Mate searching context of prey influences predator-prey space race
Torsekar, Viraj R.; Thaker, Maria (2020), Mate searching context of prey influences predator-prey space race, Dryad, Dataset, https://doi.org/10.5061/dryad.nzs7h44p8
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.
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.
Broad scale data includes 4 different categories:
- Bushes with calling males present (C)
- Bushes with non-calling males present (NC)
- Bushes with females present (F)
- Cricket-absent bushes (E)
Column 'cooc' contains data on whether a green lynx spider was present on the bush (1) or not (0).
Space use data at the fine scale is uploaded separately for males crickets (calling and non-calling) and females (phonotactic and non-phonotactic). Each file has 4 different treatments:
- Calling males with predator present (CE) and absent (CC)
- Non-calling males with predator present (NCE) and absent (NCC)
- Phonotactic females with predator present (PE) and absent (PC)
- Non-phonotactic females with predator present (NPE) and absent (NPC)
Turning angles (labelled as 'angle') of each cricket were measured as the angle subtended between the present location of a cricket and the next location of the cricket relative to the current location of the spider. The distance between the cricket and spider when this movement was recorded is labelled as 'distance'. Similarly, the time at which this movement was recorded is labelled as 'time' and the respective cricket and spider identitfy is labelled as 'male.id' and 'gls.id' respectively.