The Peppermint Stick-Insect (Megacrania batesii) occurs in rainforest habitats in the Solomon Islands and in far-north Queensland, Australia. This species is fascinating because, like many stick-insects, it has an unusually flexible reproductive system called "facultative parthenogenesis". Any female is able to reproduce sexually if she mates and gets sperm to fertilize her eggs. Fertilised eggs develop with equal probability into female or male offspring (presumably depending on whether the sperm carries an X or Y sex-chromosome). But if a female doesn't mate by the time she is ready to start laying eggs (~ 20 days after moulting into the adult stage), she will lay unfertilised eggs that develop into viable offspring (all females) through a process called "parthenogenesis". This form of asexual reproduction involves a kind of self-fertilisation, where the ovule fuses with another meiotic product to restore diploidy (two sets of homologous chromosomes). Even when females mate, they still often produce at least some of their daughters parthenogenetically. While parthenogenesis allows for normal development, it results in a dramatic loss of heterozygosity.
We are studying this species to understand how the remarkable ability to switch between sexual and asexual reproductive modes affects fitness, population persistence, and evolution. Conveniently, reproductive mode can be manipulated experimentally simply by housing females with or without male partners.
Intriguingly, although every individual female is capable of both sexual and asexual reproduction, we have found that wild populations of this species are either sexual (with a roughly even sex-ratio and near-exclusively sexual reproduction) or asexual (all-female and exclusively parthenogenetic). We have found that these sexual and asexual populations occur in a spatial mosaic throughout the known natural range of this species in Australia (Miller et al. 2024a). Remarkably, sexual and asexual populations sometimes occur in very close proximity and without any obvious barriers to dispersal or differences in habitat (Miller et al. 2024b).
We are especially interested in using this system to investigate the role of sexual conflict in influencing reproductive mode. If females successfully resist mating, they will end up reproducing asexually. If resistance fails, sexual reproduction will occur. At the population level, the outcome of sexual conflict can therefore determine reproductive mode. If males fail to mate often enough, few male offspring will be produced in the next generation. Eventually, males could go extinct. Likewise, although a single unmated female can found a new all-female population, such a population will only persist as an asexual population if it's able to avoid invasion by males that disperse from neighbouring sexual populations. Indeed, we have found abundant evidence of resistance to sex in this species (Wilner et al. 2025).
Sexual behaviours also suggest strong sexual conflict. In sexual populations, males guard females almost continuously (Boldbaatar et al. 2024). Males engage in spectacular battles over (often, literally on top of) females, and females sometimes struggle violently against male suitors (see videos below).
Megacrania batesii is a specialist on Pandanus and Benstonea host-plants, and exhibits a range of adaptations to life on these host-plants (see photos below). Its body is perfectly shaped to fit into the groove that runs down the centre of a Pandanus or Benstonea leaf, and its bluish-green colour blends beautifully into the shade between host-plant leaves. This species gets its common name from the peppermint-scented defensive spray that it shoots with remarkably good aim from movable cannon-like structures on its "shoulders" (see the video below). The defensive fluid, a milky substance consisting mainly of glucose and the alkaloid actinidine, is synthesized from chemical precursors that the stick-insects get from feeding on their host-plants. The defensive fluid is stored in paired thoracic glands, ready to be deployed against predators or researchers. The fluid is highly irritating to vertebrate eyes, but also has fungicidal properties that might protect the insects from infection. Weirdly, males spray this fluid at each other when fighting over a female!