Mate searching context of prey influences predator-prey space race

Predation risk is a strong driver of prey distribution and movement. However, fitness-influencing behaviours, such as mating, can alter risk and influence predator-prey space-use dynamics. In tree crickets, Oecanthus henryi, mate searching involves acoustic signalling by immobile males and phonotactic movement by females. Space-use patterns in tree crickets relative to their primary predators, green lynx spiders (Peucetia viridans), should therefore depend on their current mate-searching state; whether males are calling or non-calling and whether females are phonotactic or non-phonotactic. We first measured the degree of spatial anchoring of crickets to specific bushes in the field, and whether that influenced the probability of broad-scale spatial overlap with spiders. In the absence of spiders, all crickets, independent of sex or male calling status, were found to be spatially anchored to specific types of bushes and not uniformly distributed on the landscape. At the broad spatial scale, spiders were more likely to be found on bushes with female crickets, and to a lesser degree, calling male crickets. At a finer spatial scale within a bush, movement strategies of crickets not only varied depending on the presence or absence of a spider, but also on their current mate searching state. Phonotactic females showed clear predator avoidance, whereas calling and non-calling males moved towards the spider instead of away, similar to predator-inspection behaviour seen in many taxa. As the strongly-selected sex, males are more likely to undertake risky mate searching activities, which includes inspection of predator positions. Overall, we found that all crickets were predictably anchored at the landscape scale, but their sex and mate seeking behaviour influenced the degree of overlap with predators, and their antipredator movement strategies. Reproductive strategies within a prey species, therefore, can alter predator-prey space race at multiple spatial scales.

Methods
Broad-scale space use

Broad-scale space use of crickets and spiders was examined by performing field observations between 1900 hours and 2115 hours in the Ullodu (Karnataka, India) field when O. henryi are active in natural populations. We assessed whether spiders spatially overlap with crickets by comparing spider occurrences on bushes with crickets present and absent. For sampling ‘cricket-present’ bushes, we first localised crickets in the natural environment, either by their calls or using a combination of 5x5 metre quadrat sampling and opportunistic searches. Once localised, all crickets were observed for a minimum of 30 minutes and assigned to specific categories depending on their mate searching behaviour. Males that were calling more than 20% of the time were classified as ‘calling males’ (N = 35 found), as opposed to ‘non-calling males’ (N = 42) which did not call at all. The 20% cut-off was chosen to avoid infrequent callers. Since there was no definitive way to categorise female behaviours from field observations, we did not further classify females as ‘phonotactic’ or ‘non-phonotactic’ at this scale (N = 43 females found). All crickets were caught, marked using nontoxic paint markers with a unique tricolour code, and released on the same bush to avoid resampling on successive nights. For sampling ‘cricket-absent’ bushes, we then randomly chose a bush that was at a distance (within 0.5 m to 10 m, at multiples of 0.5 m) and angle (from 0 to 360 degrees, at multiples of 3 degrees) from each ‘cricket-present’ bush. All cricket-present (N = 120 total) and cricket-absent (N = 145 total) bushes were carefully searched for the presence of spiders after the observation period, and if found, spiders were caught and their size measured to confirm their ability to capture crickets (Torsekar et al. 2019). This method ensured that we are searching for spiders in both cricket-present and cricket-absent bushes at approximately the same time in the night, avoiding any sampling errors assuming spider movement between bushes within a night.

Fine-scale space use

Fine-scale space use patterns were studied by comparing movement decisions of crickets within a bush when a spider was present (predator trials) and when absent (control) in outdoor enclosures. Crickets and spiders were collected from wild populations in and around Peresandra, Karnataka, India (13°35'25.3"N 77°46'50.4"E). All spiders were starved for 48 hours before the trials to ensure similar levels of predator motivation (Torsekar et al. 2019). For all trials, one cricket was released on a bush at 1500 hours and allowed 4 hours to habituate. Fine-scaled movement observations started at 1900 hours and ended at 2100 hours. All categories of crickets were tested: calling males  (N = 29), non-calling males  (N = 40), phonotactic females (N = 42), non-phonotactic females (N = 32). Male crickets that called more than 20% of the time between 1900-2100 hours were categorised as ‘calling males’ and those not calling were ‘non-calling males’. To stimulate a phonotactic female, we played conspecific male calls from a speaker positioned 60 cm away from the female’s position. Calls were played continuously during the trial and only trials in which females elicited a phonotactic response and approached within 20 cm of the speaker were included in the ‘phonotactic females’ category. For the non-phonotactic female category, speakers were present but silent, and thus did not elicit the movement of females.

For all four categories of crickets, we measured fine-scaled movement responses in the presence and absence of a predator. For predator trials, one spider was released at 1900 hours. For control trials, a spider-sized part of a bush branch was arbitrarily tagged as the reference for cricket movement measurements. For the predator trials, the cricket and spider were alternately scan-sampled every 30 sec, for a total of about 120 minutes, and all movement decisions by both were recorded as a change in direction relative to the previous location. After each trial, points of direction changes were sequentially numbered on the bush and converted to polar coordinates from a fixed reference point. This involved measuring the height from the ground, as well as the distance and angle subtended between each tagged point and the reference point. The reference point, common for all tagged points on a bush, was the centre of a fixed and levelled survey precision compass (Survey Compass 17475780, conceptualised by Francis Barker and Sons Ltd., sold and serviced by Lawrence and Mayo, India). The subtended angles were measured using the survey precision compass, and the distances and heights were measured using a metre tape. The same procedure was followed for control trials, with the location of crickets sampled every 30 sec, for a minimum of 45 minutes.

捕食风险是驱动猎物分布与移动行为的核心因素。然而，诸如交配这类影响个体适合度（fitness）的行为，会改变捕食风险水平，并进而调控捕食者-猎物的空间利用动态。对于树蟋（Oecanthus henryi）而言，其求偶搜索过程由静止雄性的声学信号通讯，与雌性的趋声性（phonotaxis）移动两部分组成。因此，该物种相对于其主要捕食者——绿色猞猁蛛（Peucetia viridans）的空间利用模式，应取决于当前的求偶状态：即雄性处于鸣唱还是非鸣唱状态，雌性处于趋声还是非趋声状态。
本研究首先量化了野外条件下树蟋对特定灌丛的空间锚定程度，并探究该锚定行为是否会影响其与蜘蛛发生大范围空间重叠的概率。在无蜘蛛的生境中，无论性别与雄性鸣唱状态如何，所有树蟋均表现出对特定灌丛类型的空间锚定行为，而非在景观尺度上均匀分布。在大范围空间尺度上，蜘蛛更倾向于栖息在有雌性树蟋的灌丛中，其次是鸣唱雄性栖息的灌丛。在灌丛内部的精细空间尺度上，树蟋的移动策略不仅随蜘蛛的存在与否发生变化，同时也受当前求偶状态的调控：趋声雌性表现出明显的捕食者规避行为，而鸣唱与非鸣唱雄性则会主动朝向蜘蛛移动，这与诸多类群中观察到的捕食者探查行为一致。作为受强烈性选择作用的性别，雄性更倾向于开展高风险的求偶活动，其中就包括探查捕食者的位置。总体而言，本研究发现所有树蟋在景观尺度上均表现出可预测的空间锚定行为，但其性别与求偶行为会影响其与捕食者的空间重叠程度，同时也决定了其反捕食移动策略。因此，猎物种内的繁殖策略，可在多个空间尺度上改变捕食者-猎物的空间博弈动态。

方法
1. 大范围空间利用
通过在印度卡纳塔克邦乌洛杜（Ullodu）的野外开展野外观察（每日19:00至21:15，该时段为树蟋自然种群的活动高峰期），探究树蟋与蜘蛛的大范围空间利用模式。本研究通过对比有蟋蟀灌丛与无蟋蟀灌丛中蜘蛛的出现情况，评估蜘蛛与树蟋的空间重叠概率。
在“有蟋蟀灌丛”采样中，研究人员首先通过鸣唱声或结合5m×5m样方取样与机会性搜寻，在自然环境中定位树蟋。定位完成后，对所有树蟋进行至少30分钟的观察，并根据其求偶行为进行分类：鸣唱时长占比超过20%的雄性被归类为“鸣唱雄性”（共发现35只），完全不鸣唱的雄性则为“非鸣唱雄性”（共发现42只），设置20%的阈值是为了排除偶发鸣唱的个体。由于野外观察无法明确区分雌性的行为状态，因此在该尺度下未进一步将雌性划分为“趋声雌性”与“非趋声雌性”（共发现43只雌性）。所有树蟋均被捕捉，使用带有独特三色编码的无毒油漆笔进行标记后，放回原灌丛，以避免连续夜间采样时的重复取样。
在“无蟋蟀灌丛”采样中，研究人员从每个“有蟋蟀灌丛”出发，按距离（0.5m至10m，以0.5m为间隔）与角度（0°至360°，以3°为间隔）随机选取一处灌丛作为对照。在观察时段结束后，对所有“有蟋蟀灌丛”（共120处）与“无蟋蟀灌丛”（共145处）进行仔细搜查，记录蜘蛛的存在情况；若发现蜘蛛，则捕捉并测量其体型，以确认其具备捕食树蟋的能力（Torsekar等，2019）。该方法确保了在夜间大致相同的时间点，对有蟋蟀与无蟋蟀灌丛中的蜘蛛进行搜寻，规避了因蜘蛛夜间在灌丛间移动而带来的采样误差。

2. 精细空间利用
通过在室外围栏中开展对照实验，对比树蟋在灌丛内的移动决策：当捕食者存在时（捕食组）与捕食者不存在时（对照组）的行为差异。树蟋与蜘蛛均采集自印度卡纳塔克邦佩雷桑德拉（Peresandra）及其周边的野生种群（坐标：13°35'25.3"N 77°46'50.4"E）。所有蜘蛛在实验前均禁食48小时，以保证捕食动机处于相近水平（Torsekar等，2019）。
在所有实验中，将一只树蟋放置于灌丛上，于15:00释放并给予4小时的适应时间。精细移动观察于19:00开始，至21:00结束。本研究对所有类别的树蟋均进行了测试：鸣唱雄性（N=29）、非鸣唱雄性（N=40）、趋声雌性（N=42）、非趋声雌性（N=32）。其中，19:00至21:00时段内鸣唱时长占比超过20%的雄性被归类为“鸣唱雄性”，完全不鸣唱的为“非鸣唱雄性”。为刺激趋声雌性，研究人员在距离雌性定位点60cm处的扬声器播放同种雄性的鸣唱声，实验期间持续播放，仅将雌性产生趋声反应并移动至扬声器20cm范围内的实验纳入“趋声雌性”组。对于“非趋声雌性”组，扬声器处于开启状态但未播放声音，因此不会触发雌性的移动行为。
针对上述四类树蟋，本研究均测量了其在捕食者存在与不存在两种条件下的精细移动响应。在捕食组实验中，于19:00释放一只蜘蛛；在对照组实验中，随机标记灌丛上一处与蜘蛛体型相当的枝条作为蟋蟀移动测量的参照点。对于捕食组实验，每30秒对蟋蟀与蜘蛛进行交替扫描取样，总时长约120分钟，记录二者所有的移动决策（即相对于前一位置的方向变化）。每次实验结束后，将灌丛上的方向变化点按顺序编号，并以固定参照点为基准转换为极坐标，该过程包括测量每个标记点距地面的高度，以及与参照点之间的距离和夹角。所有灌丛标记点共用的参照点，为一台固定且调平的精密测量罗盘（Survey Compass 17475780，由Francis Barker and Sons Ltd.设计，由印度Lawrence and Mayo公司销售及维修）。夹角使用该精密测量罗盘进行测量，距离与高度则使用米尺进行测量。对照组实验采用相同流程，每30秒对蟋蟀的位置进行一次取样，总时长至少为45分钟。