A Technique for the Temporary Application and Augmentation of Pigment Patterns in Fish

Secondary sexual characteristics, such as pigment patterns in male fish, have significant roles in female mating preferences. Vertical body-bar pigmentation, one such characteristic, is found in a variety of Xiphophorus species. Prior investigations have revealed that these bars function as a signal attracting females and deterring rival males in at least two species. Furthermore, it has been shown that X. cortezi females prefer vertical bar symmetry as well as more bars in conspecific males. Because of the shortcomings of prior techniques used for pigment elimination and permanent color marking, a temporary method for augmenting pigment patterns with an artificial dye was developed. This technique was tested using the known preference that female X. cortezi have for more bars in conspecific males. The results corroborate previous findings as female X. cortezi preferred males with more artificial vertical bars in comparison with males with fewer. Therefore, the method of applying temporary, artificial pigment patterns using an antiseptic dye would appear to be an effective way to control and manipulate secondary sexual pigmentation for behavioral investigations in fish.     

A polymorphism in female preference for a polymorphic male trait in the swordtail fish Xiphophorus cortezi

Xiphophorus cortezi males are polymorphic for the pigment pattern vertical bars. In this study, we determined whether X. cortezi females are polymorphic in their preference for this trait by examining both within- and between-individual variation in female preference. There was significantly more variation in female preference within than between individuals using both video animations and live males as stimuli; repeatability measures were 0.86 and 0.5, respectively. Some females had a strong preference for males with bars, some for males without bars, and some had either a weak preference or no preference at all. We also found a significant difference in the strength of preference for bars between females with and without bars, suggesting the potential for a genetic correlation between preference and trait. Finally, we examined female preference for bar symmetry in the same females tested for preference for bars. We detected a significant preference for bar symmetry in the population of females as a whole, and a positive relationship between the strength of this preference and a preference for the presence/absence of bars among the subset of females that preferred ‘no bars’. We discuss these results in light the possibility that these two preferences are interrelated. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.      

Experience Plays a Role in Female Preference for Symmetry in the Swordtail Fish Xiphophorus malinche

Variation in female mating preferences was previously detected in wild-caught Xiphophorus cortezi and Xiphophorus malinche females: smaller (presumed younger) females preferred symmetrical males, while larger (presumed older) females preferred asymmetrical males. We examined the influence of experience on this variation in female preference by determining if X. malinche females would express a preference for symmetry as virgins, shift their preferences for bar symmetry as they got larger (older) and if experience with males of different bar number symmetry could explain the variation in female preference previously detected. Virgin females exhibited no preference for vertical bar number symmetry when tested in the young- or old-age classes. However, young virgins spent more time with the opposite treatment in the second when compared with first test, indicating an ability to detect the difference between symmetry and asymmetry, and potentially a preference to mate with multiple males. When females were reared in one of three treatments, housed with symmetrical, barless or both symmetrical and asymmetrical males, we detected both a treatment and tank effect on strength of preference for symmetry, suggesting that barring pattern and some other aspect of the social environment influenced the development of this mating preference. Finally, we detected no effect of age class on mean strength of preference for symmetry; however, there was a statistically different relationship between female size and strength of preference for symmetry across the two age classes, suggesting that the preference function for symmetry may not be linear in relation to female size.     

Further examination of female preference for vertical bars in swordtails: preference for 'no bars' in a species without bars

Xiphophorus pygmaeus males do not have a vertical bar pigment pattern and so female preference for the bars was determined by giving females a choice between pairs of X. multilineatus males without bars (all bars removed by freeze-branding) and with bars (freeze-branded between the bars). Female X. pygmaeus had a significant preference for male X. multilineatus without bars over males with bars, and this preference was significantly different from the preference for bars detected in X. nigrensis and X. multilineatus . The implications of the results are discussed in regard to models for the evolution of female preference.     

Female preference for trait symmetry in addition to trait size in swordtail fish

Interest in fluctuating asymmetries, random deviations from perfect bilateral symmetry, has spread to studies of sexual selection because of the intriguing idea that females could use the degree of asymmetry of a male trait to assess the genetic quality of potential mates. The evidence that females prefer males with symmetrical sexual signals, however, remains controversial. A problem that applies to most previous studies is that preference for trait size can be misinterpreted as preference for symmetry, even when overall trait size is held constant, if females assess trait size by the largest minimum on one side. If overall trait size is equal between males, the asymmetrical males will have the maximum and minimum trait size, and so preference to mate with symmetrical males could actually reflect a preference to avoid males with the minimum trait on one side. Xiphophorus cortezi females preferred males with symmetrical bar numbers when the minimum number of bars was held constant. The strength of female preference for the symmetrical males was negatively correlated with the strength of preference the same females had for bar number. These results clearly demonstrate that females preferred trait symmetry in addition to trait size.     

Larger swordtail females prefer asymmetrical males

Many organisms, including humans, find symmetry more attractive than asymmetry. Is this bias towards symmetry simply a by-product of their detection system? We examined female preference for symmetry of the pigment pattern vertical bars in the swordtail fishes Xiphophorus cortezi and Xiphophorus malinche. We found a relationship between preference for symmetry and female size, with larger and thus older females spending significantly more time with the asymmetrical video animation as compared to the symmetrical video animation. The preference for asymmetry we report demonstrates that even if females can detect symmetrical males better, this does not preclude subsequent selection on females to prefer symmetrical or asymmetrical males. In addition, because the preference was correlated with female size, past studies may have missed preference for either asymmetry and/or symmetry by not examining the relationship between female preference and size/age or by measuring a limited size/age distribution of females. In both of the species of swordtail fishes examined, a high proportion of males are asymmetrical by more than one bar. We suggest that female preference may be maintaining fluctuating asymmetries in these fishes.     

Vertical bars on male Xiphophorus multilineatus: a signal that deters rival males and attracts females

We examined the function of the vertical bar pattern on maleswordtails (Xiphophorus multilinneatus) as a signal in bothmale-male competition and female choice. This pattern had previouslybeen described as an aggressive signal because males intensifiedthe bars during male-male encounters in the laboratory. Ourfield observations supported this observation and also showedthat bars intensified when males courted females. The intensityof bars was correlated with access to females in the field.Within the size range of males that have bars, however, neitherbar number nor male size appeared to influence access to females.We used freeze-branding to remove the bars from males in thelaboratory so that we could control for characters correlatedwith bar intensity, and tested males and females separatelyso that we could separate the influence of these two componentsof sexual selection. We compared the responses of males andfemales to males that had their bars removed and control malesfreeze-branded between the bars. Test males responded more aggressivelyto males without bars as compared to control males. In addition,females showed a preference for control males over males thathad their bars removed. These results suggest that the barsmay function as a signal that deters rival males and attractsfemales.     

Bias in the heritability of preference and its potential impact on the evolution of mate choice

The evolution of mate choice is a function of the heritability of preference. Estimation in the laboratory is typically made by presenting a female with a limited number of males. We show that such an approach produces a downwardly biased estimate, which we term the heritability of choice. When preference is treated as a threshold trait then less biased estimates are obtained particularly for preferences based on the relative value of the preferred trait. Because females in the wild typically survey on average less than five males we argue that the heritability of choice may be more meaningful than the heritability of preference. The restricted number of males surveyed can lead to a reduction in the phenotypic variance of the preferred trait in the group of males selected by the females if the phenotypic variance in preference is equal to or less than the phenotypic variance in the referred trait. If the phenotypic variance in preference exceeds that of the preferred trait then the opposite occurs. A second effect of the restricted number of males sampled is that females are likely to mate initially with males that are not the most preferred. The failure to find the most preferred male may account for the common observation of multiple matings and extra-pair copulations. We suggest that current explanations for polyandry need to take this failure into account.     

Independent Mating Preferences for Male Body Size and Coloration in Female Trinidadian Guppies

Although females in numerous species generally prefer males with larger, brighter and more elaborate sexual traits, there is nonetheless considerable intra‐ and interpopulation variation in mating preferences amongst females that requires explanation. Such variation exists in the Trinidadian guppy, Poecilia reticulata, an important model organism for the study of sexual selection and mate choice. While female guppies tend to prefer more ornamented males as mates, particularly those with greater amounts of orange coloration, there remains variation both in male traits and female mating preferences within and between populations. Male body size is another trait that is sexually selected through female mate choice in some species, but has not been examined as extensively as body coloration in the guppy despite known intra‐ and interpopulation variation in this trait among adult males and its importance for survivorship in this species. In this study, we used a dichotomous‐choice test to quantify the mating preferences of female guppies, originating from a low‐predation population in Trinidad, for two male traits, body length and area of the body covered with orange and black pigmentation, independently of each other. We expected strong female mating preferences for both male body length and coloration in this population, given relaxation from predation and presumably relatively low cost of choice. Females indeed exhibited a strong preference for larger males as expected, but surprisingly a weaker (but nonetheless significant) preference for orange and black coloration. Interestingly, larger females demonstrated stronger preferences for larger males than did smaller females, which could potentially lead to size‐assortative mating in nature.     

Quantitative genetic models of sexual selection by male choice

There are many examples of male mate choice for female traits that tend to be associated with high fertility. I develop quantitative genetic models of a female trait and a male preference to show when such a male preference can evolve. I find that a disagreement between the fertility maximum and the viability maximum of the female trait is necessary for directional male preference (preference for extreme female trait values) to evolve. Moreover, when there is a shortage of available male partners or variance in male nongenetic quality, strong male preference can evolve. Furthermore, I also show that males evolve to exhibit a stronger preference for females that are more feminine (less resemblance to males) than the average female when there is a sexual dimorphism caused by fertility selection which acts only on females.     

Female disdain for swords in a swordtail fish

Studies of mate choice evolution tend to focus on how female mating preferences are acquired and how they select for greater elaboration of male traits. By contrast, far less is known about how female preferences might be lost or reversed. In swordtail fish Xiphophorus, female preference for the sword ornament is an ancestral trait. Xiphophorus birchmanni, however, is one species that has secondarily lost the sword. Using synthetic animation playback of "virtual" males, we found that female X. birchmanni preferred a swordless conspecific over a sworded heterospecific. Moreover, when offered the choice between a conspecific without a sword and one with a digitally attached sword, females preferred the former. These results suggest female preferences need not always select for elaboration of male traits, and they provide a plausible explanation for the lack of introgression of a sexual trait in a naturally occurring hybrid zone.     

Ejaculate investment and attractiveness in the stalk‐eyed fly,Diasemopsis meigenii

The phenotype‐linked fertility hypothesis proposes that male fertility is advertised via phenotypic signals, explaining female preference for highly sexually ornamented males. An alternative view is that highly attractive males constrain their ejaculate allocation per mating so as to participate in a greater number of matings. Males are also expected to bias their ejaculate allocation to the most fecund females. We test these hypotheses in the African stalk‐eyed fly, Diasemopsis meigenii. We ask how male ejaculate allocation strategy is influenced by male eyespan and female size. Despite large eyespan males having larger internal reproductive organs, we found no association between male eyespan and spermatophore size or sperm number, lending no support to the phenotype‐linked fertility hypothesis. However, males mated for longer and transferred more sperm to large females. As female size was positively correlated with fecundity, this suggests that males gain a selective advantage by investing more in large females. Given these findings, we consider how female mate preference for large male eyespan can be adaptive despite the lack of obvious direct benefits.     

The evolution of female mating preferences: differentiation from species with promiscuous males can promote speciation

Females of many species are frequently courted by promiscuous males of their own and other closely related species. Such mating interactions may impose strong selection on female mating preferences to favor trait values in conspecific males that allow females to discriminate them from their heterospecific rivals. We explore the consequences of such selection in models of the evolution of female mating preferences when females must interact with heterospecific males from which they are completely postreproductively isolated. Specifically, we allow the values of both the most preferred male trait and the tolerance of females for males that deviate from this most preferred trait to evolve. Also, we consider situations in which females base their mating decisions on multiple male traits and must interact with males of multiple species. Females will rapidly differentiate in preference when they sometimes mistake heterospecific males for suitable mates, and the differentiation of female preference will select for conspecific male traits to differentiate as well. In most circumstances, this differentiation continues indefinitely, but slows substantially once females are differentiated enough to make mistakes rare. Populations of females with broader preference functions (i.e., broader tolerance for males with trait values that deviate from females' most preferred values) will evolve further to differentiate if the shape of the function cannot evolve. Also, the magnitude of separation that evolves is larger and achieved faster when conspecific males have lower relative abundance. The direction of differentiation is also very sensitive to initial conditions if females base their mate choices on multiple male traits. We discuss how these selection pressures on female mate choice may lead to speciation by generating differentiation among populations of a progenitor species that experiences different assemblages of heterospecifics. Opportunities for differentiation increase as the number of traits involved in mate choice increase and as the number of species involved increases. We suggest that this mode of speciation may have been particularly prevalent in response to the cycles of climatic change throughout the Quaternary that forced the assembly and disassembly of entire communities on a continentwide basis.     

Variation between and within the sexes in body size preferences

Although it is often assumed that males and females have mating preferences for larger individuals of the other sex, potential underlying differences between male and female preferences for body size are not commonly investigated. Here, sexual differences in body size preferences are examined in the poeciliid fish, Brachyrhaphis rhabdophora. Females preferred larger males to smaller males, but preference did not appear to be affected by female size. One population-level analysis for males did not indicate an overall preference for larger females. A closer examination, however, revealed an effect of male size on preference; larger males preferred larger females, while smaller males preferred smaller females. It appears then that females, regardless of size, share a preference for large males, but males differ in their behaviour, depending on their body size. In addition, while the degree of difference in size between paired females did not appear to affect male preference, the degree of difference in size between paired males strongly affected female preference; the greater the difference, the more strongly females preferred the larger male. Thus, intersexual selection is found to operate in both sexes, but how it operates appears to differ. Intrasexual and intersexual differences in mating behaviour may be missed when evaluating population-wide preferences. That is, there can be underlying differences in how the sexes respond and the consequences of such differences should be considered when investigating mate choice. The results are considered in terms of the evolution of mating preferences, alternative mating strategies, assortative mating, the maintenance of trait variation in a population, and current methods to evaluate mating preferences.     

Repeatability of Male Mate Choice in the Mosquitofish, Gambusia holbrooki

Male mate choice has evolved in many species in which female fecundity increases with body size. In these species, males are thought to have been selected to favour mating with large females over smaller ones, thereby potentially increasing their reproductive success. While male mate choice is known to occur, it is less well studied than female mate choice and little is known about variation in mating preference among individual males. Here, we presented individual male eastern mosquitofish ( Gambusia holbrooki ) with paired females that differed in body size, and we quantified their mate preference on two consecutive days, allowing us to assess repeatability of preferences expressed. When males were allowed to view paired stimulus females, but not to acquire chemical or tactile cues from them, they exhibited a strong preference for large females over smaller ones. However, individual males were not consistent in the strength of their preference and repeatability was not significant. When individual males were allowed to fully interact with pairs of females, the males again exhibited a preference for large females over smaller ones, as revealed by a greater number of attempted copulations with large females than with smaller ones. In the latter social context, individual male preference was significantly repeatable. These results indicate that male eastern mosquitofish from our Florida study population possess, on average, a mating preference for larger females and that this preference is repeatable when males socially interact freely with females. The significant repeatability for mating preference, based on female body size, obtained for male mosquitofish in the current study is consistent with the presence of additive genetic variation for such preferences in our study population and thus with the opportunity for the further evolution of large body size in female mosquitofish through male mate choice.     

Assortative Pairings in Diamond Firetails (Stagonopleura guttata) are not the Result of Mutual Mate Choice for an Ornament

Positive assortative mating occurs when individuals with similar phenotypes mate more frequently with each other than is expected by chance. In species in which both the males and females are ornamented, assortative pairings could arise from mutual mate choice on the same trait. We test this mechanism of mate choice and assortative pairing in the Diamond Firetail (Stagonopleura guttata), an Australian estrildid finch in which both sexes are ornamented with red bills, red rumps and white flank spots. We have previously shown sex differences in the degree of ornamentation as females have more flank spots than males. These white flank spots are used during sexual display, being fully displayed by courting males and by females when approaching a displaying male. Here, we experimentally test whether mutual mate preference is based on the number of flank spots. There was no evidence for a direct mutual preference for spot number. Given a choice of potential mates with a natural or experimentally manipulated number of flank spots, males preferred females with more spots, while female preference was not solely based on flank spots. Intriguingly, in both wild and captive Diamond Firetails, we found the number of flank spots in pairs was correlated suggesting a basis for positive assortative pairing. Nevertheless, we conclude that assortative pairing in Diamond Firetails is not due to mutual choice of mates based on the number of flank spots. We discuss different selection pathways for this trait in each sex.     

Preference for conspecifics evolves earlier in males than females in a sexually dimorphic radiation of fishes

Speciation by sexual selection is generally modeled as the coevolution of female preferences and elaborate male ornaments leading to behavioral (sexual) reproductive isolation. One prediction of these models is that female preference for conspecific males should evolve earlier than male preference for conspecific females in sexually dimorphic species with male ornaments. We tested that prediction in darters, a diverse group of freshwater fishes with sexually dimorphic ornamentation. Focusing on the earliest stages of divergence, we tested preference for conspecific mates in males and females of seven closely related species pairs. Contrary to expectation, male preference for conspecific females was significantly greater than female preference for conspecific males. Males in four of the 14 species significantly preferred conspecific females; whereas, females in no species significantly preferred conspecific males. Relationships between the strength of preference for conspecifics and genetic distance revealed no difference in slope between males and females, but a significant difference in intercept, also suggesting that male preference evolves earlier than females’. Our results are consistent with other recent studies in darters and suggest that the coevolution of female preferences and male ornaments may not best explain the earliest stages of behavioral isolation in this lineage.     

Sexual harassment in live-bearing fishes (Poeciliidae): comparing courting and noncourting species

Sexual harassment by males has been reported from several live-bearingfishes (Poeciliidae) and has been shown to inflict costs onfemales. For example, poeciliid females have reduced feedingopportunities when accompanied by a male because females dedicateattention to avoiding male copulation attempts. Poeciliid speciesdiffer considerably in male mating behavior, such as the presenceor absence of courtship. Courting males display in front ofthe females, but males attempting to sneak-copulate approachfemales from behind, that is, in the blind portion of theirvisual field, and force copulations, which can be viewed asa male persistence trait. We predicted that poeciliid femalesneed to be more vigilant in the presence of noncourting males,and costs of harassment by noncourting males might be stronger.In a comparative approach we examined the costs of male sexualharassment for females as reduced feeding time in 9 speciesof live-bearing fishes, including courting (Poecilia latipinna,Poecilia reticulata, Xiphophorus cortezi, Xiphophorus variatus)and noncourting species (Poecilia mexicana [surface- and cave-dwellingform], Poecilia orri, Gambusia affinis, Gambusia geiseri, Heterandriaformosa). In all species examined except for the cave form ofP. mexicana, focal females spent significantly less time feedingin the presence of a male than when together with another female.The time females spent feeding was found to significantly declinewith increasing male mating activity (sum of all sexual behaviors),but there was no support for the idea that females would spendmore time feeding in the presence of courting males comparedwith noncourting ones.     

Female choice linked to male dorsal fin height in a shortfin molly

Sexual selection is a possible mechanism of speciation. This could be true even in systems where female mate choice has not been clearly observed, because pre-existing biases may be expressed if female decision-making results in male trait evolution. In some mollies, males have enlarged dorsal fins and courtship display is the prevailing mating process. In others, male dominance is thought to play a greater role. We tested females of a species in the latter group, Poecilia mexicana, for consistent preference related to dorsal fin morphology. We found that females were biased toward larger dorsal fins. This latent preference could be an important driver in trait evolution.     

Does the expression of a male plumage trait in female Kentish plovers (Charadrius alexandrinus) signal individual quality?

A small percentage (4.5%) of females Kentish plovers (Charadrius alexandrinus) observed breeding in southern Spain expressed a plumage trait characteristic of males: the forecrown (frontal) black bar. The expression of this male trait may be age-related, and reversible, i.e., within individuals it may be present in some years but not in other years. Females with frontal bars were not in better condition and nor did they initiate breeding earlier than in years when they lacked frontal bars. Nevertheless, when females had frontal bars they laid larger eggs. I suggest that the frontal bar on females could be an epiphenomenon resulting from hormonal imbalances of females as they age.