BEHAVIORAL BIOLOGY, 13,197-202 (1975), Abstract No. 4223

Behavioral Individuality in the Cichlid Fish, Tilapia mossambica DAVID P. BARASH

Departments of Psychology and Zoology, Univeristy of Washington, Seattle, Washington 98195

Behavioral individuality with regard to courtship and aggression was evaluated and demonstrated among males of the Cichlid fish, Tilapia mossambica. Individuals fall into natural categories within which behavioral differences are not apparent, suggesting a behavioral polymorphism. These categories appear to represent heritable discontinuities; possible ways in which these polymorphisms are maintained are discussed.

Students of animal behavior are beginning to recognize the existence and significance of individual differences in behavior (Hirsch, 1963, 1967). Yet there have been few attempts to evaluate the degree of behavioral individu n i t y normally present in a population of any species, and those behavioral traits analyzed have usually been the most convenient, rather than the most biologically important. In addition, the genetic basis o f aggressive and sexual behavior is in great need of study. This paper describes behavioral individuality in a small sample of male Tilapia mossambica (family: Cichlidae), suggests a genetic basis for the observed individual differences and provides hypotheses as to the factors responsible for the maintenance of the observed heterogeneity.

METHODS AND RESULTS The initial subjects were seven male Tilapia mossambica, each derived from a different source, including university, institutional and commercial stock. The size o f Tilapia is known to influence its behavior, particularly its aggressiveness (Neff, 1964); therefore, the subjects selected were all of comparable size, ranging from 8.8-9.2 cm (total length). Their ages were not known. Each subject was kept visually isolated in a 5-gal tank for 30 days before testing began. The tests involved successive introduction of each of 2 wooden models, 9.0 cm long, one painted to represent a female T. mossambica and the other a male in "Dark II" coloration (Neil, 1964). The order 197 Copyright © 1975 by Academic Press, Inc. All rights of reproduction in any fo~m reserved.

TABLE 1 Median Scores for Behavioral Tests of the Seven Original Subjects and their Offspring a Behaviors Number of frontal displays (a)

Seconds in lateral display (b)

Number of nips (c)

Seconds in Dark I1 or Black (d)

Latency (sec) for initial tilt (e)

1 la lb lc ld le

3 2 3 2 2.5 1.5

133.5 140 128.5 171 134.5 159.5

6.5 5.5 6 7 7.5 6.5

40.5 20.5 33 56 20.5 37

74 80 60.5 44.5 101.5 104

2 2a 2b 2c 2d 2e

2.5 2.5 2 2.5 2 2.5

160 170.5 123 134 160.5 151

7 7 6.5 5 6.5 6.5

120.5 96.5 110 133.5 102.5 109

60 70 65.5 42 31.5 33.5

3 3a 3b 3c 3d 3e

2.5 2.5 2.5 3 2 3

142.5 155,5 103 127 128.5 144

5 5.5 4 6 7 5

94 80 90.5 122 123.5 86

51.5 64.5 38.5 29 42 36

4 4a 4b 4c 4d 4e

1 1.5 1 0.5 1 1

83 74.5 79.5 89 97 62.5

2 2 2.5 1.5 2 3.5

286 285.5 284.5 277 294.5 300

15 22.5 19 6.5 8.5 20

5 5a 5b 5c 5d 5e

1.5 1.5 1.5 2 1 1.5

64,5 54 50.5 77.5 79 89

1 2 2.5 1.5 1 1.5

294 291 290.5 274 281.5 293.5

17 20.5 12 14.5 6 7.5

6 6a 6b 6c 6d 6e

0 0 0.5 0 0 0

0 0 0 20 15 0

0 0 0 0 0 0

22 18.5 34 39.5 42.5 20.5

93.5 110.5 44 66.5 70 83

7 7a 7b 7c 7d 7e

0 0 0 0 0 0

0 17.5 0 0 0 0

0 0 0 0 0 0

66 45.5 21.5 57 58.5 48

110 94.5 83.5 21.5 78.5 91.5

Subjects

aUnlettered numbers indicate sires, subscript letters indicate offspring; each datum represents 30 5-min tests for each individual.

INDIVIDUALITY IN FISH

199

of presentation of the two models was randomly determined each day for each subject. Each model was introduced for a 5-rain test period on each of 30 days during which time aggressive responses (male model) and courtship responses (female model) were recorded as follows: aggressive-(a)frequency of frontal displays, (b)number of seconds (out of a possible 300) spent in lateral display and @)number of nip sequences directed at the model; courtship-(d) number of seconds (out of a possible 300) spent in either "dark II or "black" coloration and (e) latency in seconds between introduction of the model and the initial appearance of "Tilting" behavior. Details of these behavior and color patterns have already been carefully described (Baerends and Baerends van Roon, 1950; Nell, 1964). High scores for a, b and c indicate high aggression; high scores for d and low scores for e indicate strong courtship. The data (Table 1) were analyzed using a series of Kolmogorov-Smirnov two-sample tests (Siegel, 1956; a non-parametric test was selected because the assumption of normality does not appear to be justified). Two-tailed tests revealed that males 1, 2 and 3 are statistically indistinguishable with regard to behaviors a, b, and c. The same applied to males 4 and 5, and males 6 and 7. The seven subjects thus distributed themselves into three distinct categories of aggressive behavior, within which no individual differences were discernible. However, comparisons of data between individuals of the three categories (d 1 vs d4, d 2 vs d7, etc.) revealed significant differences in all cases ( P < 0 . 0 5 that the data from any two such individuals are drawn from the same statistical population), excepting a lower distinctiveness for behavior A ( P < 0 . 1 0 for males 5 or 5 vs males 6 or 7). The data for courtship intensity (behaviors d and e), revealed a similar pattern of three rather discontinuous categories with similarity among individuals within the categories and distinctness between individuals from different categories. However, there was no correlation between individual tendencies for aggressiveness and for courtship intensity, since males 4 and 5 (intermediate in aggressiveness) were the maximum courters. The other 2 groups consisted of males 1, 6 and 7, and males 2 and 3. Individuals within these courtship groups were statistically equivalent with regard to behaviors d and e. Similarly, the differences between category members was significant (P < 0.05) in all cases. Having ascertained a degree of behavioral individuality, I attempte d to evaluate the heritability of these differences by obtaining and testing Fl'S. As with the males, the females represented a variety of genetic backgrounds. I randomly selected five male offspring of each subject and isolated them in 7-gal aquaria at 100 days of age. After 30 days isolation these Fl's were tested for aggression and courtship intensity, exactly as their sires had been previously.

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DAVID P. BARASH

The results are included in Table 1. Considering the results for each behavior separately, I tested the hypothesis that the six scores in each case (sire plus five offspring) had been drawn from the same continuous population, using the Kruskal-WaUis one-way analysis of variance, corrected for ties (Siegel, 1956; because of the data's high variance, determination of formal heritabilities were not feasible). The null hypothesis could not be rejected, at any confidence level. Once again, as with the sires, Kolmogorov-Smirnov tests revealed significant differences (P

Behavioral individuality in the cichlid fish, Tilapia mossambica.

BEHAVIORAL BIOLOGY, 13,197-202 (1975), Abstract No. 4223 Behavioral Individuality in the Cichlid Fish, Tilapia mossambica DAVID P. BARASH Department...
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