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Induction of parental behavior in the blue gourami, Trichogaster trichopterus (Pisces, Belontiidae) Kramer, Donald Lawrence

Abstract

The blue gourami, Trichogaster trichopterus, is a small, freshwater tropical fish, native to Southeast Asia. Parental behavior is normally shown only by males after spawning. Females and non-parental males eat small numbers of test eggs which they are given, while parental males retrieve them to their nests. The goal of this thesis was to investigate the factors responsible for the sudden change in behavior from egg-eating to parental care which takes place at the time of spawning. In the initial section, the patterns of parental behavior are described, and quantitative data on the development and maintenance of parental behavior in male fish spawning for the first time are presented. The experimental studies reveal that stimuli from the eggs are an important factor in the induction of parental behavior. Many naive males developed parental behavior within 0.5-2.5 min when given eggs spawned by other fish. This response was dependent upon the number of eggs presented: a large proportion of fish became parental when given 2000-3000 eggs, a smaller proportion did so when given 500-1000 eggs, and none did so when given 100 eggs. However, even 100 eggs evidently affected parental responsiveness because some fish eventually developed complete parental behavior when repeatedly given 100 eggs. These results indicate that eggs can induce parental behavior, not that they do so in the natural spawning situation. However, tests of males spawning with females whose oviducts were plugged indicated that exposure to eggs was an important component in the natural development of parental behavior. Spawning by itself did not induce fish to become parental, but it did reduce the number of eggs necessary for parental behavior to develop. Besides spawning and stimuli from the eggs, androgens also play a role in the development of parental behavior. It was shown that, after spawning, males performed more parental behavior than females, although detailed observations of females revealed that they had some capacity to perform parental behavior. The masculinization of females by means of methyl testosterone implants allowed them to develop parental behavior in response to large numbers of eggs. Whether castration of males eliminated their capacity to perform parental behavior could not be determined with certainty. The presence of young facilitates the maintenance of parental responsiveness to eggs when males with developing broods are compared with males whose broods have been removed. Apparently, physical contact with the young is necessary because males lose their parental responsiveness when their broods are placed in baskets which allow only visual and chemical contact. An important result of this study is the demonstration of the significance of stimulus strength in the induction of parental responsiveness. This factor has not often been taken into consideration in other studies of parental behavior. By varying stimulus strength, repetition of stimuli, and sexual experience, patterns of the induction of parental behavior were found in gouramis which were similar to those observed in other studies on a variety of mammals, birds, and fish. That is, parental behavior developed either almost immediately or gradually with the presentation of young to non-parental fish, or it was shown only if the young were presented during the "sensitive period" after spawning. The finding of such different patterns in a single species as a result of varying the strength of stimuli from the eggs suggested that the motivating effects of eggs and young on parental responsiveness may represent a common pattern underlying the control of parental behavior in a variety of vertebrate species. This possibility and the relationship between hormones and stimuli from the young in the control of parental behavior are discussed in the final chapter.

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