Male swordtails court with an audience in mind

Females are usually considered to be the target of male courtship behaviour. In nature, however, social interactions rarely occur without other observers; thus, it is conceivable that some male courtship behaviours are directed not towards females, but rather towards male rivals. The northern swordtail, Xiphophorus birchmanni, is a freshwater fish found in high densities in natural streams. Males court by swimming close to and in parallel with the female, raising their large sail-like dorsal fin, and quivering briefly. Here, we show that females prefer males that display small dorsal fins to those with large ones, and that males are less aggressive to other males with large dorsal fins. Male swordtails also raise their dorsal fins more frequently when courting in the presence of other males. These results suggest that, despite female avoidance of large dorsal fins, males that raise their fin during courtship benefit by intimidating potential competitors; the intended receivers of this signal are thus males, not females. Intrasexual selection can therefore offset the forces of intersexual selection, even in a courtship display.     

Diet alters species recognition in juvenile toads

Whether environmental effects during juvenile development can alter the ontogeny of adult mating behaviour remains largely unexplored. We evaluated the effect of diet on the early expression of conspecific recognition in spadefoot toads, Spea bombifrons. We found that juvenile toads display phonotaxis behaviour six weeks post-metamorphosis. However, preference for conspecifics versus heterospecifics emerged later and was diet dependent. Thus, the environment can affect the early development of species recognition in a way that might alter adult behaviour. Evaluating such effects is important for understanding variation in hybridization between species and the nature of species boundaries.     

Hungry females show stronger mating preferences

Female mating decisions that are based on condition-dependenttraits, such as male nutritional state, may be associated witha female's own condition. In the swordtail fish, Xiphophorusbirchmanni, females prefer the chemical cues of well-fed malesto cues of food-deprived males. Here we show that this preferenceis significantly stronger in females when they were experimentallyfood deprived than when they were well fed. Our results suggestthat if females have limited access to food resources, and arethemselves food deprived, they will attend to cues indicatingmale nutritional condition more than when environmental conditionsallow for greater access to food. Furthermore, not only is theslope of the preference function condition dependent but also,in all trials, the latency to respond to the presented stimuliwas shorter in food-deprived females, suggesting that responsivenessto environmental cues is condition dependent as well. Undernatural conditions, females of many species likely experiencevariation in resource availability. Thus, we predict that covariancebetween the strength of female preferences and resource availabilitymay be widespread and may represent a common source of femalepreference variation within and between populations.     

Correlates of male mating success in the lekking great snipe (Gallinago media): results from a four-year study

To investigate behavioral or morphological traits importantas mate choice cues, we measured selection differentials (s)as the covariances between each trait and male mating success,and directional selection gradients (J3) from multiple linearregression of the standardized traits on male mating success.Data from two leks in four consecutive years were included,and the annual data were analyzed separately. The main findingsare: (1) the distribution of male mating success proved to beless skewed than those found in many other lekking species,(2) only a few traits yielded significant selection gradients,(3) the importance of age on male mating success changed acrossyears, (4) females may use traits with a high variance as matechoice cues, and (5) individual males achieved similar matingsuccesses between years. Attendance and age were the traitsmost consistently correlated with male mating success, but notraits showed significant selection gradients in all years.Our results indicate that variable sexual selection pressuresexisted between years, but the high correlation found betweenthe mating success of individual males in successive seasonsalso indicates that permanent differences in male traits areimportant. Key words: lek, mate choice, sexual selection.     

Mate preference for multiple cues: interplay between male and nest size in the sand goby, Pomatoschistus minutus

Female mating preferences are often based on more than one cue.In empirical studies, however, different mate choice cues aretypically treated separately ignoring their possible interactions.In the current work, we studied how male body size and sizeof the male's nest jointly affect mate preferences of femalesand gobies, Pomatoschistus minutus. The females were givena binary choice between males that differed either in body sizeor size of their nest or both. We found that neither body sizenor size of the nest alone affected male attractiveness, buttogether these 2 cues had a significant effect. Specifically,large males were more popular among females when they had alarge nest than when they occupied a small nest. The resultssuggest that if interaction effects between multiple mate choicecues are not considered, there is a danger of ignoring or underestimatingthe importance of these cues in sexual selection by female choice.     

The large-male advantage in brown antechinuses: female choice, male dominance, and delayed male death

Male-biased dimorphism in body size is usually attributed tosexual selection acting on males, through either male competitionor female choice. Brown antechinuses (Antechinus stuartii) aresexually dimorphic in size, and heavier males are known to siremore offspring in the wild. We investigated four possible mechanismsthat might explain this large-male reproductive advantage. Wetested if there is a female preference for large males, a femalepreference for dominant males, if larger males compete moreeffectively for mates, and if there is a survival advantagefor large males during the mating season. We established nestinggroups of males in captivity and conducted mate choice trialsin which males from nesting groups either could or could notinteract. We assessed male dominance rank and recorded survivaltimes after mating. Females did not prefer larger males directly.The results suggest that the other three mechanisms of sexualselection tested account for the large-male advantage: largemales competed more successfully for mates, so were sociallydominant; females rejected subordinates (males they saw losingtwice in contests to previous mates); and dominant males survivedfor longer after their first mating. Females judged male rankbased on direct observation of male competitive interactionsat the time of mating and apparently could not distinguish rankfrom male scent. Effects of size and dominance on male reproductivesuccess are not confounded by age because male antechinusesare semelparous.     

Does courtship behavior contribute to species-level reproductive isolation in field crickets?

Reproductive behavior influences gene flow within and amongspecies; thus, sexual selection may be a major contributor tothe maintenance of species, and possibly their formation. HereI experimentally manipulate the courtship interactions of thefield crickets Gryllus rubens and G. texensis to examine thepotential of close-range courtship interactions to limit interspecificgene flow. Responses of males to females and of females to malecourtship song and males per se were examined for four pairedsympatric and allopatric populations. Male G. rubens were morelikely to court conspecific females, but male G. texensis courtedfemales of both species equally. If paired with conspecificmales, female G. rubens and G. texensis both preferred conspecificcourtship song. In none of these comparisons were the responsesof males or females from allopatry different from those in sympatry.There was an asymmetry of courtship response across both sexand species: male G. rubens were more discriminating than maleG. texensis, whereas female G. texensis were more discriminatingthan female G. rubens. Despite significant preferences for conspecifics,the net effect of courtship interactions would appear to limitinterspecific gene flow only weakly. These results are consistentwith courtship behavior evolving under the influence of sexualselection and only indirectly promoting species integrity.     

Juvenile infection and male display: testing the bright male hypothesis across individual life histories

Current tests of the bright male hypothesis focus on assaysof adult disease resistance and their relation to male traitdevelopment and female choice. We suggest that if parasiteshave significant harmful effects on juvenile stages of a host,then females selecting males that effectively signal juvenileparasite resistance may gain a significant "good genes" benefit.Currently, there is no information on juvenile and adult infectionor resistance in the same male and whether adult male displayssignal juvenile parasite resistance. In the present study, wemeasure infection of the ectoparasitic louse, Myrsidea ptilonorhynchi,in individual male satin bowerbirds (Ptilonorhynchus violaceus)both as juveniles and nine or more years later as adults. Wetest hypotheses that examine the role of juvenile parasite infectionin mediating sexual selection. We found that (1) juvenile infectionis higher than adult infection in the same individuals, (2)adult males able to hold display sites have lower juvenile infection,and (3) juvenile and adult infection in the same individualsare not significantly correlated. In addition, comparisons amonga larger set of individuals from a single year show that bloodand ectoparasite infections are highly correlated, and bothdecrease with male age and are inversely related to male courtshipsuccess. These results, combined with the evidence that femalesmate exclusively with bower-holding males support the hypothesisthat females use adult male display traits to identify maleswith a high level of juvenile disease resistance. We suggestthat effective tests of the bright male hypothesis should include(1) assessment of infection resistance in both subadult andadult life history stages, (2) tests of whether differencesin age-specific resistance are indicated in adult male displays,and (3) tests to determine if females attend to these traitsin mate choice. Although these requirements increase the difficultyof testing the bright male hypothesis, they are necessary fora more accurate assessment of the effects of parasites on maledisplay and female choice.     

Mechanisms of incomplete prezygotic reproductive isolation in an intertidal snail: testing behavioural models in wild populations

Two morphs (ecotypes) of the marine snail Littorina saxatilis coexist along Galician exposed rocky shores. They hybridize, but gene flow is impeded by a partial prezygotic reproductive barrier, and we have earlier suggested that this is a case of incipient sympatric speciation. To assess the mechanisms of prezygotic reproductive isolation, we estimated deviations from random mating (sexual selection and sexual isolation) of sympatric snails in 13 localities on the shore, and performed mate choice experiments in the laboratory. We also investigated the microdistribution of both morphs over patches of barnacles and blue mussels in the hybridization zone. We used computer simulations to separate the mechanisms contributing to reproductive isolation. On the shores sampled, male–female pairs were strongly assortative both with respect to morphs (mean Yule's V = 0.77) and size (mean Pearson's r = 0.47). In the laboratory, males of both morphs mounted other snails and mated other males and juveniles at random. However, mature females of equal sizes mated assortatively with respect to morph. The two morphs were nonrandomly distributed over barnacle and mussel patches in the hybridization zone. Monte Carlo simulations showed that this microdistribution could explain about half the morph and size relationships in male–female pairs, while a simple rejection mechanism, rejecting the first 1–3 mates if they were of contrasting morphs, accounted for the remaining part of the reproductive isolation, and for parts of the size relationships found between mates. A size discriminant mate choice mechanism may also, to a lesser extent, contribute to the sexual isolation. Sexual selection was observed for female size (larger ones being favoured) and among certain morphs, but distinct biological mechanisms may cause these processes.     

Direct mate choice maintains diversity among sympatric cichlids in Lake Victoria

Mate choice may play an important role in animal speciation. The haplochromine cichlids of Lake Victoria are suitable to test this hypothesis. Diversity in ecology, coloration and anatomy evolved in these fish faster than postzygotic barriers to gene flow, and little is known about how this diversity is maintained. It was tested whether recognizable forms are selection-maintained morphs or reproductively isolated species by investigating in the field reproductive timing, location of spawning sites, and mate choice behaviour. There was a large interspecific overlap in timing of breeding and location of spawning sites, which was largest in members of the same genus. Behavioural mate choice of such closely related taxa was highly assortative, such that it is likely that they are sexually isolated species and that direct mate choice is the major force that directs gene flow and maintains form diversity. The results differ from what is known about recent radiations of other lacustrine fish groups where speciation seems to be driven by diverging microhabitat preferences or diverging timing of reproduction, but are in agreement with predictions from models of speciation by diverging mate preferences.     

Sequential mate encounters: female but not male body size influences female remating behavior

Whether and how individuals choose sequentially among matesis an important but largely neglected aspect in sexual selectionstudies. Here, we explore female remating behavior in the cellarspider Pholcus phalangioides. We focus on body size as one ofthe most important traits involved in mate choice. Large andsmall females (n = 216) were double mated with large or smallmales in all eight possible combinations. All females copulatedwhen virgin, but only 82% accepted a second male. The chanceof a female remating was not significantly predicted by thebody size of the second or first male or by the size differencebetween the two. In contrast, a previous study demonstrateda male size effect in that larger males monopolized femalesuntil egg laying when two males of different sizes were present.We suggest that sequential encounters are more common undernatural conditions than male monopolization of females becauseestimates of concurrent multiple paternity together with observationsin a natural population do not favor mate guarding as the predominantmating strategy in this species. It follows from our study thatthe intensity of sexual selection on male size may be greatlyoverestimated when using a competitive laboratory setting fora species in which females generally encounter mates in a sequentialfashion. Female remating probability was significantly predictedby female size, with large females remating with higher probabilitythan small females. Thus, when mating with large females, malesmay gain higher fertilization success through increased femalefecundity but also face a higher sperm competition risk.     

Character displacement and the evolution of mate choice: an artificial neural network approach

Interactions with heterospecifics can promote the evolution of divergent mating behaviours between populations that do and do not occur with heterospecifics. This process--reproductive character displacement--potentially results from selection to minimize the risk of mating with heterospecifics. We sought to determine whether heterospecific interactions lead to divergence of female preferences for aspects of conspecific male signals. We used artificial neural network models to simulate a mate recognition system in which females co-occur with different heterospecifics in different populations. Populations that evolved conspecific recognition in the presence of different heterospecifics varied in their preferences for aspects of conspecific male signals. When we tested networks for their preferences of conspecific versus heterospecific signals, however, we found that networks from allopatric populations were usually able to select against heterospecifics. We suggest that female preferences for aspects of conspecific male signals can result in a concomitant reduction in the likelihood that females will mate with heterospecifics. Consequently, even females in allopatry may discriminate against heterospecific mates depending on the nature of their preferences for conspecifics. Such a pattern could potentially explain cases where reproductive character displacement is expected, but not observed.