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First published online October 10, 2003

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Burrow surveillance in fiddler crabs I. Description of behaviour

Jan M. Hemmi^* and J. Zeil

Visual Sciences, Research School of Biological Sciences, Australian National University, GPO Box 475, Canberra, ACT 2600, Australia

View larger version (23K): [in a new window]   Fig. 1. Dummies, experimental set-up and measurement of response strength. (A) The dummy's size and appearance from the side. Bold dimensions indicate the most frequently used dummy. (B) The dummy was moved along a straight line (dummy track) between two tent pegs. The observer controlled the dummy from a distance by means of a fishing line wrapped around the tent pegs and attached to both sides of the dummy. The small rectangle represents the recording area of the video camera, centred on the focal crab's burrow. (C) The crab's response strength was measured as the distance the crab moved towards its burrow during a dummy presentation (d0-d1) relative to the initial distance before the response (d0).

View larger version (25K): [in a new window]   Fig. 9. Response probability and the movement direction of dummies. (A) Definition of the four track segments close to the burrow that were used in the analysis and the movement direction of the dummies in these segments. (B) The number of responses in each segment (histogram) and the probability of response per segment as fitted by the statistical model (dots joined by a solid line). Each segment has been divided into ten 2 cm-wide bins sorted according to their distance from the closest point (CP). The distances are sorted in the same way as a dummy would move through them during an experiment. The dummy-burrow distance is decreasing for segments 1 and 3 and increasing for segments 2 and 4. The response probability clearly differs between the four segments (GLMM; d.f.=3, Wald/d.f.=11.95, P<0.001).

View larger version (73K): [in a new window]   Fig. 2. Four examples of crab responses to the approaching dummy. The four images show a bird's eye view of the mudflat at the time the dummy enters the field of view. Consecutive dummy positions are marked with black dots, and crab positions with white dots (t=240 ms). The onset of a crab's response is marked by a small white ring around the dummy's position at that point in time. Burrows are marked by large white circles. The graphs below the four images show the distance of the dummy (black dots) and of the crab (grey dots) from the crab's burrow over time. The crab's velocity towards its own burrow (cm s-1) is shown as a continuous line at the bottom, whereby negative velocity values indicate that the crab travelled towards the burrow. Time samples during which the movements of the crabs meet the criteria for a response are marked by a black dot below the velocity trace. Further explanations can be found in the text.

View larger version (28K): [in a new window]   Fig. 3. How the response strength (see Fig. 1C) depends on crab-burrow distance. The solid line indicates the value of response strength needed for a crab to move from its current position to within 2 cm of the burrow. At a distance of 2 cm, the crabs are likely to have tactile contact with the burrow entrance.

View larger version (48K): [in a new window]   Fig. 4. Summary of the data set used in the analysis for all experiments in which the extended dummy track intersected the vertical line defined by the crab's home vector on the crab side of the burrow (crab side = 1; see inset in B). Consecutive dummy positions are shown in a coordinate system defined by the crab's home vector. The data from different experiments were moved and rotated such that the crab's home vector always points vertically down towards the burrow, which was placed at the centre of each panel (grey circle). The trajectory of the dummy is shown from the moment it became visible in the recording area until the crab responded to the dummy or until the dummy reached it's closest point to the burrow. The thick, vertical, black line above the burrow shows the range of crab starting positions for all the experiments accumulated in the respective panel. (A-C) All trials in which the crabs responded to the dummy; (D-F) all trials in which the crabs did not respond to the dummy, or where they responded late, after the dummy had reached its closest approach to the burrow. The three rows of panels sort the dummy presentations according to the crab-burrow distance, with increasing distance from top to bottom (A,D: 5-15 cm; B,E: 15-25 cm; C,F: 25-55 cm). The position of the dummy at the time a crab responded has been marked by an enlarged symbol: a dot indicates that the response occurred on the dummy's first approach, a square indicates that the crab responded after the dummy changed direction at the end of its track and was moving back towards its starting position. In addition, a black symbol indicates that the crab responded while the dummy was approaching the burrow, whereas a grey symbol indicates that the dummy was moving away from the burrow when the response occurred. As the tracks are only shown up to their closest point to the burrow, some of the dots and squares do not lie directly on the printed section of a path in panels D, E and F. All tracks are shown to move from right to left to increase the clarity of the figure. To achieve this, tracks were mirrored at the vertical home vector where necessary.

View larger version (52K): [in a new window]   Fig. 5. Summary of the data used in the analysis for all the experiments where the extended dummy track intersected the vertical line defined by the crab's home vector on the side of the burrow opposite to the crab (crab side = 0; see inset in B). All other conventions as in Fig. 4.

View larger version (14K): [in a new window]   Fig. 6. Definition of track angle, track distance and crab side, the three parameters used to describe the dummy trajectory relative to the burrow and the crab's home vector. `Crab side' measures whether the dummy track intercepts the line defined by the crab's home vector on the crab side of the burrow (crab side = 1) or on the opposite side (crab side = 0). The variable is needed because there are always two paths that have a particular combination of track angle and track distance.

View larger version (40K): [in a new window]   Fig. 7. The effects of the four significant terms that influence the crab's probability of response (see Table 1). (A) The distance of the dummy track from the crab's burrow (track distance; see Fig. 6). The histogram shows the number of experiments where the crabs responded (black) and the number of experiments where the crabs did not respond (grey) for a number of different track distances (right scale). The dotted line connects the probability of response for each bin of the histogram (left scale). The solid line shows the probability of response as fitted by the statistical model. (B) The effects of track angle and crab side (see Fig. 6). The vertical bars show the probability of response based on the raw data for the different approach directions, and the solid lines are fitted values based on the statistical model (grey, crab side = 0; black, crab side = 1). (C) The effect of repeated dummy presentations on the probability of response. The dotted line shows the probability of response over the number of successive dummy presentations. The model fit is shown by the grey dots joined by a solid line. The number of experiments for each value of presentation repeat, i.e. the number of crabs contributing to this value, is indicated by the histogram at the bottom of the figure. The two shades of grey group the values of presentation repeat that correspond to the six levels fitted in the statistical model.

View larger version (40K): [in a new window]   Fig. 11. The parameters that influence a crab's decision of when to initiate its response to the approaching dummy. (A) The influence of crab-burrow distance on the dummy-burrow distance at the time of the response. The data set has been split according to the track's distance from the crab's burrow: small burrow track distances of 20 cm are shown as open circles, and the corresponding model fit (track distance = 10 cm) as a dotted line. Larger track distances of >20 cm and the model fit for a track distance of 20 cm are shown as filled dots and a solid line, respectively. (B) Response distance and the direction of approach of the dummy. The response distance is only slightly influenced by different track angles. The solid lines show the direction of dummy paths, and the large circles mark dummy positions at the time of response as fitted by the statistical model. The fitted response distance is shown for each of the nine approach directions (track angle) and the two values of crab side (see Fig. 6). Increasing track angles are shown in progressively lighter greys. Fitted values were calculated at the mean of all other parameters in the statistical model. The mean crab-burrow distance is 23.4 cm. Solid and dotted circles represent the mean response distance ± 2 S.E.M. (C) The effect of repeated dummy presentations on the response distance. The dotted line shows the mean response distances over successive dummy presentations. The grey dots joined by a solid black line show the model fits for the six categories of presentation repeat. The grey bars at the bottom of the panel show the number of experiments (crabs) contributing to each estimate. The two shades of grey indicate which presentation repeats correspond to the six categories of the variable presentation repeat used in the statistical modelling.

View larger version (22K): [in a new window]   Fig. 8. The effect of dummy-burrow distance on the probability of response for three different ranges of track distance. The binning used for the dummy-burrow distance was: 5-8 cm, 8-12 cm, 12-20 cm, and 5 cm-wide bins for distances larger than 20 cm. For each track distance, binning started at the upper end of the distance range (e.g. at 20-25 cm for 12-20 cm track distances). To make the results for different dummy-burrow distances directly comparable, we adjusted the probability for slightly different bin widths in such a way that all values reflect the probability that a crab responds during a 5 cm movement of the dummy. For the meaning of the grey area, see text.

View larger version (17K): [in a new window]   Fig. 10. Histogram of dummy-burrow distance (response distance). Black bars represent response distance distribution for track distances of 12 cm; grey bars represent response distance distribution for track distances of >12 cm.