The herding behaviour was seen 155 times, and the driver was identified in 145 bouts (93.5% of the time). Nineteen of the bouts with an identified driver involved a single dolphin driving without other dolphins present. These bouts were not included in analyses comparing drivers and nondrivers and are considered separately. Average bout duration was 19.9s (range of 11.0–28.0s). The interval between successive bouts on the same day ranged from 30 to 2441s (mean 281s).
The A group accounted for 60 and the B group for 66 of the remaining 126 bouts. In all 60 A-group bouts the same dolphin (TLFN) was the driver, and in all 66 B-group bouts the same dolphin (PNT) was the driver. This finding is significantly different from a distribution derived from a hypothesis that the driving individual is randomly selected for each bout.
The A group drove only at Seahorse Key, and the B group drove only around the Corrigan Reef area. TLFN drove using a series of driving fluke slaps at the beginning of each herding bout, but PNT did not. Members of the B group were observed to hit fishes into the air with their flukes (‘fish kick’; see Wells et al. 1987
) while feeding during the bouts, after the driver had ceased herding (n
=14 bouts out of 30 recorded bouts, or 47% of bouts). ‘Fish kicking’ was not observed in the A group.
The A group had a stable membership in all 60 bouts, consisting of only three dolphins: TLFN, SFSK and VFSK. The B group varied in size from two to six dolphins, including the driver, PNT.
In all 14 bouts observed with a single dolphin driving, the individual was identified as CECR. CECR drove in the same area as the B group and was observed on several occasions to be one of the non-driving members of the B group. CECR did not drive fishes while others were present.
Significantly more fishes leaped on average during the A-group bouts than the B-group bouts (; meanA-group=16.4±16.0, meanB-group=8.7±9.3, U=191, nA-group=28, nB-group=22, p=0.022), between A group and the single animal CECR (: meanA-group=16.4±16.0, meanCECR=0.3±0.7, U=390, nA-group=28; nCECR=14, p<0.001) and between B group and CECR (: meanB group=8.7 ±9.3, meanCECR=0.3±0.7, U=269, nB-group=22; nCECR=14, p<0.001).
Mann–Whitney U-test results of fishes leaping per bout compared between groups.
When the data from both groups were pooled, average driver fish-capture success did not differ significantly for nondriver fish-capture success (; sign test [all drivers–all nondrivers]: npositive=24, nties=17, nnegative=17, p=0.21). The A-group driver captured, on average, significantly more fishes than the A-group nondrivers (; sign test [A-group drivers–A-group nondrivers]: npositive=17, nties=5, nnegative=6, p=0.011). There was no significant difference between the average capture successes of the B-group driver compared to the B-group nondrivers (; sign test [B-group drivers−B-group nondrivers]: npositive=8, nties=12, nnegative=10, p=0.815).
The A-group driver captured, on average, significantly more fishes than did the B-group driver (; meanA-group driver: 1.14±0.97, meanB-group driver: 0.37±0.61, U=214; nA-group driver=28, nB-group driver=30, p=0.001), and the A-group nondrivers captured, on average, significantly more fishes than did the B-group nondrivers (; meanA-group nondrivers=0.69±0.10, meanB-group nondrivers=0.38±0.41, U=291; nA-group nondrivers=28, nB-group nondrivers=30, p=0.04).
Mann–Whitney U-test results of fish capture success compared between groups.
During 14 bouts, CECR was observed to capture four fishes, at a mean rate of 0.29±0.19 fishes captured per bout. The A-group driver caught significantly more fishes, on average, than CECR (: meanA-group driver 1.14±0.97, meanCECR=0.29±0.19, U=304; nA-group driver=28; nCECR=14, p<0.005), but the B-group driver did not (: meanB-group driver=0.37±0.61, meanCECR=0.29±0.19 U=236; nB-group driver=30; nCECR=14, p>0.2).
To test for independence of observations within the datasets, a Wald–Wolfowitz runs test was used (StatistiXL, v. 1.4, 2004). The null hypothesis of independence was rejected in only one case—that of the B-group fish capture success of nondrivers (). In this case the p-value of independence was ca. 0.05. It should be noted that there were long periods when no fishes were caught within the datasets of group B, and the analysis is very sensitive to this timeframe. If there had been just one capture that had interrupted the lack of fish captures, the assumption of independence would have been met.