From quiet-night-in to party animal: sex ratio and density affect male/female aggregations in a ‘harem’ polygynous insect

Highlights

• Tree wētā aggregations in roosts vary depending on sex ratio and population density.

• Males with larger heads are associated with more females.

• Males avoid roosting with other males unless females are present in the roosts also.

Varying forms of polygyny are observed across many animal groups. In some species, a male defends a group of females from other males, and successful defence leads to greater reproductive success. This is often referred to as harem polygyny and is most observed in mammals. A female-biased sex ratio has been associated with harem polygynous species. In such populations, formation of ‘harems’ may be an inevitable consequence of the relative lack of available males, rather than multiple females actively choosing to mate with a subset of specific males. Although a rare mating system for insects, harem polygyny has been described in several orders, including tree wētā (Orthoptera: Anostostomatidae: Hemideina) in New Zealand. Aggregations of multiple females have been found with a single male in their diurnal roosts. We aimed to determine how the sex ratio and local density of Hemideina thoracica affected the formation of harems in laboratory conditions. ‘Harems’ were observed when the sex ratio was female biased, but no more than would be expected by chance arrangement of individuals, although females preferred to associate with males rather than be alone. Conversely, when the sex ratio was male biased, females preferred to be alone. The number of females associated with each male was lowest when the sex ratio was even and local density was low. However, males and females did not associate randomly: when local density was high aggregations of multiple males with multiple females were observed more often than expected by chance, suggesting that, in contrast to classic ‘harem’ behaviour, males accepted other males being present as long as females were also found in the roost. Additionally, females were more likely to be associated with males than in low-density conditions. Individuals in this study showed much more flexibility in their mating behaviour than what would be expected in a harem polygynous species.