A theory of mate choice based on heterozygosity

In theories of mate choice that rely on genetic benefits, thenature of the"good genes" involved has received little attention.A review of genetic studies of mate choice in a variety of speciesand situations suggests that individual heterozygosity is moreimportant than previously realized. Females are predicted tovalue heterozygosity in their offspring and under some conditionsin their males. The expression of vigor, condition-sensitiveornaments, and symmetry in males may be a direct reflectionnot of "good genes" but of individual heterozygosity at keyloci or at many loci. Like sexuality itself, mate choice basedon heterozygosity and genic diversity may be an adaptation thatfavors the production of diverse and superior competitors. Femalechoice is made meaningful by sexuality, and the adaptive valueof choice probably depends on some of the same factors thatmaintain sexuality     

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.     

Repeatability of mate choice in female red jungle fowl

We staged female mate choice trials between pairs of males andrepeated the process for each female to determine the repeatabilityof female preference for males in red jungle fowl (Gallus gallus)in the first and second half of the breeding season. We measuredmale morphological traits (the size and color of the comb andthe brightness of the hackle feathers) that females are knownto use in choosing a mate. In the first half of the breedingseason, females showed repeatability in their choices of matewith respect to the male's comb characters. Females did notshow a repeatable preference with respect to male hackle feathers,and we found no repeatability of mate choice in the second halfof the season. Females seem to primarily look at the male'scomb when choosing a mate, and other ornaments seem only ofsecondary importance.[Behav Ecol 7: 243-246 (1996)]     

No female mate choice in Mallee dragon lizards, Ctenophorus fordi

Aspects of sexual selection were studied in a sexually monomorphic Australian agamid lizard (Ctenophorus fordi), in particular with respect to the sensory exploitation hypothesis. In enclosure trials, females were offered the choice between large vs. small males and, in a different experiment, males with blue vs. normal head color. The rationale for these experiments was: firstly, to establish if females actively solicit copulations; secondly, if so, do females solicit copulations non-randomly with respect to male size (because large males may have access to food resources); thirdly, if male coloration is manipulated to match traits of congeneric, conspicuous and sexually dimorphic species, do females show preference for this novel trait (in accordance with the sensory exploitation hypothesis). The corresponding manipulations were also made in a free-living population where the distribution of females on the home ranges of color-manipulated males were monitored. Blue-headed males were accepted as mating partners both in the staged mating trials and in the natural population. Females appeared not to express any kind of active or passive mate choice (rejection); in only one out of 62 trials did a female approach a male herself rather than being approached by the male(s). There was no discrimination against any male category regardless of size or color within a female's receptive period and the manipulation of male head color in the natural population did not result in spatial re-distribution of females. Thus, a female appears to mate unselectively within her receptive period. Rejection behaviors were used only outside of the receptive period to communicate, to all males, that the female is not receptive.     

Familiarity adds to attractiveness in matters of siskin mate choice

There is currently considerable controversy in evolutionary ecology revolving around whether social familiarity brings attraction when a female chooses a mate. The topic of familiarity is significant because by avoiding or preferring familiar individuals as mates, the potential for local adaptation may be reduced or favoured. The topic becomes even more interesting if we simultaneously analyse preferences for familiarity and sexual ornaments, because when familiarity influences female mating preferences, this could very significantly affect the strength of sexual selection on male ornamentation. Here, we have used mate-choice experiments in siskins Carduelis spinus to analyse how familiarity and patterns of ornamentation (i.e. the size of wing patches) interact to influence mating success. Our results show that females clearly prefer familiar individuals when choosing between familiar and unfamiliar males with similar-sized wing patches. Furthermore, when females were given the choice between a highly ornamented unfamiliar male and a less ornamented familiar male, half of the females still preferred the socially familiar birds as mates. Our finding suggests that male familiarity may be as important as sexual ornaments in affecting female behaviour in mate choice. Given that the potential for local adaptation may be favoured by preferring familiar individuals as mates, social familiarity as a mate-choice criterion may become a potential area of fruitful research on sympatric speciation processes.     

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.     

Male choice generates stabilizing sexual selection on a female fecundity correlate

We know very little about male mating preferences and how they influence the evolution of female traits. Theory predicts that males may benefit from choosing females on the basis of traits that indicate their fecundity. Here, we explore sexual selection generated by male choice on two components of female body size (wing length and body mass) in Drosophila serrata. Using a dietary manipulation to alter female size and 828 male mate choice trials, we analysed linear and nonlinear sexual selection gradients on female mass and wing length. In contrast to theoretical expectations and prevailing empirical data, males exerted stabilizing rather than directional sexual selection on female body mass, a correlate of fecundity. Sexual selection was detected only among females with access to standard resource levels as an adult, with no evidence for sexual selection among resource-depleted females. Thus the mating success of females with the same body mass differed depending upon their access to resources as an adult. This suggests that males in this species may rely on signal traits to assess body mass rather than assessing it directly. Stabilizing rather than directional sexual selection on body mass together with recent evidence for stabilizing sexual selection on candidate signal traits in this species suggests that females may trade-off resources allocated to reproduction and sexual signalling.     

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.     

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.     

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.     

Mate choice opportunity leads to shorter offspring development time in a desert insect

We describe indirect genetic benefits of mate choice in two allopatric populations of cactophilic Drosophila mojavensis. By manipulating mate choice opportunity, we show that greater mate choice among sexually mature adults leads to shorter offspring egg-to-adult development times; the extent of this reduction was influenced by population origin and by host plant environment. We performed multiple-choice mating trials with individually marked flies to investigate whether differential male mating success was a consequence of female choice, male interaction, or both. We demonstrate that male copulation frequency was not random and instead, was determined by female choice. Virgin females in these trials were no less discriminating than females that had been previously exposed to males. These results suggest that there are indirect benefits of female mate choice that are population and environment specific, consistent with the hypothesis of ecologically influenced 'good genes' sexual selection.     

The use of multiple cues in mate choice

An increasing number of studies find females to base their mate choice on several cues. Why this occurs is debated and many different hypotheses have been proposed. Here I review the hypotheses and the evidence in favour of them. At the same time I provide a new categorisation based on the adaptiveness of the preferences and the information content of the cues. A few comparative and empirical studies suggest that most multiple cues are Fisherian attractiveness cues or uninformative cues that occur alongside a viability indicator and facilitate detection, improve signal reception, or are remnants from past selection pressures. However, much evidence exists tor multiple cues providing additional information and serving as multiple messages that either indicate general mate quality or enable females that differ in mate preferences to choose the most suitable male. Less evidence exists for multiple cues serving as back-up signals. The importance of receiver psychology, multiple sensory environments and signal interaction in the evolution of multiple cues and preferences has received surprisingly little attention but may be of crucial importance. Similarly, sexual conflict has been proposed to result in maladaptive preferences for manipulative cues, and in neutral preferences for threshold cues, but no reliable evidence exists so far. An important factor in the evolution of multiple preferences is the cost of using additional cues. Most theoretical work assumes that the cost of choice increases with the number of cues used, which restricts the conditions under which preferences for multiple cues are expected to evolve. I suggest that in contrast to this expectation, the use of multiple cues can reduce mate choice costs by decreasing the number of mates inspected more closely or the time and energy spent inspecting a set of mates. This may be one explanation for why multiple cues are more common than usually expected. Finally I discuss the consequences that the use of multiple cues may have for the process of sexual selection, the maintenance of genetic variation, and speciation.     

Female assessment of male functional fertility during mate choice in a promiscuous fish

Sexual selection should favour females that can assess the functional fertility of available sexual partners and avoid mating with recently mated, sperm‐depleted males. Our current understanding of the sensory mechanism(s) underlying female assessment of males based on their functional fertility and avoidance of sperm‐depleted males is incomplete. Female Trinidadian guppies (Poecilia reticulata) are known to avoid mating with males that they had previously observed mating with other females. Here, we investigated experimentally the proximate sensory cues that they use to distinguish between paired size‐ and colour‐matched mated and unmated males in the absence of visual public information on their prior mating histories. When only water‐borne chemical cues from the males were available, females avoided the previously mated male and preferred the unmated one, but they chose randomly when only male visual cues (and no chemical cues) were available. They also preferred unmated over mated males when freely swimming with them in a more sensorially complex environment with multiple male cues (i.e., visual, chemical and mechanical cues) concurrently available. Females exhibited no preference for either stimulus males when both were unmated, irrespectively of the sensory environment. These novel results suggest that, in the absence of prior visual public information on the recent mating histories of males, female guppies use olfactory cues putatively emitted by mated males to avoid mating with them. The source and nature of the implicated olfactory cues and the fitness benefits gained by female guppies in sexually preferring males that have not recently mated remain unknown and warrant further research.     

Female mate choice and male red coloration in a natural three-spined stickleback (Gasterosteus aculeatus) population

Under laboratory conditions, female three-spined sticklebacks(Gasterosteus aculeatus L.) show a mating preference for intenselyred-colored males. We verified this female choice in the fieldby observing a freshwater stickleback population in its naturalhabitat. During the egg collection phase, individual courtingmales were localized with the aid of a dummy of a ripe female,caught and photographed under standardized conditions, and released.After males had stopped collecting eggs, we counted the numberof eggs in the nests. The more intense a male's red breedingcoloration, the more eggs he received. Simultaneous female choiceexperiments in the laboratory suggested that ripe females ofthis population preferred redder males. Breeding activitiesof the males in the field were clustered and seem to be synchronizedwithin clusters. At one of the breeding sites, more intensered males were in better physical condition, but this was notthe case at another site. Although several synchronized breedingcycles were observed, the majority of males seem to completeonly one breeding cycle.     

The empirical question of thresholds and mechanisms of mate choice

Summary Theoretical discussions concerning how animals might best sample and select mates have suggested that individuals could base decisions either on a sample of mates (sampled-based decisions) or on a threshold of comparison (threshold-based decisions). Recent theoretical work demonstrates that threshold-based mating decisions generate higher expected fitness than sample-based mating decisions when search costs exist. Empirical results from most unmanipulated systems, however, either conclude that females make sample-based decisions or are inconclusive. A few experimental studies designed to detect mating thresholds purport to demonstrate threshold-based choice but an examination of these studies indicates such conclusions were premature. We believe that few examples of threshold-based choice exist because protocols designed to identify mating thresholds were often inconsistent with models of threshold choice. We suggest that future empirical work strive not to document mating thresholdsper se. Rather, future work might best reveal decision rules by manipulating the distribution of quality among potential mates; such manipulations predict uniquely how females using sample-based and threshold-based decision rules should behave.