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Facultative change in mating decisions can result from both exogenous and endogenous factors. The hermaphroditic roundworm, Caenorhabditis elegans, is a model system for many areas of biology, yet, aspects of its reproductive biology are not well understood. Here, we present data suggesting that the reproductive behaviour of hermaphrodites changes with self-sperm status. Self-sperm containing hermaphrodites are more likely to eject male ejaculate after insemination than self-sperm depleted hermaphrodites, presumably reducing the likelihood of fertilization by male sperm. Self-sperm depleted hermaphrodites are more likely to mate, and mate in a shorter period of time than self-sperm containing hermaphrodites. This increase in the willingness to mate is correlated with self-sperm depletion, but not hermaphrodite age. Hermaphrodites that have exhausted their self-sperm also show a lower rate of sprinting away from males than self-sperm containing hermaphrodites, indicating a lower level of mating resistance. The combined results suggest that self-sperm containing hermaphrodites are more likely to show precopulatory mate avoidance as well as postcopulatory mechanisms to avoid fertilization by males than sperm-depleted hermaphrodites, presumably to delay outcrossing until self-sperm is exhausted. Thus, there appears to be a shift in hermaphrodite mating behaviour based on reproductive status.

The evolution of complex signals via sexual selection is becoming a primary focus in mate choice studies. Male swordtail fish, Xiphophorus helleri, have a secondary sexual trait, the sword, which is a composite trait consisting of four components favoured by sexual selection via female mate choice. The components include three coloured stripes and size. Although a complete sword is most attractive to female green swordtails, females prefer partial swords containing complete black stripes to incomplete swords lacking black coloration. Female swordtails also prefer longer swords. We addressed how subcomponents of the sword affect female mate choice. Digitally altered videos were used to investigate whether females respond differently to swords with proximal black stripes than to swords with distal black stripes. Although females preferred complete swords to both of the stimuli with partial stripes, they preferred males with distally striped swords to males with proximally striped swords. These results are discussed in light of hypotheses concerning the evolution of mating preferences for multicomponent traits. This study contributes to the growing body of information regarding the response of females to multicomponent visual traits.

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.

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.

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

Female poeciliid fish of the sister genera Xiphophorus and Priapella share a mating preference favouring a sword, despite phylogenetic evidence that the sword arose in Xiphophorus . This conclusion is based on the presence of a preference favouring a sword in three unsworded species and one sworded species. Here I investigate the response to a sword in the poeciliid fish Heterandria bimaculata by testing females with conspecific males that differ in the presence or absence of a sword. Female H. bimaculata were found to prefer unsworded males, thus sworded males were discriminated against. I also tested the strength of female response; females did not show a differential response to males based on sword length. The preference for unsworded males could indicate that when new male traits arise, biases might oppose trait evolution, even in cases in which similar male traits in related taxa are favoured. Explanations for the female preference for unsworded males are discussed.

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.

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.

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

Swordtail fish exhibit a striking sexual dimorphism in the structure of the caudal fin; males have a colourful, elongated caudal appendage, termed a sword, that is absent in females. In laboratory tests of female preference for male sword length in the green swordtai, Xiphophorus helleri, females were given the choice between a pair of males differing naturally or experimentally in sword length to test the hypothesis that the elaboration of this trait is partially a consequence of female choice. Five tests with different pairs of males were conducted in which sword differences between paired males varied from 6 to 24 mm. Female preferences were measured by scoring the amount of time a female spent in association with each male. In all five tests, females preferred males with longer swords. Female preference was shown to be an increasing function of difference in sword length: as the difference in sword length increased, the strength of the preference increased. Characters correlated with sword length were eliminated as sources of preference by experimentally altering sword lengths; preferences were reversed when the relative sword lengths were reversed. These results suggest that sexual selection by female choice may have played a role in the evolution of the sword and that female preference is based on sword length itself, and not on a character correlated with sword length.