Alexandra L. Basolo

Sensory ecology, receiver biases and sexual selection

During courtship, signals are sent between the sexes, and received signals contain information that forms the basis of decision making. Much is known about signal content, but less is known about signal design-what makes signals work efficiently? A consideration of design not only gives new insights into the evolution of signals (including novelty), but also allows the development of specific and testable predictions about the direction of evolution. Recently there has been increased interest in signal design, but this has resulted in some apparently divergent views in the literature.

Phylogenetic Evidence for the Role of a Pre-Existing Bias in Sexual Selection

Females of the genus Xiphophorus, which includes unsworded platyfish and sworded swordtails, share a mating preference which favours a sword despite phylogenetic evidence that the sword was not present in the evolutionary history of platyfish. A recent molecular phylogeny, however, proposes that the platyfish arose from within the swordtails. If this is the case, the preference for a sword in platyfish may be a retained ancestral preference rather than a bias that evolved before the first appearance of the sword. To determine whether or not the preference favouring a sword is an ancestral bias present before the evolution of the sword, I tested sword preferences in the sister genus, Priapella, which lacks a sword: female P. olmecae were found to prefer conspecific males with artificial swords to those without swords. These results suggest that a pre-existing bias favouring a sword arose before the divergence of these two genera, and thus before the appearance of a sword. In addition, the strength of the preference exhibited by P. olmecae females for a sword was found to vary with sword length; as the length of the sword was increased, the strength of the preference increased. Female P. olmecae, therefore, prefer males with longer swords to males with shorter swords. This increasing preference with sword length is similar to the preference of green swordtails, suggesting that the preference has a common basis in the two groups. More generally, this work further establishes the pre-existing bias model as a viable explanation for the evolution of female preferences and male traits.

A further examination of a pre-existing bias favouring a sword in the genus Xiphophorus

Models of sexual selection have recently been broadened to include the pre-existing bias model. This model suggests that female preferences that result in the evolution of male traits can be established prior to the appearance of a male trait. Previous work suggests that in the genus Xiphophorus, which consists of swordless platyfish and sworded swordtails, female southern platyfish, X. maculatus, prefer conspecific males with surgically attached swords, despite phylogenetic evidence that the common ancestor of platyfish and swordtails was swordless. In the present study, female X. maculatuswere found to prefer conspecific males with a naturally occurring component of the sword, an abbreviated lower caudal stripe. To investigate the possibility that the previously suggested pre-existing bias favouring a sword was merely an extension of a preference for this stripe specific to X. maculatus, whether female variable platyfish, X. variatus, also demonstrate a preference for males with swords was tested; this species has neither a sword nor any component of the sword. Female X. variatuswere found to prefer conspecific males with swords to those without. This work suggests that X. variatus and X. maculatus share a preference favouring a sword, despite the lack of a sword in these species. ? 1995 The Association for the Study of Animal Behaviour

Shift in investment between sexually selected traits: tarnishing of the silver spoon

Studies of resource allocation strategies have concentrated on the influence of natural selection on the evolution of life history traits. To a lesser degree, the effects of trade-offs between natural and sexual selection on the evolution of allocation strategies have also been considered. Trade-offs between sexually selected traits that are important to females but that appear to differ in cost, however, have not been considered. Female green swordtails, Xiphophorus helleri, prefer males with longer swords to males with shorter swords, and in this study they demonstrated a preference for larger males to smaller males. Furthermore, sexually mature males invested differentially in body and sword growth depending on resource availability; males that had an unlimited amount of food invested in both body and sword growth, but males shifted to a food-restricted regime halted investment in body growth and invested only in sword growth. These results suggest that males shift their pattern of investment in two sexually selected traits when food becomes restricted. In general, variable environmental conditions may favour such conditional investment strategies in species in which there is more than one preferred male trait and the costs of the traits differ. Copyright 1998 The Association for the Study of Animal Behaviour Copyright 1998 The Association for the Study of Animal Behaviour.

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.

An evaluation of video playback using Xiphophorus helleri

Video playback is being increasingly used as a technique for behavioural research. The importance of critically evaluating the effectiveness of video playback is clear, as available video technology is not designed for nonhuman visual systems. We discuss several aspects concerning the perception of video images that could lead to inconclusive or erroneous results. Researchers should verify that behaviour observed in response to video playback is comparable to behaviour observed in response to live animals. We conducted such a verification using live and video playback methods to measure female response to swords of varying lengths in the green swordtail, Xiphophorus helleri. Using both methods, female response appeared to be an increasing function of male sword length. Females did not differ in their response to live and video versions of noncourting, noninteractive males, however, females tended to prefer video playbacks of males with longer swords, a result that has also been found in experiments using live males. These results suggest that females express the same qualitative mating preference, but not necessarily the same quantitative preference, for sword length when viewing video stimuli. Several methodological factors that may contribute to an apparent difference in the strength of the preference are discussed. Despite these differences, both methods produced comparable results; female response to sworded males tended to increase as sword length increased. These experiments demonstrate that video playback is an effective method to measure female preferences accurately in X. helleri and provide an example of how video playback can be evaluated in other species. Copyright 2000 The Association for the Study of Animal Behaviour.

Male-male competition and the sword in male swordtails, Xiphophorus helleri

Male green swordtails, Xiphophorus helleri, express a secondary sexual trait, the sword, which is favoured by female mate choice. However, the role of the sword as a signal in male–male competition has received little attention. We studied the effect of both male body size and sword length on the outcome of male–male competition. Although body size and natural sword length were positively correlated, only body size had a significant effect on competitive success. To examine the independent effect of sword length on competitive success, males were matched for body size and natural swords were replaced with plastic caudal attachments simulating swords. The attachments were of equal size but were painted to approximate swords of different lengths. When matched for body size, males with longer apparent swords won significantly more contests than males with shorter apparent swords. These data suggest that, controlling for body size, longer-sworded males experience greater competitive success. Thus, in green swordtails, it appears that both male–male competition and female choice may have played a role in the evolution of longer swords.

THE RELATIVE IMPORTANCE OF DIFFERENT DIRECT BENEFITS IN THE MATE CHOICES OF A FIELD CRICKET

Abstract Discussions about the evolution of female mating preferences have often suggested that females should express multiple strong preferences when different male traits are correlated with different mating benefits, yet few studies have directly tested this hypothesis by comparing the strength of female preferences for male traits known to be correlated with different benefits. In the variable field cricket, Gryllus lineaticeps, females receive fecundity and fertility benefits from mating with males with higher chirp rates and life-span benefits from mating with males with longer chirp durations. Although females prefer higher chirp rates and longer chirp durations when the other trait is held constant, it is possible that they give priority to one of these song traits when both vary. In this study, we examined the relative importance of chirp rate and chirp duration in female mate choice using single-stimulus presentations of songs that varied in both chirp rate and chirp duration. Females expressed both directional and stabilizing preferences based on chirp rate, responding most strongly to a chirp rate approximately one standard deviation above the population mean. Females did not express preferences based on chirp duration, and did not express correlational preferences. These results suggest that females may give priority to the reproductive benefits provided by males that produce higher chirp rates.

Repeatable female preferences, mating order and mating success in the poeciliid fish, Heterandria formosa

Abstract. Sexual selection via female choice can afford preferred males comparably higher mating success than those males that lack preferred traits. In addition, many models of sexual selection assume that both male traits and female preferences are heritable. In this study we test whether females of the poeciliid fish, Heterandria formosa, have repeatable pre-copulatory preferences for larger males. We also test whether female pre-copulatory preferences are always reliable indicators of male mating success. When given a choice between a large and a small male, females prefer larger males, and the repeatability of this preference is high. Although there are no overall differences in male mating success between large and small males, large males have a higher mating success when they are the first to mate than when they are the second to mate. Likewise, preferred males also have higher mating success when they are the first to mate than when they mate second. Therefore, the repeatable female preferences observed in this study only predict male mating success when the preferred male mates first. These results illustrate that even significantly repeatable female preferences do not translate into male mating success, which is an assumption of many examinations of the importance of female choice in sexual selection.

The conformation of a female preference for a composite male trait in green swordtails

Female green swordtails, Xiphophorus helleri, possess a preference for the multicomponent male sword. Previous work suggests that this female preference is a result of a pre-existing receiver bias favouring a sword and has contributed to sword evolution via intersexual selection. We use video presentations to investigate how females respond to components of this composite trait. Complete swords in this species have at least four individual components. Based on female response to video stimuli depicting males with intact swords and males in which sword components were digitally manipulated, it appears that males possessing complete swords elicit the strongest female response. Additionally, we found that females prefer stimuli with black sword coloration to those lacking black coloration. The results suggest that multiple components of the sword stimulate females, and that these components operate in conjunction to make the sword structure more attractive to females.  2002 The Association for the Study of Animal Behaviour