Sexual dimorphism, behaviour and evolution
Tree wētā are a sexually dimorphic, endemic genus (Hemideina spp., Orthoptera) in New Zealand with seven species currently recognised. Living in habitats ranging from lowland forests to urban gardens to alpine rock tors, wētā have survived the introduction of mammalian predators in New Zealand and exhibit many interesting behaviours. I have investigated nutrient intake in males and females (Wehi et al. 2013a) and whether males are more susceptible to predation, resulting in skewed sex ratios (e.g. Wehi et al. 2011).
In this video made by Massey University I talk more about wētā and their mating systems.
Find out more about wētā in this article; why males fight, who eats them, and why we need ‘back-up populations’ to save species. “Absurd Creature of the Week: This Bug Is Big as a Gerbil. Fortunately It Loves Carrots“.
I’ve also worked on their diet (Wehi and Hicks 2010) and movement patterns in forest remnants (Wehi et al. 2013b, Brown et al. 2018). Although tree wētā are fairly commonplace, remarkably little is known about their ecology. I continue to work on sexual dimorphism and foraging in this genus. I am also currently working on the ecology of Hemideina maori, a species that lives mostly, but not exclusively, in the mountainous rock tors of South Island, and survives the winter under snow. The Hellaby Trust has supported some of this work.
Wētā move between cavities in forest trees during the summer (Wehi et al. 2013b), and appear to favour cavities that are not exposed to artificial lighting at night (Wehi et al. 2015) when they are active. Male wētā are able to distinguish females by cuticular scent alone, but are poor at identifying species-specific females (Wehi et al. 2017) as has been shown in some other crickets. As species distributions shift with climate change, this lack of species recognition leads to a risk of species hybridisation where tree weta species co-exist.
Adult males have large jaws that are used as weaponry to fight other males can be found with harems of females, with whom they mate. Although we suspected that adult males might be more visible to predators such as owls or rats because of their behaviour, we found no difference in male and female numbers in a meta-analysis of sex ratio at different locations (Wehi et al. 2011). However, such large jaws in males may lead to differences in foraging patterns (Brown et al. 2018).
Tree wētā are frequently used as a monitoring species in ecological restoration, but their susceptibility to baits that are targeted at killing mammalian predators such as rats is largely unknown. We have begun exploring this susceptibility by analysing bait feeding preferences (Morgan et al. 2017) and behaviour change (Parli et al. 2020).
- Parli A, Besson A, Wehi PM, Johnson S 2020. Sub-lethal exposure to a vertebrate pesticide bait alters behaviour in an orthopteran. Journal of Insect Conservation (2020): 1-12 10.1007/s10841-020-00222-6
- Brown MBJG, Gemmill CEC, Miller S & Wehi PM 2018. Diet selectivity in a terrestrial forest invertebrate, the Auckland tree wētā, across three habitat zones. Ecology and Evolution 8: 2495–2503. 10.1002/ece3.3763
- Wehi PM, Monks A & Morgan-Richards M 2017. Tree wētā are attracted to cuticular scent cues but do not distinguish according to sex or among two closely-related species. Ethology 123: 825–834. 10.1111/eth.12652
- Morgan DKJ, Clapperton BK, Gillanders JA & Wehi PM 2017. The palatability of carrot surface-coated with repellents or dyed blue or green to a terrestrial macroinvertebrate. New Zealand Journal of Zoology 44: 212–224. 10.1080/03014223.2017.1289473
- Wehi PM, Jorgensen M & Morgan D 2015. Predictors of relative abundance of tree wētā (Hemideina thoracica) in an urban forest remnant. New Zealand Journal of Ecology 39: 280–285.
- Wehi PM, Raubenheimer D & Morgan-Richards M 2013a. Tolerance for nutrient imbalance in an intermittently feeding herbivorous cricket, the Wellington tree wētā PLOS ONE 8(12): e84641. 10.1371/journal.pone.0084641
- Wehi PM, Jorgensen M & Morgan-Richards M 2013b. Sex- and season-dependent behaviour in a flightless insect, the Auckland tree wētā (Hemideina thoracica). New Zealand Journal of Ecology 37: 75–83.
- Wehi PM, Nakagawa S, Trewick S & Morgan-Richards M. 2011. Does predation result in adult sex ratio skew in a sexually dimorphic genus? Journal of Evolutionary Biology 24:2321-2328 [featured on the cover, issue 24 (11)]. 10.1111/j.1420-9101.2011.02366.x
- Griffin MJ, Trewick SA, Wehi PM & Morgan-Richards M 2011. “Invertebrate mice”: Exploring the concept of niche convergence in a land without rodents. New Zealand Journal of Ecology 35: 302-307.
- Wehi PM & Hicks BJ 2010. Isotopic fractionation and trophic enrichment in the Auckland tree wētā, a large herbivorous insect. Journal of Insect Physiology 56: 1877–1882. 10.1016/j.jinsphys.2010.08.005