Ecology of predator-prey interactions in the context of mate searching

Abstract

Animals communicating in the context of mate searching benefit by increasing their likelihood of encountering mates. Mate searching begins the important process of individuals of opposite sexes forming pairs, which is necessary for achieving mating success and improving their chances of attaining reproductive success. But mate searching individuals also experience costs of being conspicuous to unintended receivers such as eavesdropping predators. Prey respond to and manage such predation risk by typically reducing activity or moving away from the risk. Such alteration in behaviour can have potential costs on foraging and mating opportunities. Costs of such behavioural management of risk on an ecological timescale have been studied extensively in the foraging context but have not received enough attention in the reproduction context. Hence, my thesis aims at understanding the ecology of predator-prey interactions in the context of mate searching, using the tree cricket Oecanthus henryi as a model system. I first estimated the relative predation risk experienced by communicating and noncommunicating, male and female crickets from their primary predators, green lynx spiders, at multiple spatial scales. I then manipulated predation risk in enclosure experiments and observed how it affects communication and survival, to compare their relative fitness consequences. Finally, I examined how crickets and spiders use space at two different spatial scales, in order to explore whether crickets behaviourally manage the risk they experience while searching for mates. Before observing how prey respond to predation risk, it is important to estimate what risk they face in nature. And since mate searching typically has two strategies of pair formation, including signalling and responding to signals, with each generating different cues, it is vital to know whether the risk these strategies face are different. Hence, I estimated predation risk of mate searching in natural cricket populations, at multiple spatial scales at which predators and prey interact with each other. I partitioned predation risk into constituent probabilities of prey cooccurring, encountering and being eaten by their predator. Each probability was estimated separately for mate-searching males and females and compared with males and females not motivated to search for mates, as controls. Predation risk was found to be similar across all comparisons, suggesting that mate searching in O. henryi does not face costs from predation. Another result of estimating predation risk was that probability of mortality from predation was observed to be very low at a within-night timescale. Although low, crickets could potentially perceive this risk to be either large, or maybe even negligible. Hence, to explore how crickets respond to predation risk in the context of mate searching, I manipulated predation risk and observed differences in mate searching behaviour in male and female crickets, and their survival. Mating success of individuals was also recorded to investigate whether modification of matesearching behaviour and survival bears fitness consequences. This experiment was conducted inside field enclosures and predation risk was manipulated by varying cricket-spider ratios inside the enclosure. Males altered mate-searching behaviours including likelihood of calling, calling effort and likelihood of movement whereas female mate searching behaviour remained unaffected with changing predation risk. Survival for both was strongly affected. Finally, in addition to survival of both male and female crickets affecting their mating success, male likelihood to move also negatively affected mating success. Hence, varying predation risk does affect mate searching in O. henryi males as they shift from calling to moving, and this shift has negative fitness consequences. In addition, males and females differ in their response to predation risk even though the risk they face in the field is similar. Since male crickets’ management of predation risk has adverse fitness consequences, I was interested in investigating whether crickets can instead manage predation risk by controlling their spatial distribution and potentially move away from predators. Hence, I studied space use patterns of predators and prey at two spatial scales: broad scale or across bushes and fine scale or within bushes. I hypothesised that fine scale evasion of spiders by crickets will depend on their broad scale overlap with spiders. I found that at the broad scale, space use by spiders is not affected by the presence or absence of crickets on the bush. Hence, at the fine scale, I expected crickets, regardless of their mate searching strategy, to exhibit similar predator evasion movement behaviour. Instead, both calling and non-calling males turned more towards spiders on average, when close to the spiders, whereas responding and non-responding females moved the same when a spider was present or absent. Males, predicted to be the more risk-taking sex, could be inspecting predators to gather information on spider location. Therefore, cricket space use at the fine scale appears to be driven by the sex and context of the individual, rather than their spatial overlap with spiders at the broad scale.