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Waning of the aggressive response to male models in the three-spined stickleback (Gasterosteus aculeatus L.)
Anim . Behav., 1969, 17, 224-228
WANING OF THE AGGRESSIVE RESPONSE TO MALE MODELS IN THE THREE-SPINED STICKLEBACK (GASTEROSTEUS ACULEATUS L .) By HARMAN V . S . PEEKE*, EVERETT J . WYERSt, & MICHAEL J . HERZ* University of Southern California two stimuli and spent time in the centre compartment avoiding visual contact with them . The purpose of this investigation was to study the course of aggressive responsivity of male three-spined sticklebacks to daily presentations of wooden models of male sticklebacks . Models were used, rather than live stimuli, because they allowed more complete stimulus control by the experimenter, and hence allowed a constant manner of presentation from day to day avoiding the variability that live males might provide . The five models were similar except for different amounts of red colour on the ventral anterior surface . It has long been held that red in that anatomical locus is an important component of the releasing stimulus for aggression in the stickleback (Tinbergen 1951) . It was predicted that varying this one important component of the releaser would produce a series of similar models with varying ability to elicit aggression . Differential ability to elicit aggression would allow assessment of the interaction, if any of the effects of repeated elicitation of aggressive responses and the initial releasing strength of the models . The completion of the nest construction by the male three-spined stickleback is considered the transition from the nest building phase of the reproductive cycle to the sexual phase, at which time the male will respond with aggression towards other males intruding upon his territory . However, a male may be maintained in the sexual phase for study if, after the completion of his nest, he is denied the opportunity to court a female successfully. Therefore a study involving changes in aggressive behaviour over time should ideally start and finish within the sexual phase, during which a relatively high but stable level of aggression is maintained (Sevenster 1961) . Such behaviour is normally quite stable . Methods The male three-spined sticklebacks used in this study were collected from a small land-locked creek in southern California in the fall and winter . This source is always well populated with sticklebacks of both sexes, a relatively constant proportion of which appeared to be in
Thorpe (1963) has recently stated that, `There has as yet been no critical work on habituation in fishes . . .' (p . 306) . Since the appearance of Thorpe's review several investigators have demonstrated response decrement of repeated elicited responses in the fish . The `tail flip' response of the goldfish which serves to locomote the fish rapidly away from sources of stimulation has been habituated both in response to a pressure wave set up by a tap on the side of the fish's container and to a shadow passing overhead (Rodgers et al. 1963) . A similar type of response has been habituated to a shadow stimulus in Lebistes reticulatus (Russell 1967) . There is also evidence suggesting habituation of aggressive responses in fish. Triplett (1901), studying the perch, Morone americanus, found that they would stop attempting to attack minnows that were protected by a clear glass partition and, after complete habituation of the predator response, the glass partition could be removed without jeopardy to the minnows . More recently, Baenninger (1966) has presented data from the Siamese fighting fish (Betta splendens) that he interprets as evidence for habituation of conspecific aggression . He placed Bettas in a tank with a model of another Betta at one end and a real Betta in an adjoining tank at the other . In the centre was an area which did not allow a view of either stimulus. The measure of aggression was the amount of time spent in the centre of the apparatus . Over a period of 16 hr the Bettas spent successively more time in the centre section . This result was interpreted to mean that the fish was no longer motivated to view either stimulus because his aggressive motivation had waned . An alternative explanation of these results is that as time went on the fish became more and more fearful of the *Now at the University of California San Francisco Medical Center (Department of Psychiatry) and Laboratory of Psychobiology, Langley Porter Neuropsychiatric Institute, San Francisco . f Now at the State University of New York, Stony Brook (Department of Psychology) . 224
PEEKE et al. : AGGRESSIVE RESPONSE IN THREE-SPINED STICKLEBACKS
breeding condition . In the laboratory the fish were kept in crowded colony tanks 61 cm x 61 cm x 30 cm (15-17 ° C) in order to retard the development of reproductive behaviour. No plants were provided, but the bottom was covered with sand and gravel . All fish were fed daily with either Tubifex worms or brine shrimp (Artemia salina) or both . The experiments were carried out in banks of smaller tanks, 26 cm x 50 cm x 30 cm, separated by opaque dividers . Each tank was filled to a depth of 6-8 cm with fine sand and gravel . the left side and left rear of the bottom were planted with elodea (Anacharis canadesis) and milfoil (Myriophyllum spicatum) . A small quantity of thread algea was also provided which led to an abundant growth of algea on the bottom and sides of the aquaria. This growth was removed only from the front . Lighting was provided for 16 hr per day from a 60-W incandescent bulb placed directly above the centre of each tank, 60 cm from the water's surface . An outside-of-tank air-driven filter, using spun glass as the filtration agent, functioned continuously providing some filtration and circulation of the water. Within 1 to 3 days males selected from the colony tanks and placed individually in smaller experimental tanks would construct a nest* ; that day was designated `day N.' On its day N each fish was presented with each of five wooden *The actual construction of the nest usually takes only several hours, and can take much less . The difference among fish in days to nest construction reflects differences in readiness to build rather than speed of construction .
models of a male stickleback . The models were constructed of balsa wood, with a shape similar to the body shape (without fins) of a male stickleback ; the eyes were represented by black circular dots . The models differed only in the extent of red colour (Munsel colour system between 5R 4/10 and 5R 5/10) on the ventral surface of the model (the rest of the model was of the natural wood colour, protected by a coat of clear lacquer) . The five models ranged from one with no red area (model No . 1), through three models with progressively more ventral anterior red area (models Nos . 2, 3 and 4), to a final one that was all red (model No . 5) . Each of the models was presented for 2 min each day for 12 successive days . The order in which the five models were to be presented was determined randomly, with the restriction that the same order could not be used more than once per group. The models were presented in the same sequence each day with the inter-model time equal to the time necessary to remove one model from the tank and replace it with another, usually no more than 15 sec . The models were attached by a thin stiff wire to a portable motor which was suspended above the tank on a clear Plexiglas platform . The motor propelled the models through a circular (counter-clockwise) path with a diameter of 15 . 5 cm at a rate of one revolution every 10 sec . The circular pathway was approximately halfway between the surface of the water and the gravel substrate . The model was always presented in the right end of the tank such that the model would miss the right wall of the tank by less than 1 cm . Although previous published work from other
A,j/////p°.
I MODEL I
MODEL
2
MODEL 3
MODEL
41
4
.~///////P
MODEL 5
/ l
I I WOOD
BLACK
17mm I ".4
RED
225
I I I I jt 49mm
I 1 j I I I >1
Fig . 1 . Series of five wooden models used to release aggression,
226
ANIMAL BEHAVIOUR, 17, 2
laboratories (e.g . Sevenster 1961) indicated that the aggressive tendency should remain stable over the duration of such an experiment once the fish had entered to the sexual stage, additional groups were utilized to control for the possibility that any change in the frequency of aggressive responses might simply reflect decreases in aggression following the completion of nest construction . This control procedure consisted of initiating the 12 days of model presentation for different groups of fish either 3, 6 or 9 days after nest completion . Thus there were four groups : group N, presented with the models for the first time within 24 hr of nest completion, and group N + 3, N + 6, and N + 9, first stimulated 3, 6 or 9 days post-nest construction respectively . The behaviour of each subject was carefully observed before, during and after model presentations and the frequency of biting was recorded on an event recorder. `Biting' is defined here as any open-mouthed contact with the model . Such contact was followed by a closure of the jaws forming a typical bite. The force of such bites was usually sufficient to displace the model by several millimetres against the stiffness of the wire staff. The two experimenters responsible for observation of the fish during model presentation scored one fish in parallel, and found agreement on well over 90 per cent of the responses recorded, suggesting high inter-observer reliability. In general the same experimenter observed a given fish on all 12 days . Results Initial Response to the Models In total, thirty-seven fish constructed nests and were used for this study . Of these, ten showed marked avoidance of the stimuli from the moment they were introduced into the tank and did not approach the model or strike at it over the period of 12 days . Several fish were included in this category although they bit once or twice at only one of the models on only one day. These 'non-responders' all shared a similar behaviour that the experimenters noted on protocol sheets . As the model was introduced into the tank these fish would immediately dart behind a row or clump of plants, assume a posture with the head higher than the tail, and the tail slightly curled to one side or the other . They would not usually change position unless startled by the experimenter and then would re-assume the posture behind some other cover. What differentiated these fish from fish that
stopped responding to the models after a few days of stimulation is that the latter would not seek cover and would continue to orient toward the model, and would make slow and incomplete approach movements . They also would not assume the head-high, tail-curled stereotyped posture . Three fish showed the typical 'nonresponder' behaviour on the first day (one in group N and two in group N + 9), but then began approaching and biting at the wooden model on the second . Once the agonistic behaviour began, such fish did not revert to their previous posture . The distribution of 'non-responders' among the groups was 2 of 13 for group N, 2 of 8 for group N + 3, and 3 of 8 for group N + 6 and N + 9 . The descriptions and analyses of the data are based on 27 fish which attacked the models ; 11 in group N, 6 in group N + 3, and 5 each in groups N + 6 and N + 9 . Post-nest Construction Drive Fluctuation The purpose of assigning fish to the groups starting 3, 6 or 9 days after nest completion was to assess the change in the aggression tendency as a simple function of the number of days after nest completion . Separate KruskallWallis one-way analysis of variances by ranks (Siegal 1956) were performed for each model across the four groups, using as the measure of `initial aggression' the highest number of bites at a given model recorded on any of the first 3 days . This measure, rather than the number of bites on day 1, was used to avoid problems associated with initial reluctance of some fish to approach the models until after the 1st day, or, in a few cases, after the 2nd day . There were no significant differences among the four groups for any of the five models . As a further test of possible differences among these groups, the total number of bites was lumped together for all five models and summed across all days for each group . This analysis should show a difference if there was any common trend in the individual model analyses reported above . Again, the Kruskall-Wallis test failed to yield a significant value . From these tests it can be concluded that the aggressive tendency does not change as a simple function of post-nest construction time . Differential Releasing Characteristics of the Models If the ability of the models to release aggressive behaviour were dependent upon the amount or placement of the red colour on the ventral anterior surface, then we should expect either
PEEKE et al. : AGGRESSIVE RESPONSE IN THREE-SPINED STICKLEBACKS
differences among the models on the measure of initial aggression (see above), or different rates of change as a function of repeated stimulus presentations . To test for differences in `initial aggression' (defined above) elicited by the models, a Friedman two-way analysis of variance by ranks (Siegal 1956) was computed (subjects x models) with all four post-nest construction groups lumped together insofar as there was no difference among them . The test did not show any difference among models . The same test was computed using the total number of responses across all twelve days as the measure of aggression elicited by the models . This manner of testing should show differential resistance to habituation, despite the result found above, that they started at approximately the same level . Again, however, no differential model effect was found . Changes in Aggressive Behaviour as a Function of Daily Model Presentations The number of bites directed at each of the five models clearly decreased as a function of the number of days of model presentation (Fig . 2) .
N W D
1
1
2
3
4
5
6
7
8
9
10
11
12
DAY Fig. 2. Mean number of bites at each of the models (Nos. 1 to 5) over 12 successive days of daily 2 min presentations,
22 7
The decrease in the number of bites directed at the model from the `initial level' to the 'terminal level' (highest number of bites recorded on a single day during the first 3 days to the highest number recorded on a single one of the last 3 days) is significant for each of the five models (P<0 . 001) in each case) as shown by Wilcoxon matched pairs, signed ranks tests (Siegal 1956) . Again, all four groups were lumped together for these analyses . It is clear that the aggressive response to crude models of conspecific males wanes over days as a function of continued presentation. Discussion Aggressive behaviour in the male three-spined stickleback, measured as the frequency of bites directed at the stimuli, wanes as a result of continued presentation of crude models of conspecific males. This is in agreement with the findings of Baenninger (1966) and Clayton & Hinde (1968) with the Siamese fighting fish, that conspecific aggression does wane with repeated or constant exposure . Baenninger produced his result in 16 hr of constant stimulus presentation while the present study administered the stimuli for short periods each day for 12 days . The results of the two studies however appear, at least superficially, similar . Caution must be exercised to avoid generalizing results of this and similar studies to more natural situations . Such rapid habituation of conspecific aggressive responses would seriously disrupt the normal reproductive cycle of the stickleback if mere repetitions of males wandering into the resident male's territory were sufficient to reduce the aggressive tendency . It has been suggested that habituation is very stimulus specific (Thorpe 1963) and perhaps the rate at which the response to a stimulus habituates is a positive function of the constancy of the stimulus from trial to trial . Such habituation would appear to have little survival value, but in nature the character of the releasing stimuli would likely vary a great deal from `trial' to `trial', and the occurrence of intermittent consummatory acts (conspecific territorial fighting) would further protect against habituation of such responses . In the present study efforts were made to present stimuli that were extremely constant in an attempt to maximize the probability of finding habituation of aggression in a species of fish noted for its aggressive tendencies during its reproductive cycle . The generality of this phenomenon should be enhanced by a demonstration of habituation of aggression to a
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ANIMAL BEHAVIOUR, 17, 2
conspecific male using as a stimulus a real, nuptially coloured male stickleback . Indeed, such a demonstration exists (Peeke 1966) . The importance, phyletic generality and some possible mechanisms of habituation are discussed elsewhere and the reader is referred to these references (Russell 1967 ; Thorpe 1963 ; Peeke 1966). The amount of red on the ventral anterior surface of the wooden model did not significantly influence the ability of this particular shape of model to elicit aggressive responses . This is in apparent contradiction to the reports of Tinbergen (1951), who found that red on the lower portion of an extremely crude model appears sufficient to release aggressive displays . It must be stated, however, that the models used in the present study may have been insufficient releasers . Had a more detailed model been used that would elicit more aggression even without a red underside, then perhaps amount and location of red colour might have interacted with shape and detail to produce the previously reported result linking a red underside to aggression elicitation ability . That the observed habituation of aggressive responses over successive model presentations can be attributed to the inadequate releasing quality of the stimuli appears unlikely . Beanninger (1966) and Clayton & Hinde (1968) also obtained evidence for habituation of components of aggressive displays in Betta splendens to a mirror image, a stimulus which could be considered a better releaser than the models used in the present situation . Peeke (1966) found that aggressive responses toward real male sticklebacks confined in a clear tube decreased at a faster rate than did responses to a stationary model, suggesting that habituation may be independent of the reinforcing quality of the stimulus . Summary Territorial three-spined sticklebacks were pre-
sented with crude wooden models of conspecific males . The models differed only in the amount of red colour on the anterior ventral surface . Each model was presented for 2 min each day for 12 days . It was found that the amount of red colour did not significantly affect the amount of aggression elicited by the model . The aggressive response ('biting') decreased markedly from the high level observed on the initial presentations to a low level of responding at the termination of the experiment . Acknowledgments This research was supported in part by Research Grant G-18891 (to E . J . Wyers) from the National Science Foundation, and in part by USPHS predoctoral fellowship No . MH 22269 (to M . J . Herz) . REFERENCES Baenninger, R . (1966) . Waning of aggressive motivation in Betta splendens . Psychonom . Sci., 4, 241-242.
Clayton, F. L. & Hinde, R . A . (1968) . The habituation and recovery of aggressive display in Betta splendens . Behaviour, 30, 97-106 . Peeke, H . V. S . (1966) . Habituation of aggression in the three-spined stickleback (Gasterosteus aculeatus L .) . Doctoral Dissertation, University of Southern California. Rodgers, W . L., Melzack, R . & Segal, J . R . (1963). Tail-flip response in the goldfish. J. comp. physiol . Psychol ., 56, 917-923 . Russell, E . M . (1967) . Changes in the behaviour of Lehistes reticulatus upon repeated shadow stimulus Anim. Behav ., 15, 574-585 . Sevenster, P. (1961) . A causal analysis of displacement activity, fanning, in Gasterosteus aculeatus. Behaviour Suppl., 9 .
Siegal, S . (1956). Nonparametric Statistics for the Behavioral Sciences. New York : McGraw-Hill . Thorpe, W . H . (1963). Learning and Instinct in Animals . Cambridge, Mass . : Harvard University Press. Tinbergen, N . (1951) . The Study of Instinct . Oxford : Oxford University Press. Triplett, N . (1901). The educability of the perch . Am . J. Physiol., 12, 354-360. (Received 14 March 1968 ; revised 20 June 1968 ; Ms. number : A695)
WANING OF THE AGGRESSIVE RESPONSE TO MALE MODELS IN THE THREE-SPINED STICKLEBACK (GASTEROSTEUS ACULEATUS L .) By HARMAN V . S . PEEKE*, EVERETT J . WYERSt, & MICHAEL J . HERZ* University of Southern California two stimuli and spent time in the centre compartment avoiding visual contact with them . The purpose of this investigation was to study the course of aggressive responsivity of male three-spined sticklebacks to daily presentations of wooden models of male sticklebacks . Models were used, rather than live stimuli, because they allowed more complete stimulus control by the experimenter, and hence allowed a constant manner of presentation from day to day avoiding the variability that live males might provide . The five models were similar except for different amounts of red colour on the ventral anterior surface . It has long been held that red in that anatomical locus is an important component of the releasing stimulus for aggression in the stickleback (Tinbergen 1951) . It was predicted that varying this one important component of the releaser would produce a series of similar models with varying ability to elicit aggression . Differential ability to elicit aggression would allow assessment of the interaction, if any of the effects of repeated elicitation of aggressive responses and the initial releasing strength of the models . The completion of the nest construction by the male three-spined stickleback is considered the transition from the nest building phase of the reproductive cycle to the sexual phase, at which time the male will respond with aggression towards other males intruding upon his territory . However, a male may be maintained in the sexual phase for study if, after the completion of his nest, he is denied the opportunity to court a female successfully. Therefore a study involving changes in aggressive behaviour over time should ideally start and finish within the sexual phase, during which a relatively high but stable level of aggression is maintained (Sevenster 1961) . Such behaviour is normally quite stable . Methods The male three-spined sticklebacks used in this study were collected from a small land-locked creek in southern California in the fall and winter . This source is always well populated with sticklebacks of both sexes, a relatively constant proportion of which appeared to be in
Thorpe (1963) has recently stated that, `There has as yet been no critical work on habituation in fishes . . .' (p . 306) . Since the appearance of Thorpe's review several investigators have demonstrated response decrement of repeated elicited responses in the fish . The `tail flip' response of the goldfish which serves to locomote the fish rapidly away from sources of stimulation has been habituated both in response to a pressure wave set up by a tap on the side of the fish's container and to a shadow passing overhead (Rodgers et al. 1963) . A similar type of response has been habituated to a shadow stimulus in Lebistes reticulatus (Russell 1967) . There is also evidence suggesting habituation of aggressive responses in fish. Triplett (1901), studying the perch, Morone americanus, found that they would stop attempting to attack minnows that were protected by a clear glass partition and, after complete habituation of the predator response, the glass partition could be removed without jeopardy to the minnows . More recently, Baenninger (1966) has presented data from the Siamese fighting fish (Betta splendens) that he interprets as evidence for habituation of conspecific aggression . He placed Bettas in a tank with a model of another Betta at one end and a real Betta in an adjoining tank at the other . In the centre was an area which did not allow a view of either stimulus. The measure of aggression was the amount of time spent in the centre of the apparatus . Over a period of 16 hr the Bettas spent successively more time in the centre section . This result was interpreted to mean that the fish was no longer motivated to view either stimulus because his aggressive motivation had waned . An alternative explanation of these results is that as time went on the fish became more and more fearful of the *Now at the University of California San Francisco Medical Center (Department of Psychiatry) and Laboratory of Psychobiology, Langley Porter Neuropsychiatric Institute, San Francisco . f Now at the State University of New York, Stony Brook (Department of Psychology) . 224
PEEKE et al. : AGGRESSIVE RESPONSE IN THREE-SPINED STICKLEBACKS
breeding condition . In the laboratory the fish were kept in crowded colony tanks 61 cm x 61 cm x 30 cm (15-17 ° C) in order to retard the development of reproductive behaviour. No plants were provided, but the bottom was covered with sand and gravel . All fish were fed daily with either Tubifex worms or brine shrimp (Artemia salina) or both . The experiments were carried out in banks of smaller tanks, 26 cm x 50 cm x 30 cm, separated by opaque dividers . Each tank was filled to a depth of 6-8 cm with fine sand and gravel . the left side and left rear of the bottom were planted with elodea (Anacharis canadesis) and milfoil (Myriophyllum spicatum) . A small quantity of thread algea was also provided which led to an abundant growth of algea on the bottom and sides of the aquaria. This growth was removed only from the front . Lighting was provided for 16 hr per day from a 60-W incandescent bulb placed directly above the centre of each tank, 60 cm from the water's surface . An outside-of-tank air-driven filter, using spun glass as the filtration agent, functioned continuously providing some filtration and circulation of the water. Within 1 to 3 days males selected from the colony tanks and placed individually in smaller experimental tanks would construct a nest* ; that day was designated `day N.' On its day N each fish was presented with each of five wooden *The actual construction of the nest usually takes only several hours, and can take much less . The difference among fish in days to nest construction reflects differences in readiness to build rather than speed of construction .
models of a male stickleback . The models were constructed of balsa wood, with a shape similar to the body shape (without fins) of a male stickleback ; the eyes were represented by black circular dots . The models differed only in the extent of red colour (Munsel colour system between 5R 4/10 and 5R 5/10) on the ventral surface of the model (the rest of the model was of the natural wood colour, protected by a coat of clear lacquer) . The five models ranged from one with no red area (model No . 1), through three models with progressively more ventral anterior red area (models Nos . 2, 3 and 4), to a final one that was all red (model No . 5) . Each of the models was presented for 2 min each day for 12 successive days . The order in which the five models were to be presented was determined randomly, with the restriction that the same order could not be used more than once per group. The models were presented in the same sequence each day with the inter-model time equal to the time necessary to remove one model from the tank and replace it with another, usually no more than 15 sec . The models were attached by a thin stiff wire to a portable motor which was suspended above the tank on a clear Plexiglas platform . The motor propelled the models through a circular (counter-clockwise) path with a diameter of 15 . 5 cm at a rate of one revolution every 10 sec . The circular pathway was approximately halfway between the surface of the water and the gravel substrate . The model was always presented in the right end of the tank such that the model would miss the right wall of the tank by less than 1 cm . Although previous published work from other
A,j/////p°.
I MODEL I
MODEL
2
MODEL 3
MODEL
41
4
.~///////P
MODEL 5
/ l
I I WOOD
BLACK
17mm I ".4
RED
225
I I I I jt 49mm
I 1 j I I I >1
Fig . 1 . Series of five wooden models used to release aggression,
226
ANIMAL BEHAVIOUR, 17, 2
laboratories (e.g . Sevenster 1961) indicated that the aggressive tendency should remain stable over the duration of such an experiment once the fish had entered to the sexual stage, additional groups were utilized to control for the possibility that any change in the frequency of aggressive responses might simply reflect decreases in aggression following the completion of nest construction . This control procedure consisted of initiating the 12 days of model presentation for different groups of fish either 3, 6 or 9 days after nest completion . Thus there were four groups : group N, presented with the models for the first time within 24 hr of nest completion, and group N + 3, N + 6, and N + 9, first stimulated 3, 6 or 9 days post-nest construction respectively . The behaviour of each subject was carefully observed before, during and after model presentations and the frequency of biting was recorded on an event recorder. `Biting' is defined here as any open-mouthed contact with the model . Such contact was followed by a closure of the jaws forming a typical bite. The force of such bites was usually sufficient to displace the model by several millimetres against the stiffness of the wire staff. The two experimenters responsible for observation of the fish during model presentation scored one fish in parallel, and found agreement on well over 90 per cent of the responses recorded, suggesting high inter-observer reliability. In general the same experimenter observed a given fish on all 12 days . Results Initial Response to the Models In total, thirty-seven fish constructed nests and were used for this study . Of these, ten showed marked avoidance of the stimuli from the moment they were introduced into the tank and did not approach the model or strike at it over the period of 12 days . Several fish were included in this category although they bit once or twice at only one of the models on only one day. These 'non-responders' all shared a similar behaviour that the experimenters noted on protocol sheets . As the model was introduced into the tank these fish would immediately dart behind a row or clump of plants, assume a posture with the head higher than the tail, and the tail slightly curled to one side or the other . They would not usually change position unless startled by the experimenter and then would re-assume the posture behind some other cover. What differentiated these fish from fish that
stopped responding to the models after a few days of stimulation is that the latter would not seek cover and would continue to orient toward the model, and would make slow and incomplete approach movements . They also would not assume the head-high, tail-curled stereotyped posture . Three fish showed the typical 'nonresponder' behaviour on the first day (one in group N and two in group N + 9), but then began approaching and biting at the wooden model on the second . Once the agonistic behaviour began, such fish did not revert to their previous posture . The distribution of 'non-responders' among the groups was 2 of 13 for group N, 2 of 8 for group N + 3, and 3 of 8 for group N + 6 and N + 9 . The descriptions and analyses of the data are based on 27 fish which attacked the models ; 11 in group N, 6 in group N + 3, and 5 each in groups N + 6 and N + 9 . Post-nest Construction Drive Fluctuation The purpose of assigning fish to the groups starting 3, 6 or 9 days after nest completion was to assess the change in the aggression tendency as a simple function of the number of days after nest completion . Separate KruskallWallis one-way analysis of variances by ranks (Siegal 1956) were performed for each model across the four groups, using as the measure of `initial aggression' the highest number of bites at a given model recorded on any of the first 3 days . This measure, rather than the number of bites on day 1, was used to avoid problems associated with initial reluctance of some fish to approach the models until after the 1st day, or, in a few cases, after the 2nd day . There were no significant differences among the four groups for any of the five models . As a further test of possible differences among these groups, the total number of bites was lumped together for all five models and summed across all days for each group . This analysis should show a difference if there was any common trend in the individual model analyses reported above . Again, the Kruskall-Wallis test failed to yield a significant value . From these tests it can be concluded that the aggressive tendency does not change as a simple function of post-nest construction time . Differential Releasing Characteristics of the Models If the ability of the models to release aggressive behaviour were dependent upon the amount or placement of the red colour on the ventral anterior surface, then we should expect either
PEEKE et al. : AGGRESSIVE RESPONSE IN THREE-SPINED STICKLEBACKS
differences among the models on the measure of initial aggression (see above), or different rates of change as a function of repeated stimulus presentations . To test for differences in `initial aggression' (defined above) elicited by the models, a Friedman two-way analysis of variance by ranks (Siegal 1956) was computed (subjects x models) with all four post-nest construction groups lumped together insofar as there was no difference among them . The test did not show any difference among models . The same test was computed using the total number of responses across all twelve days as the measure of aggression elicited by the models . This manner of testing should show differential resistance to habituation, despite the result found above, that they started at approximately the same level . Again, however, no differential model effect was found . Changes in Aggressive Behaviour as a Function of Daily Model Presentations The number of bites directed at each of the five models clearly decreased as a function of the number of days of model presentation (Fig . 2) .
N W D
1
1
2
3
4
5
6
7
8
9
10
11
12
DAY Fig. 2. Mean number of bites at each of the models (Nos. 1 to 5) over 12 successive days of daily 2 min presentations,
22 7
The decrease in the number of bites directed at the model from the `initial level' to the 'terminal level' (highest number of bites recorded on a single day during the first 3 days to the highest number recorded on a single one of the last 3 days) is significant for each of the five models (P<0 . 001) in each case) as shown by Wilcoxon matched pairs, signed ranks tests (Siegal 1956) . Again, all four groups were lumped together for these analyses . It is clear that the aggressive response to crude models of conspecific males wanes over days as a function of continued presentation. Discussion Aggressive behaviour in the male three-spined stickleback, measured as the frequency of bites directed at the stimuli, wanes as a result of continued presentation of crude models of conspecific males. This is in agreement with the findings of Baenninger (1966) and Clayton & Hinde (1968) with the Siamese fighting fish, that conspecific aggression does wane with repeated or constant exposure . Baenninger produced his result in 16 hr of constant stimulus presentation while the present study administered the stimuli for short periods each day for 12 days . The results of the two studies however appear, at least superficially, similar . Caution must be exercised to avoid generalizing results of this and similar studies to more natural situations . Such rapid habituation of conspecific aggressive responses would seriously disrupt the normal reproductive cycle of the stickleback if mere repetitions of males wandering into the resident male's territory were sufficient to reduce the aggressive tendency . It has been suggested that habituation is very stimulus specific (Thorpe 1963) and perhaps the rate at which the response to a stimulus habituates is a positive function of the constancy of the stimulus from trial to trial . Such habituation would appear to have little survival value, but in nature the character of the releasing stimuli would likely vary a great deal from `trial' to `trial', and the occurrence of intermittent consummatory acts (conspecific territorial fighting) would further protect against habituation of such responses . In the present study efforts were made to present stimuli that were extremely constant in an attempt to maximize the probability of finding habituation of aggression in a species of fish noted for its aggressive tendencies during its reproductive cycle . The generality of this phenomenon should be enhanced by a demonstration of habituation of aggression to a
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conspecific male using as a stimulus a real, nuptially coloured male stickleback . Indeed, such a demonstration exists (Peeke 1966) . The importance, phyletic generality and some possible mechanisms of habituation are discussed elsewhere and the reader is referred to these references (Russell 1967 ; Thorpe 1963 ; Peeke 1966). The amount of red on the ventral anterior surface of the wooden model did not significantly influence the ability of this particular shape of model to elicit aggressive responses . This is in apparent contradiction to the reports of Tinbergen (1951), who found that red on the lower portion of an extremely crude model appears sufficient to release aggressive displays . It must be stated, however, that the models used in the present study may have been insufficient releasers . Had a more detailed model been used that would elicit more aggression even without a red underside, then perhaps amount and location of red colour might have interacted with shape and detail to produce the previously reported result linking a red underside to aggression elicitation ability . That the observed habituation of aggressive responses over successive model presentations can be attributed to the inadequate releasing quality of the stimuli appears unlikely . Beanninger (1966) and Clayton & Hinde (1968) also obtained evidence for habituation of components of aggressive displays in Betta splendens to a mirror image, a stimulus which could be considered a better releaser than the models used in the present situation . Peeke (1966) found that aggressive responses toward real male sticklebacks confined in a clear tube decreased at a faster rate than did responses to a stationary model, suggesting that habituation may be independent of the reinforcing quality of the stimulus . Summary Territorial three-spined sticklebacks were pre-
sented with crude wooden models of conspecific males . The models differed only in the amount of red colour on the anterior ventral surface . Each model was presented for 2 min each day for 12 days . It was found that the amount of red colour did not significantly affect the amount of aggression elicited by the model . The aggressive response ('biting') decreased markedly from the high level observed on the initial presentations to a low level of responding at the termination of the experiment . Acknowledgments This research was supported in part by Research Grant G-18891 (to E . J . Wyers) from the National Science Foundation, and in part by USPHS predoctoral fellowship No . MH 22269 (to M . J . Herz) . REFERENCES Baenninger, R . (1966) . Waning of aggressive motivation in Betta splendens . Psychonom . Sci., 4, 241-242.
Clayton, F. L. & Hinde, R . A . (1968) . The habituation and recovery of aggressive display in Betta splendens . Behaviour, 30, 97-106 . Peeke, H . V. S . (1966) . Habituation of aggression in the three-spined stickleback (Gasterosteus aculeatus L .) . Doctoral Dissertation, University of Southern California. Rodgers, W . L., Melzack, R . & Segal, J . R . (1963). Tail-flip response in the goldfish. J. comp. physiol . Psychol ., 56, 917-923 . Russell, E . M . (1967) . Changes in the behaviour of Lehistes reticulatus upon repeated shadow stimulus Anim. Behav ., 15, 574-585 . Sevenster, P. (1961) . A causal analysis of displacement activity, fanning, in Gasterosteus aculeatus. Behaviour Suppl., 9 .
Siegal, S . (1956). Nonparametric Statistics for the Behavioral Sciences. New York : McGraw-Hill . Thorpe, W . H . (1963). Learning and Instinct in Animals . Cambridge, Mass . : Harvard University Press. Tinbergen, N . (1951) . The Study of Instinct . Oxford : Oxford University Press. Triplett, N . (1901). The educability of the perch . Am . J. Physiol., 12, 354-360. (Received 14 March 1968 ; revised 20 June 1968 ; Ms. number : A695)