Refuge size variation and potential for sperm competition in Wellington tree weta

Alternative mating strategies in the Wellington tree weta expressed via genetic polymorphism for precocial male maturation

The expression of alternative mating strategies is generally hypothesized to be a plastic response of the genome to the environment or a genetic polymorphism. The Wellington tree weta (Hemideina crassidens) is an orthopteran insect in which males express three alternative mating morphotypes related to the size of mandibular weaponry. My common garden experiment in the laboratory shows that male weapon size at maturity in this species is nether condition- nor environment-dependent; therefore, supporting the hypothesis that it is genetically polymorphic. Males that matured at the 8th instar had the shortest development time and the smallest weaponry, males that matured at the 10th instar had the longest development time and the largest weaponry, and males that matured at the 9th instar were intermediate to the other two morphs. These morph-related differences in development time suggest that male alternative mating strategies in this species are expressed via precocial male maturation, a rare mode of strategy expression in insects. Additionally, adult lifespans differed significantly among morphs with 8th instar males having the shortest, and 9th instar males having the longest lifespans. I discuss how these differences in lifespan align with morph-specific investment in pre- and post-copulatory processes to equalize fitnesses among morphs.

The impact of small groups on pre- and postcopulatory sexual selection in polyandrous populations

Sexual selection is a key evolutionary force but varies widely between populations. Two key factors that influence sexual selection are the extent to which females copulate with multiple males (polyandry) and variation in the social environment. Increasing research demonstrates populations are structured by complex socio-sexual networks, and the structure of these networks can influence sexual selection by shaping the relationship between male precopulatory mating success and the intensity of postcopulatory competition. However, comparatively less attention has been dedicated to the influence of group structure on sexual selection and how differences in the size of groups may impact on the relative force of pre- and postcopulatory sexual selection in polyandrous populations. The presence of groups (i.e., group structure) and the size of groups varies widely in nature and forms an implicit part of much experimental sexual selection research under laboratory conditions. Here I use simulations of mating competition within populations that vary in the size of groups they contain, to show that variation in group size, and in particular small groups, can influence sexual selection. Specifically, I show that null expectations for the operation of pre- and postcopulatory sexual selection is governed by the size of groups within populations because smaller group sizes constrain the structure of sexual networks leading to reinforcing episodes of pre- and postcopulatory sexual selection. Given broad variation in group structure in nature and the tendency for experimental sexual selection research to study replicate small groups, these effects have implications for our understanding of the operation of sexual selection in polyandrous populations.

Investigating host plant association, calling activity, and sexual dimorphism in Indian Gryllacropsis sp. (Orthoptera: Anostostomatidae)

Both sexes of Indian weta Gryllacropsis sp. communicate acoustically. Females lack an external ovipositor making it difficult to differentiate between the sexes in the field. There is limited ecological information on the species as it is found high up on the trunks of evergreen trees, well camouflaged, and active only at night. The present study was conducted to gain ecological information on this little known yet intriguing species. We tested the hypotheses that (a) calling activity of Indian weta is uniformly distributed throughout the year and (b) there is no difference in morphometric measurements between the sexes. The study was conducted in Bhagwan Mahavir Wildlife Sanctuary and Mollem National Park, Goa, India. Visual scanning of tree trunks followed by vegetation sampling, psychoacoustic sampling, and morphometric analyses was carried out. Resource selection function values, obtained for a total of 52 tree species from 1984 individuals, were less than 0.1 for all plant species indicating no preference by the wetas. Peak calling activity was observed in the month of November (Rayleigh's test, Z = 7.90, p < .01). Discriminant function analysis on morphometric characters of males and females (Wilk's λ = 0.32 approx. F (4, 21) = 11.24 p < .0001, classification accuracy = 96.15%) provided clear distinction between males and females. Contribution of body weight was significant (standardized canonical discriminant function coefficients = +1) and could be used for identification of sexes in the field. These polyphagous insects provide insights on understanding ecological specialization due to host plant association, signal evolution, and mating behavior.

Equal fitness among alternative mating strategies in a harem polygynous insect

Alternative mating strategies are widespread among animal taxa, with strategies controlled by a genetic polymorphism (Mendelian strategy) being rarer in nature than condition-dependent developmental strategies. Mendelian strategies are predicted to have equal average fitnesses and the proportion of offspring produced by a strategy should equal the equilibrium proportion of individuals representing the strategy in a population. Developmental strategies are not expected to produce offspring in equilibrium proportions; however, whether the alternative phenotypes should have equal average fitness is debated. The Wellington tree wētā (Hemideina crassidens) (Orthoptera: Anostostomatidae) is a harem polygynous insect in which intense sexual competition has favoured the evolution of three alternative mating strategies that differ in weapon size and the ability to fight for control of harems. Here, we use molecular genotyping to test the hypothesis that the alternative strategies in this species are maintained by having equal relative fitness and that morphs produce offspring in equilibrium proportions. As expected, the average relative fitness of the three strategies did not significantly differ and the proportion of offspring produced by each morph is equal to the frequency of that morph in the population. Our results support the hypothesis that the alternative male morphs in H. crassidens represent Mendelian strategies.

Siring success in kangaroos: size matters for those in the right place at the right time

In polygynous species, male reproductive success is predicted to be monopolized by a few dominant males. This prediction is often not supported, suggesting that ecological and alternative mating tactics influence siring success. The spatiotemporal distribution of individuals and the number of males competing for each receptive female are often overlooked because they are difficult to monitor in wild animals. We examined how spatial overlap of female–male pairs, the time spent by a male on the breeding site, number of competitors, and morphological traits influence siring probability in eastern gray kangaroos (Macropus giganteus). We compared home range overlap for 12 208 dam–male pairs and 295 known dam–sire pairs to define local competitive groups and to estimate every male’s opportunity to sire the young of each female. We compared models considering morphological traits relative to the entire population or to local competitive groups. Including local competition improved model performance because it estimated the intensity of competition and compared each male’s morphological traits to those of its competitive group. Regardless of size, males can increase their probability to sire a young by increasing their mating opportunity relative to the mother. We underline the importance of considering spatial structure to assess the intensity of competition in species where males cannot equally access all females in a population. The estimation of mating opportunity and intensity of local competition improves our understanding of how morphological traits affect siring success when each mating event involves a different set of competing males, a characteristic of most wild species.