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Prey catching in the archer fish: does the fish use a learned correction for refraction?
Behavioural Processes 52 (2000) 21 – 34 www.elsevier.com/locate/behavproc
Prey catching in the archer fish: does the fish use a learned correction for refraction? P.J.A. Timmermans *, J.M.H. Vossen Department Comparati6e and Physiological Psychology, NICI, Uni6ersity of Nijmegen, P.O. Box 9104, 6500 HE Nijmegen, The Netherlands Received 6 January 2000; received in revised form 19 May 2000; accepted 9 June 2000
Abstract An answer to the question of how the archer fish hits an aerial insect, despite the refraction of light at the surface of the water has not yet been found. The aims of the present studies are to find out: (1) whether the fish applies a learned correction with the virtual image as a point of reference; (2) whether deprivation of practice in squirting affects performance. For the first aim the accuracy of squirts was measured in 30 subjects. Contrary to suggestions from the literature, elevation failures were prominent but the frequencies of over- and under-squirting did not differ, which does not support the idea that the fishes applied a learned correction for refraction by using feedback from the efficacy of squirts. For the second aim, five experimental subjects were deprived of practice, whereas six control subjects got daily practice, during 6 months. The only significant difference, found thereafter, was that during the first session experimental subjects aimed more often before squirting than control subjects did, but hitting was not affected. A number of subjects developed abnormal mandibles which inevitably led to squirting too high. Our findings do not support the hypothesis that the archer fish uses learned corrections for refraction. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Archer fish; Prey catching; Aiming
1. Introduction Archer fishes bring down insects with a squirt of water from their mouth. During squirting the tip of the mouth just breaks the surface but the eyes stay well below. For obvious reasons the attention paid to this peculiar way of prey catching became * Corresponding author. E-mail address: p – [email protected] (P.J.A. Timmermans).
focused primarily on two questions: how the fish ejects a squirt of water (see e.g. Elshoud and Koomen, 1985), and how it can hit its target despite the shift of the prey’s image caused by the refraction of light at the surface of the water (see e.g. Dill, 1977). The first question was elaborated by Milburn and Alexander (1976), and Elshoud and Koomen (1985). Remarkably, the refraction problem was put forward not earlier than in 1952 (Meyers, 1952), but Meyers did not elaborate on it, and it still has not been solved.
0376-6357/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 6 3 5 7 ( 0 0 ) 0 0 1 0 7 - 8
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Several mechanisms for coping with refraction, which remarkably, all are behavioural, have been suggested and some have been studied. Hediger and Heusser (1961), by means of high speed filming, falsified their hypothesis that the archer fish’s squirt consists of a very fast series of droplets during the ejection of which the fish adjusts its aim with the aid of visual feedback from the trajectories of droplets, like a machine gunner does with tracers. In his extensive papers Lu¨ling (1958, 1963) assumed that the archer fish (Toxotes jaculatrix) encounters two problems: refraction of light, and parallax between line of sight and direction of the squirt. For the greater part, the fish would avoid both problems by squirting nearly vertically. However, this idea is contradicted by the Verwey (1928) report, Lu¨ling’s own illustrations (Lu¨ling, 1958, 1963, 1964, 1969), and by the Herald (1965) photograph, all showing T. jaculatrix squirting at angles between 70 and 80°. The Bekoff and Dorr (1976) confirmation of Lu¨ling’s theory, is not supported by their finding that targets at a height of 30 cm often were missed by 15 – 20 cm. For T. chatareus Timmermans (1975) observed that aiming angles varied between 60 and 110°. The aiming angle is the angle between the water surface and the line from the fish’s snout to the target, at the moment of squirting. The angle of the squirt may differ a few degrees from the aiming angle. Dill (1977) affirmed that other angles than 90° were used successfully, and in his thorough analysis demonstrated that the refraction archer fishes have to cope with is not trivial. According to Lu¨ling (1958), most misses scored by T. jaculatrix were on the azimuth (left or right of the target) not in elevation (over or under the target). We feel, that in fish squirting approximately vertical, that this indeed could be the case, but as we have seen T. chatareus, and probably also T. jaculatrix often use other aiming angles than 90°. According to Dill (1977) T. chatareus does make elevation failures, which is supported by our observations. It is clear that the best chances to find an answer to the question of how the archer fish succeeds in hitting, are offered by studying the development of squirting in young specimens.
However, the smallest specimens to be obtained (1.5 cm) already are squirting successfully, and there are no reports on breeding in captivity to be found. We succeeded in getting archer fishes to spawn in captivity, but the eggs did not hatch (see also Timmermans and Maris, 2000). Therefore, we chose to work with grown subjects. We did two experiments. In the first one we assessed the distribution of elevation failures; in the second one we assessed whether deprivation of squirting affected performance. Our line of reasoning in Experiment I is as follows. The virtual image of the target is perceived above the position of the real target. Roughly speaking, this refraction effect could be corrected for in two ways: (1) The archer fish is somehow programmed (as a consequence of genetical selection, or learning during an early sensitive period) to hit a target despite refraction; (2) The archer fish keeps adapting its aiming behaviour to the effects of preceding squirts (operant conditioning). Lu¨ling (1958) suggested that his fish used the location of the impact of misses on the target board as negative feedback (use of cues that the preceding aiming was wrong), and moreover held them capable to make use of the relative positions of target and misses to readjust their aiming for the next trial. But Lu¨ling did not put this idea to a test, and the only example he described is not convincing: squirt 1 miss to the left; squirt 2 miss even more to the left; squirt 3 again to the left but less than 1; squirt 4 hit. Moreover, adjustment of aiming based on impacts of misses, like humans use it, presupposes considerable cognitive capacities. Furthermore, the prey’s background in the natural habitat will be rather variable, so impacts of misses not always will offer useful feedback, or will even be absent. A simpler mechanism could be the use of positive feedback (use of cues that the preceding aiming was right), i.e. to repeat squirts that are rewarded with prey, but this idea has not been tested either. To proceed with our argumentation: In case 1, where the fishes are somehow programmed to hit a target despite refraction, the target itself remains the aim, and there is no adjustment of aiming based on its effects. In this case one would expect an equal number of misses above and below the
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target in a sample of subjects. Misses will occur, because aiming remains a noisy process. On the individual level one would expect, among others, differences in the location of the target within the range of impacts. In case 2, however, where the fishes are using feedback to adjust their aiming, refraction remains a potential cause of systematic bias. The fishes should try to repeat what they did when they (accidentally) hit the target although they aimed at the virtual image, or they should try to avoid preceding misses. In any case, they should learn to aim somewhere below the virtual image, i.e. no longer to use the virtual image itself as the aim, although the virtual image remains their only point of reference, as there are no reliable points of reference in the background, for example, like we have at our disposal when using a bull’s eye with concentric circles. One therefore would expect misses above the target to be more frequent than below it. The purpose of Experiment I was to test whether misses above the target were more frequent than misses below the target. A significantly larger number of misses above than below the target would contradict the hypothesis of a programmed correction for refraction. During the observations it soon appeared that a considerable number of subjects, that were consistently squirting too high, had elongated mandibles. We therefore decided also to pay some attention to this phenomenon. In Experiment II our line of reasoning is as follows. If archer fishes learn (by trial and error) how to aim, it is conceivable that deprivation of practice could reduce the chances to hit, or otherwise affect performance, also in subjects that already learned how to aim. If, on the other hand, the way of aiming, once it has developed, is not adapted to its consequences when it is used, it is to be expected also to be insensitive to disuse. The purpose of Experiment II was to compare the performance of a group that had got daily exercise with the performance of a group that had been deprived of squirting for a long time. In order not to miss subtle effects, we filmed the tests so we could take a great number of measures.
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2. Materials and methods
2.1. Subjects and housing All subjects were imported and obtained from an aquarium store. They were kept solitarily in glass tanks (50× 30× 30 cm), separated by opaque partitions to exclude mutual influences on behaviour during the tests. Subjects were fed morning and evening with a measured portion of: small crickets, small meal-worms or daphnia that were thrown into the water. During experiments the fish were fed only once daily, so that they were deprived of food for 924 h before practice and test sessions in their home aquarium. Small dead crickets were used as targets and as rewards during these sessions. The tanks were aerated except during practice and test sessions, when pumps were turned off to ensure a calm water surface. Water temperature was kept at 25°C, pH between 6.5–7.5°, and hardness at 6.5 DH. Fluorescent lights were on from 20:00 to 08:00 h. In Experiment I, 30 subjects were used with an average length of 4.3 cm at the start. In Experiment II, 11 subjects, of initially 2–2.5 cm, were used. During 6 months six control animals got daily practice (squirting at food items), and five experimental animals were deprived of squirting at food items.
2.2. Experimental set up The experimental setup (Fig. 1a) used for practice sessions in Experiment I and II, and for test sessions in Experiment I, permitted to offer targets (dead crickets, 3–4 mm) at various heights on a vertical white target board (150× 10 cm). A triangular ridge in the middle of the board prevented scoring indirect hits by means of ricochets, because drops hitting the ridge were split and sideways deflected by the triangle. On the ridge, at intervals of 5 cm a thin 3 cm long steel wire with a loop (3 mm ø) could be inserted. On this loop the target was offered, so that it fell into the water when it was hit by the fish. In order to facilitate recording, the observer was sitting in front of the aquarium, while keep-
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P.J.A. Timmermans, J.M.H. Vossen / Beha6ioural Processes 52 (2000) 21–34
ing his eye and the target in line with the centre of a cross (on target height) on the wall behind the aquarium. Thus he could reliably observe whether a squirt hit the target, whether squirt and target
overlapped (missing in front or behind the target at the right elevation), whether the squirt went over, or under the target. To further enhance reliability, only a narrow slit (1.5 cm) was left
Fig. 1. (a) Schematic view of home aquarium with setup for practice in Experiment I and II, and for tests in Experiment I: 1, opaque cover; 2, slit for squirting (1,5 cm wide); 3, target board with loop (adjustable in height) to offer cricket on; 4, plastic fringe curtain to be used for shelter; (c –d) transection of target board showing triangular ridge to prevent hits from ricochets. (b) Setup, for tests in Experiment II, to be put upon home aquarium after removal of target board. 1, opaque cover; 2, slit for squirting (1.5 cm wide) with flap to close it between trials; 3, support (adjustable in height) for wire (4) with cricket (target) that is dropped out sight behind screen (5) when the subject has squirted; 6, cricket (reward) that is pushed into aquarium at the moment the target disappears behind screen.
P.J.A. Timmermans, J.M.H. Vossen / Beha6ioural Processes 52 (2000) 21–34
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Table 1 Schematic procedure of Experiment I Practice and tests during 12 successive weeks Practice
Test
Weekdays 1–6, three crickets with 5 cm interspace were On weekday 7, four crickets were offered successively, during 1 offered simultaneously. If all crickets were hit within 3 min each, at the height of the lowest target of the cluster of min, next day the height of the cluster was increased 5 cm, the prior practice day. Squirts were scored as: hit, right if not, the height was decreased 5 cm. elevation, too high, or too low.
open in the cover of the aquarium, parallel to the front pane (see Fig. 1a). This narrow slit forced the subjects to take position parallel to the front pane. Thus all squirts were in the same plane, i.e. they went at right angles through the direction in which the observer was watching. For the tests of Experiment II a novel setup was used, because the setup (target board) used for practice of the control group during 6 months, would have given the controls an advantage over the experimental group. The novel set up (see Fig. 1b) could be put on the aquaria after removal of the target board. The experimenter could open the slit in the cover from a distance. The target (cricket) was fixed to a thin 10 cm long wire with a pivot joint on the vertical support on the rim of the tank. When the pivot was released, the wire with the target (fixed cricket) fell behind a little screen, disappearing out of sight of the fish, while at the same time a relay pushed another cricket (reward) into the aquarium, right below the place were the fixed target just disappeared. In a pilot test, fishes reacted to this procedure like they did when they squirted a cricket down, i.e. they rushed forward and took the food. A spotlight shining in the direction in which the fish was to squirt illuminated the squirt. The fish was illuminated by a spotlight above the camera, which was placed at right angles in front of the aquarium.
2.3. Procedures 2.3.1. Experiment I: are misses abo6e the target more frequent than below? Since in natural conditions archer fishes will gain experience in squirting, practice and test
sessions were taken in the course of 12 weeks (see Table 1). Practice sessions were given on weekdays 1–6. Each session three crickets (3–4 mm) were offered simultaneously, one on each of three successive loops (5 cm apart) on the target board. If all three targets were hit within 3 min, the height of the cluster of three targets was increased 5 cm on the next day, in case not all three targets were hit target height was decreased 5 cm. Targets that were hit fell into the water and were eaten, targets that had not been hit were thrown into the water at the end of the session, but not immediately after the fish had squirted. So, hitting was reinforced immediately, whereas missing and notsquirting were not reinforced, and the total amount of food was independent of performance. To be sure that the subjects saw the targets, each session started by dropping a cricket into the slit in the cover of the aquarium by means of tweezers. The timing of the session started after the subjects had taken this lure. Each week on the 7th day there were test sessions following the same procedure as practice sessions, except that this time four fixed targets were offered successively, during trials of maximally 1 min each, 5 cm below the lowest target of the preceding practice session. After a subject had squirted four times, or when the 1 min trial had passed, a cricket was thrown into the water, and the next trial was started. Each squirt was scored as hit, on right elevation, too high, or too low. Right elevation means: the squirt’s trajectory is seen to overlap the target (see experimental setup), i.e. the squirt passes in front of, or behind the target, or it hits the target. Since the diameter of target and squirt amounted to 3–4 and 5–7
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P.J.A. Timmermans, J.M.H. Vossen / Beha6ioural Processes 52 (2000) 21–34
mm, respectively, the ‘right elevation zone’ was maximally 13–18 mm, depending on individual variation. Consequently, too high and too low means respectively: over and under the right elevation zone. Right elevation was subdivided into hits and misses. So, a miss, in this case, was a failure on the azimuth. A hit was a squirt on the target.
2.4. Experiment II: does depri6ation of squirting affect performance? The following procedures will be explained successively: assignment to groups; treatment; habituation; testing (see Table 2). During the first week 11 subjects were tested individually by successively offering three (loose) crickets on the target board at a height of 10 cm, and three at 15 cm, and registering the number of hits (when hit, the cricket was lifted of the loop and dropped into the water), and misses per cricket during 3 min. According to the ratio hit: miss the subjects were evenly assigned to the experimental (n=5) and to the control group (n = 6). The control group got practice sessions (see Exp. I) on 6 days each week during 27 weeks. The experimental group got no practice, and since the aquaria had an opaque cover, there were no opportunities to obtain food by means of squirting. In the course of 3 weeks before testing, all subjects were habituated to the setup by putting the apparatus (see Fig. 1b) upon the aquaria each day during 15 min, but without cricket. The intensity of the spotlights was increased gradually. In
each of these habituation sessions the flap that covered the slit in the cover was opened three times, and each time a cricket fell into the aquarium. After 3 weeks all subjects ate the crickets within 15 min. Meanwhile the treatment of the preceding weeks continued. Tests were taken on 3 successive days with the target (cricket) at 35 cm on day 1, at 45 cm on day 2, and at 55 cm on day 3. The height of 55 cm was the maximal height the worst control subject had attained during exercise. Lower targets were used to decrease the risk of getting no response from experimental subjects. The order in which the subjects were tested was randomised over days. A test session started by gradually increasing the light intensity, and putting the target on the wire and the reward into place (see Fig. 1b). After 5 min the first trial was started by opening the flap, while a cricket (lure) dropped into the water. The moment the subject took this lure was recorded, and when the subject rose in the direction of the target on the wire, the camera was started and two successive squirts were filmed. After the second squirt the wire with the fixed target was dropped while the reward was pushed into the water (see Fig. 1b). The trial was ended by closing the flap. Thus the fishes were rewarded for squirting, not specifically for hitting. For each target height four such trials were given successively. When a fish did not take the lure within 10 min, or did not squirt within 10 min after taking the lure, the session was cancelled and started anew the next day. Subjects that did not cooperate thrice, were discarded.
Table 2 Schematic procedure of Experiment II Assignment week 1
Treatment weeks 2–27
Habituation weeks 25–27
Test week 28
According to the ratio hits: misses, at target heights of 10 cm and 15 cm, 5 SsExperimentals, and 6 Sscontrols.
Controls got 6 days practice During 15 min daily, all Ss got All Ss were filmed: day 1, weekly (procedure see Table 1). test setup upon home tank, and target at 35 cm, day 2, Experimentals got no practice. spotlights on. target at 45 cm, day 3, target at 55 cm. Eight squirts daily.
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2.5. Data-analysis Some subjects in Experiment I did not always produce four squirts before the trial was over, therefore several parameters were expressed as percentages of the number of squirts per subject (see Table 3). Differences between groups were tested by means of the Mann – Whitney U-test, or the Fisher exact probability test, differences between dependant samples were tested by means of the Sign test (Siegel and Castellan, 1988). Differences in mouth angles between groups, and between individuals were tested by means of UNIANOVA (Maxwell and Delaney, 1990). 3. Results
3.1. Experiment I: are misses abo6e the target more frequent than below? Table 3 presents for each subject (column 1); (1) test parameters, which include, as percentages from the number of squirts per subject: hits, (column 2), squirts at right elevation (column 3), too high squirts (column 4),6 too low squirts (column 5), and as percentages of squirts at right elevation, hits (column 6). (2) Subject characteristics, which include: target height increase during practice (column 7), whether the mandibles were normal or too long (column 8), and whether or not the subject completed all sessions or broke off in the course of the first 6 weeks of the 12 weeks test period (column 9). The data in Table 3 have been arranged according to the rank order of the percentages of hits from squirts per subject (column 2), and the subjects have been divided into a ‘low success’ category (hits B12%), and a ‘high success’ category (hits \ 20%). The main question was about the distribution of squirts with respect to elevation (right, too high, too low), but the high incidence of subjects with deviating subject characteristics, and the apparent break up into two categories (low and high success), are reasons to deal with the deviating subjects first. Two subjects, number 11, which had turbid eyes, and number 10, which had elongated mandibles, refused to squirt from the start and were not implicated in the analysis of the results.
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Six subjects had elongated mandibles, eight subjects did not complete all sessions (i.e. stopped squirting before half of the 12 weeks observation period was over), and 14 subjects did not achieve increase in target height during practice. The great majority of these subjects belonged to the low success category, which, next to the low percentage of hits, differs from the high success category in the following respects. More subjects in the low success category squirted more than 60% too high than subjects in the high success category did, but the categories do not differ with respect to squirting too low (Fisher exact test, resp. PB 0.001; PB 0.38). More subjects from the low success category achieved no target height increase, and did not complete all sessions than subjects from the high success category (Fisher exact test, resp. PB 0.001; PB0.002). All subjects with elongated mandibles belonged to the low success category (Fisher exact test, P B 0.001). The six subjects with elongated mandibles squirted significantly more often too high than the normal subjects did (M.W.U. test, n1 = 6; n2 = 22; z= 3,63; PB 0.00016). Only one subject (30) with elongated mandibles completes all sessions, though it only scores 1.2% of hits. Because of these differences the low success category will not further be analysed with respect to the question about the number of too high and too low squirts. The reasons to disregard subjects with elongated mandibles, and subjects not completing their sessions, are self evident. The reason to disregard subjects not achieving target height increase, is our previous finding in two studies that young archer fishes, in the course of 12 weeks practice sessions, achieved target height increase of 28 cm, respectively, 26 cm on an average. For the same reason three subjects from the high success category (1, 3, & 13) were disregarded, subject 3, moreover, not having completed its sessions. The remaining subjects in the high success category (n= 14) completed all sessions, achieved target height increase, and had normal mandibles. In the paragraphs to follow we elaborate the results concerning the main question, which was about the distribution of too low and too high squirts, with the results from these 14 selected
High success
Low success
2 13 12 4 1 27 16 24 29 28 9 3 6 23 8 21 22
11 10 7 20 30 17 14 5 15 25 19 26 18
Subjects
20.5 22.2 22.6 23.0 23.7 24.4 26.7 33.3 33.6 36.2 36.2 37.5 38.2 39.0 41.0 43.5 56.5
0.0 0.0 0.0 0.0 1.2 1.8 4.9 6.6 7.7 7.8 9.5 11.6 11.8 23.3 28.6 46.2 26.9 27.6 32.9 46.7 39.2 39.3 41.3 55.3 68.8 40.0 39.0 43.6 65.9 56.5
0.0 0.0 0.0 10.0 4.6 5.4 19.5 19.2 17.9 11.5 19.0 19.7 14.4 54.8 0.0 4.2 73.1 6.6 36.6 18.7 0.0 29.9 21.3 30.9 18.7 18.2 7.3 0.0 27.1 25.6
0.0 0.0 69.4 80.0 94.8 67.8 78.1 5.3 78.7 88.5 76.2 60.5 0.0 21.9 71.4 49.6 0.0 65.8 30.5 34.6 60.8 30.8 37.4 13.8 12.5 41.8 53.7 56.4 7.0 17.9
0.0 0.0 30.6 10.0 0.6 26.8 2.4 75.5 3.4 0.0 4.8 19.8 85.6
% Too low
% Hits from right elevation
93.7 77.8 49.0 85.7 85.7 74.0 60.6 85.0 85.7 87.8 70.8 54.5 91.3 100 94.1 66.0 100
0.0 0.0 0.0 0.0 20.0 33.3 25.0 34.5 42.8 66.7 50.0 58.8 81.8 10 0 20 10 0 10 15 15 30 15 15 0 20 10 15 35 15
0 0 0 0 0 0 0 0 0 0
0 0
Normal Normal Normal Normal Normal Normal Normal Normal Normal Normal Normal Normal Normal Normal Normal Normal Normal
Normal Too long Too long Too long Too long Normal Too long Normal Normal Too long Too long Normal Normal
Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No Yes Yes Yes Yes Yes
No No No No Yes Yes No Yes No No No No Yes
Mandible: Practice and test normal/too long sessions completed: yes/no
Target height increase during practice (cm)
% Too high
% Hits
% Right elevation
Subject characteristics
Test parameters
Table 3 Test parameters and subject characteristics of subjects with low and high success
28 P.J.A. Timmermans, J.M.H. Vossen / Beha6ioural Processes 52 (2000) 21–34
P.J.A. Timmermans, J.M.H. Vossen / Beha6ioural Processes 52 (2000) 21–34
subjects in the high success category. Six subjects more often squirted too high than too low, and eight subjects more often too low than too high, but this discrepancy is not significant (Sign test, n= 14; x = 6; P= 0.395). Table 3 shows that subjects differed much from each others with respect to too high squirts, too low squirts, and squirts at right elevation. In most subjects the distribution is unbalanced: in nine subjects the percentages too high and too low differed more than factor 2, two of them (8 & 24) never scored too high, and one (4) never too low. Altogether there are no indications whatsoever that the fishes had a tendency to squirt too high, because there were six subjects with a higher too high than too low score, versus eight subjects with a lower too high than too low score. Squirting predominantly too high or too low did not affect the percentage of squirting at the right elevation (M.W.U. test, n1 =6; n2 =8; U= 24; P=0.525). The median percentage of squirting at right elevation is 44 for or subjects squirting predominantly too high, and 40.5 for subjects squirting predominantly too low. Although not measured directly, the findings show that marksmanship on the azimuth also varied greatly. As to be expected, most subjects also made failures on the azimuth, which becomes apparent from the percentages (column 6, high success) of hits from squirts at right elevation (‘hits’ were squirts on the target; ‘at right elevation’ were squirts at target height, i.e. on target+ left and right of it), which in only two subjects (22 and 23) amount 100, and from the finding that the percentages of hits (column 2, high success) are lower than the percentages of squirts at right elevation (column 3, high success), the medians amount, 34.5 and 40.5%, respectively (Sign test, n = 12; x =0; P =0.001). The finding that the percentage of squirts at right elevation (column 3, high success), with a median of 40.5, is lower than the percentage of hits from squirts at right elevation (column 6, high success), with a median of 85.7, (Sign test, n=14; x =2; P =0.006), suggests that it is more difficult to achieve right elevation than right azimuth (on right elevation). Because there are no data on the ranges of elevation angles and azimuthal angles, it can only be concluded that (proportionally) the right elevation angle was
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achieved less often than the right azimuthal angle on the right elevation.
3.2. Experiment II: does depri6ation of squirting affect performance? Differences in performance between controls (n= 6), which got daily practice, and experimentals (n= 5), which were deprived of practice, were minor. Nevertheless we choose to elaborate the outcome of a number of measures (hits, latencies, intervals, and angles), because it concerns a unique and detailed comparison, and apart from that, we could assess individual mouth angles, which are an interesting measure of aiming in general. One control subject refused to cooperate during testing. One other control subject had developed an elongated mandible and will be treated separately. The four remaining subjects in the control group and four subjects in the experimental group scored hits. Neither for separate target heights, nor for the lumped data, there was a difference between the experimental and control group in the number of hits, no matter whether the numbers of subjects that did hit, or the numbers of whole groups are compared (M.W.U. test, n1 = 5 and n2 = 4; U= 2; P= 0.20). It is remarkable that also in the experimental group, hitting occurred already during the first session (height 35), two subjects even hitting already at the first squirt, and in seven out of the eight squirts of this session. In both groups, subjects that scored hits rarely missed more than 3 cm, whereas the experimental subject that scored no hits at all, systematically squirted too high. The experimental group and the control group did not differ in latencies to the first squirt of a trial, either for each target height separately or for the lumped data (M.W.U. test, n1 = 5 and n2 = 4; resp. height 35: U=10, P= 0.55; height 45 and 55: U= 6, P= 0.34; total: U= 9, P= 0.55). The intervals between the first and the second squirt of trials (see Fig. 2) seem to be somewhat longer in the experimental group than in the control group, but this difference approaches significance only for target height 55 (M.W.U. test,
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P.J.A. Timmermans, J.M.H. Vossen / Beha6ioural Processes 52 (2000) 21–34
Fig. 2. Medians and semi-interquartile ranges of intervals between squirt 1 and squirt 2 of trials, for three target heights.
n1 = 5 and n2 = 4; U =3; P =0.056). The interval between first and second squirt is much shorter than the latency to the first squirt, i.e. a median interval of 5.5 s versus a median latency of 24.1 s. The reason is, that when the first squirt does not yield prey, which was the case in the procedure we applied, archer fishes tend to give the second squirt from the same location, within a few seconds, which is the time needed for aiming anew (aiming: with the snout at the surface, as an apparent pivot point, the fish pitches to enlarge the angle between body axis and surface immediately before squirting). This tendency brings us to the question of differences in aiming, as far as the ratio aiming: squirting is concerned. For the lumped data, aiming was followed by squirting in 81.3% of the cases in controls, but in experimentals in 65.2%. Only at target height 35 (first session), the difference is significant (M.W.U. test, n1 = 5 and n2 =4; U = 1.5, 0.016BP B 0.032). Fig. 3 shows the frequency of aiming without squirting. The fish’s habit to quickly squirt again from the same position in case of no reward, led to a discrepancy between the intervals ‘first aimingfirst squirt’ and ‘second aiming-second squirt’; the first interval being the longest (for both groups together, Sign test, n = 8; x =1; P = 0.02). This
Fig. 3. Medians and semi-interquartile ranges of frequency of aiming without squirting, for three target heights (* 0.016 B P B0.032).
difference (see Fig. 4) is significant in experimentals (Sign test, n= 5; x= 0, P= 0.031). In controls no difference is apparent, and n is too small for statistical testing. However, the intervals do not differ significantly between the groups in any session, nor in the three sessions together.
Fig. 4. Medians and semi-interquartile ranges of intervals between aiming and squirting, for squirt 1 and squirt 2 (* P= 0.031).
P.J.A. Timmermans, J.M.H. Vossen / Beha6ioural Processes 52 (2000) 21–34
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Fig. 5. Schematic distribution of individual mouth angles with their medians (m).
The film enabled us to measure positions and angles. There were no differences between the control and experimental group with respect to the place from where squirting occurred (distance from the target’s perpendicular), and with respect to the squirting angle (angle between the line from the snout through the squirt, and the surface of the water). Also the mouth angle (angle between the body axis and the squirt at the dorsal side of the fish) did not differ between groups (UNIANOVA, df=1; F= 1,230; P= 0.34). The mouth angle appeared to vary considerably between subjects (UNIANOVA, df= 7; F = 113 373; P=0.000) but to be relatively constant within subjects (see Fig. 5). The range of both groups together (n=9) stretched from 142 – 166°, the individual ranges amounted to 8° in one subject, 10° in six subjects, and 11° in two subjects. It is interesting to note that the subject with the elongated mandible had a considerable smaller mouth angle, ranging from 110 to 128°, which is in accordance with the deformation of the mandible that changes the squirt’s direction.
4. Discussion The main question to be answered by Experiment I was whether misses above the target are more frequent than misses below the target. In the Introduction we hypothesised that if misses predominantly are too high this could indicate that the refraction effect is being corrected for by means of feedback from the consequences of previous squirts. It soon became clear that individual characteristics differed too much to analyse the data in a strait forward fashion. The reason why a considerable number of subjects were squirting too high more often than too low, obviously had nothing to do with the virtual image, but was caused by an elongation of their mandible, a phenomenon to be dealt with later on. Moreover, a great deal of subjects did not achieve increase of target height during practice, like we found in preceding experiments, and most of these subjects stopped squirting before the first 6 weeks of the 12 weeks observation period were over.
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It was decided to further elaborate the data only with 14 so called successful subjects which fulfilled three criteria: apparently normal mandibles, a complete record of practice and test sessions, and increase in target height in the course of the practice period. The data from these subjects clearly show that the range of trajectories of squirts was unbalanced where elevation is concerned, but the number of subjects mainly squirting too high did not differ from the number squirting mainly too low, and too high and too low ‘squirters’ did not differ in percentages of squirting correct. So, as far as elevation failures are concerned, there are no indications that the fishes were using feedback from effects of squirts, as Lu¨ling (1958, 1963) suggested, or corrected for refraction with the virtual image as a point of reference. The fishes rather seemed to be programmed to squirt in a certain range that overlapped the target. For reasons to follow, it was a pity that we did not assess the exact dimensions of that range. On the azimuth too, there apparently was a distribution of trajectories. Lu¨ling (1958) suggested that the right angle of elevation is more easy to achieve than the right azimuthal angle. However, our data suggest the opposite to be true, since they reveal that the percentage of squirts at right elevation is lower than the percentage of hits (right azimuth at right elevation) from squirts at right elevation. Imagine the target to lie on a plane perpendicular to the trajectory of the squirts, then the squirts pass through that plane in a range around the target. According to Lu¨ling’s view, this range is an ellipse the major axis of which is horizontal. Our data give no direct information about the shape of the range, but are suggestive of an elliptic range with a vertical major axis, because the percentage of squirts at right elevation is lower than the percentage of hits from squirts at right elevation. So it seems, that the right azimuthal angle is easier to achieve than the right elevation. Because the fishes use variable aiming angles deviating from 90° (Dill, 1977), in a range of 60–110° (Timmermans, 1975), this is what one would expect as a consequence of gravity, even when a refraction effect is left aside. We now pay attention to the fish with elon-
gated mandibles, which all overshot the target. Such fish were encountered earlier (Elshoud and Koomen, 1985). In our ‘long-jaws’ it was directly visible, and verified by film in Experiment II, that the protruding tip of the lower jaw forced the jet of water higher up. The behaviour of ‘long-jaws’ possibly provides more direct clues with respect to the question whether archer fish can learn to adjust their aim by means of positive feedback (cues that the preceding aiming was right). The use of positive feedback, presupposes that the range of trajectories of squirts overlaps the target’s position, so that accidental hits occur now and than. The subjects that developed into ‘longjaws’ at first did score hits. As their deformation developed gradually, they could have learned to adapt their aiming to its effects, but they did not, failed to hit anymore, and stopped squirting. That this refusal was caused by non-reward, was revealed by the finding that these fish could easily be shaped to resume squirting by successively rewarding them for approaching, aiming, and squirting. That squirting can be brought under stimulus control, already was reported by Waxman and McCleave (1978), and by Goldstein and Hall (1990). Unfortunately it is not clear what caused the elongation of mandibles, anyhow, swimming against the glass played no part. The question to be answered by Experiment II was whether deprivation of squirting would affect performance. We found the main measures, hitting and speed (latency to squirting) to be unaffected. Neither did we find differences in the position from which the fish squirted, and in the squirting angles and the mouth angles. The only difference we found was, that during the first session, experimental (practice deprived) subjects more often aimed without squirting than controls (that got daily practice). Probably, this phenomenon is related with the finding that in experimentals the interval between aiming and squirting was longer for the first squirt than for the second one. However, as we have seen already, these differences did not lead to a difference in latency to squirt. To resume, the achievement of archer fishes that had been deprived of practice during six months was no less
P.J.A. Timmermans, J.M.H. Vossen / Beha6ioural Processes 52 (2000) 21–34
than that of controls, but during the first session, practice deprived subjects interrupted significantly more attempts than controls did. The film did not reveal differences in performance between attempts that were interrupted and attempts that were carried on. Maybe more detailed images will give an answer to this question. The width of the range of individual mouth angles (angle between the body axis and the squirt, at the dorsal side of the fish) of 8 – 11° that we found, is much smaller than the range of 30° reported by Lu¨ling (1958, 1963). Differences between our subjects led to a total range of 142 – 167° for our sample (n =9). Lu¨ling reported a range of 140–170°. So, it could be that Lu¨ling’s range refers to the range from his sample (n = 7) rather than to a common range all subjects used, as his text suggests. The concurrence between Lu¨ling’s range and ours suggests that, in this respect, there is no difference between the species T. jaculatrix, used by Lu¨ling, and the species we used, T. chatareus. It is not clear why one of our experimental subjects squirted too high in all trials. Although this subject had no extreme range of mouth angles, it cannot be excluded that a deformation of the mandibles was developing, but it could also be that the fish was aiming at a target beyond the camera’s view. Rewarding the fishes after each second squirt, as we did, cannot be expected to counteract squirting at a wrong target. To conclude, in Experiment I we hypothesised that if archer fishes would use feedback from preceding squirts to adapt their aiming, too high squirts probably would prevail over too low squirts, and in Experiment II we hypothesised that if archer fishes would have to learn how to aim correctly, performance would be affected by deprivation of practice. Essentially, the findings from the experiments revealed that (1) squirting above the target does not prevail, that (2) chances to hit are not lowered by disuse. The accidental occurrence of a number of specimens with elongated mandibles, moreover, revealed that (3) the fish do not adapt their aiming to the deviation of the squirt caused by their deformation. These findings all suggest that aiming once developed is resistant to change, in other words, the fishes seemed programmed to squirt in a certain range.
33
Since all findings were obtained with fish that squirted effectively already at the start of the observations, we can not decide whether the programming is genetic or acquired through early experience. The resistance to change we found, i.e. the failure to find indications of a learning process, suggests a genetic program.
Acknowledgements We thank technicians of the psychology laboratory for instrumentation, P. Cloosterman for filming, A.J.D.M. van Eil for collecting and analysing data, A.J.H. Willems for statistic assistance, and M.Th.M. Janssen for taking care of the animals.
References Bekoff, M., Dorr, R., 1976. Predation by ‘shooting’ in archerfish, Toxotes jaculatrix: accuracy and sequences. B. Psych. Soc. 7, 167 – 168. Dill, L.M., 1977. Refraction and the spitting behavior of the archerfish (Toxotes chatareus). Behav. Ecol. S. 2, 169 – 184. Elshoud, G.C.A., Koomen, P., 1985. A biomechanical analysis of spitting in archerfishes (Pisces, Perciformes, Toxidae). Zoomorphology 105, 240 – 252. Goldstein, S.R., Hall, D., 1990. Variable ratio control of the spitting response in the archer fish (Toxotes jaculator). J. Com. Psych. 104, 373 – 376. Hediger, H., Heusser, H., 1961. Zum ‘Schiessen’ des Schu¨tzenfisches, Toxotes jaculatrix. Natur und Volk 91, 237 – 243. Herald, E.S., 1965. How accurate is the archer fish? Pac. Discov. 9, 12 – 13. Lu¨ling, K.H., 1958. Morphologisch-anatomische und histologische Untersuchungen am Auge des Schu¨tzenfisches Toxotes jaculatrix (Pallas 1766) (Toxotidae), nebst Bemerkungen zum Spuckgehaben. Z. Morph. O8 kol. Tiere 47, 529 – 610. Lu¨ling, K.H., 1963. The archerfish. Sci. Am. 209, 100 – 129. Lu¨ling, K.H., 1964. Der Schu¨tzenfisch Toxotes jaculatrix und sein Benehmen. Acta Soc. Zool. Bohemoslov. ZVAZEK XXVIII 3, 250 – 260. Lu¨ling, K.H., 1969. Spitting at prey by Toxotes jaculatrix and Colisa lalia. Bonn. Zool. Beitra¨ge 20, 416 – 422. Maxwell, S.E., Delaney, H.D., 1990. Designing Experiments and Analying Data. Brooks and Cole, Pacific Grove, pp. 455 – 488. Meyers, G.S., 1952. How the shooting apparatus of the archerfish was discovered. Aquar. J. 23, 210 – 214.
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Milburn, O., Alexander, R. McN, 1976. The performance of the muscles involved in spitting by the archerfish Toxotes. J. Zool. Lond. 180, 243–251. Siegel, S., Castellan, N.J., 1988. Nonparametric Statistics for the Behavioral Sciences. McGraw-Hill, New York. Timmermans, P.J.A., 1975. The prey catching behaviour of the archerfish. Toxotes Neth. J. Zool. 25, 381.
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Timmermans, P.J.A., Maris, E., 2000. Does the bright spot on the back of young archer fishes serve group coherence? Neth. J. Zool. 50, 4. Verwey, J., 1928. Iets over de voedingswijze van Toxotes jaculator. De Trop. Nat. 17, 162 – 166. Waxman, H.M., McCleave, J.D., 1978. Autoshaping in the archerfish (Toxotes chatareus). Behav. Biol. 22, 541 – 544.
Prey catching in the archer fish: does the fish use a learned correction for refraction? P.J.A. Timmermans *, J.M.H. Vossen Department Comparati6e and Physiological Psychology, NICI, Uni6ersity of Nijmegen, P.O. Box 9104, 6500 HE Nijmegen, The Netherlands Received 6 January 2000; received in revised form 19 May 2000; accepted 9 June 2000
Abstract An answer to the question of how the archer fish hits an aerial insect, despite the refraction of light at the surface of the water has not yet been found. The aims of the present studies are to find out: (1) whether the fish applies a learned correction with the virtual image as a point of reference; (2) whether deprivation of practice in squirting affects performance. For the first aim the accuracy of squirts was measured in 30 subjects. Contrary to suggestions from the literature, elevation failures were prominent but the frequencies of over- and under-squirting did not differ, which does not support the idea that the fishes applied a learned correction for refraction by using feedback from the efficacy of squirts. For the second aim, five experimental subjects were deprived of practice, whereas six control subjects got daily practice, during 6 months. The only significant difference, found thereafter, was that during the first session experimental subjects aimed more often before squirting than control subjects did, but hitting was not affected. A number of subjects developed abnormal mandibles which inevitably led to squirting too high. Our findings do not support the hypothesis that the archer fish uses learned corrections for refraction. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Archer fish; Prey catching; Aiming
1. Introduction Archer fishes bring down insects with a squirt of water from their mouth. During squirting the tip of the mouth just breaks the surface but the eyes stay well below. For obvious reasons the attention paid to this peculiar way of prey catching became * Corresponding author. E-mail address: p – [email protected] (P.J.A. Timmermans).
focused primarily on two questions: how the fish ejects a squirt of water (see e.g. Elshoud and Koomen, 1985), and how it can hit its target despite the shift of the prey’s image caused by the refraction of light at the surface of the water (see e.g. Dill, 1977). The first question was elaborated by Milburn and Alexander (1976), and Elshoud and Koomen (1985). Remarkably, the refraction problem was put forward not earlier than in 1952 (Meyers, 1952), but Meyers did not elaborate on it, and it still has not been solved.
0376-6357/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 6 3 5 7 ( 0 0 ) 0 0 1 0 7 - 8
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Several mechanisms for coping with refraction, which remarkably, all are behavioural, have been suggested and some have been studied. Hediger and Heusser (1961), by means of high speed filming, falsified their hypothesis that the archer fish’s squirt consists of a very fast series of droplets during the ejection of which the fish adjusts its aim with the aid of visual feedback from the trajectories of droplets, like a machine gunner does with tracers. In his extensive papers Lu¨ling (1958, 1963) assumed that the archer fish (Toxotes jaculatrix) encounters two problems: refraction of light, and parallax between line of sight and direction of the squirt. For the greater part, the fish would avoid both problems by squirting nearly vertically. However, this idea is contradicted by the Verwey (1928) report, Lu¨ling’s own illustrations (Lu¨ling, 1958, 1963, 1964, 1969), and by the Herald (1965) photograph, all showing T. jaculatrix squirting at angles between 70 and 80°. The Bekoff and Dorr (1976) confirmation of Lu¨ling’s theory, is not supported by their finding that targets at a height of 30 cm often were missed by 15 – 20 cm. For T. chatareus Timmermans (1975) observed that aiming angles varied between 60 and 110°. The aiming angle is the angle between the water surface and the line from the fish’s snout to the target, at the moment of squirting. The angle of the squirt may differ a few degrees from the aiming angle. Dill (1977) affirmed that other angles than 90° were used successfully, and in his thorough analysis demonstrated that the refraction archer fishes have to cope with is not trivial. According to Lu¨ling (1958), most misses scored by T. jaculatrix were on the azimuth (left or right of the target) not in elevation (over or under the target). We feel, that in fish squirting approximately vertical, that this indeed could be the case, but as we have seen T. chatareus, and probably also T. jaculatrix often use other aiming angles than 90°. According to Dill (1977) T. chatareus does make elevation failures, which is supported by our observations. It is clear that the best chances to find an answer to the question of how the archer fish succeeds in hitting, are offered by studying the development of squirting in young specimens.
However, the smallest specimens to be obtained (1.5 cm) already are squirting successfully, and there are no reports on breeding in captivity to be found. We succeeded in getting archer fishes to spawn in captivity, but the eggs did not hatch (see also Timmermans and Maris, 2000). Therefore, we chose to work with grown subjects. We did two experiments. In the first one we assessed the distribution of elevation failures; in the second one we assessed whether deprivation of squirting affected performance. Our line of reasoning in Experiment I is as follows. The virtual image of the target is perceived above the position of the real target. Roughly speaking, this refraction effect could be corrected for in two ways: (1) The archer fish is somehow programmed (as a consequence of genetical selection, or learning during an early sensitive period) to hit a target despite refraction; (2) The archer fish keeps adapting its aiming behaviour to the effects of preceding squirts (operant conditioning). Lu¨ling (1958) suggested that his fish used the location of the impact of misses on the target board as negative feedback (use of cues that the preceding aiming was wrong), and moreover held them capable to make use of the relative positions of target and misses to readjust their aiming for the next trial. But Lu¨ling did not put this idea to a test, and the only example he described is not convincing: squirt 1 miss to the left; squirt 2 miss even more to the left; squirt 3 again to the left but less than 1; squirt 4 hit. Moreover, adjustment of aiming based on impacts of misses, like humans use it, presupposes considerable cognitive capacities. Furthermore, the prey’s background in the natural habitat will be rather variable, so impacts of misses not always will offer useful feedback, or will even be absent. A simpler mechanism could be the use of positive feedback (use of cues that the preceding aiming was right), i.e. to repeat squirts that are rewarded with prey, but this idea has not been tested either. To proceed with our argumentation: In case 1, where the fishes are somehow programmed to hit a target despite refraction, the target itself remains the aim, and there is no adjustment of aiming based on its effects. In this case one would expect an equal number of misses above and below the
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target in a sample of subjects. Misses will occur, because aiming remains a noisy process. On the individual level one would expect, among others, differences in the location of the target within the range of impacts. In case 2, however, where the fishes are using feedback to adjust their aiming, refraction remains a potential cause of systematic bias. The fishes should try to repeat what they did when they (accidentally) hit the target although they aimed at the virtual image, or they should try to avoid preceding misses. In any case, they should learn to aim somewhere below the virtual image, i.e. no longer to use the virtual image itself as the aim, although the virtual image remains their only point of reference, as there are no reliable points of reference in the background, for example, like we have at our disposal when using a bull’s eye with concentric circles. One therefore would expect misses above the target to be more frequent than below it. The purpose of Experiment I was to test whether misses above the target were more frequent than misses below the target. A significantly larger number of misses above than below the target would contradict the hypothesis of a programmed correction for refraction. During the observations it soon appeared that a considerable number of subjects, that were consistently squirting too high, had elongated mandibles. We therefore decided also to pay some attention to this phenomenon. In Experiment II our line of reasoning is as follows. If archer fishes learn (by trial and error) how to aim, it is conceivable that deprivation of practice could reduce the chances to hit, or otherwise affect performance, also in subjects that already learned how to aim. If, on the other hand, the way of aiming, once it has developed, is not adapted to its consequences when it is used, it is to be expected also to be insensitive to disuse. The purpose of Experiment II was to compare the performance of a group that had got daily exercise with the performance of a group that had been deprived of squirting for a long time. In order not to miss subtle effects, we filmed the tests so we could take a great number of measures.
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2. Materials and methods
2.1. Subjects and housing All subjects were imported and obtained from an aquarium store. They were kept solitarily in glass tanks (50× 30× 30 cm), separated by opaque partitions to exclude mutual influences on behaviour during the tests. Subjects were fed morning and evening with a measured portion of: small crickets, small meal-worms or daphnia that were thrown into the water. During experiments the fish were fed only once daily, so that they were deprived of food for 924 h before practice and test sessions in their home aquarium. Small dead crickets were used as targets and as rewards during these sessions. The tanks were aerated except during practice and test sessions, when pumps were turned off to ensure a calm water surface. Water temperature was kept at 25°C, pH between 6.5–7.5°, and hardness at 6.5 DH. Fluorescent lights were on from 20:00 to 08:00 h. In Experiment I, 30 subjects were used with an average length of 4.3 cm at the start. In Experiment II, 11 subjects, of initially 2–2.5 cm, were used. During 6 months six control animals got daily practice (squirting at food items), and five experimental animals were deprived of squirting at food items.
2.2. Experimental set up The experimental setup (Fig. 1a) used for practice sessions in Experiment I and II, and for test sessions in Experiment I, permitted to offer targets (dead crickets, 3–4 mm) at various heights on a vertical white target board (150× 10 cm). A triangular ridge in the middle of the board prevented scoring indirect hits by means of ricochets, because drops hitting the ridge were split and sideways deflected by the triangle. On the ridge, at intervals of 5 cm a thin 3 cm long steel wire with a loop (3 mm ø) could be inserted. On this loop the target was offered, so that it fell into the water when it was hit by the fish. In order to facilitate recording, the observer was sitting in front of the aquarium, while keep-
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ing his eye and the target in line with the centre of a cross (on target height) on the wall behind the aquarium. Thus he could reliably observe whether a squirt hit the target, whether squirt and target
overlapped (missing in front or behind the target at the right elevation), whether the squirt went over, or under the target. To further enhance reliability, only a narrow slit (1.5 cm) was left
Fig. 1. (a) Schematic view of home aquarium with setup for practice in Experiment I and II, and for tests in Experiment I: 1, opaque cover; 2, slit for squirting (1,5 cm wide); 3, target board with loop (adjustable in height) to offer cricket on; 4, plastic fringe curtain to be used for shelter; (c –d) transection of target board showing triangular ridge to prevent hits from ricochets. (b) Setup, for tests in Experiment II, to be put upon home aquarium after removal of target board. 1, opaque cover; 2, slit for squirting (1.5 cm wide) with flap to close it between trials; 3, support (adjustable in height) for wire (4) with cricket (target) that is dropped out sight behind screen (5) when the subject has squirted; 6, cricket (reward) that is pushed into aquarium at the moment the target disappears behind screen.
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Table 1 Schematic procedure of Experiment I Practice and tests during 12 successive weeks Practice
Test
Weekdays 1–6, three crickets with 5 cm interspace were On weekday 7, four crickets were offered successively, during 1 offered simultaneously. If all crickets were hit within 3 min each, at the height of the lowest target of the cluster of min, next day the height of the cluster was increased 5 cm, the prior practice day. Squirts were scored as: hit, right if not, the height was decreased 5 cm. elevation, too high, or too low.
open in the cover of the aquarium, parallel to the front pane (see Fig. 1a). This narrow slit forced the subjects to take position parallel to the front pane. Thus all squirts were in the same plane, i.e. they went at right angles through the direction in which the observer was watching. For the tests of Experiment II a novel setup was used, because the setup (target board) used for practice of the control group during 6 months, would have given the controls an advantage over the experimental group. The novel set up (see Fig. 1b) could be put on the aquaria after removal of the target board. The experimenter could open the slit in the cover from a distance. The target (cricket) was fixed to a thin 10 cm long wire with a pivot joint on the vertical support on the rim of the tank. When the pivot was released, the wire with the target (fixed cricket) fell behind a little screen, disappearing out of sight of the fish, while at the same time a relay pushed another cricket (reward) into the aquarium, right below the place were the fixed target just disappeared. In a pilot test, fishes reacted to this procedure like they did when they squirted a cricket down, i.e. they rushed forward and took the food. A spotlight shining in the direction in which the fish was to squirt illuminated the squirt. The fish was illuminated by a spotlight above the camera, which was placed at right angles in front of the aquarium.
2.3. Procedures 2.3.1. Experiment I: are misses abo6e the target more frequent than below? Since in natural conditions archer fishes will gain experience in squirting, practice and test
sessions were taken in the course of 12 weeks (see Table 1). Practice sessions were given on weekdays 1–6. Each session three crickets (3–4 mm) were offered simultaneously, one on each of three successive loops (5 cm apart) on the target board. If all three targets were hit within 3 min, the height of the cluster of three targets was increased 5 cm on the next day, in case not all three targets were hit target height was decreased 5 cm. Targets that were hit fell into the water and were eaten, targets that had not been hit were thrown into the water at the end of the session, but not immediately after the fish had squirted. So, hitting was reinforced immediately, whereas missing and notsquirting were not reinforced, and the total amount of food was independent of performance. To be sure that the subjects saw the targets, each session started by dropping a cricket into the slit in the cover of the aquarium by means of tweezers. The timing of the session started after the subjects had taken this lure. Each week on the 7th day there were test sessions following the same procedure as practice sessions, except that this time four fixed targets were offered successively, during trials of maximally 1 min each, 5 cm below the lowest target of the preceding practice session. After a subject had squirted four times, or when the 1 min trial had passed, a cricket was thrown into the water, and the next trial was started. Each squirt was scored as hit, on right elevation, too high, or too low. Right elevation means: the squirt’s trajectory is seen to overlap the target (see experimental setup), i.e. the squirt passes in front of, or behind the target, or it hits the target. Since the diameter of target and squirt amounted to 3–4 and 5–7
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mm, respectively, the ‘right elevation zone’ was maximally 13–18 mm, depending on individual variation. Consequently, too high and too low means respectively: over and under the right elevation zone. Right elevation was subdivided into hits and misses. So, a miss, in this case, was a failure on the azimuth. A hit was a squirt on the target.
2.4. Experiment II: does depri6ation of squirting affect performance? The following procedures will be explained successively: assignment to groups; treatment; habituation; testing (see Table 2). During the first week 11 subjects were tested individually by successively offering three (loose) crickets on the target board at a height of 10 cm, and three at 15 cm, and registering the number of hits (when hit, the cricket was lifted of the loop and dropped into the water), and misses per cricket during 3 min. According to the ratio hit: miss the subjects were evenly assigned to the experimental (n=5) and to the control group (n = 6). The control group got practice sessions (see Exp. I) on 6 days each week during 27 weeks. The experimental group got no practice, and since the aquaria had an opaque cover, there were no opportunities to obtain food by means of squirting. In the course of 3 weeks before testing, all subjects were habituated to the setup by putting the apparatus (see Fig. 1b) upon the aquaria each day during 15 min, but without cricket. The intensity of the spotlights was increased gradually. In
each of these habituation sessions the flap that covered the slit in the cover was opened three times, and each time a cricket fell into the aquarium. After 3 weeks all subjects ate the crickets within 15 min. Meanwhile the treatment of the preceding weeks continued. Tests were taken on 3 successive days with the target (cricket) at 35 cm on day 1, at 45 cm on day 2, and at 55 cm on day 3. The height of 55 cm was the maximal height the worst control subject had attained during exercise. Lower targets were used to decrease the risk of getting no response from experimental subjects. The order in which the subjects were tested was randomised over days. A test session started by gradually increasing the light intensity, and putting the target on the wire and the reward into place (see Fig. 1b). After 5 min the first trial was started by opening the flap, while a cricket (lure) dropped into the water. The moment the subject took this lure was recorded, and when the subject rose in the direction of the target on the wire, the camera was started and two successive squirts were filmed. After the second squirt the wire with the fixed target was dropped while the reward was pushed into the water (see Fig. 1b). The trial was ended by closing the flap. Thus the fishes were rewarded for squirting, not specifically for hitting. For each target height four such trials were given successively. When a fish did not take the lure within 10 min, or did not squirt within 10 min after taking the lure, the session was cancelled and started anew the next day. Subjects that did not cooperate thrice, were discarded.
Table 2 Schematic procedure of Experiment II Assignment week 1
Treatment weeks 2–27
Habituation weeks 25–27
Test week 28
According to the ratio hits: misses, at target heights of 10 cm and 15 cm, 5 SsExperimentals, and 6 Sscontrols.
Controls got 6 days practice During 15 min daily, all Ss got All Ss were filmed: day 1, weekly (procedure see Table 1). test setup upon home tank, and target at 35 cm, day 2, Experimentals got no practice. spotlights on. target at 45 cm, day 3, target at 55 cm. Eight squirts daily.
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2.5. Data-analysis Some subjects in Experiment I did not always produce four squirts before the trial was over, therefore several parameters were expressed as percentages of the number of squirts per subject (see Table 3). Differences between groups were tested by means of the Mann – Whitney U-test, or the Fisher exact probability test, differences between dependant samples were tested by means of the Sign test (Siegel and Castellan, 1988). Differences in mouth angles between groups, and between individuals were tested by means of UNIANOVA (Maxwell and Delaney, 1990). 3. Results
3.1. Experiment I: are misses abo6e the target more frequent than below? Table 3 presents for each subject (column 1); (1) test parameters, which include, as percentages from the number of squirts per subject: hits, (column 2), squirts at right elevation (column 3), too high squirts (column 4),6 too low squirts (column 5), and as percentages of squirts at right elevation, hits (column 6). (2) Subject characteristics, which include: target height increase during practice (column 7), whether the mandibles were normal or too long (column 8), and whether or not the subject completed all sessions or broke off in the course of the first 6 weeks of the 12 weeks test period (column 9). The data in Table 3 have been arranged according to the rank order of the percentages of hits from squirts per subject (column 2), and the subjects have been divided into a ‘low success’ category (hits B12%), and a ‘high success’ category (hits \ 20%). The main question was about the distribution of squirts with respect to elevation (right, too high, too low), but the high incidence of subjects with deviating subject characteristics, and the apparent break up into two categories (low and high success), are reasons to deal with the deviating subjects first. Two subjects, number 11, which had turbid eyes, and number 10, which had elongated mandibles, refused to squirt from the start and were not implicated in the analysis of the results.
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Six subjects had elongated mandibles, eight subjects did not complete all sessions (i.e. stopped squirting before half of the 12 weeks observation period was over), and 14 subjects did not achieve increase in target height during practice. The great majority of these subjects belonged to the low success category, which, next to the low percentage of hits, differs from the high success category in the following respects. More subjects in the low success category squirted more than 60% too high than subjects in the high success category did, but the categories do not differ with respect to squirting too low (Fisher exact test, resp. PB 0.001; PB 0.38). More subjects from the low success category achieved no target height increase, and did not complete all sessions than subjects from the high success category (Fisher exact test, resp. PB 0.001; PB0.002). All subjects with elongated mandibles belonged to the low success category (Fisher exact test, P B 0.001). The six subjects with elongated mandibles squirted significantly more often too high than the normal subjects did (M.W.U. test, n1 = 6; n2 = 22; z= 3,63; PB 0.00016). Only one subject (30) with elongated mandibles completes all sessions, though it only scores 1.2% of hits. Because of these differences the low success category will not further be analysed with respect to the question about the number of too high and too low squirts. The reasons to disregard subjects with elongated mandibles, and subjects not completing their sessions, are self evident. The reason to disregard subjects not achieving target height increase, is our previous finding in two studies that young archer fishes, in the course of 12 weeks practice sessions, achieved target height increase of 28 cm, respectively, 26 cm on an average. For the same reason three subjects from the high success category (1, 3, & 13) were disregarded, subject 3, moreover, not having completed its sessions. The remaining subjects in the high success category (n= 14) completed all sessions, achieved target height increase, and had normal mandibles. In the paragraphs to follow we elaborate the results concerning the main question, which was about the distribution of too low and too high squirts, with the results from these 14 selected
High success
Low success
2 13 12 4 1 27 16 24 29 28 9 3 6 23 8 21 22
11 10 7 20 30 17 14 5 15 25 19 26 18
Subjects
20.5 22.2 22.6 23.0 23.7 24.4 26.7 33.3 33.6 36.2 36.2 37.5 38.2 39.0 41.0 43.5 56.5
0.0 0.0 0.0 0.0 1.2 1.8 4.9 6.6 7.7 7.8 9.5 11.6 11.8 23.3 28.6 46.2 26.9 27.6 32.9 46.7 39.2 39.3 41.3 55.3 68.8 40.0 39.0 43.6 65.9 56.5
0.0 0.0 0.0 10.0 4.6 5.4 19.5 19.2 17.9 11.5 19.0 19.7 14.4 54.8 0.0 4.2 73.1 6.6 36.6 18.7 0.0 29.9 21.3 30.9 18.7 18.2 7.3 0.0 27.1 25.6
0.0 0.0 69.4 80.0 94.8 67.8 78.1 5.3 78.7 88.5 76.2 60.5 0.0 21.9 71.4 49.6 0.0 65.8 30.5 34.6 60.8 30.8 37.4 13.8 12.5 41.8 53.7 56.4 7.0 17.9
0.0 0.0 30.6 10.0 0.6 26.8 2.4 75.5 3.4 0.0 4.8 19.8 85.6
% Too low
% Hits from right elevation
93.7 77.8 49.0 85.7 85.7 74.0 60.6 85.0 85.7 87.8 70.8 54.5 91.3 100 94.1 66.0 100
0.0 0.0 0.0 0.0 20.0 33.3 25.0 34.5 42.8 66.7 50.0 58.8 81.8 10 0 20 10 0 10 15 15 30 15 15 0 20 10 15 35 15
0 0 0 0 0 0 0 0 0 0
0 0
Normal Normal Normal Normal Normal Normal Normal Normal Normal Normal Normal Normal Normal Normal Normal Normal Normal
Normal Too long Too long Too long Too long Normal Too long Normal Normal Too long Too long Normal Normal
Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No Yes Yes Yes Yes Yes
No No No No Yes Yes No Yes No No No No Yes
Mandible: Practice and test normal/too long sessions completed: yes/no
Target height increase during practice (cm)
% Too high
% Hits
% Right elevation
Subject characteristics
Test parameters
Table 3 Test parameters and subject characteristics of subjects with low and high success
28 P.J.A. Timmermans, J.M.H. Vossen / Beha6ioural Processes 52 (2000) 21–34
P.J.A. Timmermans, J.M.H. Vossen / Beha6ioural Processes 52 (2000) 21–34
subjects in the high success category. Six subjects more often squirted too high than too low, and eight subjects more often too low than too high, but this discrepancy is not significant (Sign test, n= 14; x = 6; P= 0.395). Table 3 shows that subjects differed much from each others with respect to too high squirts, too low squirts, and squirts at right elevation. In most subjects the distribution is unbalanced: in nine subjects the percentages too high and too low differed more than factor 2, two of them (8 & 24) never scored too high, and one (4) never too low. Altogether there are no indications whatsoever that the fishes had a tendency to squirt too high, because there were six subjects with a higher too high than too low score, versus eight subjects with a lower too high than too low score. Squirting predominantly too high or too low did not affect the percentage of squirting at the right elevation (M.W.U. test, n1 =6; n2 =8; U= 24; P=0.525). The median percentage of squirting at right elevation is 44 for or subjects squirting predominantly too high, and 40.5 for subjects squirting predominantly too low. Although not measured directly, the findings show that marksmanship on the azimuth also varied greatly. As to be expected, most subjects also made failures on the azimuth, which becomes apparent from the percentages (column 6, high success) of hits from squirts at right elevation (‘hits’ were squirts on the target; ‘at right elevation’ were squirts at target height, i.e. on target+ left and right of it), which in only two subjects (22 and 23) amount 100, and from the finding that the percentages of hits (column 2, high success) are lower than the percentages of squirts at right elevation (column 3, high success), the medians amount, 34.5 and 40.5%, respectively (Sign test, n = 12; x =0; P =0.001). The finding that the percentage of squirts at right elevation (column 3, high success), with a median of 40.5, is lower than the percentage of hits from squirts at right elevation (column 6, high success), with a median of 85.7, (Sign test, n=14; x =2; P =0.006), suggests that it is more difficult to achieve right elevation than right azimuth (on right elevation). Because there are no data on the ranges of elevation angles and azimuthal angles, it can only be concluded that (proportionally) the right elevation angle was
29
achieved less often than the right azimuthal angle on the right elevation.
3.2. Experiment II: does depri6ation of squirting affect performance? Differences in performance between controls (n= 6), which got daily practice, and experimentals (n= 5), which were deprived of practice, were minor. Nevertheless we choose to elaborate the outcome of a number of measures (hits, latencies, intervals, and angles), because it concerns a unique and detailed comparison, and apart from that, we could assess individual mouth angles, which are an interesting measure of aiming in general. One control subject refused to cooperate during testing. One other control subject had developed an elongated mandible and will be treated separately. The four remaining subjects in the control group and four subjects in the experimental group scored hits. Neither for separate target heights, nor for the lumped data, there was a difference between the experimental and control group in the number of hits, no matter whether the numbers of subjects that did hit, or the numbers of whole groups are compared (M.W.U. test, n1 = 5 and n2 = 4; U= 2; P= 0.20). It is remarkable that also in the experimental group, hitting occurred already during the first session (height 35), two subjects even hitting already at the first squirt, and in seven out of the eight squirts of this session. In both groups, subjects that scored hits rarely missed more than 3 cm, whereas the experimental subject that scored no hits at all, systematically squirted too high. The experimental group and the control group did not differ in latencies to the first squirt of a trial, either for each target height separately or for the lumped data (M.W.U. test, n1 = 5 and n2 = 4; resp. height 35: U=10, P= 0.55; height 45 and 55: U= 6, P= 0.34; total: U= 9, P= 0.55). The intervals between the first and the second squirt of trials (see Fig. 2) seem to be somewhat longer in the experimental group than in the control group, but this difference approaches significance only for target height 55 (M.W.U. test,
30
P.J.A. Timmermans, J.M.H. Vossen / Beha6ioural Processes 52 (2000) 21–34
Fig. 2. Medians and semi-interquartile ranges of intervals between squirt 1 and squirt 2 of trials, for three target heights.
n1 = 5 and n2 = 4; U =3; P =0.056). The interval between first and second squirt is much shorter than the latency to the first squirt, i.e. a median interval of 5.5 s versus a median latency of 24.1 s. The reason is, that when the first squirt does not yield prey, which was the case in the procedure we applied, archer fishes tend to give the second squirt from the same location, within a few seconds, which is the time needed for aiming anew (aiming: with the snout at the surface, as an apparent pivot point, the fish pitches to enlarge the angle between body axis and surface immediately before squirting). This tendency brings us to the question of differences in aiming, as far as the ratio aiming: squirting is concerned. For the lumped data, aiming was followed by squirting in 81.3% of the cases in controls, but in experimentals in 65.2%. Only at target height 35 (first session), the difference is significant (M.W.U. test, n1 = 5 and n2 =4; U = 1.5, 0.016BP B 0.032). Fig. 3 shows the frequency of aiming without squirting. The fish’s habit to quickly squirt again from the same position in case of no reward, led to a discrepancy between the intervals ‘first aimingfirst squirt’ and ‘second aiming-second squirt’; the first interval being the longest (for both groups together, Sign test, n = 8; x =1; P = 0.02). This
Fig. 3. Medians and semi-interquartile ranges of frequency of aiming without squirting, for three target heights (* 0.016 B P B0.032).
difference (see Fig. 4) is significant in experimentals (Sign test, n= 5; x= 0, P= 0.031). In controls no difference is apparent, and n is too small for statistical testing. However, the intervals do not differ significantly between the groups in any session, nor in the three sessions together.
Fig. 4. Medians and semi-interquartile ranges of intervals between aiming and squirting, for squirt 1 and squirt 2 (* P= 0.031).
P.J.A. Timmermans, J.M.H. Vossen / Beha6ioural Processes 52 (2000) 21–34
31
Fig. 5. Schematic distribution of individual mouth angles with their medians (m).
The film enabled us to measure positions and angles. There were no differences between the control and experimental group with respect to the place from where squirting occurred (distance from the target’s perpendicular), and with respect to the squirting angle (angle between the line from the snout through the squirt, and the surface of the water). Also the mouth angle (angle between the body axis and the squirt at the dorsal side of the fish) did not differ between groups (UNIANOVA, df=1; F= 1,230; P= 0.34). The mouth angle appeared to vary considerably between subjects (UNIANOVA, df= 7; F = 113 373; P=0.000) but to be relatively constant within subjects (see Fig. 5). The range of both groups together (n=9) stretched from 142 – 166°, the individual ranges amounted to 8° in one subject, 10° in six subjects, and 11° in two subjects. It is interesting to note that the subject with the elongated mandible had a considerable smaller mouth angle, ranging from 110 to 128°, which is in accordance with the deformation of the mandible that changes the squirt’s direction.
4. Discussion The main question to be answered by Experiment I was whether misses above the target are more frequent than misses below the target. In the Introduction we hypothesised that if misses predominantly are too high this could indicate that the refraction effect is being corrected for by means of feedback from the consequences of previous squirts. It soon became clear that individual characteristics differed too much to analyse the data in a strait forward fashion. The reason why a considerable number of subjects were squirting too high more often than too low, obviously had nothing to do with the virtual image, but was caused by an elongation of their mandible, a phenomenon to be dealt with later on. Moreover, a great deal of subjects did not achieve increase of target height during practice, like we found in preceding experiments, and most of these subjects stopped squirting before the first 6 weeks of the 12 weeks observation period were over.
32
P.J.A. Timmermans, J.M.H. Vossen / Beha6ioural Processes 52 (2000) 21–34
It was decided to further elaborate the data only with 14 so called successful subjects which fulfilled three criteria: apparently normal mandibles, a complete record of practice and test sessions, and increase in target height in the course of the practice period. The data from these subjects clearly show that the range of trajectories of squirts was unbalanced where elevation is concerned, but the number of subjects mainly squirting too high did not differ from the number squirting mainly too low, and too high and too low ‘squirters’ did not differ in percentages of squirting correct. So, as far as elevation failures are concerned, there are no indications that the fishes were using feedback from effects of squirts, as Lu¨ling (1958, 1963) suggested, or corrected for refraction with the virtual image as a point of reference. The fishes rather seemed to be programmed to squirt in a certain range that overlapped the target. For reasons to follow, it was a pity that we did not assess the exact dimensions of that range. On the azimuth too, there apparently was a distribution of trajectories. Lu¨ling (1958) suggested that the right angle of elevation is more easy to achieve than the right azimuthal angle. However, our data suggest the opposite to be true, since they reveal that the percentage of squirts at right elevation is lower than the percentage of hits (right azimuth at right elevation) from squirts at right elevation. Imagine the target to lie on a plane perpendicular to the trajectory of the squirts, then the squirts pass through that plane in a range around the target. According to Lu¨ling’s view, this range is an ellipse the major axis of which is horizontal. Our data give no direct information about the shape of the range, but are suggestive of an elliptic range with a vertical major axis, because the percentage of squirts at right elevation is lower than the percentage of hits from squirts at right elevation. So it seems, that the right azimuthal angle is easier to achieve than the right elevation. Because the fishes use variable aiming angles deviating from 90° (Dill, 1977), in a range of 60–110° (Timmermans, 1975), this is what one would expect as a consequence of gravity, even when a refraction effect is left aside. We now pay attention to the fish with elon-
gated mandibles, which all overshot the target. Such fish were encountered earlier (Elshoud and Koomen, 1985). In our ‘long-jaws’ it was directly visible, and verified by film in Experiment II, that the protruding tip of the lower jaw forced the jet of water higher up. The behaviour of ‘long-jaws’ possibly provides more direct clues with respect to the question whether archer fish can learn to adjust their aim by means of positive feedback (cues that the preceding aiming was right). The use of positive feedback, presupposes that the range of trajectories of squirts overlaps the target’s position, so that accidental hits occur now and than. The subjects that developed into ‘longjaws’ at first did score hits. As their deformation developed gradually, they could have learned to adapt their aiming to its effects, but they did not, failed to hit anymore, and stopped squirting. That this refusal was caused by non-reward, was revealed by the finding that these fish could easily be shaped to resume squirting by successively rewarding them for approaching, aiming, and squirting. That squirting can be brought under stimulus control, already was reported by Waxman and McCleave (1978), and by Goldstein and Hall (1990). Unfortunately it is not clear what caused the elongation of mandibles, anyhow, swimming against the glass played no part. The question to be answered by Experiment II was whether deprivation of squirting would affect performance. We found the main measures, hitting and speed (latency to squirting) to be unaffected. Neither did we find differences in the position from which the fish squirted, and in the squirting angles and the mouth angles. The only difference we found was, that during the first session, experimental (practice deprived) subjects more often aimed without squirting than controls (that got daily practice). Probably, this phenomenon is related with the finding that in experimentals the interval between aiming and squirting was longer for the first squirt than for the second one. However, as we have seen already, these differences did not lead to a difference in latency to squirt. To resume, the achievement of archer fishes that had been deprived of practice during six months was no less
P.J.A. Timmermans, J.M.H. Vossen / Beha6ioural Processes 52 (2000) 21–34
than that of controls, but during the first session, practice deprived subjects interrupted significantly more attempts than controls did. The film did not reveal differences in performance between attempts that were interrupted and attempts that were carried on. Maybe more detailed images will give an answer to this question. The width of the range of individual mouth angles (angle between the body axis and the squirt, at the dorsal side of the fish) of 8 – 11° that we found, is much smaller than the range of 30° reported by Lu¨ling (1958, 1963). Differences between our subjects led to a total range of 142 – 167° for our sample (n =9). Lu¨ling reported a range of 140–170°. So, it could be that Lu¨ling’s range refers to the range from his sample (n = 7) rather than to a common range all subjects used, as his text suggests. The concurrence between Lu¨ling’s range and ours suggests that, in this respect, there is no difference between the species T. jaculatrix, used by Lu¨ling, and the species we used, T. chatareus. It is not clear why one of our experimental subjects squirted too high in all trials. Although this subject had no extreme range of mouth angles, it cannot be excluded that a deformation of the mandibles was developing, but it could also be that the fish was aiming at a target beyond the camera’s view. Rewarding the fishes after each second squirt, as we did, cannot be expected to counteract squirting at a wrong target. To conclude, in Experiment I we hypothesised that if archer fishes would use feedback from preceding squirts to adapt their aiming, too high squirts probably would prevail over too low squirts, and in Experiment II we hypothesised that if archer fishes would have to learn how to aim correctly, performance would be affected by deprivation of practice. Essentially, the findings from the experiments revealed that (1) squirting above the target does not prevail, that (2) chances to hit are not lowered by disuse. The accidental occurrence of a number of specimens with elongated mandibles, moreover, revealed that (3) the fish do not adapt their aiming to the deviation of the squirt caused by their deformation. These findings all suggest that aiming once developed is resistant to change, in other words, the fishes seemed programmed to squirt in a certain range.
33
Since all findings were obtained with fish that squirted effectively already at the start of the observations, we can not decide whether the programming is genetic or acquired through early experience. The resistance to change we found, i.e. the failure to find indications of a learning process, suggests a genetic program.
Acknowledgements We thank technicians of the psychology laboratory for instrumentation, P. Cloosterman for filming, A.J.D.M. van Eil for collecting and analysing data, A.J.H. Willems for statistic assistance, and M.Th.M. Janssen for taking care of the animals.
References Bekoff, M., Dorr, R., 1976. Predation by ‘shooting’ in archerfish, Toxotes jaculatrix: accuracy and sequences. B. Psych. Soc. 7, 167 – 168. Dill, L.M., 1977. Refraction and the spitting behavior of the archerfish (Toxotes chatareus). Behav. Ecol. S. 2, 169 – 184. Elshoud, G.C.A., Koomen, P., 1985. A biomechanical analysis of spitting in archerfishes (Pisces, Perciformes, Toxidae). Zoomorphology 105, 240 – 252. Goldstein, S.R., Hall, D., 1990. Variable ratio control of the spitting response in the archer fish (Toxotes jaculator). J. Com. Psych. 104, 373 – 376. Hediger, H., Heusser, H., 1961. Zum ‘Schiessen’ des Schu¨tzenfisches, Toxotes jaculatrix. Natur und Volk 91, 237 – 243. Herald, E.S., 1965. How accurate is the archer fish? Pac. Discov. 9, 12 – 13. Lu¨ling, K.H., 1958. Morphologisch-anatomische und histologische Untersuchungen am Auge des Schu¨tzenfisches Toxotes jaculatrix (Pallas 1766) (Toxotidae), nebst Bemerkungen zum Spuckgehaben. Z. Morph. O8 kol. Tiere 47, 529 – 610. Lu¨ling, K.H., 1963. The archerfish. Sci. Am. 209, 100 – 129. Lu¨ling, K.H., 1964. Der Schu¨tzenfisch Toxotes jaculatrix und sein Benehmen. Acta Soc. Zool. Bohemoslov. ZVAZEK XXVIII 3, 250 – 260. Lu¨ling, K.H., 1969. Spitting at prey by Toxotes jaculatrix and Colisa lalia. Bonn. Zool. Beitra¨ge 20, 416 – 422. Maxwell, S.E., Delaney, H.D., 1990. Designing Experiments and Analying Data. Brooks and Cole, Pacific Grove, pp. 455 – 488. Meyers, G.S., 1952. How the shooting apparatus of the archerfish was discovered. Aquar. J. 23, 210 – 214.
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Milburn, O., Alexander, R. McN, 1976. The performance of the muscles involved in spitting by the archerfish Toxotes. J. Zool. Lond. 180, 243–251. Siegel, S., Castellan, N.J., 1988. Nonparametric Statistics for the Behavioral Sciences. McGraw-Hill, New York. Timmermans, P.J.A., 1975. The prey catching behaviour of the archerfish. Toxotes Neth. J. Zool. 25, 381.
.
Timmermans, P.J.A., Maris, E., 2000. Does the bright spot on the back of young archer fishes serve group coherence? Neth. J. Zool. 50, 4. Verwey, J., 1928. Iets over de voedingswijze van Toxotes jaculator. De Trop. Nat. 17, 162 – 166. Waxman, H.M., McCleave, J.D., 1978. Autoshaping in the archerfish (Toxotes chatareus). Behav. Biol. 22, 541 – 544.