William E. Wagner

Asymmetries in Mating Preferences Between Species: Female Swordtails Prefer Heterospecific Males

In male swordtails (Xiphophorus nigrensis) there are three size classes that derive from allelic variation at the pituitary locus on the Y chromosome. Progeny analysis and preference tests suggest that females prefer to mate with larger males. In the closely related X. pygmaeus, there is no allelic variation at this locus; this species consists of males similar in size only to smaller X. nigrensis males. In addition to being smaller than most X. nigrensis males, these X.pygmaeus males also lack both the swordtail and a major component of the courtship display common in most X. nigrensis males. Usually, female X. pygmaeus prefer to mate with heterospecific males rather than conspecifics, regardless of body size and the presence of a swordtail. However, the smallest X. nigrensis males lack the same courtship component as do the X. pygmaeus males, and in this comparison female X. pygmaeus show no preference. Although sexual selection, through its action on divergence of courtship displays, has been implicated as a factor leading to speciation, in this case sexual selection could lead to the congealing of gene pools between heterospecifics.

Sexual selection on alleles that determine body size in the swordtail Xiphophorus nigrensis

SummaryIn a natural population, we document changes in the frequencies of alleles influencing body size and size-correlated alternative male reproductive tactics, and we examine the possible role of sexual selection in producing these changes. Male swordtails (Xiphophorus nigrensis, Rio Choy) exhibit three body size classes (small, intermediate, and large) that primarily derive from allelic variation (s, I, L) at the Y-linked pituitary (P) locus. Some XX individuals are male. They are small and can be fathered either by XX or XY males. We compared the frequencies of Y-linked P alleles across two generations in a natural population. There was a significant decrease in the Y-s genotype relative to Y-I and Y-L genotypes. Laboratory experiments suggest that the disadvantage of Y-s resulted, at least in part, from female preference for larger courting males relative to small chasing males. All larger courting males are of the Y-I or Y-L genotypes. Although the frequency of Y-linked P alleles changed across generations, there was no change in the distribution of male body size classes. The stability of the phenotypic distribution across generations, despite changes in P allele frequencies, was maintained by the production of small XX sons by Y-I and Y-L males.

Deceptive or honest signalling of fighting ability ? A test of alternative hypotheses for the function of changes in call dominant frequency by male cricket frogs

Previous data on intrasexual competition in Blanchards cricket frog, Acris crepitans blanchardi, demonstrated that males assess the size of an opponent, an important component of fighting ability, based on the dominant frequency of an opponents calls. Males of this species, however, often lower their dominant frequency in response to the calls of larger opponents, suggesting the conditional production of deceptive signals of size. In this study, playback experiments were used to test honest and deceptive signalling hypotheses for the function of changes in call dominant frequency. Calls that decreased in dominant frequency were more effective at repelling opponents than were calls that either increased, or did not change, in dominant frequency. A decrease in call dominant frequency therefore appears to convey an advantage in contests between male cricket frogs. During the period when males changed their dominant frequency, size became significantly less predictable from call dominant frequency. Furthermore, the extent to which males changed their dominant frequency was independent of size. Changes in call dominant frequency thus do not appear to function as honest signals of size. Because the extent to which males lowered their dominant frequency was positively correlated with their probability of attacking an opponent, and because animals producing deceptive signals should not attack opponents, changes in call dominant frequency do not appear to function as deceptive signals in size. Instead, changes in call dominant frequency appear to function as signals of size-independent fighting ability. The results presented suggest that male cricket frogs signal information about their size through initial call dominant frequency, and that they signal information about other aspects of their fighting ability through changes in call dominant frequency.

Graded aggressive signals in Blanchard's cricket frog: vocal responses to opponent proximity and size

Blanchards cricket frog, Acris crepitans blanchardi, possesses a complex communication system showing extensive variation within and among individual males. Potential social determinants of variation in male calling behaviour were experimentally examined through playback experiments of synthetic advertisement calls. Specifically, the following were investigated: (1) how male calling behaviour changes in response to the presence of an intruding caller; (2) how calling behaviour varies with intruder proximity; and (3) how calling behaviour varies with absolute and relative intruder size. Aggressive calling is defined as the type of signal produced during wrestling contests over calling sites. Nearly all aspects of male calling behaviour varied continuously with playback sound pressure level, becoming progressively more aggressive as sound pressure level increased, that is, as the simulated intruder drew nearer. Male calling behaviour therefore appeared to be graded in response to intruder distance, increasing in aggressiveness with decreasing distance. Males responded more aggressively to playbacks of low frequency calls (simulating a large intruder) than to playbacks of high frequency calls (simulating a small intruder). However, vocal responses did not vary with male size or size relative to that of the simulated intruder. Vocal escalation independent of a males absolute and relative size may be a consequence of ambiguity about relative strength, or it may be a mechanism through which males attempt to conceal their relative strength from opponents.

Sexual selection in the Gulf Coast toad,Bufo valliceps: female choice based on variable characters

Abstract Sexual selection on male size and calling behaviour was studied for 2 years in one population and for 3 years in a second population of the Gulf Coast toad. A large-male mating advantage was detected only at one site during 1 year. Measures of calling behaviour were available for 3 years at one of the sites: on a nightly basis in 2 years, males with higher call rates were significantly more likely to mate than males with lower call rates. However, there was no relationship between mating success and either call duration or calling effort on a nightly bias in any year. In discrimination experiments, females preferred higher call rates and calls of longer duration, but not calls of lower frequency. Biased male mating success with respect to call rate therefore appears to have been partially a consequence of female mate choice. Call duration is positively correlated with male size, while dominant frequency is negatively correlated with male size in this species. Because females did not discriminate between high- and low-frequency calls, and because there was no bias in mating success with respect to call duration, it appears unlikely that female choice resulted in the large-male mating advantage observed in 1 year. Despite a nightly bias in mating success with respect to call rate that appears to have resulted from female mate choice, there was no detectable sexual selection on average call rate in any year. Instead, the number of nights of chorusing activity was the only detectable direct target of selection in all 3 years at one of the sites. The repeatability of call rate within males was low, ranging from 0·08 to 0·33. Simulations suggest that within-male variation in a trait can reduce the strength of sexual selection on the trait. Lack of selection on call rate, despite female choice based on call rate, may thus have been partially a consequence of within-male variation in this trait.

Secondary reduction of preference for the sword ornament in the pygmy swordtail Xiphophorus nigrensis (Pisces: Poeciliidae)

Female mating preferences can be secondarily lost for a number of reasons. We examined the preference of female pygmy swordtails, Xiphophorus nigrensis, for the sword, a conspicuous extension of the caudal fin in some males. Females failed to show a preference for conspecific males with swords when presented with live males naturally varying in sword length, with live males of manipulated sword length, and with synthetic animations of males expressing natural variation in sword length. Females showed a significant bias against swords when presented with synthetic animations bearing supernormal sword characteristics. The reduced preference for swords, relative to closely related fish, may result from an increase in the cost of choice due to predation risk, selection against mating with heterospecifics, or changes in the spatial and contrast properties of the conspecific signal. uf6d9 2002 The Association for the Study of Animal Behaviour

A negative correlation between trait and mate preference inXiphophorus pygmaeus

Variation in female mating preferences was compared with variation in male traits across populations of Xiphophorus pygmaeus. Females of this species prefer the large courting males of X. nigrensis to their own small, non-courting males. It was predicted that if large, courtingX. pygmaeus males were to occur, they would be preferred. The discovery, distribution and subsequent spread, of large X. pygmaeus males provided an unusual opportunity to analyse how sexual selection might act on a trait not currently fixed in a species. The present study showed variation between populations in female preference for large male body size that was negatively correlated with the presence of large males. This variation in preference persisted over a 5-year period. Female preference for larger body size and courtship are evolutionarily decoupled from the male traits of large size and courtship in X. pygmaeus. This pattern is not consistent with indirect selection models (e.g. runaway selection), which predict that female mating preferences and male traits evolve in concert due to a genetic correlation between preference and trait. ? 1996 The Association for the Study of Animal Behaviour

Variation in Advertisement and Release Calls, and Social Influences on Calling Behavior in the Gulf Coast Toad (Bufo valliceps)

We examined variation in calls and calling behavior in two populations of Bufo valliceps in south-central Texas. Both pulse rate of advertisement calls and call rate were positively related to body temperature, whereas call duration was negatively correlated with body temperature. Advertisement call frequency was not significantly correlated with toad temperature, but was negatively correlated with SVL. Of the temporal advertisement call variables, only call duration was significantly correlated (positively) with SVL. Release call frequency was negatively correlated with SVL. Field observations revealed significantly greater variation among than within males for call rate, call duration, and calling effort. However, despite the consistency of individual calling behavior, calling activity varied with social context. Calling effort was weakly, positively correlated with male density. Call rate and calling effort varied nonlinearly with nearest neighbor distance. Both variables were highest at intermediate distances (2.5-7.5 m), decreasing as nearest neighbors became very close ( 7.5 m).

Chorus Organization in the Gulf Coast Toad (Bufo valliceps): Male and Female Behavior and the Opportunity for Sexual Selection

We examined variation in mating system structure, male and female behavior, and the opportunity for sexual selection over multiple years in two populations of the Gulf Coast toad, Bufo valliceps, in south-central Texas. There was substantial variation in mating system structure among years and among sites. Duration of calling activity ranged between seven and 29 nights, chorus sizes ranged between two and 65 males, and male densities ranged between 0.01 and 0.52 males/m of shoreline. Overall, operational sex ratio was negatively related to both chorus size and male density. Males did not exhibit satellite behavior, and active searching behavior was rare. Females usually appeared to freely select mates, and single males were never observed attempting to displace an amplectant male. Male mating success was strongly skewed in all years at both sites; within a year >60% of males did not mate, but some males mated with as many as four females. The opportunity for selection arising from variance in male mating success varied both among years and among sites within a year. Our observations suggest that the potential for sexual selection by female choice is high, despite the somewhat explosive breeding season of this species.

Males and females evolve riskier traits in populations with eavesdropping parasitoids

Predation and/or parasitism often limits the evolution of conspicuous male traits and female preferences because conspicuous traits can attract predators or parasites and it is costly for females to associate with males that attract predators or parasites. As a result, males and females in high-risk populations are expected to evolve safer mating behaviors compared to individuals from low-risk populations. We tested this antagonistic selection hypothesis in the field cricket Gryllus lineaticeps. Males produce chirped songs, and both female crickets and the eavesdropping parasitoid fly Ormia ochracea prefer faster chirp rates. The flies attack the field crickets late in the breeding season and parasitized crickets die. We used a common garden rearing design to test for evolved differences in songs and preferences between high- and low-risk populations. In contrast to predictions of the antagonistic selection hypothesis, males from high-risk populations produced faster (riskier) chirp rates and females preferred faster chirps. We suggest that late-season parasitism selects for increased investment in reproductive traits to maximize reproduction before the advent of parasitoid activity (“late-season parasitism hypothesis”), which would at least explain riskier female preferences and potentially riskier male songs in the high-risk populations. Predation and parasitism may thus have diverse and unexpected effects on the evolution of reproductive behavior, depending upon the temporal pattern of predator- or parasite-induced mortality.Significance statementMating signals are typically conspicuous and not only attract partners but also predators and parasites. Even the silent mating partner may experience predation or parasitism by associating with the signaler. Under these circumstances, it is commonly assumed that natural and sexual selection act in opposite directions, effectively limiting the evolution of conspicuous signals and preferences. We demonstrate that an eavesdropping parasitic fly caused the evolution of preferences, and potentially songs, in a field cricket in the opposite, more conspicuous, direction than predicted by antagonistic selection. We argue that the temporal pattern of parasitism in relation to the reproductive season likely causes this unexpected evolutionary pattern. We propose the late-season parasitism hypothesis as an alternative to the antagonistic selection hypothesis, which might better explain more conspicuous mating trait values in other species that experience seasonal predation or parasitism.