Sex- and season-dependent behaviour in a flightless insect, the Auckland tree weta (Hemideina thoracica)

Abstract

In a polygynous mating system, males frequently compete by locating and defending sites with resources essential to female survival and reproduction. We investigated seasonal changes in site occupancy in a sexually dimorphic, harem-forming insect, the Auckland tree weta (Hemideina thoracica). First we established artificial cavities as diurnal refuge cavities and potential harem guarding sites. We then examined cavity occupancy changes, and, based on our knowledge of prior occupants, determined sex-specific patterns of arrival, departure, and aggregation at a population level throughout the year. Both season and the sex of prior occupants influenced weta occupancy patterns. Most observations were of single females. However, both males and females moved into cavities previously occupied by a weta of the opposite sex more often than expected by chance alone. Females avoided cavities where other females were present, except during summer when most harems formed. In early summer, male and female tree weta previously living apart began co-habiting. Generally there was little relationship between the number and sex of the weta inside cavities and female departure rates from cavities. Males who were sharing with other males departed cavities more frequently than single males, as might be expected in a polygynous species with male-male combat. Males were less likely to depart if they were sharing a cavity with a harem of more than two females during the summer-autumn period. Analysis of departure rates from artificial cavities indicates males are more mobile than females only in winter and spring. Based on our arrival and departure data, and high occupancy of artificial cavities, we suggest that female weta at this site are mobile and may search for mates during the summer. The data are consistent with a polygynandrous mating system as inferred for other tree weta species (Hemideina spp.).