Risks of finding mates in the wild: Ecological and Behavioural Determinants of Sex-biased Predation

Abstract

Sex-biased predation occurs when one sex of a prey species is consumed more than the other, irrespective of their relative availability (sex-ratio). Some of the potential factors leading to sex- biased predation are sex differences in morphology and/or behaviour of prey species. In the context of mate-finding, we often see sex-specific behavioural strategies, such as signalling and searching. The relative risk of predation on the two sexes during mate-finding depends on who signals, who searches, the risks associated with these behaviours, and the degree of involvement in these activities. Prey wing remains found in the roosts of a bat predator, Megaderma spasma, reveal interesting patterns for two katydid genera, Mecopoda and Onomarchus. Mecopoda sp. wing remains are male-biased in the breeding season and female- biased in the non-breeding season, while Onomarchus uninotatus wing remains are female- biased year-round. These two katydids differ in their mate-finding strategies: in Mecopoda sp., which are found close to the ground, males signal acoustically and females search silently to locate them. Onomarchus uninotatus is an arboreal katydid, that uses an acoustic-vibratory multimodal duet for mate-finding within trees, with both sexes signalling and searching to find mates. To explore the factors driving the sex-biased predation by M. spasma on Mecopoda sp., I used a combination of field observations and enclosure experiments with wild-caught live animals. The males and females of Mecopoda sp. were compared with respect to (i) their availability, i.e., natural sex-ratio across the seasons, (ii) the predation risks associated with different behaviours, and (iii) the prevalence of their risky behaviours in the wild. The results show that the relative availability of the sexes does not explain the male-biased predation on Mecopoda in the breeding season; whereas in the non-breeding season, very few males are available, which could lead to the female-biased predation. Males perform high-risk mate-finding behaviours, such as calling and flight, with a high prevalence in the wild. Although flight is equally risky for both sexes, females rarely fly. To understand bat predation risk on male and female O. uninotatus and its possible role as a selection pressure driving the evolution of multimodal duetting, outdoor enclosure experiments were performed with live bats and katydids. Bat predation risks were compared (i) between the signalling and searching strategies of each sex, and (ii) between male and female signalling. The findings indicate that bat predation risk fails to explain the evolution of vibrational signalling in O. uninotatus females, as searching by walking is as safe as signalling, leading to an overall low predation risk on females within a tree. However, their duetting behaviour lowers the risk on males, who can shift from high-risk signalling to low- risk searching by walking, once females start signalling. Finally, I focused on bat predation risk of searching behaviour in O. uninotatus, which potentially takes flights across trees. Comparisons were made between males and females for (i) bat predation risk of flight using enclosure experiments and (ii) across-tree movement patterns using radio-telemetry. I found that flight is equally risky for both sexes, but females move across trees 1.6 times more often and 1.8 times greater distances, which could make them more vulnerable to predation, offering a possible explanation for the female-biased prey remains found in M. spasma roosts. Overall, this thesis provides a comprehensive examination of the predation risks associated with signalling and searching behaviours of katydids. The interplay between sex-specific behaviours and ecological factors can explain sex-biased predation patterns