Condition-dependent signalling and mating behaviour in the tree cricket, Oecanthus henryi

Abstract

Male reproductive success in systems exhibiting sexually selected traits can be driven by male mating effort in sexual signals and in the production of nuptial gifts. Male mating effort can be determined by various biotic and abiotic factors, such as nutritional quality, age, body size, mating status and temperature, contributing to the immediate condition of the male. Less preferred males in certain systems can also evolve condition-dependent alternate tactics to improve their reproductive fitness. In systems that involve long-distance acoustic signaling by males for mate attraction, in addition to male mating effort, female motivation to acoustically localize a potential mate can also impact male reproductive success. Female motivation can again be governed by female condition, depending upon nutritional status, age, size and mating status. Crickets of the order Orthoptera are excellent model systems to study all of the above. Males typically engage in energy-demanding long-distance acoustic signalling to attract conspecific females. Intraspecific variation in male investment in signalling and signal quality can be a key factor in determining the attractiveness of males to conspecific females and hence their reproductive fitness. Male mating effort in signalling can show condition-dependent trade-offs with other life history traits such as lifespan. Moreover, several orthopteran species also produce nuptial gifts that females feed on during mating. As most orthopterans are also polygamous, the replenishment time of nuptial meal and spermatophore can constrain the number of mating opportunities procured by males, thereby influencing their fitness. Male investment in number of matings and quality of nuptial gift can show condition-dependent variation. Although condition-dependent male investment in nuptial gifts and multiple matings has been the focus of many sexual selection studies, to the best of our knowledge, how various aspects of mating (re-mating propensity, latency to re-mate, investment in nuptial gift and male reproductive success) are modulated by male condition has not yet been investigated by following males across multiple matings. Therefore, in this thesis, I studied condition-dependent signalling and mating behaviour in an acoustically communicating tree cricket species, Oecanthus henryi in an attempt to elucidate the determinants of: a) male mating effort in signalling and mating, b) female responsiveness to signals and courtship, c) alternative signalling tactics in males (baffling) and d) male reproductive success (matings). In the first chapter, I studied the condition dependence of lifetime signaling of O. henryi by following individual males throughout their life in a semi-natural setup. Males reared on better quality adult diet had a higher lifetime and nightly signaling effort. Higher signaling effort implies better mate attraction potential. In the second chapter, I explored the condition-dependent male investment in nuptial feeding and corresponding reproductive success in terms of spermatophore attachment duration (SPAD). Nuptial feeding duration was found to be longer in males supplemented with highly nutritive diet. SPAD was affected by diet quality, age and male body size. Males on high-quality diet reaped more reproductive benefits with increasing age. Smaller males received the least reproductive benefit across age and diet. Oecanthus henryi males not only invest in energetically costly acoustic signals and nuptial gifts, but additionally exhibit a unique tool-making behavior called baffling, which confers them a reproductive advantage by increasing their signal loudness. In the third chapter, I investigated the condition-dependence of this signaling strategy and found that smaller males on better diet had higher baffling probability. This result suggests that baffling is an energetically expensive alternate tactic used by less-preferred males to improve their reproductive fitness. Nightly baffling propensity on a lifetime scale showed patterns of age-dependent decline. The senescence patterns of baffling did not differ significantly across dietary conditions. In the final chapter of my thesis, I aimed to understand the effect of female dietary condition and mating status on female mate search and mating behaviour. Interestingly, female mating status, and not diet, turned out to be a key driving factor in this context. Despite a high remating propensity, mated females showed significantly low motivation to perform phonotaxis. Besides, both nuptial feeding duration and spermatophore attachment duration decreased in mated females. Thus, for a male it is more advantageous to mate with an unmated female. This highlights the importance of the proportion of virgin females in a wild population for male reproductive success.