Why are female color polymorphisms rare in territorial damselflies?

In nonterritorial damselflies, females often come in multiple color morphs, perhaps because females with rare colors experience reduced sexual harassment, and thus have a frequency‐dependent fitness advantage, compared to females of the most common color morph, but such polymorphisms are rare in territorial species. We consider three hypotheses to explain the rarity of female color polymorphisms in territorial species: (a) misdirected male aggression, (b) poor male mate recognition, and (c) low mating harassment rates. The first hypothesis has some empirical support, and can account for the absence of andromorphs (i.e., females that resemble males), but does not explain the absence of multiple heteromorphs. We tested the second hypothesis by presenting females of two novel color morphs (green‐ or red‐banded abdomens) to territorial male Hetaerina capitalis. Females of both novel color morphs elicited fewer sexual responses than control females, and the red morph occasionally elicited aggressive responses. These results indicate that novel female color morphs would experience reduced mating harassment in this species, contradicting the hypothesis that male mate recognition is too poorly developed to reduce harassment of novel female morphs. By process of elimination, the third hypothesis, that harassment rates are too low in territorial species to provide rare female morphs a fitness advantage, is favored, but remains untested. Our findings also suggest that the common practice of color‐marking odonates for behavioral research is likely to interfere with mate choice, as has long been known to be the case in birds.     

Maintenance of a female-limited polymorphism in Ischnura ramburi (Zygoptera: Coenagrionidae)

Colour polymorphisms can be maintained in a population if all morphs have equal fitness on average, if fitness is frequency dependent or if fitness functions cross for some environmental or social variable. We studied female-limited colour polymorphism in the Rambur's forktail damselfly, Ischnura ramburi, in which one female morph looks like the male. The most commonly cited hypotheses to explain this polymorphism involve an advantage to andromorphs of avoiding costly matings through male mimicry. An alternative hypothesis argues that males learn the most common morph and that the polymorphism is maintained by a rare-morph advantage of mating avoidance, irrespective of male mimicry. We tested predictions of the male mimicry hypothesis, learned mate recognition hypothesis (LMR) and two new hypotheses. We used censuses and a mark-resight study to estimate density, sex ratio, morph frequency and mating frequencies. We observed interactions to test for male mimicry and female competition and to evaluate the frequency of mating attempts. Andromorphs were less likely than gynomorphs to receive mating attempts in encounters with males, but did not mate less frequently, or attack males or interrupt oviposition by other females more frequently. Contrary to the LMR hypothesis, the rarer morph was more likely to receive mating attempts. Andromorph frequency was greater in older females than in younger females, suggesting higher mortality or dispersal of gynomorphs. Our results support a modification of the male mimicry hypothesis, the signal detection hypothesis. Together with past studies, our results suggest that the female morphs may be alternative mating avoidance strategies.     

Reflectance spectra and mating patterns support intraspecific mimicry in the colour polymorphic damselfly <Emphasis Type="Italic">Ischnura elegans</Emphasis>

Coexistence of female colour morphs in animal populations is often considered the result of sexual conflict, where polymorphic females benefit from reduced male sexual harassment. Mate-searching males easily detect suitable partners when only one type of female is present, but become challenged when multiple female morphs coexist, which may result in frequency-dependent mate preferences. Intriguingly, in damselflies, one female morph often closely resembles the conspecific male in body coloration, which has lead to hypotheses regarding intra-specific male-mimicry. However, few studies have quantitatively evaluated the correspondence between colour reflectance spectra from males and male-like females, relying instead on qualitative visual assessments of coloration. Using colour analyses of reflectance spectra, we compared characteristics of the body coloration of ontogenetic male and female colour morphs of the damselfly Ischnura elegans. In addition, we evaluated whether males appear to (1) discriminate between immature and mature female colour morphs, and (2) whether male-like females experience reduced male mating attention and low mating frequencies as predicted from male-mimicry. Spectral reflectance data show that immature female morphs differ substantially in coloration from mature individuals. Mating frequencies were much lower for immature than mature female morphs. For the male-like female morph, measures of colour were statistically indistinguishable from that of both immature and mature conspecific males. Mating frequencies of male-like females were lower than those of other mature female morphs under field and experimental conditions. Together, our results indicate that males may use the observed spectral differences in mate choice decisions. Furthermore, male-like females may be regarded as functional mimics that have reduced attractiveness and lowered rates of sexual harassment by mate-searching males.     

Male harassment drives females to alter habitat use and leads to segregation of the sexes

Sexual conflict is ubiquitous across taxa. It often results in male harassment of females for mating opportunities that are costly for females, in some cases reducing reproductive success and increasing mortality. One strategy that females may employ to avoid sexual harassment is to segregate spatially from males. In fact, we do find sexual segregation in habitat use in species that have high levels of sexual conflict; however, the role of sexual harassment in driving such segregation remains poorly understood. Here, we demonstrate experimentally in a population of wild Trinidadian guppies Poecilia reticulata that male sexual harassment drives females into habitats that they otherwise do not prefer to occupy. In support of the social factors hypothesis for sexual segregation, which states that social factors such as harassment drive sexual segregation, this female behaviour leads to segregation of the sexes. In the presence of males, females actively select areas of high predation risk, but low male presence, and thus trade off increased predation risk against reduced sexual harassment.     

A model of sexual selection and female use of refuge in a coercive mating system

In many non-monogamous systems, males invest less in progeny than do females. This leaves males with higher potential rates of reproduction, and a likelihood of sexual conflict, including, in some systems, coercive matings. If coercive matings are costly, the best female strategy may be to avoid male interaction. We present a model that demonstrates female movement in response to male harassment as a mechanism to lower the costs associated with male coercion, and the effect that female movement has on selection in males for male harassment. We found that, when females can move from a habitat patch to a refuge to which males do not have access, there may be a selection for either high, or low harassment male phenotype, or both, depending on the relationship between the harassment level of male types in the population and a threshold level of male harassment. This threshold harassment level depends on the relative number of males and females in the population, and the relative resource values of the habitat; the threshold increases as the sex ratio favours females, and decreases with the value of the refuge patch or total population. Our model predicts that selection will favour the harassment level that lies closest to this threshold level of harassment, and differing harassment levels will coexist within the population only if they lie on the opposite sides of the threshold harassment. Our model is consistent with empirical results suggesting that an intermediate harassment level provides maximum reproductive fitness to males when females are mobile.     

Female prereproductive coloration reduces mating harassment in damselflies

Conspicuous female coloration can evolve through male mate choice or via female-female competition thereby increasing female mating success. However, when mating is not beneficial, such as in pre-reproductive females, selection should favor cryptic rather than conspicuous coloration to avoid male detection and the associated harassment. Nevertheless, conspicuous female coloration occurs in many prereproductive animals, and its evolution remains an enigma. Here, I studied conspicuous female coloration in Agriocnemis femina damselflies, in which the conspicuous red color of the immature females changes to a less conspicuous green approximately a week after their emergence. I measured body size, weight, and egg numbers of the female morphs and found that red females are smaller and lighter and do not carry developed eggs. Finally, I calculated the occurrence frequency and mating frequency of red and green females in several populations over a three-year period. The results demonstrate that red females mated less frequently than green females even when red females were the abundant morph in the populations. I concluded that conspicuous female coloration is likely to function as a warning signal of sexual unprofitability, thereby reducing sexual harassment for females and unprofitable mating for males.     

Is egg carrying attractive? Mate choice in the golden egg bug (Coreidae,Heteroptera)

In several species of fish, females select males that are already guarding eggs in their nests. It is a matter of debate as to whether a female selects a good nest site for her offspring (natural selection) or a male for his attractiveness (sexual selection). The golden egg bug, Phyllomorpha laciniata Vill, resembles fish in the sense that mating males carry more eggs than single males, but in the bugs, female mate choice is decoupled from egg site choice. The sexual selection hypothesis predicts that if females select males using male egg load as a cue for male quality, they should not mate with a male when eggs are removed, regardless of his mating attempts. When individual females were enclosed with an egg-loaded male and an unloaded male, they mated equally often with both males, although the loaded males courted more. In addition, when only successful males were used, females mated equally often with the loaded male and the unloaded male irrespective of sex ratio. Male choice rather than female choice affected mating frequency when sex ratio was equal. Therefore, females do not select the male by the eggs he carries, but successful males may receive many eggs due to egg dumping by alien females while they mate or as a consequence of mate guarding.     

Sexual conflict and the tragedy of the commons

It is widely understood that the costs and benefits of mating can affect the fecundity and survival of individuals. Sexual conflict may have profound consequences for populations as a result of the negative effects it causes males and females to have on one another's fitness. Here we present a model describing the evolution of sexual conflict, in which males inflict a direct cost on female fitness. We show that these costs can drive the entire population to extinction. To males, females are an essential but finite resource over which they have to compete. Population extinction owing to sexual conflict can therefore be seen as an evolutionary tragedy of the commons. Our model shows that a positive feedback between harassment and the operational sex ratio is responsible for the demise of females and, thus, for population extinction. We further show that the evolution of female resistance to counter harassment can prevent a tragedy of the commons. Our findings not only demonstrate that sexual conflict can drive a population to extinction but also highlight how simple mechanisms, such as harassment costs to males and females and the coevolution between harassment and resistance, can help avert a tragedy of the commons caused by sexual conflict.     

Convenience polyandry or convenience polygyny? Costly sex under female control in a promiscuous primate

Classic sex roles depict females as choosy, but polyandry is widespread. Empirical attempts to understand the evolution of polyandry have often focused on its adaptive value to females, whereas 'convenience polyandry' might simply decrease the costs of sexual harassment. We tested whether constraint-free female strategies favour promiscuity over mating selectivity through an original experimental design. We investigated variation in mating behaviour in response to a reversible alteration of sexual dimorphism in body mass in the grey mouse lemur, a small primate where female brief sexual receptivity allows quantifying polyandry. We manipulated body condition in captive females, predicting that convenience polyandry would increase when females are weaker than males, thus less likely to resist their solicitations. Our results rather support the alternative hypothesis of 'adaptive polyandry': females in better condition are more polyandrous. Furthermore, we reveal that multiple mating incurs significant energetic costs, which are strikingly symmetrical between the sexes. Our study shows that mouse lemur females exert tight control over mating and actively seek multiple mates. The benefits of remating are nevertheless not offset by its costs in low-condition females, suggesting that polyandry is a flexible strategy yielding moderate fitness benefits in this small mammal.     

Sex in the morning or in the evening? Females adjust daily mating patterns to the intensity of sexual harassment

Selection on males to mate at a higher rate than females often results in male harassment of females and counteracting female responses. When the reproductive value of copulation changes over time, these mating strategies are expected to be time dependent. Here, we demonstrate that variation in the intensity of male harassment leads to drastic changes in female daily mating patterns. In feral populations of fowl Gallus gallus domesticus, male harassment is intense, particularly in the evening when inseminations are most likely to result in fertilization. We experimentally manipulated the intensity of male harassment through similar-sized groups of different sex ratios. Male mating propensity was always higher than females', particularly in male-biased groups and in the evening, when males were closer to and more likely to approach females. Females counteracted male harassment by escalating resistance to mating and--crucially--by shifting their daily mating pattern: in strongly female-biased groups with relaxed sexual harassment, females solicited sex in the evening, while in male-biased groups, they solicited sex in the morning, thus avoiding harassment in the evening. Together, these results indicate that intersexual conflict may occur not only over mating rates but also over when in the day to copulate.     

Sexual selection favours male parental care, when females can choose

Explaining the evolution of male care has proved difficult. Recent theory predicts that female promiscuity and sexual selection on males inherently disfavour male care. In sharp contrast to these expectations, male-only care is often found in species with high extra-pair paternity and striking variation in mating success, where current theory predicts female-only care. Using a model that examines the coevolution of male care, female care and female choice; I show that inter-sexual selection can drive the evolution of male care when females are able to bias mating or paternity towards parental males. Surprisingly, female choice for parental males allows male care to evolve despite low relatedness between the male and the offspring in his care. These results imply that predicting how sexual selection affects parental care evolution will require further understanding of why females, in many species, either do not prefer or cannot favour males that provide care.     

Social preferences based on sexual attractiveness: a female strategy to reduce male sexual attention

Male sexual harassment of females is common across sexually reproducing species and can result in fitness costs to females. We hypothesized that females can reduce unwanted male attention by constructing a social niche where their female associates are more sexually attractive than themselves, thus influencing the decision-making of males to their advantage. We tested this hypothesis in the Trinidadian guppy (Poecilia reticulata), a species with high levels of male sexual harassment. First, we confirmed that non-receptive females were harassed less when they were paired with a more sexually attractive (receptive) female than with another non-receptive female. We then found that, indeed, females exploit this as a strategy to reduce sexual harassment; non-receptive females actively preferred to associate with receptive over non-receptive females. Importantly, when given access only to chemosensory cues, non-receptive females still showed this preference, suggesting that they use information from chemical cues to assess the sexual attractiveness of potential female partners. Receptive females in contrast showed no such preferences. Our results demonstrate that females can decrease male harassment by associating with females that are more sexually attractive than themselves and that they perform active partner choices based on this relative attractiveness. We propose that this strategy is likely to represent an important pathway by which females can construct social niches that influence the decision-making of others to their advantage; in this case, to reduce the sexual harassment they experience.     

The evolution of female‐limited polymorphisms in damselflies: a signal detection model

Female‐limited intraspecific colour variation is a widely distributed trait within damselflies. Typically, one morph resembles the male (the andromorph) whereas one, or sometimes more, do not [the heteromorph(s)]. While several selective explanations have been offered, such as decreased harassment by males balanced by predation or lack of mating success, field data indicate that andromorphs and heteromorphs mate at equal frequencies in the field, and survive equally well. In this paper, I use a signal detection model to characterize the properties of a new male‐mimicry hypothesis, in which andromorphs are not only more similar to males, but are also encountered more by males. I show that this combination of frequency‐dependent and frequency‐independent factors readily combine to generate a balanced polymorphism. The model explains why morphs have similar mean mating frequencies, why the experimentally observed mating preferences of males vary between ponds, and why the frequency of andromorphs tends to rise with sex ratio.     

Cues for Mate Recognition and the Effect of Prior Experience on Mate Recognition in Enallagma Damselflies

In many coenagrionid damselflies, sexually mature females exhibit color polymorphisms, with some females resembling conspecific males. Although it has been suggested that the latter function as male mimics, this does not appear to be the case for those in the genus Enallagma. We found that sexually dimorphic coloration of the female abdomen and thorax are important cues for sexual recognition by males. We demonstrate for the first time in the Odonata, that males learn to recognize andromorphs as potential mates. After 2 days in an enclosure, sexually mature males exposed to only andromorphic females initiated more sexual interactions with tethered andromorphs than with heteromorphs, the majority morph in the natural population. Exposure to only heteromorphic females tended to decrease males' sexual responses to andromorphs, but not significantly so. Because the frequency of female morphs often varies within a population, learned mate recognition would be advantageous for males that search for mates. Our results lead to a novel, frequency-dependent hypothesis for the occurrence and maintenance of female-limited color polymorphisms.     

Female polymorphism, frequency dependence, and rapid evolutionary dynamics in natural populations

Rapid evolutionary change over a few generations has been documented in natural populations. Such changes are observed as organisms invade new environments, and they are often triggered by changed interspecific interactions, such as differences in predation regimes. However, in spite of increased recognition of antagonistic male-female mating interactions, there is very limited evidence that such intraspecific interactions could cause rapid evolutionary dynamics in nature. This is because ecological and longitudinal data from natural populations have been lacking. Here we show that in a color-polymorphic damselfly species, male-female mating interactions lead to rapid evolutionary change in morph frequencies between generations. Field data and computer simulations indicate that these changes are driven by sexual conflict, in which morph fecundities are negatively affected by frequency- and density-dependent male mating harassment. These frequency-dependent processes prevent population divergence by maintaining a female polymorphism in most populations. Although these results contrast with the traditional view of how sexual conflict enhances the rate of population divergence, they are consistent with a recent theoretical model of how females may form discrete genetic clusters in response to male mating harassment.     

Differences in Mating Propensity Between Immature Female Color Morphs in the Damselfly Ischnura elegans (Insecta: Odonata)

Female-limited color polymorphisms occur in a variety of animal taxa where excessive male sexual harassment may explain the coexistence of multiple female color morphs. In the color polymorphic damselfly Ischnura elegans, mature and immature female color morphs coexist at the mating site where males are in search for suitable mating partners. Here, we study male preference and female mating propensity for the two immature female morphs. As would be expected, compared to mature morphs, both immature female morphs mate much less. Within immature females, one morph consistently mates more frequently compared to the other morph, a pattern that is similar for the ontogenetically corresponding mature female morphs. Preference experiments with the two differently colored immature female morphs, however, did not indicate male mate preference for either morph. Low mating frequencies of immature females at natural sites in combination with relatively high attractiveness of immature models in terms of male preference indicate that female behavior influences female mating success.     

Sociosexuality as predictor of sexual harassment and coercion in female and male high school students

Sexual harassment and coercion have mainly been considered from a sex difference perspective. While traditional social science theories have explained harassment as male dominance of females, the evolutionary perspective has suggested that sex differences in the desire for sex are a better explanation. This study attempts to address individual differences associated with harassment from an evolutionary perspective. Considering previous research that has found links between sociosexual orientation inventory (SOI) and harassment, we consider whether this association can be replicated in a large, representative sample of high school students (N=1199) from a highly egalitarian culture. Expanding the previous studies which mainly focused on male perpetrators and female victims, we also examine females and males as both perpetrators and as victims. We believe that unrestricted sociosexuality motivates people to test whether others are interested in short-term sexual relations in ways that sometimes might be defined as harassment. Furthermore, unrestricted individuals signal their sociosexual orientation, and while they do not desire all individuals that react to these signals with sexual advances, they attract much more sexual advances than individuals with restricted sociosexual orientations, especially from other unrestricted members of the opposite sex. This more or less unconscious signaling thus makes them exploitable, i.e., harassable. We find that SOI is a predictor for sexual harassment and coercion among high school students. The paper concludes that, as expected, unrestricted sociosexuality predicts being both a perpetrator and a victim of both same-sex and opposite-sex harassment.     

Natural selection and genetic variation for female resistance to harm from males

The sexual conflict hypothesis predicts that males evolve traits that exploit the higher parental investment of females, which generates selection for females to counter-evolve resistance. In Drosophila melanogaster it is now established that males harm females and that there is genetic variation among males for the degree of this harm. Genetic variation among females for resistance to harm from males, and the corresponding strength of selection on this variation, however, have not been quantified previously. Here we carryout a genome-wide screen for female resistance to harm from males. We estimate that the cost of interactions with males depresses lifetime fecundity of females by 15% (95% CI: 8.2-22.0), that genetic variation for female resistance constitutes 17% of total genetic variation for female adult fitness, and that propensity to remate in response to persistent male courtship is a major factor contributing to genetic variation for female resistance.     

Do female garter snakes evade males to avoid harassment or to enhance mate quality?

Females of many species behave in ways that make it difficult for males to locate, court, and inseminate them. Two hypotheses have been advanced to explain such behavior: either a female thereby minimizes costs of harassment (sexual conflict model) or by playing "hard to get" she discourages inferior suitors (indirect mate choice model). Our studies of garter snakes (Thamnophis sirtalis parietalis) at a communal den in Manitoba support an interpretation of sexual conflict rather than indirect mate choice. Female snakes dispersed rapidly from the den through areas with relatively few males rather than waiting for additional courtship. Many females dispersed without mating. Experimental (pheromonal) manipulation of the intensity of courtship accelerated rates of female dispersal rather than delaying dispersal, as would be predicted if females wait to obtain matings. The behaviors of females escaping from courting groups were maximally effective in losing their suitors regardless of the number of courting males or whether or not the female was capable of mating (recently mated females cannot mate again because of a mating plug). In total, our data are most consistent with the hypothesis that female garter snakes at communal dens evade males to escape harassment rather than to enhance mate quality.     

Proximity‐dependent Response to Variably Complex Mating Signals in Túngara Frogs (Physalaemus pustulosus)

The plasticity of animal behavior allows individuals to maximize fitness in a wide range of contexts. Both production of and preference for mating signals are context‐dependent according to internal factors such as hormonal state, and external factors such as predation risk. In many species, male‐to‐female proximity also defines an important context for mating communication. Males often possess short‐distance courtship signals, and females often exhibit distance‐related variation in signal response. Such variation in response may occur when a signal’s relevance changes with male‐to‐female distance, but it may also result from perceptual constraints that are unrelated to fitness. Túngara frogs produce variably complex advertisement calls, and sexual selection theory predicts that females should prefer calls of greater complexity. Preference tests, however, have not demonstrated consistent trends for preference between calls of variable complexity. We tested whether proximity to males influences female response to variable signal complexity and found that both preference and memory for signal complexity are proximity‐dependent.