Aggression and fighting ability are correlated in the swordtail fish Xiphophorus cortezi: the advantage of being barless

Males of the swordtail fish Xiphophorus cortezi are polymorphic for the pigment pattern vertical bars. Previous studies indicate that barred males exhibit higher levels of aggression towards males with bars than those without, while barless males fail to exhibit differential levels of aggression to either morph. In this study I matched barred and barless males for size and paired them in dyadic contests in order to determine if either morph was more dominant and if so, if dominance was the result of higher aggression levels. I found that barless males had higher bite frequencies and were able to win a majority of the contests while barred males consistently escalated to biting first, even though in most cases they were ultimately the losers. In order to determine whether the observed aggression levels and fighting abilities were inherent to being barless or a consequence of responses to the bars themselves, the dyads were re-paired once after barless males were given temporary bars and once after barred males had their bars removed. Thus, each morph encountered his opponent in both a barred and barless state. Regardless of bar state, naturally barless males continued to be more aggressive and more dominant than their barred counterparts. In addition, naturally barred males only won contests in which they bit more. These results indicate that for this species, aggression is an important component of winning contests when opponents are roughly the same size. As a result, naturally barless males as a whole appear to have higher resource holding potential (RHP) than naturally barred males of the same size because of their greater aggression levels.     

Sexual selection and species recognition in the pygmy swordtail, Xiphophorus pygmaeus: conflicting preferences

Sexual selection and species recognition play important roles in mate choice; however, sexual selection preferences may overlap with traits found in heterospecifics, producing a conflict between sexual selection and species recognition. We examined female preferences in Xiphophorus pygmaeus for male traits that could provide both types of information to determine how females use multiple cues when preferences for these cues would conflict. We also examined X. pygmaeus behavior in the field to determine if females have the opportunity to choose mates. As no male-male competition was observed in the field, and females occasionally chased males from feeding areas, females apparently have the opportunity to exercise mate choice in their natural habitat. In the laboratory, female X. pygmaeus used body size as a sexual selection cue, preferring large heterospecifics (X. cortezi) to small conspecifics. Females also preferred barless X. cortezi over barred X. cortezi when males were size matched. Because X. pygmaeus males do not have bars, this preference suggests that X. pygmaeus females use vertical bars in species recognition, and that large body size and vertical bars are conflicting cues. However, X. pygmaeus females did not have a preference for males of either species when sexual selection and species recognition cues were presented concurrently. This result was surprising, because preferences for species recognition cues are often assumed to be stronger than sexual selection cues. We suggest that females may be using additional species-specific cues in mate choice to prevent hybridization.     

Body size preferences in the pot-bellied seahorse <Emphasis Type="Italic">Hippocampus abdominalis:</Emphasis> choosy males and indiscriminate females

Male seahorses (genus Hippocampus) provide all post-fertilization parental care, yet despite high levels of paternal investment, these species have long been thought to have conventional sex roles, with female mate choice and male–male competition. Recent studies of the pot-bellied seahorse (Hippocampus abdominalis) have shown that sex-role reversal occurs in high-density female-biased populations, indicating that male mating preferences may lead to sexual selection on females in this species. Egg size, egg number, and offspring size all correlate positively with female body size in Hippocampus, and by choosing large mating partners, male seahorses may increase their reproductive success. While male brood size is also positively correlated with body size, small H. abdominalis males can carry exceptionally large broods, suggesting that the fecundity benefits of female preference for large partners may be limited. We investigated the importance of body size in reproductive decisions of H. abdominalis, presenting focal individuals of both sexes with potential mating partners of different sizes. Mating preferences were quantified in terms of time spent courting each potential partner. Male seahorses were highly active throughout the mate-choice trials and showed a clear behavioral preference for large partners, while females showed significantly lower levels of activity and equivocal mating preferences. The strong male preferences for large females demonstrated here suggest that sexual selection may act strongly on female body size in wild populations of H. abdominalis, consistent with predictions on the importance of female body size for reproductive output in this species. An erratum to this article can be found at     

Female preferences for multiple attributes in the acoustic signals of the Italian treefrog, <Emphasis Type="Italic">Hyla intermedia</Emphasis>

The advertisement call of frogs and toads is an example of multiple message signal because different acoustic properties encode different kinds of biologically significant information. In the Italian treefrog, Hyla intermedia, pulse rate and frequency have been found to be under stabilizing female preferences and to encode information important for mate recognition, whereas the number of calls per call group have been found to be under directional preferences and, thus, to be important for mate quality assessment. In this study, we investigate preferences for calls that differ simultaneously in frequency, pulse rate, and number of calls per call group, and we ask how these properties interact with each other in influencing female mating decisions. Results of two-choice phonotaxis experiments provide no evidence to support the hypothesis that females process multi-attribute signals in a hierarchical way. In contrast, the pattern of preferences is consistent with the ‘preference function’ hypothesis, that is, with the hypothesis that females rank signals along an ordinal scale of values and choose accordingly. Pulse rate and frequency influence mating preferences more than does the number of calls per call group. The interaction between pulse rate and frequency is not additive but multiplicative: small differences in either pulse rate or frequency that, alone, have no effects on female choice, interact synergistically so that their combination has strong influence on female preferences. A preference repeatability test shows strong among-female differences in preference for multi-attribute signals. We suggest that this result reveals not only a variation in attribute values among females, but also a variation in the way females weight and combine attribute values into a single preference score.     

Do female association preferences predict the likelihood of reproduction?

Sexual selection acting on male traits through female mate choice is commonly inferred from female association preferences in dichotomous mate choice experiments. However, there are surprisingly few empirical demonstrations that such association preferences predict the likelihood of females reproducing with a particular male. This information is essential to confirm association preferences as good predictors of mate choice. We used green swordtails (Xiphophorus helleri) to test whether association preferences predict the likelihood of a female reproducing with a male. Females were tested for a preference for long- or short-sworded males in a standard dichotomous choice experiment and then allowed free access to either their preferred or non-preferred male. If females subsequently failed to produce fry, they were provided a second unfamiliar male with similar sword length to the first male. Females were more likely to reproduce with preferred than non-preferred males, but for those that reproduced, neither the status (preferred/non-preferred) nor the sword length (long/short) of the male had an effect on brood size or relative investment in growth by the female. There was no overall preference based on sword length in this study, but male sword length did affect likelihood of reproduction, with females more likely to reproduce with long- than short-sworded males (independent of preference for such males in earlier choice tests). These results suggest that female association preferences are good indicators of female mate choice but that ornament characteristics of the male are also important.     

The role of information in mate-choice copying in female sailfin mollies (Poecilia latipinna)

Female mate choice is a complex process involving both genetic and social factors. Extrinsic cues may play a role in determining how these factors interact. Mate-choice copying is a socially influenced mate-choice strategy in which females observe other females during mate choice and choose the same male as those females. Previous studies have shown that female sailfin mollies (Poecilia latipinna) prefer larger over smaller males, and this preference is assumed to be genetically based. In this study we tested, first, whether sailfin molly females changed their mate preference in favour of smaller males when they could obtain more information by observing two model females sequentially for 5 min each or one model female for 20 min next to the smaller male. Second, we tested if females that had changed their preferences in favour of smaller males maintained this learned preference afterwards. In copying experiments, females changed their preferences in favour of smaller males both when they could observe two model females each for 5 min near by a smaller male and when they could observe one model female for 20 min near the smaller male. In the latter case, females maintained this learned preference for smaller males up to 5 weeks after the copying experiment. This shows that mate-choice copying has a long-lasting effect on mate-choice decisions in sailfin molly females and that mate-choice copying can serve as a mechanism for cultural inheritance of mate preferences in females.     

Reinforcement of male mate preferences in sympatric killifish species Lucania goodei and Lucania parva

Reinforcement occurs when reduced hybrid fitness leads to the evolution of a stronger prezygotic isolation. Populations sympatric with closely related species, where hybridization occurs, are predicted to have stronger mate preferences than allopatric populations. The reinforcement of male mate preference is thought to be rarer than the reinforcement of female preference, but this inference may be biased by the lack of studies on male preference. We tested male mate preferences from sympatric and allopatric populations of two closely related species of killifish: Lucania goodei and Lucania parva. We found that sympatric males had greater preferences for conspecific females than allopatric males. Furthermore, conspecific preferences in allopatric populations were weakest when these populations were geographically distant (>50?km) from those of heterospecifics. Our data suggest that reinforcement has contributed to male conspecific preference and speciation in Lucania.     

Mate preferences in <Emphasis Type="Italic">Drosophila</Emphasis> infected with <Emphasis Type="Italic">Wolbachia</Emphasis>?

The maternally inherited bacterium Wolbachia pipientis generates strong reproductive incompatibilities between uninfected females and infected males (cytoplasmic incompatibility), significantly reducing both female and male reproductive success. Such fitness costs are thought to place selective pressure on hosts to evolve pre-copulatory preferences for mating with compatible mates, thereby enabling them to avoid the reproductive incompatibilities associated with Wolbachia. Therefore, uninfected females are predicted to prefer mating with uninfected males, whereas infected males are predicted to prefer mating with infected females. Despite these predictions, previous investigations of pre-copulatory mate preferences in Wolbachia-manipulated Drosophila have not found evidence of female preference for uninfected or compatible males. However, none of these studies utilised a design where focal individuals are provided with a simple choice in a relatively non-competitive situation. We examined both female and male pre-copulatory mate preference based on mate infection status in Drosophila simulans and D. melanogaster using simple choice assays involving between 30–50 replicates per treatment. Although we found no evidence of female pre-copulatory mate preferences in either species, male D. simulans exhibited some preference for mating with females of the same infection status. However, this preference was not evident when we repeated the experiment to confirm this finding. Consequently, we conclude that neither male nor female D. melanogaster and D. simulans exhibit significant Wolbachia-associated pre-copulatory mate preferences.     

Male preference for colourful females affected by male size in a marine fish

It is becoming increasingly clear that mate preferences are not static, but can vary as a function of ecological conditions and the state of the choosing individual. This applies not only to females, the sex that has usually been the subject in research on mate preferences, but also to males. Under certain conditions, males should be selective in their choice of breeding partner. In the two-spotted goby, Gobiusculus flavescens, a small marine fish, breeding females develop conspicuous yellow-orange bellies, which they actively display to males during courtship. We have recently shown that males prefer more colourful females as mates. In this study, we test if the size of a male affects his preference for colourful females. Using three-compartment mate-choice aquaria, we recorded the interest shown by a male in two females differing in coloration but similar in size. Large and small males were equally eager to court females, but only large males showed a greater interest in the more colourful females. We suggest that small males are unselective because they usually obtain few mating opportunities, as a result of being unsuccessful in mate attraction or male contest competition. This study provides the first demonstration that the size of a male affects his preference for female colour.     

Trichinella spiralis infection in voles alters female odor preference but not partner preference

Females may choose mates based on secondary sex traits that reflect disease resistance. Accordingly, females should be able to distinguish between unparasitized and parasitized males, and should prefer to mate with unparasitized individuals. Mate and odor preferences for uninfected males or males infected with the nematode, Trichinella spiralis, were examined among prairie voles (Microtus ochrogaster) and meadow voles (M. pennsylvanicus). In a 15-min odor preference test, only female meadow voles distinguished between bedding from parasitized and unparasitized conspecific males, and preferred to spend time with bedding from unparasitized males. Although T. spiralis infection influenced odor preference in female meadow voles, there was no effect of infection status on mate preference among either species. Testosterone and corticosterone concentrations were not different between parasitized and unparasitized males. However, among prairie voles, males that spent an increased amount of time with females during the mate preference test had elevated testosterone concentrations. Taken together, these data suggest that (1) female meadow voles can discriminate between unparasitized and parasitized males, (2) the effects of infection on steroid hormone concentrations may be masked by the effects of social interactions, and (3) parasites may represent a selective constraint on partner preference in voles; however, the life cycle of parasites may influence female preference and should be considered in studies of female preference. Received: 23 April 1998 / Accepted after revision: 25 October 1998     

An experimental examination of female preference patterns for components of the male advertisement call in the quacking frog,<Emphasis Type="Italic"> Crinia georgiana</Emphasis>

Even though females prefer particular components of a male display, their preferences may not be expressed during mate choice. Here, we quantified female preference patterns in the frog Crinia georgiana for call rate, pulse number and dominant frequency using two-speaker trials. Females typically chose signals emitted at a higher rate, with more pulses (when variation was extreme) and with an average dominant frequency. The results for pulse number and call rate agree with a previous field study which found that these two call components explained significant variation in male mating success. In contrast, the strong preferences for average dominant frequencies detected here disagree with the previous field study which failed to find any relationship between dominant frequency and male mating success. To explain the discrepancy we investigated whether the patterns of female preference for dominant frequency changed when another property, call rate, was varied simultaneously. Most females chose the call with an average dominant frequency when offered a choice between a call with an average dominant frequency and call rate or a non-average dominant frequency (±2 SD) and high call rate. When the differences in dominant frequency were reduced (±1 SD), females showed no clear preference for either signal. Accordingly, female preference patterns for one call component can vary with the expression of another. These results do not explain why dominant frequency did not account for any variation in male mating success under field conditions.Communicated by T. Beschlitz     

Preference for male traits in female wolf spiders varies with the choice of available males,female age and reproductive state

Individual variation in female preference for male traits may influence mate choice, especially if benefits and costs of choosiness vary with the range of available males or reproductive timing. We examined variation in female preference for male leg tuft size in Schizocosa ocreata (Hentz) wolf spiders with video playback. Dichotomous (simultaneous) choice test experiments included all possible combinations of four stimuli (i.e., modified versions of the same video male stimulus): average tuft size (control), reduced (−25%), enlarged (+25%), and no tufts (removed). Females exhibited a directional preference for larger tuft size independent of the nature of the choice (except for reduced tufts vs no tufts where no difference was seen). Female preference in the short term (over a period of 4 days) was also highly repeatable for control vs reduced tufts, but not for control vs enlarged tufts. Responses of females in ‘no-choice’ presentations of a single (control) male stimulus varied with age post-maturity; females were less receptive in weeks 1 and 2, highly receptive at week 3, and less thereafter. Mated females were least receptive and most aggressive towards a male stimulus. Females offered choices repeatedly at different ages post-maturity consistently preferred the control male vs reduced tufts over all 3 weeks but varied in their preference for enlarged tufts vs control male. In the first 2 weeks, females preferred the enlarged tuft male stimulus, but showed no preference by the third week. Females tested in week 4 showed no preference in either choice. Results suggest that the potential interaction between female preference for male traits and female reproductive timing may be a critical consideration in mate choice.     

Mustached males in a tropical poeciliid fish: emerging female preference selects for a novel male trait

One possible mechanism for the (co-)evolution of seemingly novel male traits and female preferences for them is that males exploit pre-existing female biases, and livebearing fishes (Poeciliidae) have been at the forefront of this research for almost two decades. Here, using 13 poeciliid species from four different genera, we tested whether mustache-like rostral filaments found in males of some Mexican molly (Poecilia sphenops) populations could have evolved due to exploitation of a pre-existing female bias. While Mexican mollies were the only species with a significant female association preference for mustached males, we also did not find any species exhibiting significant aversion for mustached males; rather, variance in female preference scores was large throughout. For example, more than 25% of females spent twice as much time with the mustached male compared to the non-mustached male in most species, but even 31% of Mexican molly females spent more time near the non-mustached male. Hence, a comparison of the strength of preference was inconclusive. We discuss the possibility that the female preference of P. sphenops for mustached males could be due to a female pre-existing bias (sensu lato), even if population means were not significant for species other than P. sphenops. This highlights the importance of distinguishing between population means and individual preferences when interpreting mate choice, and thus, adds depth to the concept of mating preferences as a motor for evolutionary change.     

Indirect fitness benefits are not related to male dominance in a killifish

Recent theoretical and empirical studies have shown that male dominance is often at odds with female mate preference and that indirect (genetic) benefits of mate choice may not be related to male dominance. We tested whether female preference corresponded to male dominance and whether mating with dominant males conveyed benefits to offspring fitness in a small freshwater fish, the African annual killifish Nothobranchius korthausae (Cyprinodontiformes), a species without parental care. The experimental design used controlled for the effect of male age, possibility of sperm and egg depletion, and accounted for a potential that females express their preference through maternal effects by manipulation of egg mass during ovulation. By sequentially mating females with males of known dominance, we found that female N. korthausae showed no mate preference in terms of egg numbers deposited with respect to male dominance or body size and no congruent mate preference to specific males was detected. However, males sired offspring with consistently higher hatching success and the effect was repeatable across individual females. Thus, some males provided females with indirect benefits related to additive genetic quality (“good genes”) and expressed via increased hatching rate, but this benefit was not related to male dominance status or body size.     

A female preference for experienced males in the almond moth, Cadra cautella

Male mating status can affect female reproductive output if male ejaculate investment declines over consecutive matings. Accordingly, females are predicted to mate preferentially with virgin males. In mildly polyandrous lepidopterans, female fitness is less affected by reduced male investment than in more polyandrous species, and so the predictions for female mating preferences are less clear. We examined female mating preferences in the mildly polyandrous almond moth, Cadra cautella, in which ejaculate size does not affect female reproductive output. First, we allowed females to mate with virgin or once-mated males, in which the males were presented individually or simultaneously. We recorded the latency to mating and, in the case of the simultaneously presented trials, the identity of the successful, copulating male. We found that females mated more frequently with mated males (when simultaneously presented with both males), yet females did not differ in the time taken to initiate copulation with any male. We further examined if this mated male advantage was due to differential mate detection or locomotory behaviour of the male treatments. We tested the ability of virgin and mated males to locate a receptive female within a wind tunnel using long-distance pheromone cues and recorded their activity budget. We found no difference in the ability of mated or virgin males to locate or approach a receptive female, or in their activity levels. These data suggest a female preference for mated males in this species, a preference that may minimise other potential costs of mating.     

Male mate choice favors more colorful females in the gift-giving cabbage butterfly

While the phenomenon of male mate choice has attracted considerable attention in the last two decades, whether this sexual selection mechanism could drive the evolution of female ornaments remains poorly understood. Here, we used experimental manipulation of female wing coloration to investigate male mate choice in Pieris rapae, a gift-giving butterfly. Further, we tested whether males’ nutritional status influenced their mating preferences by subjecting larvae to short periods of starvation. We found that males showed significantly more mating approaches toward control females with more colorful wings (higher pteridine content), and that this preference was strongest in low-nutrition males. Additionally, a study of field-collected females revealed that pteridine-based wing coloration was positively correlated with female egg load, which suggests such ornaments may signal female quality. Pteridine-based ornaments are widespread in nature, however their potential as honest signals in male mate choice remains largely unexplored. This work furthers our understanding of how male mate choice and female ornamentation may evolve in species whose mating systems include nutritional nuptial gifts.     

Responses of female lizards,<Emphasis Type="Italic"> Lacerta monticola</Emphasis>, to males' chemical cues reflect their mating preference for older males

Female mate choice based on visual traits appears to be rare in lizards. Field observations suggest that females of the lizard Lacerta monticola preferred to mate with larger/older males. Although older males are usually green and larger, and younger males brown and smaller, there is some overlap in size and coloration between age classes. Thus, visual cues may not always be reliable indicators of a male's age. We hypothesized that female mate-choice preferences may be based on males' pheromones, which might transmit information about characteristics such as age. In a laboratory experiment, we analyzed the effect of age of males on attractiveness of their scents to females. When we offered scents of two males of different age, females associated preferentially with scents of older males. This suggested that females were able to assess the age of males by chemical signals alone, and that females preferred to be in areas scent-marked by older males. Thus, females may increase their opportunities to mate with males of high quality, or may avoid harassment by sneaking young males. This result agreed with field observations on females mating with old males, and rejection of advances by young males. Our results also suggested that female preference for older males may depend on their own body size. Large females showed a strong preference for older males, whereas smaller females were not so selective. This, together with males' preference for large females, might lead to size-assortative matings. We suggest that the quality and/or quantity of male pheromones could communicate to the female heritable male genetic quality (i.e. age) and thereby serve as the basis of adaptive female choice in lizards.Communicated by W.E. Cooper     

Phenotypic differentiation in female preference related to geographic variation in male predation risk in the Trinidad guppy (Poecilia reticulata)

Summary Populations of the Trinidad guppy range from areas with high levels of predation by other species of fish to areas with little or no piscine predation. Previous studies have shown that variation among populations in male coloration can be explained by a balance between female preference for brighter males and natural selection against bright males. High levels of male courtship activity may also increase both predation risk and mating success. Therefore, in high-predation areas, females that mate with males that court frequently as well as those that choose bright males would presumably produce male offspring with low survivorship. Consistent with this variation in natural selection, we observed that females from high-predation populations were less likely to choose bright and frequently courting males than females from low-predation populations. This result supports the hypothesis that female preference is evolving as a character genetically correlated with the preferred male character, in which case higher levels of natural selection acting against the male character should be related to lower levels of female preference.     

Negative correlation between male ornament size and female preference intensity in a wild guppy population

Although traditional sexual selection theories for the evolution of ornamental male traits often assume consistency in female preferences for the traits over time, recent theories predict plasticity in female mate preferences and the contribution of plastic female preferences to the maintenance of polymorphism in male sexual ornaments. However, the plasticity of female preferences and its influences on male ornaments are almost unknown in natural populations. Here we examined both the intensity of female preferences and the exaggeration of a male ornament (relative area of orange spots) in a wild population of the guppy (Poecilia reticulata) every few months over a period of 3 years. Results indicated that female preference intensity was negatively correlated with the relative area of orange spots of males. In addition, we found a positive correlation between female preference intensity and the relative area of orange spots of males of their offspring generation. Because the relative area of orange spots of male guppies is a heritable trait, female preferences might have a strong influence on male orange spot size in the next generation. This study provides the first evidence of plastic female preferences depending on the scarcity of males with large ornaments in a natural population. Results of this study imply a possible contribution of female preferences for the maintenance of variation in male ornamentation.     

The efficacy of female choice in chimpanzees of the Taï Forest, Côte d’Ivoire

Male–male competition has historically been considered the major force driving sexual selection. However, female choice and inter-sexual conflict are increasingly recognized as important influences affecting differential mating and reproductive success. Many females exhibit preferences for particular males; however, male strategies may conflict with females’ ability to obtain their mate preferences. To influence paternity, females must affect both (1) whether or not sexual interactions occur, particularly during the periovulatory period (POP) and (2) the outcome of sexual interactions. This study focuses on the effectiveness of female choice in wild chimpanzees (Pan troglodytes verus). Over 2,600 h of data were collected on two habituated chimpanzee communities in the Taï National Park, Côte d’Ivoire. Female mate preferences were measured by quantifying proceptive and resistance behavior toward males in both the periovulatory period and non-POP phases of estrus. The efficacy of female preference was measured both (1) by measuring success rates of female proceptivity and resistance behaviors and (2) by determining how well measures of female mate preference (proceptivity and resistance rates) predict male mating success. Though male chimpanzees are clearly dominant to females, the results indicate that females could effectively resist male solicitations and, in most cases, unwanted copulations were averted. Both female proceptivity and resistance rates correlate (positively and inversely, respectively) with male mating success in POP. Outside POP, female proceptivity rates corresponded with male mating success, but resistance rates did not. Males (irrespective of rank) that were preferred by females obtained higher mating success compared to other males during the POP, suggesting that females were effective in their mate choice and that, despite clear male dominance, female choice influences paternity in wild chimpanzees.