reproduction of males sufficiently to produce a female-biased operational sex ratio. By contrast, many systems exhibiting partial sex role reversal (i.e. males choosy and competitive) are not associated with elevated levels of male parental investment, reduced male reproductive rates, or reduced male bias in the operational sex ratio. Instead, large female mate quality variance resulting from factors such as strong last-male sperm precedence or large variance in female fecundity may select for both male choosiness and competitiveness in such systems. Thus, partial and complete sex role reversal do not merely represent different points along a continuum of increasing male parental investment, but may evolve via different evolutionary pathways.     

Preferred males are not always good providers: female choice and male investment in tree crickets

Female tree crickets (Oecanthus nigricornis) prefer large malesbut do not receive larger glandular courtship gifts from thesemales. This finding is puzzling from both the male and femaleperspectives, because females should prefer males providingmore direct benefits, and because males who provide larger giftsachieve higher insemination success. We tested for differencesin the quality of male secretions and found that larger malesprovided more proteinaceous food gifts than did rivals, whichcould explain why they are preferred by females. The preferencein turn could cause depletion of food gift reserves in favoredmales, because natural remating rates are high and because evena single feeding bout negatively affects glandular stores. Mostintriguingly, we showed that preferred males can adaptivelydecrease the size of courtship food-gifts provided (in orderto conserve gifts for future mating events) when they perceivethat the probability of multiple future mating opportunitiesis high. Thus, the elevated mating rates of preferred males(both before and after a focal mating event) could account forthe small size of their courtship food-gifts.     

The parental investment model and minimum mate choice criteria in humans

Trivers' parental investment model states that individuals facinghigher levels of parental investment will become increasinglychoosy in their choice of mates. For humans, this leads to twopredictions. First, both males and females will be choosierin relationships more likely to lead to the production of children.Second, females will be choosier than are males, because theirminimum risk of parental investment is higher. Previous studiesof human mate choice found support for these predictions, withone curious exception: male choosiness was lower for short-termsexual relationships involving no relationship commitment (one-nightstands) than for short-term relationships involving no sexualactivity (single dates). Because the risk of parental investmentwould be higher in a one-night stand, this suggests that truerisk of parental investment was not the underlying factor governingchoosiness levels, either because study subjects assigned differentlevels of sexual activity to the relationships than were intendedby the investigators of the study or because perceived riskis more important in human mate choice than real risk. To confirmthat male/female differences in choosiness criteria exist inhumans, and to evaluate the effect that different expected levelsof real or perceived parental investment may have on choosiness,we studied mate choosiness in the context of five types of relationshipsthat reflected explicitly defined, increasing levels of riskof parental investment for both males and females. The subjectswere 468 undergraduate students, mostly between the ages of18–24. By using questionnaires, male and female participantsrated their minimum requirements in a potential mate for 29personal characteristics with respect to level of relationship.Our results confirm the major predictions of the parental investmentmodel for humans but suggest that sex differences in choosinessare better explained by perceived rather than real risk of parentalinvestment.     

Direct selection on mate choice: female redlip blennies pay more for better mates

Sexual selection may be reduced by costs of mate choice suchas predation risk and energetic expense. Despite their theoreticalimportance, such costs have rarely been quantified or relatedto the quality of mate obtained. We performed such a study witha wild population of redlip blennies in Barbados. In this coralreef fish, females traveled 0.2–12 m from their feedingterritories to the territories of nesting males, where theyspawned and left their eggs in the males' care. Previous studieshave shown that larger and older males are better parents andare preferred by females. Here, we found that long-distancetrips by females were made primarily toward larger males, andthe extra distance traveled beyond the nearest male was correlatedwith the improvement in size of male obtained. Comparisons withfeeding budgets while the females remained on their home territoriessuggested that costs from lost time and energy were minor, andno predation attempts on females were observed. However, onlonger trips the females faced more aggressive attacks fromdamselfishes, whose territories they crossed. In contrast, noattacks were observed from damselfish when the blennies remainedon their own territories. These attacks appeared to be responsiblefor a greater incidence of scars on females than on males. Thisdifference between the sexes became more pronounced as spawningprogressed, and then decreased as females healed during thenonspawning interval. Thus, female blennies endure a cost ofinterspecific harassment which is correlated with the qualityof mate obtained. This cost may restrain sexual selection inthis species.     

How universal are human mate choices? Size does not matter when Hadza foragers are choosing a mate

It has been argued that size matters on the human mate market: both stated preferences and mate choices have been found to be non-random with respect to height and weight. But how universal are these patterns? Most of the literature on human mating patterns is based on post-industrial societies. Much less is known about mating behaviour in more traditional societies. Here we investigate mate choice by analysing whether there is any evidence for non-random mating with respect to size and strength in a forager community, the Hadza of Tanzania. We test whether couples assort for height, weight, body mass index (BMI), per cent fat and grip strength. We test whether there is a male-taller norm. Finally, we test for an association between anthropometric variables and number of marriages. Our results show no evidence for assortative mating for height, weight, BMI or per cent fat; no evidence for a male-taller norm and no evidence that number of marriages is associated with our size variables. Hadza couples may assort positively for grip strength, but grip strength does not affect the number of marriages. Overall we conclude that, in contrast to post-industrial societies, mating appears to be random with respect to size in the Hadza.     

Human mate choice and the wedding ring effect

Individuals are often restricted to indirect cues when assessing the mate value of a potential partner. Females of some species have been shown to copy each other’s choice; in other words, the probability of a female choosing a particular male increases if he has already been chosen by other females. Recently it has been suggested that mate-choice copying could be an important aspect of human mate choice as well. We tested one of the hypotheses, the so-called wedding ring effect—that women would prefer men who are already engaged or married—in a series of live interactions between men and women. The results show that women do not find men signaling engagement, or being perceived as having a partner, more attractive or higher in socioeconomic status. Furthermore, signs of engagement did not influence the women’s reported willingness to engage in short-term or long-term relationships with the men. Thus, this study casts doubt on some simplified theories of human mate-choice copying, and alternative, more complex scenarios are outlined and discussed. Tobias Uller works on broad issues in evolutionary biology, such as life-history evolution and evolutionary genetics. Christoffer Johansson recently received his Ph.D. with a dissertation on biomechanics of swimming birds. Their collaborative work on humans is focused on mate choice.     

A Comparative Bayes tactic for mate assessment and choice

Models of mate choice tactics have assumed that females randomlyencounter males when collecting information and the informationis perfect. Empirical observations of four bird species showthat females selectively visit males and repeat visits to malesbefore mating. This suggests that the assumptions of previousmodels have been too restrictive. An alternative model of informationgathering and mate choice, which relaxes the assumptions ofrandom encounters and perfect information, is presented. Inthis Comparative Bayes model, the decision of when and fromwhom to collect information is made using Bayesian estimatesof each male's quality. Predictions from the model are that:(1) the occurrence of mate assessment will increase as initialuncertainty about the quality of males increases, as the costof gathering information decreases, and as the signal perceivedby the female becomes a better representation of males' actualqualities; (2) the occurrence of repeat visits to males willbe highest when signals from males are of medium reliability;and (3) the decision of which male to assess will depend onthe estimated qualities of males, prior certainty about eachmale's quality, the reliability of each male's signal, and thecosts of assessment. Simulations compare the fitness outcomesof the Comparative Bayes tactic to other mate choice tactics.The fitness from the Comparative Bayes tactic is significantlyhigher than from the fixed threshold tactic and than from thebest-of-n tactic when the cost of assessment is low. [BehavEcol 7: 451–460 (1996)]     

Indirect mate choice, direct mate choice and species recognition in a bower-building cichlid fish lek

Sexual selection arising through female mate choice typically favours males with larger, brighter and louder signals. A critical challenge in sexual selection research is to determine the degree to which this pattern results from direct mate choice, where females select individual males based on variation in signalling traits, or indirect mate choice, where male competition governs access to reproductively active females. We investigated female mate choice in a lekking Lake Malawi cichlid fish, Hemitilapia oxyrhynchus, in which males build and aggressively defend sand 'bowers'. Similar to previous studies, we found that male reproductive success was positively associated with bower height and centrality on the lek. However, this pattern resulted from males holding these territories encountering more females, and thus their greater success was due to indirect mate choice. Following initial male courtship, an increase in the relative mating success of some males was observed, but this relative increase was unrelated to bower size or position. Crucially, experimentally manipulating bowers to resemble those of a co-occurring species had no appreciable effect on direct choice by females or male spawning success. Together, these results suggest indirect mate choice is the dominant force determining male-mating success in this species, and that bowers are not signals used in direct mate choice by females. We propose that, in this species, bowers have a primary function in intraspecific male competition, with the most competitive males maintaining larger and more central bowers that are favoured by sexual selection due to higher female encounter rates.     

Genes, copying, and female mate choice: shifting thresholds

Recent experimental work on guppies (Poecilia reticulata) hasexamined the strength of genetic and cultural (copying) factorsin determining female mate choice. Using females from a populationwith a heritable preference for the amount of orange body colorpossessed by males, prior work discovered that a threshold differencein orange color among males existed below which females wouldchoose a less orange male if they observed another female choosethat male, but above which they consistently preferred the moreorange of the males, regardless of whether they viewed anotherfemale prefer the less orange male. I tested whether this thresholdcan be shifted by increasing the amount of mate-copying informationavailable to a female. I demonstrate that when a female hasthe opportunity to see two different model females independentlyprefer the less orange of two males or a single female neara drab male for a longer period of time (twice as long as inprior work), the observer female prefers this drab male evenwhen males dramatically differ in orange coloration.     

Phenotypic plasticity in female mate choice behavior is mediated by an interaction of direct and indirect genetic effects in Drosophila melanogaster

Female mate choice is a complex decision‐making process that involves many context‐dependent factors. In Drosophila melanogaster, a model species for the study of sexual selection, indirect genetic effects (IGEs) of general social interactions can influence female mate choice behaviors, but the potential impacts of IGEs associated with mating experiences are poorly understood. Here, we examined whether the IGEs associated with a previous mating experience had an effect on subsequent female mate choice behaviors and quantified the degree of additive genetic variation associated with this effect. Females from 21 different genetic backgrounds were housed with males from one of two distinct genetic backgrounds for either a short (3 hr) or long (48 hr) exposure period and their subsequent mate choice behaviors were scored. We found that the genetic identity of a previous mate significantly influenced a female's subsequent interest in males and preference of males. Additionally, a hemiclonal analysis revealed significant additive genetic variation associated with experience‐dependent mate choice behaviors, indicating a genotype‐by‐environment interaction for both of these parameters. We discuss the significance of these results with regard to the evolution of plasticity in female mate choice behaviors and the maintenance of variation in harmful male traits.     

Runaway sexual selection without genetic correlations: social environments and flexible mate choice initiate and enhance the fisher process

Female mating preferences are often flexible, reflecting the social environment in which they are expressed. Associated indirect genetic effects (IGEs) can affect the rate and direction of evolutionary change, but sexual selection models do not capture these dynamics. We incorporate IGEs into quantitative genetic models to explore how variation in social environments and mate choice flexibility influence Fisherian sexual selection. The importance of IGEs is that runaway sexual selection can occur in the absence of a genetic correlation between male traits and female preferences. Social influences can facilitate the initiation of the runaway process and increase the rate of trait elaboration. Incorporating costs to choice do not alter the main findings. Our model provides testable predictions: (1) genetic covariances between male traits and female preferences may not exist, (2) social flexibility in female choice will be common in populations experiencing strong sexual selection, (3) variation in social environments should be associated with rapid sexual trait divergence, and (4) secondary sexual traits will be more elaborate than previously predicted. Allowing feedback from the social environment resolves discrepancies between theoretical predictions and empirical data, such as why indirect selection on female preferences, theoretically weak, might be sufficient for preferences to become elaborated.     

Bias in studies of heterozygosity and mate choice

A key question for molecular and behavioural ecologists who study mating systems is to understand why, in many species, females choose to mate with extra-pair males. Recently a possible explanation, 'genetic compatibility', has gained increasing empirical support (for a comprehensive review, see Kempenaers 2007 ). Genetic compatibility hypotheses assume that females seek extra-pair mates with alleles that complement their own. Typically, this will be achieved by mating with a male of a different genotype than her own, in order to maximise the heterozygosity of her offspring. Because numerous studies have indicated positive associations between heterozygosity and fitness (see Coltman & Slate 2003 ), it follows that mating with 'compatible' males will result in heterozygous, and therefore fit, offspring. Most empirical support for genetic compatibility has been obtained with microsatellite markers that have first been used to assess parentage and then to estimate relatedness and/or individual heterozygosity. A problem with this approach is a possible bias that favours the detection of extra-pair paternity when the extra-pair male has a genotype different from that of the female and her social mate. This in turn could lead to the erroneous conclusion that extra-pair males are less related to the female than within-pair males. In this issue of Molecular Ecology , Wetzel & Westneat 2009 (hereafter W&W), use simulation studies to assess the extent of this bias, using parameter estimates obtained from recent empirical data. They identify two forms of bias that may affect tests of the genetic compatibility hypothesis, and provide guidelines on how these biases may be avoided.     

Adult sex ratio variation: implications for breeding system evolution

Adult sex ratio (ASR) exhibits immense variation in nature, although neither the causes nor the implications of this variation are fully understood. According to theory, the ASR is expected to influence sex roles and breeding systems, as the rarer sex in the population has more potential partners to mate with than the more common sex. Changes in mate choice, mating systems and parental care suggest that the ASR does influence breeding behaviour, although there is a need for more tests, especially experimental ones. In the context of breeding system evolution, the focus is currently on operational sex ratios (OSRs). We argue that the ASR plays a role of similar importance and urge researchers to study the ASR and the OSR side by side. Finally, we plead for a dynamic view of breeding system evolution with feedbacks between mating, parenting, OSR and ASR on both ecological and evolutionary time scales.     

Does the time spent near a male predict female mate choice in a Malawian cichlid?

Gravid female Malawian Pseudotropheus cichlids spent significantly more time with males that they subsequently chose as mates, indicating that time spent near a male is a valid and accurate method of measuring female preference. Furthermore, females preferred to mate with males that had longer pelvic fins and a larger number of eggspots on their anal fins. In some instances, females chose to mate with both of the males with which they were presented, possibly because they were unable to discriminate between them.