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The Behavioral Organization of Responses To Territorial Intruders and Frightening Stimuli in Cichlid Fish (Haplochromis Spp.)

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Variation in responses to several constant external stimuli was studied amongst individual fish belonging to three closely related Haplochromis (Cichlidae) species. The purpose of the two experiments was to determine the motivational mechanisms through which responsiveness to stimuli in the environment may affect attack and display behavior to conspecifics. Therefore, the interaction between responses to two types of stimuli, a conspecific territorial intruder restrained in a small glass tube and a bright film light, were analyzed. A constant external stimulus situation allows individual differences in responses, and changes in the response over time, to be attributed to differences or changes in the activation levels of internal controlling factors for the response. In the first experiment single territorial males were kept in aquariums in which a restrained conspecific intruder was presented to them for 10-20 minutes. Their responses to the intruder were filmed on video, and analyzed with the intention of determining the main characteristics of the response that vary between individual fish. A principle components analysis of the responses of 14 fish explained 77.4% of the interindividual variance with 4 components. These components were interpreted as representing individual variation in: 1 ) amount of response directed to the intruder verses other stimuli in the area where the intruder was presented (Interest in the Intruder), 2) Avoidance responses to the intruder including the displays that are most turned-away from the intruder, i. e. , lateral display, tailbeating, and vibrating, 3) Attraction to the Shelter (inverted flower pot) in order to hide, 4) Persistence of responding to the intruder. Scores for each individual on each of these response dimensions were calculated so that a fish could be characterized relative to the others on these 4 properties of the response. Two of the components accounted for the occurrence of attack and display behavior to the intruder, Avoidance (component 2) and Persistence (component 4). The former was found to be negatively correlated with direct orientation to the intruder, and the latter to be associated with the amount of attack and display behavior directed towards it. It was concluded that two different motivational factors underlying these response dimensions contribute positively to the motor command for the response to the intruder. It was also concluded that there must be a general motivational factor affecting the reactivity to all unexpected external stimuli, for the persistence of performing behavior to the intruder, and the duration of hiding after the experimenter entered the room or a bright light was switched on, were all strongly positively correlated. In section II, analyses of changes in behavior over time were reported. After the initial 10 minute observation period, response to the intruder was recorded for an additional 10-20 minute period. Individual scores on the components in both periods were compared in order to determine trends of change in the response dimensions represented by the 4 principle components. It was found that scores on Avoidance and Shelter Attraction decrease over time in most individuals, whereas for the remaining two components they either increase or decrease. Multiple regression analysis showed that the scores on Avoidance in the second observation period are positively related to the initial period scores on both Avoidance and Shelter Attraction. Change in scores on Persistence over time is negatively related to Avoidance scores. It was concluded that there is an interrelationship between factors underlying the response dimensions Avoidance and Shelter Attraction, and that the initial level of Avoidance responding may control changes in persistent responding to the intruder via some mechanism associated with amount of direct orientation to it. In some test sessions, after the initial observation period a bright film light above the tank was switched on and left on while the response to the intruder was recorded for a subsequent and equally long period. Component scores of individuals in the second observation period could thus be compared under conditions of responding to the intruder both with and without a frightening stimulus present-the bright light. It was found that the light caused an increment in scores on Shelter Attraction that was about the same in all animals. It aiso caused an increment in scores on Avoidance responding that amounted to the scores being reset back to their initial level in the first observation period before the light came on. Further, multiple regression analysis revealed some relationships between components that were only apparent when the light was on. Shelter Attraction scores were negatively related to Persistence scores, and Avoidance scores were positively related to Interest in the Intruder scores (component 1). The results presented in section III therefore present further evidence for a common motivational factor underlying the response dimensions Avoidance and Shelter Attraction, for both were increased by the addition of the light stimulus. Also the addition of a frightening stimulus apparently has qualitative and quantitative effects on the response to the intruder. It increases Avoidance responses and the total amount of response to the intruder. The second experiment, reported in section IV, was carried out in order to determine the effect of intruder presentation on attacking and hiding responses elicited by other stimuli. This was done by keeping 10 juvenile fish in the same tank as the test fish, and observing attack behavior to them before and after a 15-minute intruder presentation. The experimenter entering the room to place or remove the intruder elicited a hiding response that could be compared before and after intruder presentation. It was found that although the persistence of attacking the little fish and intruder, and the duration (persistence) of hiding from the experimenter were all positively correlated when each stimulus first appeared, after intruder presentation the degree of change in persistent attack to the little fish and persistent hiding were not related. The former was found to increase in all fish, and the degree of increase was positively related to the amount of direct orientation to the intruder while it was present. Persistent hiding however, was found to decrease after intruder presentation, and this was negatively related to the total time spent responding to the intruder regardless of the direction of orientation to it. It was concluded therefore that the intruder stimulates motivational factors that increase persistent response to territorial intruders and that exclude persistent responding to disturbing stimuli. It was also found that the intruder releases a hiding response, for although its presentation decreases the duration of hiding immediately after the experimenter had been in the room, it causes an increase in the frequency of hiding throughout the period after its removal. These results were combined into a model for the behavioral organization of the responses to these stimuli. The model proposes that each type of stimulus, territorial intruders or disturbance of surroundings, activate motivational systems (T and D respectively) for responding to these specific classes of stimuli. These two systems compete for behavioral dominance, thus for control of selective attention. Also, stimulation from all novel stimuli, disturbing and territorial intruders, combines to determine the level of the fear system. The output of the fear system is channelled to the factors controlling motor patterns for responding to the momentarily dominant stimulus. Thus if system T is dominant, the fear system controls motor patterns for avoidance responses, and if D is dominant it controls motor patterns for hiding. Attack and display behavior to the intruder is thus determined by the level of system T and the level of the fear system. The duration of the response is partly determined by the level of the former relative to the level of system D. An additional general factor for reactivity to relatively novel stimuli (U) positively influences the output level of both systems T and D so as to maintain the dominance of the momentarily dominant system. Changes over time in the persistence of responding to the intruder are controlled by two factors-a decremental process (habituation) due to the decreasing effectiveness of the restrained intruder to stimulate system T and fear, and an incremental process resulting from positive feedback from orienting towards the restrained intruder that does not leave the territory. The model is discussed in relation to similar behavioral models in the literature, and also as to its relevance to literature concerning fear behavior in animals.

Affiliations: 1: Zoölogisch Laboratorium, University of Groningen, Haren (Gr.), The Netherlands


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