Operant and Classically-Conditioned Aggressive Behavior in Siamese Fighting Fish

Transcription

1 AM. ZOOLOGIST, 6: (1966). Operant and Classically-Conditioned Aggressive Behavior in Siamese Fighting Fish TRAVIS THOMPSON Department of Psycliiatry and Neurology University of Aiimiesota Medical School, Minneapolis, Minnesota SYNOPSIS. Male Betta splendens were conditioned to emit an operant response sequence reinforced by presentation of a model of another male Betta in aggressive display (Thompson, 1963). The color of the model with respect to the color of the subject co-varied with the rate of operant response (Thompson and Sturm, 1965). In another experiment, unconditioned aggressive display behavior elicited by the mirror image of a male Betta was brought under the control of a previously ineffective stimulus by classical conditioning. Relative rates of acquisition of four components of the display were compared. A discriminative conditioning procedure, using two different colors of light as the CS, revealed that the response was elicited specifically by the CS (Thompson and Sturm, 1966). The dramatically aggressive behavior of Siamese fighting fish (Betta splendens) has long interested behavioral scientists. Detailed observations and investigations by Lissmann (1932), Smith (1937), Braddock and Braddock (1958), Foselius (1957), and others, have elucidated major classes of variables controlling unconditioned aggressive display by male Bettas. One of the various factors known to affect the occurrence of aggressive display is the presentation of a visual image of anouier male Betta. This stimulus has been uniformly demonstrated to be a releaser for aggressive display. If a male Betta that has been exposed to the visual image of another male along one wall of an aquarium is observed for some time following exposure, one will see the subject return repeatedly to the area where the "intruding" male Betta was encountered, and, as the subject periodically swims along the aquarium wall where the intruder was presented, the fish may display aggressively. Such observations suggested that presentation of this releasing stimulus (i.e., the visual image of another male Betta) might act as a reinforcing consequence for an operant response. And further, it seemed that the display behavior This research was supported in part by research grants GB-1912 and GB-4S18 from the National Science Foundation, and has been reported elsewhere by Thompson (1963) and Thompson and Sturm (1965o, 19656). Experiments II and III reported here were done in collaboration with Thomas Sturm. (629) (behavior occurring in the absence of the appropriate unconditioned releasing stimulus), might come under the control of other stimuli which had been associated with the releaser. These considerations led to a series of experiments in which the visual image of a male Betta was conceived of both as a reinforcer for an operant response and as an unconditioned eliciting stimulus for aggressive display. In addition, the tendency for various components of the display to become conditioned to other environmental stimuli via classical conditioning was also explored. Male Betta splendens of domestic stock, 6 to 10 cm long, purchased from local aquarium supply stores, served as subjects in all the following experiments. The fish were fed a diet of tubifex worms and/or frozen brine shrimp once a day throughout the course of the experiments. When not in the experimental test chamber, each fish was maintained in a 2-liter pyrex beaker filled with conditioned tap water at 26 to 29 C. EXPERIMENT I The test tank was 30 by 20 by 25 cm and had transparent Lucite walls (Fig. 1). A 121/^-cm Lucite ring with a 71/^-cm circular aperture was suspended from a translucent Lucite tank lid, and a beam of light, focused across the aperture of the ring, fell on a photoelectric cell imbedded in the plastic of the opposite side of the ring. A response

2 630 TRAVIS THOMPSON was recorded when the fish swam all the way through the ring, first breaking the beam of light, then allowing the light beam to fall back on the photoelectric cell when the subject's transit of the ring was complete. The back wall of the tank was used for presentation of visual stimuli. Three stimuli were examined sequentially for reinforcing properties. The first was a one-way mirror attached to the back wall of the tank. The mirror was presented for 20 sec by shutting off a 75-watt lamp behind it. Subsequently, a moving model of a male B. splendens in aggressive display was tested as a reinforcing stimulus and, finally, a stationary model of a male B. splendens in aggressive display was tested. The moving model was presented across A. the wall of the tank (30 cm) for 20 sec, and, after conditioning and extinction with this stimulus as a reinforcer, the same model was presented in a stationary position for 20 sec. Figure 2 presents the composite curves of the number of responses per 24-hr session for the four fish throughout the 60 days of the first part of this experiment. The operant level was determined on the first two days (10 to 25 responses per session), and the acquisition of the response reinforced by mirror presentation after the first two days may be seen in the figure. This behavior approached stabilization after 12 to 14 days at 600 to 700 responses per session. Extinction of the response reinforced by mirror presentation began on day 17. Im- 10" 10" a. 6 watt, IIOv.a.c. lamp Hue-green fat response ring c one-way glass d. transluscent lucite e. 75 watt, IIOv.a.c. lamp red f. 45 front surface mirror ^ FIG. 1. Apparatus used for studying the reinforcing effects of visual stimuli in Betta splendens. All dimensions are in inches. (A) Side view of test tank illustrating the relationship of the one-way mirror (c) and lamp (e) to the response ring (b). (B) g. 1/2" lucite tubing h. cl-404 glass photocell Front view of test tank facing the one-way mirror. The beam of light from the 6-watt bulb (a) is directed by a 45 mirror (f) across the aperture of the response ring (b) falling upon the photo cell (h). (C) Model of male B. splendens used as reinforcer.

3 AGGRESSIVE BEHAVIOR IN FISH 631 ACQUISITION MIRROR EXT MIRROR UGHT CHANGE MOVING MODEL EXT. MMODEL STATIONARY MODEL EXT. ST. MODEL g in LU LU Q_ to 700 " " 'I III UlU II FIG sion first -T T O SUCCESSIVE 24 HOUR SESSIONS 2. Total number of responses per 24-hour ses- procedures indicated at the top of the graph is defor four fish, expressed as a range. After the scribed in the text, two sessions of operant level, the sequence of mediately after extinction began, the mean response rate began to drop (from 650 responses on day 16 to 125 responses on day 21). For the next five days (day 22 to day 26), the light was shut off after a response, but the mirror was removed. Thus, a change in illumination alone was substituted for the illumination change plus mirror presentation. The response rate increased slightly on day 22 but returned to 25 to responses for the four succeeding days, indicating that the mirror presentation, not the change in illumination, was responsible for maintaining behavior. Beginning with day 27, each response was followed by presentation of a model of a male B. splendens in aggressive display that moved across the wall of the tank. The number of responses for this reinforcer increased sharply over 10 days, stabilizing at approximately 575 responses per session. Extinction began on day 28, and reached a level of 10 to 50 responses within six days. The final manipulation involved presentation of the same model in a stationary position along the wall of the tank for 20 sec. The model was then removed until another response was made. Responding for this reinforcer reached an asymptotic rate in ten sessions (150 to 200 responses) and was extinguished to the operant level of 10 to 25 responses. These findings parallel Forselius' detailed description (1957) of the use of models and mirror to release a variety of agonistic behaviors in anabantid fishes, including Betta. These findings are consistent with Forselius' observations on the ability of these stimuli to act as releasers, and they suggest that the relative positive-reinforcing properties of visual stimuli, similar to the visual properties of a male fish in aggressive display, co-vary with the degree to which stimuli will evoke unlearned display. EXPERIMENT II The above findings led us to wonder exactly what visual properties of a male Betta in aggressive display make it an effective reinforcer for an operant response. Therefore, in the next experiment, the color of the visual reinforcer was varied and the relative reinforcing properties of these stimuli compared. Subjects were three male B. splendens: Fish A was red and 6

4 632 TRAVIS THOMPSON 1. 6 Volt Light Bulb 2. Focusing Lens 3. Reflecting Mirror 4 Photoelectric Cell 5. Gate (o.b.and c) 6. Plexiglass Partition 7. Model 8. Fin Illumination Light Bulb 9. Model Electric Train Engine 10 Car Carrying Model II. Track 12 Aquarium Tank 13 Opaque Screen 14 Live Male Betta (Subject) FIG. 3. Side and top views of experimental apparatus for operant conditioning of Siamese fighting fish. The model electric train (9 and 10) used to present the reinforcing model (7) can be seen in the stopped position. The subject (14) is about to pass through gate Sc which will cause the model to move cm long, Fish B was blue and 6 cm long, and Fish C was blue with a green dorsal fin and was 6.5 cm long. Each fish was tested for one hour at approximately the same time each day, and the one-hour session began with the first reinforced response. A fixed sequence of responses was required of the fish so that they would be in a proper orientation vis-a-vis the visual stimulus when presentation occurred. Thus, the fish had to swim through three gates in an underwater maze in a prescribed order. (The gate positions are shown in Fig. 3.) At each gate, a beam of light focused into the tank on a small mirror (^ by?4 cm ) was reflected back out onto a photoelectric cell. The mirrors were small enough not to elicit display from the fish to its image. By passing through a gate, the fish interrupted the light beam and closed a relay, and, when all three relays were closed in proper order, the visual reinforcer was quickly to position A, where it will stop for 4 sec, then continue slowly to position B, where it will increase speed as it circles around the tank on the track (11) stopping again until the next response sequence (5a -> 5b-» 5c) has been completed. All dimensions are in cm. presented. One of six colored models of a male Betta in aggressive display was mounted on an electric train in such a way that the model, but not the train, could be seen by the fish. The model appeared in lateral display on the left side of the tank, stopped in front of the third gate for four seconds (which allowed the fish enough time to display aggressively), and then moved slowly across the front of the tank from left to right. The model traveled across the front of the tank in four seconds, allowing the fish to follow and to continue presenting to the stimulus. After moving beyond the end of the tank, the train changed speed, circled back, and stopped at the left side of the tank, out of the subject's sight. In order to see the model again, the fish was required to renegotiate the maze. The reinforcing stimuli were six models made of painted balsa wood (body) and colored cellophane (fins). Scales, fin spikes,

5 AGGRESSIVE BEHAVIOR IN FISH O Si 80 LJ CC bj > 60 < Bella A (Red), Green Model PERCENTAGE OF OPERANT RESPONSES FOLLOWED BY AGGRESIVE DISPLAY Model Red Blue Green Sessions Betto A (Red) Betta B (Blue) Bet to C (Blue-Green) I SESSIONS Bella C (Blue-Green) Red Model O Si C 80 Blue Model rr g 40 l SESSIONS FIG. 4. Cumulative records of complete response sequences leading to presentation of the red, green, and blue models for each fish. The table indicates SESSIONS the percentage of reinforced operant responses which were followed by aggressive display for each fish, and fin rays were drawn on the model with black ink, based on Lissmann's description of the lateral view of male Bettas (Lissmann, 1932). A 6-v lamp was illuminated behind the model as it passed in front of the tank to increase the luminescence of the fins, and the ceiling light over the tank illuminated the body. The colors of the models were matched with colors in Munsell's Color System Atlas, and all models showed the full aggressive display. Models were run for two blocks of five sessions each to determine their reinforcing properties. The following order was employed to control for a possible order effect blue, led, green, green, red blue and two models of each color were used to control for slight differences in body shape due to hand carving; one model of each color was used in the first block and the second model of each color was used in the second five-session block. Figure 4 shows the cumulative number of responses by the three fish that led to presentation of each model. The separation between session 5 and session 6 on the graph indicates that the last five sessions were not run continuously with the first five. As the graph indicates, the red fish (A) responded at the highest rate when the green model was the reinforcing stimulus, at an intermediate rate when the blue model was used, and at the lowest rate when the red model was used. The blue fish (B)

6 634 TRAVIS THOMPSON Fish A (Red) 0L = 008RSP/l0 minutes (Calculated from 4-lhf Sessions) FishS(Bkie) OL'O25HSP/IOmim,les (Calculated from 4-lhr Sessions] FishC(Blue-Green) 0 L RSP/IO minutes (Calculated Irom 3-1 nr Sessions) Q 25 j o FIG. S. Total responses during successive 10-min periods for each model and all three subjects. Oper- 10 Minute Intervols responded most with the red model, intermediately with the green model, and least with the blue model. The blue and green fish (C) responded in approximately the same order as Fish B, except that the differential rate between the red and green models was greater than between the green and blue models. For Fish B, the differential rates were approximately equal among the three model colors. A more detailed presentation of response distribution for each fish during the 60- minute sessions is shown in Figure 5. Most responding occurred during the first 10 minutes of each session, and response rates tapered off to rates of from four to ten times the operant level throughout the remainder of each session. The overall rate differences in responses leading to the three different colored models may be more clearly seen in the individual curves. The greatest consistent differences were between the reinforcer that maintained the highest rate and the two weaker reinforcers. However, this was less evident with Fish B's performance, whose three response curves are much closer together, than the response curves produced by the other two subjects. Each fish emitted the lowest number of responses to the model which had the color most like its own, and Fish B and C emitted the highest rate of response to the color least like their own. The rate of respond- ant levels are indicated on each set of graphs for each subject (O.L.). ing to the sets of same-color models (used as controls in the second block of five sessions) was similar to the rate of responding to the set of models used in the first block of five sessions, as can be seen in the data presented earlier. The table in Figure 5 indicates the percentage of time that the fish displayed aggressively in the presence of the model. All displayed at more than 95% of the reinforcement presentations, except for the green-blue fish (Fish C), which did not display at all in the presence of either of the green models. REMARKS In the two foregoing experiments, the highest levels of response were maintained by reinforcement with the mirror image of a male Betta. Intermediate levels of: responding were maintained by a moving model of a male Betta in aggressive display, and the lowest levels of responding were maintained by the same model in a stationary position. These data suggested that the subject's coloration with respect to the model's coloration might be an important variable determining operant reinforcing strength. In the second experiment, it appeared that a fish would respond least when the coloration was like its own. These findings ostensibly contradict results

7 AGGRESSIVE BEHAVIOR IN FISH 635 Q-Wooden comportment b-ioo wott light bulb c-omon-skin paper d-red cellophane e- 7 '/z wott light bulb f - Aquarium tank g- One way mirror In the second experiment, it was found that male Bettas exhibited aggressive display 95% of the time in the presence of a visual reinforcer. We noticed that some subjects began to display following the emission of the operant, even before the reinforcing model was in view. This suggested that the display was elicited by stimuli which had been repeatedly paired with model presentation. Adler and Hogan (1963) made a similar observation when they classically conditioned gill-cover erection, one component of this complex display. They used a weak electric shock as the conditioned stimulus and a mirror as the unconditioned stimulus. Our third experiment was therefore designed to study the relative rates at which four components of the unconditioned aggressive-display sequence could be classically conditioned, and we first sought to establish the feasibility of a light as a conditioned stimulus. A discriminative conditioning procedure was used in this experiment in order to demonstrate that the four aggressive-display components were specifically elicited by the conditioned stimulus. This procedure eliminated the possibility that our results were the product of sensitization. FIG. 6. Observer's view of apparatus for classically conditioning aggressive display in Betta splendens. obtained in the first experiment, since the color of the mirror image was the same as that of the subject and should, therefore, have been associated with the lowest rate of responding. However, the mirror image is a highly complex stimulus, composed of all the movements and color changes which occur during the aggressive-display sequence, and, because of the complex nature of the mirror image, the second experiment dealt solely with one aspect of the visual reinforcing stimulus, i.e., coloration. EXPERIMENT III Part 1 A red stimulus-light was presented for 10 sec before and at the beginning of a 15-sec mirror-presentation along one wall of an aquarium tank. Two compartments, each enclosing two -watt light bulbs, were placed 14 cm apart, allowing just enough space for the aquarium tank (12 by 25 by 23 cm) to rest between the compartments (Fig. 6). The compartment walls adjacent to the aquarium were made of translucent onionskin paper, one of which was covered by a one-way mirror. Turning off the light behind the one-way mirror and turning on the light at the opposite side of the tank made the mirror highly reflective. Reversing the illumination terminated the mirror effect. A 71/^-watt bulb behind a sheet of red cellophane stretched across the end of the tank (12 by 12 cm) illuminated the interior of the tank with bright-red light during presentation of the conditioned stimulus. The subject's behavior was observed through the opposite end of the tank. Red-light and mirroipresentation were programmed by automatic electronic switching and timing devices. Data were recorded by observing the four display components during red-light presentation. Each fish was tested daily for 10 trials programmed at random intervals varying from 30 to 240 seconds, with a mean intertrial time of 120 sec. This aperiodic intertrial procedure was used to minimize development of temporal discrimination. Because aggressive display occurred occasionally, immediately after the home tank was moved, in the absence of an eliciting stimulus, the home tank was transferred to

8 636 TRAVIS THOMPSON A FRONTAL APPROACH UNDULATING MOVEMENT 2 g UNDULATING MOVFMFNT ^P "" MAXIMUM DISPLAY FIG. 7. Four photographic views of a single male Betta splendens illustrating four components of the complex aggressive display seauence from different observational positions. A. Frontal approach, viewed head-on, revealing fin erection and gill erection (GE). B and C. Two successive rapidly undulating body mum display in frontal Notice that all fins and the test apparatus 10 min before each session. The observer sat at the end of the tank and recorded the occurrence or nonoccurrence of the display for each trial on the standard score sheets. The four following display components could be distinguished: (1) frontal approach to the mirror and/or to the red stimulus light, (2) fin erection, (3) gill-cover erection, and (4) undulating movements (described in detail by Forselius, 1957). Figure 7 presents four photographic views of a male Betta that illustrate the four aggressive-display components. The upper left photograph shows the subject in frontal approach with the gill covers erected (GE). The upper right and lower left photographs illustrate the undulating swimming movements, in which the fish first orients its head toward the opponent with its body curved slightly away then bends its body in the opposite direction with a rapid thrust of the tail. The lower left photograph also shows the gill covers erected, revealing the bright-red gill membranes. The maximum fin erection, as well as the gill erection, can be seen in the lower right photograph. Two adaptation sessions of 10 trials each were run in which the red conditionedstimulus light, but not the mirror, was presented to establish that the light alone did not elicit display behavior. Conditioning was continued on this schedule, using a 15-sec mirror-presentation until the four views of the fish exhibiting movements. D. The maxiapproach viewed laterally. the gill covers are erected.

9 AGGRESSIVE BEHAVIOR IN FISH 637 EXPERIMENT I UWUUTWG MOJEN NTS. / ~ " M l COVER KCTCN C U COVER EFECTDJ FRONTAL APPROACH 1 FIG. 8. The number of occurrences of each of the four aggressive display components during each successive block of 10 trials exhibited by Fish 1 and TRIALS IN BLOCKS OF TEN 2 during Experiment I. Extinction began at the arrow (Ext). components occurred on each of 10 consecutive trials in the presence of the conditioned stimulus. In extinction, the oneway mirror was removed, and the illumination changes occurred as before. Since illumination changes accompanied mirror-presentation, it would not have been adequate to discontinue mirror-presentation by no longer switching the lights on during ex- tinction. Illumination changes may have become a conditioned stimulus for the conditioned response. Thus, in extinction the conditioned stimulus (red light) was presented and followed by illumination changes, but not by mirror-presentation. In order to control light intensity, the mirror was replaced by additional onionskin sheets until the light intensities of the two

10 638 TRAVIS THOMPSON Fish I-Red Light CS Fish 2-Red Light CS. /> % - Fin Erection o o Unduloting Movements * + Gill Cover Erection o- o Frontol " ^ " Fin Erection Unduloting Movements + * Gill Cover Erection < Frontol Approach I r- / // y // / 40 ^ FIG. 9. by Fish Trials Trials g a n at the arrow (Ext). The cumulative number of displays exhibited 1 and 2 during Experiment I. Extinction besides of the tank, as measured by a light meter, were equal. This procedure was continued until 10 consecutive trials without any occurrences of the four components had elapsed. Figure 8 shows the number of occurrences of each of the four aggressive-display components during each block of 10 trials. During the adaptation sessions, neither fish displayed aggressively in the presence of the red light. Fin erection was clearly the most rapidly acquired response, while frontal approach required the greatest number of trials to reach the criterion level. There appeared to be little difference between the rates at which Fish 1 acquired undulating movements and gill-cover erection, but Fish 2 acquired the former more rapidly than the latter. Extinction of all four responses required 30 to 40 trials for each fish. The regularity of the accelerated acquisition curves is much more apparent in the cumulated data, and comparison among the four components is facilitated, as indicated in Figure 9. The marked difference in the rate at which Fish 1 acquired fin erection versus frontal approach may be seen in relation to the very similar difference in this subject's acquisition of undulating movements versus gill-cover erection. By contrast, the four acquisition curves for Fish 2 are more clearly separated. Extinction of all four responses required from 30 to 40 trials for each fish, but the form of extinction curves is clarified when expressed cumulatively. Part 2 The results of Part 1 of this experiment indicate that classical conditioning techniques may bring four components of the unconditioned aggressive display by male Bettas under the control of a previously neutral stimulus light. The possibility remained, however, that these conditioned responses might be partly the product of sensitization. Through repeated presentation of the unconditioned stimulus (the mirror) within this environment, novel or similar simuli might have become capable of eliciting the aggressive-display behaviors. To eliminate this possibility, a positive conditioned-stimulus light was repeatedly paired with mirror-presentation, while a negative conditioned-stimulus light was repeatedly presented alone. If the aggressivedisplay behaviors occurred in the presence of the positive conditioned stimulus and not in the presence of the negative conditioned stimulus, the results must reflect discriminative classical conditioning rather than sensitization. Two experimentally naive male Bettas (Fish 3 and 4), measuring 5.0 and 6.0 cm,

11 AGGRESSIVE BEHAVIOR IN FISH 639 respectively, were used, and the equipment was the same as that employed in Part 1 of this experiment. In addition, a piece of green cellophane was placed above the red cellophane, and a second 7i/ -watt bulb was situated behind the green cellophane window. Either light could be presented to illuminate the interior of the tank with one or the other color. Fish 3 and 4 were tested daily for 20 trials each. These trials comprised 10 green-light presentations followed by mirror-presentation, and 10 red-light presentations not followed by niirror-presentation. Thus, the green light was the positive and the red light the negative conditioned stimulus. The inter-trial interval varied from 30 to 240 sec with a mean inter-trial time of EXPERIMENT II UNDULATING M&flvtNTS (1I z UJ I a! tti COVER ERECTCH ClU CWER ERECTION Q > 8 FRONTAL APPflQtH FRONTAL AffHMCH TRIALS IN BLOCKS OF TEN FIG. 10. The number of occurrences of each of the presence of the green light (the positive CS) and four aggressive display components during each the lower lighter line indicates the number of disblock of 10 trials by Fish 3 and 4 in Experiment II. plays elicited by the red light (negative CS). Ex- The upper dark line is the record of displays in the Unction began at the arrow (Ext).

12 640 TRAVIS THOMPSON Fish 3- Fish 4- ' Fin Erection 1 Unduloting Movements Gill Cover Erection Frontol Approach 1 Fin Erection Undulating Movements Gill Cover Erection Frontal Approoct) *. 40.Q ' Trials FIG. 11. The cumulative number of displays exhibited by Fish 3 and 4 during Experiment II. 120 sec. The randomized presentation of the two stimulus lights was arranged according to the Gellerman series (1933). As in Part 1, the home tank was transferred to the test apparatus 10 min before the first trial began, and responses were recorded on standardized score sheets. Extinction was conducted by presenting the green light for 10 sec, but no longer following it by mirror-presentation. During this procedure, the one-way glass was left in place. This contrasted with the extinction procedure of Part 1 of this experiment, in which the illumination changes associated with mirror-presentation also occurred during extinction. The procedure in Part 2 controlled for the possibility that the mirror might have produced, during conditioned-stimulus presentation, a slight reflection capable of eliciting the unconditioned response. The criteria for acquisition and extinction were the same as those used in Part 1. Figure 10 presents the number of occurrences of the four aggressive-display components in the presence of the positive conditioned stimulus and the negative conditioned stimulus during each block of 10 trials. With the exception of one fin erection by Fish 4 in the presence of both the positive and negative conditioned stimuli, no responses occurred during the Trials Extinction began at the arrow (Ext). adaptive trials. Fin erection and undulating movements were the first two components to be acquired by both fish, whereas gill-cover erection and frontal approach were the last two to be acquired. In the last 10 trials, in which all four components were elicited by every green-light presentation (the conditioning criterion), no responses were elicited by the red light. From 20 to 50 trials were required for the four components to reach the extinction criterion of 10 consecutive trials without any occurrence of display. These relationships are clearer when presented as the cumulative number of displays for each component, as in Figure 11. The difference in rate of acquisition of fin erection vs. undulating movements, as opposed to the difference in rate of acquisition of gill-cover erection vs. frontal approach, is marked. A comparison of the slopes of the cumulative extinction curves reveals considerable differences in the rates of extinction, which is not apparent from examining the number of displays per block of 10 trials. Gill-cover erection and frontal approach extinguished most rapidly, as revealed by their lesser slopes. The cumulative curves for all components paired with the green light were positively accelerated, while the curves for responses elicited by the red light were negatively accelerated.

13 AGGRESSIVE BEHAVIOR IN FISH 641 Discussion The three experiments reported here suggest several interesting relations between the "releaser" concept used by ethologists and the term "reinforcer" employed by experimental psychologists. A releaser can act as a positive reinforcer for an operant response, since its contingent presentation increases the frequency with which the operant occurs. Thus, a releaser has the same functional properties as other stimuli which are capable of (1) acting as unconditioned stimuli in a classical-conditioning paradigm and (2) functioning as reinforcers for operants (e.g., food). If there is anything unusual about this reinforcer (the visual image of another male Betta in aggressive display), it is that the consequences in a natural setting would almost surely be damaging to the organism. Usually, stimuli which are associated with damaging consequences do not function as positive reinforcers, but are avoided or removed. A possible interpretation of these data is that the reinforcing consequence of presenting the visual image of another male Betta is not that the visual stimulus is inherently reinforcing, but that it elicits behavior which is more probable than the operant under consideration. It has been shown that operants can be reinforced when the only consequence is the opportunity to engage in more probable behavior (Premack, 1959). Certainly, in Betta splendens, aggressive display is more probable than the swimming operant (operant level = 3-10/ hr). Another implication of these findings is that display behavior occurring in the absence of the usual releaser is not necessarily "vacuum activity" (i.e., a fixed action pattern occurring under conditions of high motivation in the absence of the appropriate releaser; Verplanck, 1957). In these experiments, four components of the aggressive display were brought under the control of a previously ineffective stimulus through classical conditioning. This obviously does not suggest that all behavior occurring in the absence of the usual releaser is conditioned, but that this is one possible mechanism by which such behavior may come to occur. REFERENCES Adler, N., and J. A. Hogan Classical conditioning and punishment of an instinctive response in Betta splendens. Animal Behaviour 11: Braddock, J. C, and Z. I. Braddock Effects of isolation and social contact upon the development of aggressive behaviour in the Siamese fighting fish, (Betta splendens). Animal Behaviour 7: Forselius, S Studies of anabantid fishes. I. Zool. Bidrag. 32: Lissmann, H. W Die Umwelt des Kampfisches. Z. Vergl. Physiol. 18: Premack, D Toward empirical behavior laws: I. Positive reinforcement. Psychol. Rev. 66: Smith, H. M The fighting fish of Siam. Nat. Hist. 39:265. Thompson, T Visual reinforcement in Siamese fighting fish. Science 141: Thompson, T., and T. Sturm. 1965a. Visual reinforcer color and operant behavior in Siamese fighting fish. J. Exptl. Anal. Behavior 8: Thompson, T., and T. Sturm Classical conditioning of Siamese fighting fish. J. Exptl. Anal. Behavior 8: