Within-season variability of fighting behaviour in an Australian alpine grasshopper

Abstract

Throughout the breeding season, changing environmental and biological conditions can lead to variation in the reproductive landscape of many species. In alpine environments temperature is a key driver of behaviour for small ectotherms such as insects, but variable biotic factors such as mate quality and availability can also influence behaviour. Kosicuscola tristis is a small semelparous grasshopper of the Australian alpine region. In a rare behaviour among grasshoppers, K. tristis males engage in vigorous fights over access to females, involving mandible displays, kicking, biting and grappling. In this study we describe the variation in fighting behaviour of K. tristis throughout the breeding season and test several hypotheses related to temperature, body size, mating behaviour, and female quality. We show that K. tristis males are more aggressive toward each other at the end of the breeding season than at the beginning. This increased aggression is associated with decreased daily average temperatures (from ~20°C to ~9°C), decreased mating activity, increased female fecundity, and an unexpected trend toward an increase in female-to-male aggression. These results suggest that K. tristis is likely under increased selective pressure to time key life cycle events with favourable biological and climatic conditions. The stochastic nature of alpine environments combined with a relatively short life span and breeding season, as well as limited mating opportunities toward the end of the season may have contributed to the evolution of this extraordinary mating system.

Male mate choice in the chameleon grasshopper (Kosciuscola tristis)

“…In previous studies, we have found that females reject males in up to 30% of trials and show aggression in up to 10% of trials (Muschett et al, ). Previous observations have also revealed evidence of females rejecting males by kicking and grappling with the male (Muschett et al, ). However, we did not observe female mate rejection in our experiments.…”

“…At last, it is likely that the prevalence of male choice varies throughout the breeding season. Muschett et al () determined male K. tristis become more aggressive towards each other at the end of the breeding season, as opportunities to mate dwindle. An assessment of the seasonal variability of this behaviour would provide much‐needed insight into the mechanisms of male choice in this species.…”

“…Females readily mate with multiple males, sometimes within minutes of a previous mating (Muschett et al, ), suggesting there is a risk of sperm competition in this species and thus potentially selection on males to avoid it, or traits that improve male competitiveness such as differential sperm allocation. While female K. tristis do not exhibit precopulatory mate choice regarding male size or colour (Umbers, Herberstein, et al, ; Umbers, Tatarnic, et al, ), they can avoid copulation by hopping away and/or by kicking and grappling with males (Muschett et al, ). This avoidance suggests that females exhibit some preference for mates but the traits females favour are not clear.…”

“…In chameleon grasshoppers (Kosciuscola tristis), males engage in fierce fights over access to females, a rare behaviour among grasshoppers. Fights involve mandible displays, biting, kicking and grappling, particularly at the end of the breeding season (Muschett, Umbers, & Herberstein, 2017;Umbers et al, 2012;Umbers, Tatarnic, et al, 2013). In opposition, early in the season, there is little to no aggression among males.…”

“…In opposition, early in the season, there is little to no aggression among males. At this time, males are found riding on adult females, presumably mate guarding, and on subadult females perhaps to ensure they are the first to mate with her once she emerges as an adult (Mahoney, Tatarnic, O'Hanlon, & Umbers, 2017;Muschett et al, 2017). Kosciuscola tristis are protandrous, adult females emerge a few weeks after the males and are quickly mated (Dearn, 1977;Green & Osborne, 1981;Umbers, Herberstein, & Madin, 2013;Umbers, Tatarnic, et al, 2013;Umbers and Muschett, pers.…”