Representation of different exact numbers of prey by a spider-eating predator

Abstract

Our objective was to use expectancy-violation methods for determining whether Portia africana, a salticid spider that specializes in eating other spiders, is proficient at representing exact numbers of prey. In our experiments, we relied on this predator's known capacity to gain access to prey by following pre-planned detours. After Portia first viewed a scene consisting of a particular number of prey items, it could then take a detour during which the scene went out of view. Upon reaching a tower at the end of the detour, Portia could again view a scene, but now the number of prey items might be different. We found that, compared with control trials in which the number was the same as before, Portia's behaviour was significantly different in most instances when we made the following changes in number: 1 versus 2, 1 versus 3, 1 versus 4, 2 versus 3, 2 versus 4 or 2 versus 6. These effects were independent of whether the larger number was seen first or second. No significant effects were evident when the number of prey changed between 3 versus 4 or 3 versus 6. When we changed prey size and arrangement while keeping prey number constant, no significant effects were detected. Our findings suggest that Portia represents 1 and 2 as discrete number categories, but categorizes 3 or more as a single category that we call 'many'.

Keywords: Portia africana; Salticidae; numerical cognition; predation; spider.

Figure 1.

Apparatus used in expectancy-violation experiments. Not drawn to scale. Side Walls, Front Wall, Ceiling and Pan not shown. Beginning of trial: test spider (Portia africana) walks out of Pit and on to top of Starting Tower. Test spiders complete successful trials by proceeding from Starting Tower along Pathway to top of Viewing Tower. Thin arrows indicate path test spider takes to reach lures; dotted arrow indicates path test spider takes to opt out of completing detour. Inset: example of a Scene with four lures visible to test spider from Starting or Viewing Tower. Cable-release device moves Scene up and down once every 30 s while the test spider is on top of Starting Tower and while test spider is on top of Viewing Tower. When test spider arrives at Island, Scene is removed and then replaced by a different Scene or else the previous Scene is returned.

Figure 2.

Latency (time elapsing between test spider (Portia africana) arriving at the top of the Viewing Tower and crossing the Access Ramp). Experimental: prey arrangement or prey size visible from top of Viewing Tower different from prey arrangement or prey size visible from top of Starting Tower. Control: prey arrangement or prey size visible from top of Viewing Tower same as prey arrangement or prey size visible from top of Starting Tower. In all experiments, prey number was constant. Boxes show medians and upper and lower quartiles, and whiskers show minimum and maximum values. All comparisons n.s. See table 1 for details of each experiment.

Figure 3.

Percentage of test spiders (Portia africana) that arrived at the Window Frame after crossing the Access Ramp. Experimental: prey arrangement or prey size visible from top of Viewing Tower different from prey arrangement or prey size visible from top of Starting Tower. Control: prey arrangement or prey size visible from top of Viewing Tower same as prey arrangement or prey size visible from top of Starting Tower. In all experiments, prey number was constant. All comparisons n.s. See table 1 for details of each experiment.

Figure 4.

Latency (time elapsing between test spider (Portia africana) arriving at the top of the Viewing Tower and crossing the Access Ramp). Experimental: prey number visible from top of Viewing Tower different from prey number visible from top of Starting Tower. Control: prey number visible from top of Viewing Tower same as prey number visible from top of Starting Tower. In all experiments, prey arrangement and prey size was constant. Boxes show medians and upper and lower quartiles, and whiskers show minimum and maximum values. See table 2 for details of each experiment.

Figure 5.

Percentage of test spiders (Portia africana) that arrived at the Window Frame after crossing the Access Ramp. Experimental: prey number visible from top of Viewing Tower different from prey number visible from top of Starting Tower. Control: prey number visible from top of Viewing Tower same as prey number visible from top of Starting Tower. In all experiments, prey arrangement and prey size was constant. See table 2 for details of each experiment.