Overall, I found a stable aggressiveness-boldness relationship in K. marmoratus:
more aggressive individuals were also bolder. And, the relationship between aggressiveness and boldness remained stable regardless of whether the fish received a wining, a losing or no contest experience. The fish‟s tendency to explore and its learning performance, however, were not significantly correlated with its aggressiveness. In addition, baseline testosterone levels positively correlated with individuals‟ aggressiveness and boldness. The fish‟s tendency to explore correlated with post-experience testosterone levels, whereas learning performance did not have significant relationship with testosterone or cortisol levels. These results suggest that baseline testosterone levels might play an important role in mediating the relationship between aggression and boldness in the fish.
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Tables
Table 1. Cox regression modeling the relationship between contest experience (W, N
& L) and hormone (testosterone and cortisol) levels (log transformed) and aggressiveness (latency to gill display log transformed), controlling for strain, body size and last fighting outcome. Cox regression models the hazard (instantaneous risk) of the event (gill display); a higher hazard of gill display (positive coefficient)
1An indicator variable for the focal individuals that received a pre-designated winning or losing experience; the baseline group comprised the individuals that received no experience.
2An indicator variable for the focal individuals that lost more than one month previously; the baseline group comprised the individuals that won a contest more than one month previously.
Table 2. Cox regressions modeling the relationship between aggressiveness (latency to gill display log transformed), contest experience (W, L & N), the interaction between aggressiveness and contest experience and hormone (testosterone and cortisol) levels (log transformed) and boldness (latency to emerge from a shelter), controlling for strain, body size and last contest outcome. Cox regression models the hazard (instantaneous risk) of an event (emerging from the shelter); a higher hazard of emerging from the shelter (positive coefficient) indicates a shorter latency to emerge from the shelter (ie higher boldness). (T: testosterone; CORT: cortisol; * P < 0.05)
1An indicator variable for the focal individuals that received a pre-designated winning or losing experience; the baseline group comprised the individuals that received no experience.
2An indicator variable for the focal individuals that lost more than one month previously; the baseline group comprised the individuals that won a contest more than one month previously.
Table 3. Cox regressions modeling the correlation between aggressiveness (the latency to gill display log transformed), contest experience (W, L & N), the interaction between aggressiveness and contest experience and hormone (testosterone and cortisol) levels (log transformed) and the tendency to explore (latency to approach a novel object), controlling for strain, body size and last contest outcome. Cox regression models the hazard (instantaneous risk) of an event (approaching a novel object); a higher hazard of exploring (positive coefficient) indicates a shorter latency approach the object, ie a greater tendency to explore. (T: testosterone; CORT: cortisol;
* P < 0.05)
1An indicator variable for the focal individuals that received a pre-designated winning or losing experience; the baseline group comprised the individuals that received no experience.
2An indicator variable for the focal individuals that lost more than one month previously; the baseline group comprised the individuals that won a contest more than one month previously.
Table 4. Logistic regressions modeling the correlation between aggressiveness (latency to gill display log transformed), contest experience (W, L & N), the interaction between aggressiveness and contest experience and hormone (testosterone and cortisol) levels (log transformed) and learning performance (the probability of entering the correct reservoir), controlling for strain, body size and last contest outcome. (T: testosterone; CORT: cortisol; * P < 0.05)
Probability of entering the correct reservoir
1An indicator variable for the focal individuals that received a pre-designated winning or losing experience; the baseline group comprised the individuals that received no experience.
2An indicator variable for the focal individuals that lost more than one month previously; the baseline group comprised the individuals that won a contest more than one month previously.
Table 5. Multiple linear regressions modeling the effect of contest experience on post-experience testosterone levels (log transformed) controlling for baseline testosterone levels (also log transformed), handling time, strain, body size and last contest outcome. (T: testosterone levels; * P < 0.05)
Post-experience T
Variable df b SE F P value
Contest experience 2 0.009 0.992
Baseline T 1 0.585 0.056 109.246 < 0.001*
Handling time 1 -0.025 0.017 2.312 0.130
Strain 4 0.396 0.811
Body Size 1 0.023 0.019 1.429 0.234
Last L1 1 0.12 0.07 3.468 0.064
1An indicator variable for the focal individuals that lost more than one month previously; the baseline group comprised the individuals that won a contest more than one month previously.
Table 6. Multiple linear regressions modeling the effect of contest experience on post-experience cortisol levels (log transformed), controlling for baseline cortisol levels (also log transformed), handling time, strain, body size and last contest outcome. (CORT: cortisol levels; * P < 0.05)
Post-experience CORT
Variable df b SE F P value
Contest experience 2 0.975 0.379
Baseline CORT 1 0.522 0.074 49.767 < 0.001*
Handling time 1 -0.037 0.047 0.645 0.423
Strain 4 2.033 0.092
Body Size 1 0.053 0.054 0.957 0.329
Last L1 1 0.02 0.18 0.012 0.914
1An indicator variable for the focal individuals that lost more than one month previously; the baseline group comprised the individuals that won a contest more than one month previously.
Figures
Figure 1. The setting for the aggressiveness test: The experimental aquarium was 16
10 20 cm3 large, with a mirror on the narrow side. It contained water 12 cm deep and 2cm of gravel. There was a mark 4 cm from the mirror on the wide side of the aquarium and, 10 cm away from the mirror a black opaque removable partition, separating the start area from the area with the mirror.
Figure 2. The setting for the boldness test: The testing aquarium was 16 10 20 cm3 large, containing water 12 cm deep and 2 cm of gravel. The aquarium contained a black plastic shelter (6 6 8 cm3) with a removable sliding door.
Figure 3. The setting for the exploratory test: The experimental aquarium was 34 12 26 cm3 large containing water 16 cm deep and2 cm of gravel. I put a new shelter at the end of the wide side and a black opaque partition 6 cm from the opposite end.
There were two marks 4 cm from the shelter on the surface of the aquarium designating the “well explored region”.
Figure 4. The setting for the spatial learning ability test (T-maze test): The T-maze
consisted a cross piece (70 cm), with a down piece (35 cm) joining the mid-point of cross piece. Both section sections were 10 cm wide, 12 cm deep and contained water 5 cm deep. There was a 22.5 22.5 22.5 cm3 reservoir at each end of the cross piece, with 1 cm of gravel, 3 shelters and water 13 cm deep. There was an opaque partition at the entrance to each reservoir and 10 cm away from the end of the down piece of the T-maze.
Figure 5. Scatter graphs plotting aggressiveness ranking against boldness ranking for fish with (a) a winning experience, (b) no contest experience and (c) a losing experience. (Statistics are Spearman rank correlation coefficients and related P values.)
Figure 6. Scatter graphs plotting aggressiveness ranking against the tendency to explore ranking with (a) a winning experience, (b) no contest experience and (c) a losing experience. (Statistics are Spearman rank correlation coefficients and related P values.)
Figure 7. Box plots comparing aggressiveness ranking between the individuals which entered the correct reservoir and the individuals which entered the wrong reservoir in the spatial learning test with (a) a winning experience, (b) no contest experience and (c) a losing experience. (P values calculated using a Wilcoxon test. The box itself contains the middle 50% of the data. The upper edge of the box indicates the 75th percentile of the data set, and the lower edge of the box indicates the 25th percentile of the data set. The short line in the box indicates the median value of the data. The upper and the lower ends of the vertical lines indicate the minimum and maximum data values, respectively.
Figure 8. Box plots comparing the ranking of latency to gill displays of the different strains. The box itself contains the middle 50% of the data. The upper edge of the box indicates the 75th percentile of the data set, and the lower edge of the box indicates the 25th percentile of the data set. The short line in the box indicates the median value of the data. The upper and the lower ends of the vertical lines indicate the minimum and maximum data values, respectively.
Figure 9. Box plots comparing the ranking of latency to emerge from shelter of different strains. The box itself contains the middle 50% of the data. The upper edge of the box indicates the 75th percentile of the data set, and the short lower edge of the box indicates the 25th percentile of the data set. The line in the box indicates the median value of the data. The upper and the lower ends of the vertical lines indicate the minimum and maximum data values, respectively.
Appendix
Table 1. The median and the range of latency to gill display (index for aggression;
lower value indicates higher aggression), latency to emerge from shelter (index for boldness; lower values indicates bolder), latency to explore novel object (index for exploratory tendency; lower values indicates greater exploratory tendency) and learning performance (the frequency of entering the correct and the wrong reservoir) for individuals with different contest experiences