Quantitative genetics of immune function and body size in the house cricket,Acheta domesticus

Female house crickets are attracted to male calling song containing a relatively high number of syllables per ‘chirp’, which tends to be produced by large males. In a previous study, we showed that this song characteristic is also positively and independently correlated with haemocyte load, an important determinant of the ability to produce an encapsulation response in insects. Females will therefore tend to select males with high encapsulation ability (and large body size) as mates. The present study demonstrates that variation in haemocyte load and body size, together with a second parameter of immune function (the ability to encapsulate a synthetic substrate), is heritable in the same population. Moreover, all three traits are shown to be positively genetically correlated. In favouring males that produce calling song with the preferred characteristics, females should therefore also tend to produce larger offspring with a greater ability to produce an encapsulation response.     

Temporal pattern changes in the calling song of the katydidMygalopsis marki Bailey in response to conspecific song (Orthoptera: Tettigoniidae)

Males of Mygalopsis markiBailey (Tettigoniidae: Orthoptera) alter the temporal structure of their song in response to other competing males. The song of males calling in aggregations has a high variance in the number of syllables per chirp, with short intervals between each chirp. In contrast, the temporal pattern of the song of isolated males is more evenly spaced, with an increase in length of the interchirp intervals and low variance in the number of syllables per chirp. In order to simulate a calling male moving closer to a male in an aggregation, a playback technique was adopted whereby the recorded calling song of a male was presented to itself via a loudspeaker in increments of 2dB. The change in song pattern of the resident male involved a reduction in the number of syllables per chirp and an increase in the interchirp interval, with the number of chirps per second remaining constant. This reduction in the output of the song, instead of not calling as a result of an acoustic contest, may still allow males to continue calling for females.     

Sexual selection reveals a cost of pathogen resistance undetected in life-history assays

Mechanisms of resistance to pathogens and parasites are thought to be costly and thus to lead to evolutionary trade-offs between resistance and life-history traits expressed in the absence of the infective agents. On the other hand, sexually selected traits are often proposed to indicate “good genes” for resistance, which implies a positive genetic correlation between resistance and success in sexual selection. Here I show that experimental evolution of improved resistance to the intestinal pathogen Pseudomonas entomophila in Drosophila melanogaster was associated with a reduction in male sexual success. Males from four resistant populations achieved lower paternity than males from four susceptible control populations in competition with males from a competitor strain, indicating an evolutionary cost of resistance in terms of mating success and/or sperm competition. In contrast, no costs were found in larval viability, larval competitive ability and population productivity assayed under nutritional limitation; together with earlier studies this suggests that the costs of P. entomophila resistance for nonsexual fitness components are negligible. Thus, rather than indicating heritable pathogen resistance, sexually selected traits expressed in the absence of pathogens may be sensitive to costs of resistance, even if no such costs are detected in other fitness traits.     

Reliable acoustic cues for female mate preference in a katydid (Scudderia curvicauda, Orthoptera: Tettigoniidae)

The call of male Scudderia curvicauda (Orthoptera: Tettigoniidae)consists of a series of phrases, and each phrase contains syllables.Females respond to the male signal with ticks that follow malephrases after a specific period of time. Pair formation takesplace after males locate the female using her response sounds.Repeated recordings of males revealed that the average numberof syllables produced per phrase was a table, within-male parameterand that this parameter was a reliable predictor of male size(pronotum length). Thus, phrase length could be a reliable cueby which females evaluate males. We presented virgin femaleswith a sequential choice of two tape-recorded male calls thatdiffered only in the mean number of syllables produced per phrase.Two different playback tapes were used, and each female wastested on each of 5 consecutive days with the same playbacktape. Females responded more often and with a greater numberof ticks to calls containing more syllables per phrase, andthis preference was maintained throughout the testing period.Male size was a poor predictor of the size of the spermatophorefood-gift produced by the male; therefore, females are probablynot selecting males for this attribute. For one of the playbacktapes, there was a significant increase in female responsivenessover several playback trials, suggesting that females may employa falling-threshold tactic with respect to mate preference.     

Condition‐Dependent Choosiness For Highly Attractive Songs In Female Canaries

Partner choice in animals has evolved under the effects of intra‐ and inter‐sexual selection. Even if a consensus between females is often observed towards some males, certain females can drastically change their mating decision. Different conditions such as early learning, mate‐choice copying, prior experience or subtle intrinsic factors can interfere with the decision. One of them is body condition of the females at the time of choosing. In this experiment, the flight feathers of female canaries Serinus canaria were cut, thus decreasing their flight ability. After a short period in a flight room, two types of artificial songs containing different tempi of the test ‘A’ phrase (16 vs. 8 syllables per second) were played to these females. It is known that only A phrases with a high tempo provoke a high level of sexual response in females. Female preference was evaluated by counting the number of Copulation Solicitation Displays (CSDs). The results show that, in both (normal and physically reduced) conditions, females prefer songs containing an A16 phrase. However, the strength of this preference is lower in females with compromised body condition than in females from the control group, suggesting a decreased choosiness in the poor body condition females.     

PROXIMATE MECHANISMS OF SEXUAL SELECTION IN WOOD FROGS

Observations and several types of field experiments on the mating behavior of wood frogs have revealed the proximate mechanisms for a size-related reproductive advantage in both males and females. For females, larger individuals produce larger clutches; for males, larger individuals can better remain clasped to females when contested by rival males and can better depose males clasped to other females. No results obtained support of the existence of mate choice in either males or females. Males were estimated to be 4.74 times as variable as females in the number of zygotes produced per individual per season; however, much of the variation in male RS resulted from a male-biased sex ratio at the breeding site rather than from sexual selection. After taking sex ratio effects into consideration, males were estimated to be only 1.63 times as variable as females. Patterns of variation in RS in males and females are associated with numerous sex-specific differences in life history and morphology. Life history differences include differential growth rates, ages at sexual maturity, and rates of mortality. Interpretation of how the body size dimorphism (females larger than males) in this species relates to sexual selection is consistent with information on how similar variations in body size influence RS for each sex, and how males and females differ in the functional relationship between body size and RS. Average RS increases more with body size in females than in males. Although body size directly influences RS for females, the possibility exists that, for males, other anatomical features correlated with body size more directly affect RS. Preliminary evidence suggests that sexual selection influences male arm length and that the male body size : RS relationship results as an incidental correlation.     

The relative importance of different direct benefits in the mate choices of a field cricket

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.     

Female life span and fertility are increased by the ejaculates of preferred males

In animals with internal fertilization, sperm competition among males can favor the evolution of male ejaculate traits that are detrimental to females. Female mating preferences, in contrast, often favor traits in males that are beneficial to females, yet little is known about the effect of these preferences on the evolution of male ejaculates. A necessary condition for female preferences to affect the evolution of male ejaculate characteristics is that females select mates based on a trait correlated with ejaculate quality. Previous work has shown that females of the variable field cricket, Gryllus lineaticeps, prefer males that produce calling songs containing faster and longer chirps. In this study, we tested the hypothesis that females receive more beneficial ejaculates from preferred males. Females were placed on either a high- or a reduced-nutrition diet then mated twice to a male of known song phenotype. Females received only sperm and seminal fluid from males during these matings. There was no effect of male song phenotype on any fitness component for females on the high-nutrition diet. Reduced-nutrition females mated to males that produced preferred song types, however, lived longer, produced more eggs, produced more fertile eggs, and had a higher proportion of their eggs fertilized than those mated to other males. The life-span benefit was positively associated with male chirp duration, and the reproductive benefits were positively associated with male chirp rate. We explored two possible mechanisms for the life span and reproductive benefits. First, a path analysis suggested that part of the effect of male chirp duration on female life span may have been indirect; females mated to males that produced longer chirps showed delayed oviposition, and females that delayed oviposition lived longer. Males that produce longer chirps may thus transfer fewer or less potent oviposition stimulants to females in their seminal fluid. Second, there was a positive correlation between male chirp rate and the number of sperm transferred to females. The fertility benefit may thus have resulted from females receiving more sperm from males that produce faster chirps. Finally, there was a negative phenotypic correlation between male chirp rate and chirp duration, suggesting that females may have to trade off the life span and reproduction benefits when selecting a mate.     

Signalling plasticity and energy saving in a tropical bushcricket

Males of the tropical bushcricket Mecopoda elongata synchronize their acoustic advertisement signals (chirps) in interactions with other males. However, synchrony is not perfect and distinct leader and follower roles are often maintained. In entrainment experiments in which conspecific signals were presented at various rates, chirps displayed as follower showed notable signal plasticity. Follower chirps were shortened by reducing the number and duration of syllables, especially those of low and medium amplitude. The degree of shortening depended on the time delay between leader and follower signals and the sound level of the entraining stimulus. The same signal plasticity was evident in male duets, with the effect that the last syllables of highest amplitude overlapped more strongly. Respiratory measurements showed that solo singing males producing higher chirp rates suffered from higher metabolic costs compared to males singing at lower rates. In contrast, respiratory rate was rather constant during a synchronous entrainment to a conspecific signal repeated at various rates. This allowed males to maintain a steady duty cycle, associated with a constant metabolic rate. Results are discussed with respect to the preference for leader signals in females and the possible benefits males may gain by overlapping their follower signals in a chorus.     

The Effect of Age on a Sexually Selected Acoustic Display

Models of sexual selection suggest that females should prefer to mate with older males because old age is evidence of heritable high viability. In a longitudinal analysis, we demonstrate that male field crickets (Gryllus campestris) alter their calling song with age. Carrier frequency, a calling song character related to growing condition and the main song component under female preference, changed towards higher sexual attractiveness with age. Body mass decreased slightly with age, while chirp rate, an indicator of current condition, remained stable. By choosing males singing at a low frequency, female field crickets would base their mate choice decision on a sexual trait that indicates superior growing conditions as juvenile and on viability, i.e. enhanced current condition as adult.     

Immune system activation affects male sexual signal and reproductive potential in crickets

Parasite-mediated sexual selection theory posits that individuals(usually females) choose mates by assessing the expression ofcostly secondary sexual signals, which provide reliable indicationsof parasite resistance. If these signals are indeed reliable,then immune-compromised males are predicted to exhibit changesin the sexual signal that are discernable by the female. Moreover,the mating pair is predicted to exhibit some reduction in reproductivefitness if the male is immune compromised. Here, we addressedthese predictions in the ground cricket, Allonemobius socius,by injecting juvenile males with lipopolysaccarides, which allowedus to activate the immune system without the introduction ofa metabolically active pathogen. As a consequence, we were ableto disentangle the cost of immune system activation from thecost of infection. We found that immune activation had a long-termeffect on male calling song and the males' ability to providepaternal resources, which can constrain male and female reproductivepotential. We also found that song interpulse interval variedsignificantly with the male's immune treatment and may thereforeprovide choosy females with a way to avoid mating with immune-compromisedmales. In short, our data support the parasite-mediated theoryof sexual selection, suggesting that female's gain direct benefitsby mating with males who are immune competent.     

Variance in female responses to the fine structure of male song in the field cricket, Gryllus integer

Although female mating preferences are a focus of current controversy,little detailed information exists on female preferences withinnatural populations. In the field cricket Gryllus integer, malecalls attract sexually receptive females, and females preferentiallymove toward male calls with longer calling bouts (periods ofcalling containing no pause greater than 0.10 s in real time).This study investigated female preferences for other variablesof the male song, including syllable period, chirp pause, andnumber of syllables per chirp. Male song was measured in thefield to determine mean values for each variable in nature.Female preferences were determined using a locomotor-compensatordevice, on which females ran in response to sequential playbacksof synthesized male song. Mean female preferences correspondedroughly to mean male song variables. Nonetheless, females variedgreatly in their responses to synthesized calls differing insyllable period, syllable number, and chirp pause. Moreover,individual females who were more selective for any one variablealso tended to be more selective for others. These results showthat females may differ from one another in their mating preferencesand degrees of selectivity, even within a single population.     

Female resistance to sexual coercion can evolve to preserve the indirect benefits of mate choice

Sexual conflict over the indirect benefits of mate choice may arise when traits in one sex limit the ability of the other sex to freely choose mates but when these coercive traits are not necessarily directly harmful (i.e. forced fertilization per se). Although we might hypothesize that females can evolve resistance in order to retain the indirect, genetic benefits (reflected in offspring attractiveness) of mating with attractive males, up to now it has been difficult to evaluate potential underlying mechanisms. Traditional theoretical approaches do not usually conceptually distinguish between female preference for male mating display and female resistance to forced fertilization, yet sexual conflict over indirect benefits implies the simultaneous action of all of these traits. Here, we present an integrative theoretical framework that draws together concepts from both sexual selection and sexual conflict traditions, allowing for the simultaneous coevolution of displays and preferences, and of coercion and resistance. We demonstrate that it is possible for resistance to coercion to evolve in the absence of direct costs of mating to preserve the indirect benefits of mate choice. We find that resistance traits that improve the efficacy of female mating preference can evolve as long as females are able to attain some indirect benefits of mating with attractive males, even when both attractive and unattractive males can coerce. These results reveal new evolutionary outcomes that were not predicted by prior theories of indirect benefits or sexual conflict.     

Sexual conflict and environmental change: trade-offs within and between the sexes during the evolution of desiccation resistance

Intralocus sexual conflict occurs when males and females experience sex-specific selection on a shared genome. With several notable exceptions, intralocus sexual conflict has been investigated in constant environments to which the study organisms have had an opportunity to adapt. However, a change in the environment can result in differential or even opposing selection pressures on males and females, creating sexual conflict. We used experimental evolution to explore the interaction between intralocus sexual conflict, sexual dimorphism and environmental variation in Drosophila melanogaster. Six populations were selected for adult desiccation resistance (D), with six matched control populations maintained in parallel (C). After 46 generations, the D populations had increased in survival time under arid conditions by 68% and in body weight by 20% compared to the C populations. The increase in size was the result of both extended development and faster growth rate of D juveniles. Adaptation to the stress came at a cost in terms of preadult viability and female fecundity. Because males are innately less tolerant of desiccation stress, very few D males survived desiccation-selection; while potentially a windfall for survivors, these conditions mean that most males’ fitness was determined posthumously. We conjectured that selection for early maturation and mating in males was in conflict with selection for survival and later reproduction in females. Consistent with this prediction, the sexes showed different patterns of age-specific desiccation resistance and resource acquisition, and there was a trend towards increasingly female-biased sexual size dimorphism. However, levels of desiccation resistance were unaffected, with D males and females increasing in parallel. Either there is a strong positive genetic correlation between the sexes that limits independent evolution of desiccation resistance, or fitness pay-offs from the strategy of riding out the stress bout are great enough to sustain concordant selection on the two sexes. We discuss the forces that mould fitness in males under a regimen where trade-offs between survival and reproduction may be considerable.     

Calling is an honest indicator of paternal genetic quality in poison frogs

Several competing hypotheses have been put forward to explain why females of many species mate preferentially with males possessing the most conspicuous signals (e.g., ornaments, displays, or songs). We performed a laboratory experiment using two species of poison frogs, Dendrobates leucomelas and Epipedobates tricolor, to test the hypothesis that male calling performance is an honest indicator of parental quality. Our analyses are based on data from behavioral observations of mating activities of captive-reared individuals (and their offspring) that were housed in terraria for four consecutive breeding seasons. Male mating success increased with male calling rate and chirp duration in both species, suggesting that females preferred males with more elaborate calls. Because calling performance improved with age in D. leucomelas, female poison frogs that prefer males with more elaborate calls in the wild may end up mating with older males that have already proven their ability to survive. Females that mated with good callers obtained higher quality offspring. Eggs fertilized by males with high calling rates and long chirp durations had higher hatching success and produced tadpoles that were more likely to metamorphose into surviving frogs. As a consequence, females that mated with males with high calling performance obtained more surviving offspring per egg, compared to females that mated with poor callers. Collectively, our findings comply with the notion that female poison frogs prefer to mate with good callers because calling performance is a reliable predictor of offspring quality. The possible influence of maternal allocation and reasons for the strong effect size compared to previous studies are discussed.     

Sexual antagonism for resistance and tolerance to infection in Drosophila melanogaster

A critical task in evolutionary genetics is to explain the persistence of heritable variation in fitness-related traits such as immunity. Ecological factors can maintain genetic variation in immunity, but less is known about the role of other factors, such as antagonistic pleiotropy, on immunity. Sexually dimorphic immunity—with females often being more immune-competent—may maintain variation in immunity in dioecious populations. Most eco-immunological studies assess host resistance to parasites rather than the host''s ability to maintain fitness during infection (tolerance). Distinguishing between resistance and tolerance is important as they are thought to have markedly different evolutionary and epidemiological outcomes. Few studies have investigated tolerance in animals, and the extent of sexual dimorphism in tolerance is unknown. Using males and females from 50 Drosophila melanogaster genotypes, we investigated possible sources of genetic variation for immunity by assessing both resistance and tolerance to the common bacterial pathogen Pseudomonas aeruginosa. We found evidence of sexual dimorphism and sexual antagonism for resistance and tolerance, and a trade-off between the two traits. Our findings suggest that antagonistic pleiotropy may be a major contributor to variation in immunity, with implications for host–parasite coevolution.     

Standing genetic variance for female resistance to harm from males and its relationship to intralocus sexual conflict

Interlocus sexual conflict theory predicts that some male adaptations are harmful to their mates. Females are therefore expected to evolve resistance to this harm. Using cytogenetic cloning techniques, we tested for heritable genetic variation among females for resistance to harm from males and determined whether propensity to remate, female body size, and intralocus conflict contributes to this variation. We found low but significant heritability for female resistance, but this variation accounted for more than half of the standing genetic variation for net fitness among females. We found no association between female resistance and female body size or level of intralocus sexual conflict. Reluctance to remate was found to be an important factor contributing to the female resistance phenotype, and we found a positive selection gradient on this trait. However, we observed only a nonsignificant positive correlation between a female's resistance and her net fitness. One factor contributing to the observed nominal level of selection on female resistance was that males cause the greatest amount of harm to females with the highest intrinsic fecundity.     

Reversed sexual conflict in a promiscuous antelope

A general tenet of sexual conflict theory is that males have higher optimum mating rates than do females and therefore should be more persistent when it comes to mating. However, in promiscuous species, females might benefit from high mating rates as a result of increased conception probability with favored males, whereas favored males benefit from mating selectively because of sperm depletion. When this results in higher optimum mating rates for females than for males, there is potential for reversed sexual conflicts between persistent females and resistant males. Here I report evidence of such a reversed sexual conflict in a promiscuous antelope, the African topi. Rather than mating randomly, favored males prefer to balance mating investment equally between females as predicted by strategic sperm allocation theory. Females, however, enhance their probability of mating with favored males through aggression toward mating pairs. Supporting the idea that aggressive females thereby harass males to mate at a rate that is suboptimal from the males' perspective, males become increasingly likely to counterattack aggressive females with whom they have already mated disproportionately, and such male counterattacks are associated with refusal to mate with the aggressive females. This study points to reversed sexual conflict as a more significant evolutionary force in promiscuous mammals than previously thought; however, such conflicts probably often go unnoticed because males, in contrast to females, can avoid mating without conspicuous resistance.     

Chirp rate is independent of male condition in a synchronising bushcricket

Males of the bushcricket Mecopoda elongata synchronise their chirps with neighbouring males, but because synchrony is imperfect, one male's chirp preceeds the other by some 50-200 ms. Since a male's intrinsic chirp rate is critical for the establishment of the leader role in a duet, and females prefer the leader in a choice situation, we investigated a possible condition dependence of this male trait. In a duet leader males are usually those calling at a higher intrinsic rate; therefore, we investigated whether calling at a higher rate indicates male condition. The calling metabolism was quantified in a respirometer; the factorial slope of males calling at a high rate was three times higher compared to males calling at lower rates. Males produce on average 3.4 singing bouts/per night, and there is a significant increase in chirp periods (CPs) with successive singing bouts. Call properties were investigated throughout a male's life; chirp period increases significantly with age. Two groups of males were reared on either a low- or a high-nutrition diet, and the influence of male condition on different song parameters was investigated. CPs in two feeding regimes did not differ significantly, although males of the low-nutrition diet group were significantly affected by nutrition with respect to mortality, a delayed last moult and reduced weight as adults. We therefore conclude that solo chirp rates do not reflect phenotypic male condition properly.