Darryl T. Gwynne

Sexual Difference Theory: Mormon Crickets Show Role Reversal in Mate Choice

Male Mormon crickets produce a large spermatophore that the female eats. Spermatophore proteins are important to female reproduction, and females compete for access to singing males. Males reject most receptive females as mates, and those accepted are more fecund than rejected individuals. This role reversal in courtship is in contrast to the behavior of the sexes in katydid species in which the males produce small spermatophores.

Sexual competition among females: What causes courtship-role reversal?

In most animals, males are the competitive sex whereas females are typically non-competitive and choosy of mates. In a variety of taxa, certain species (or populations within species) show a reversal in these typical courtship roles. Recent research with these organisms supports a central tenet of sexual selection theory: that it is the relative investment of the sexes in offspring that controls the number of males and females available for mating, and thus is the main determinant of the degree of sexual competition in each sex.

Courtship feeding and the fitness of female katydids (Orthoptera: Tettigoniidae)

Katydid males (Requena verticalis) produce spermatophores with a large sperm‐free spermatophylax, which is eaten by the female after mating. In this study, I asked 1) how do spermatophylax nutrients affect the fitness of the mated female and her progeny? 2) does this male‐produced food substitute for other food in the diet of the female, or is it a source of specialized nutrients? and 3) does an increase in the size of the spermatophylax eaten influence female reproduction in the same way as the additional spermatophylaxes that would be obtained from multiple mating?

Testing parental investment and the control of sexual selection in katydids: the operational sex ratio.

Theory predicts that relative parental investment by the sexes controls the ratio of the numbers of fertilizable females and sexually active males (the operational sex ratio, OSR) and thus sexual selection and sexual differences. This paper tests the hypothesis that variation in parental investment controls the numbers of males and females available for mating. In katydids, males and females invest parentally by providing material investment to eggs, with the male donation derived from spermatophore materials eaten by the female. Previous work with katydids in nature has shown intraspecific variation in sexual selection acting on females; only certain populations show a courtship role reversal in which females compete for access to males. These observations led to the hypothesis that the availability of food in nature determines the frequency of spermatophore production and thus the number of males available for mating. In the present study, male and female katydids were maintained on diets that differed in food quality. Both sexes were given daily access to sexually receptive members of the opposite sex. As predicted, males on a low-quality diet mated less frequently than males on a high-quality diet. The opposite trend was observed for females: females on a low-quality diet increased mating frequency, apparently to obtain additional spermatophore nutrients. These data suggest that male parental investment in the spermatophore increases in its relative importance when diet is low in quality. The effects of diet on mating frequency result in different estimates of the OSR, which accord with observations in nature: for the high-quality diet, there are fewer fertilizable females than sexually active males; similar conditions in nature would be predicted to result in a male-to-male competitive (polygynous) mating system. For the low-quality diet, there were slightly fewer fertilizable females, indicating that such a diet may result in competition among females.

FEMALE CHOICE FOR AN INDICATOR OF MALE SIZE IN THE SONG OF THE BLACK-HORNED TREE CRICKET, OECANTHUS NIGRICORNIS (ORTHOPTERA: GRYLLIDAE: OECANTHINAE)

The calling song of male crickets, including Oecanthus nigricornis (Walker), attracts females for mating and provides a model system of sexual communication. We give the first conclusive identification of a feature of cricket song that is both attractive to females and indicates a phenotypic feature (body size) that determines male mating success and female reproductive benefits. We do this by first testing for correlations between song characteristics and aspects of male phenotype that are hypothesized to indicate male quality. We show that song is a reliable indicator of male size and male age, and that large male size is associated with increased female fecundity. We then use playbacks of synthetic songs that mimic natural variation in song parameters to study song preferences and we compare preferences under different presentation regimes to determine whether choices are based on relative song quality or some fixed criterion. Females show a preference for the lower frequency songs produced by large males, but only during simultaneous playbacks. Thus female choice is based on the relative quality of calls that can be sampled simultaneously. These results provide strong support for the hypothesis that females use variation in calling song to assess male mate quality.

Mate selection by female katydids (Orthoptera: Tettigoniidae, Conocephalus nigropleurum)

Abstract Female katydids receive a large spermatophore at mating which they subsequently eat. Available evidence indicates that spermatophore nutrients are important to female reproduction. Heavier males produce larger spermatophores. When given the choice between two singing males of different weights, females always mated with the larger individual.

Courtship feeding in katydids benefits the mating male's offspring

SummaryFor species exhibiting courtship feeding it is typically argued that the food gift presented by males is a sexually-selected trait in serving to acquire fertilizations. An alternative hypothesis is that the trait is maintained by natural selection for parental investment in which the fitness of the mating males offspring is increased. Here I argue that the spermatophylax, a nutritious part of the spermatophore provided to female katydids, Requena verticalis, functions mainly as parental investment. Previous research suggested that variation in the size of the male donation in this species (1) did not influence the ability of males to transfer ejaculates and (2) resulted in variation in offspring fitness. In the present paper genetic markers and radiolabels are used to show that the offspring are fathered by the males that donate the nutrients. Although these results indicate that the large spermatophylax is maintained by selection for increased parental investment, it is likely that this male offering originated in a sexual selection context whereby males fed females in order to obtain fertilizations.

Food Quality Controls Sexual Selection in Mormon Crickets by Altering Male Mating Investment

Populations of several katydid species (Orthoptera: Tettigoniidae) show large differences in mating system; in some species males compete and females choose mates, whereas in others these courtship roles are reversed. Male katydids feed their mates during mating with a large, nutritious spermatophore. I hypothesized that courtship role reversal and increased sexual selection on females was a result of limitation in nutritious food items causing a decrease in the number of males available for mating (and thus in the sper- matophore meals available). This hypothesis was tested by varying diet in field cages of Mormon crickets, Anabrus simplex. Results showed that, in contrast to a diet supplemented with proteinaceous food items, a poor diet resulted in: (1) a decrease in the number of sexually active males, (2) an increase in sexual selection for female body size, and (3) a reversal in the typical courtship roles. The experiment ruled out alternative explanations for variation in the mating system of this species, such as differences in population density and age. The conclusion from this study is that increased sexual selection on females due to such a change in diet is a result of an increase in the relative importance of male parental investment.

Sex-biased predation and the risky mate-locating behaviour of male tick-tock cicadas (Homoptera: Cicadidae)

In this study, web-building spiders captured more male than female tick-tock cicadas, Cicadetta quadricincta. The preponderance of males in webs was not due to a male-biased adult sex ratio but instead appeared to be a consequence of mate-seeking activities. Males of this species flew frequently; they spent, on average, about half a min on a perch from which they sang before flying to another perch. They also made many short flights when responding to receptive females. Females did not show these sort of flight patterns. Male behaviour appeared to be a result of the pair-forming pattern of the species. Receptive females answered calling males by flicking their wings and males moved toward the sounds produced by these movements. The risky call-and-fly tactics of males would allow them to signal for mates over a large area.

CONDITION DEPENDENCE OF MALE LIFE SPAN AND CALLING EFFORT IN A FIELD CRICKET

Abstract Sexually selected traits are thought to impose survival costs on showy males. Recent empirical work found a negative relationship between male display and survival in a field cricket species (Orthoptera, Gryllidae, Gryllinae) where there is no evidence of a mating bias toward older males. In most species, however, male survival and ornamentation are positively correlated, and older males often have a mating success advantage over younger males. These findings suggest that male quality and survival are positively correlated, but more tests of this hypothesis are needed. We measured the condition dependence of male survival and calling effort in another grylline, Gryllus pennsylvanicus, where older males have previously been shown to have greater mating success. We varied condition by manipulating diet, and measured male life span and calling effort to assess the relative condition dependence of these traits. High- and medium-condition males survived longer than low-condition males, and high-condition males called more than medium- and low-condition males. Differences in calling effort among the condition treatments were not apparent early in life, but emerged as males aged. We discuss possible explanations for the differences between our study and contrasting results such as the previous grylline work.