Group size and aquatic vegetation modulates male preferences for female shoals in wild zebrafish, Danio rerio

Abstract

Shoaling decisions in the wild are determined by a combination of innate preferences of the individual along with the interplay of multiple ecological factors. In their natural habitat as well as in the laboratory, zebrafish is a shoaling fish. Here, we investigate the role of group size and associated vegetation in shaping shoaling preferences of wild male zebrafish. We studied the association preference of males to groups of female shoals in a multi-choice test design. We found that males made greater proportion of visits to an 8-female group compared to 2 and 4-female groups. However, males spent similar proportions of time across the three female-containing groups. When artificial vegetation was incorporated along with female number as an additional factor, we found that males prefer high and moderately vegetated patches compared to low or no-vegetation groups, irrespective of the number of females in these patches. Based on experiments using a novel multi-choice design, our results show that preference for group size can change due to interaction of two separate factors. This work is a first attempt to understand the role of aquatic flora in determining shoaling preferences in zebrafish, using an experimental paradigm consisting of a gradation in female and vegetation densities.

Figure 1

Diagrammatic representation of the arena for the density experimental set-up. The central chamber (indicated by a circle) represents the area where the test males were released and the corner square chamber (separated by transparent mesh) contained females of varying density. The distance of each patch from the central chamber was 40 cm.

Figure 2

Diagrammatic representation of the arena the vegetation experimental set-up. The central chamber (indicated by a circle) represents the area where the test males were released and the corner square chambers (separated by transparent mesh) contained females of varying density and each patch was associated with variable number of plastic plants representing vegetation cover.

Figure 3

(a) Boxplots showing the proportion of visits (I_visit) by the male to the various chambers (patches) containing females in varying density. The chamber containing no females received a significantly lesser number of visits compared to the other three chambers. The eight female patch received higher proportion of visits compared to two and four female patches. Differing alphabets indicate statistical significance (p < 0.05), while similar alphabets indicate no statistical difference. The lower and upper ends of the box plots represent the 1st and 3rd quartiles, the horizontal line within each boxplot is the median, and the upper and lower whiskers are the ×1.5 the interquartile range. Outlines are shown as open circles. (b) Boxplots showing proportion of total time (I_time) spent by the male in each of the chambers. Males spent significantly longer time in the three female-containing chambers compared to the chamber with no females. They spent similar proportions of time in 2, 4 and 8 female patches. Differing alphabets indicate statistical significance between the two plots (p < 0.05). The lower and upper ends of the box plots represent the 1st and 3rd quartiles, the horizontal line within each boxplot is the median, and the upper and lower whiskers are the ×1.5 the interquartile range. Outlines are shown as open circles.

Figure 4

(a) Boxplots showing the proportion of visits (I_visit) by the male to the various chambers containing the females in varying density in vegetation presence (E1 set). High (H) and medium (M) chambers having eight females (and high plant density) and four females (and medium plant density) respectively had a higher proportion of visits by the male compared to the low (L) (two females and low plant density) and null (N) (one female and no plants) chambers. Similar alphabets placed above the plots indicate no statistical difference whereas dissimilar alphabets indicate significant statistical difference (p < 0.05). The lower and upper ends of the box plots represent the 1st and 3rd quartiles, the horizontal line within each boxplot is the median, and the upper and lower whiskers are the ×1.5 the interquartile range. Outlines are shown as open circles. (b) Boxplots showing the proportion of time (I_time) spent by the male in each of the chamber (E1 set). Males spent a significantly greater proportion of time in medium (M) chamber compared to the other three patches. Similar alphabets placed above the plots indicate no statistical difference whereas dissimilar alphabets indicate significant statistical difference (p < 0.05). The lower and upper ends of the box plots represent the 1st and 3rd quartiles, the horizontal line within each boxplot is the median, and the upper and lower whiskers are the ×1.5 the interquartile range. Outlines are shown as open circles.

Figure 5

(a) Boxplots showing the proportion of visits by the male to the various chambers (E2 set). Males visited the medium (M) chamber (four females and medium plant density) significantly more than the high (H) (two females with high plant density), low (L) (eight females with low plant density) or Null (N) (one female with no plants) chambers. H patch also had greater proportion of visits compared to N patch. Similar alphabets placed above the plots indicate no statistical difference whereas dissimilar alphabets indicate significant statistical difference (p < 0.05). The lower and upper ends of the box plots represent the 1st and 3rd quartiles, the horizontal line within each boxplot is the median, and the upper and lower whiskers are the ×1.5 the interquartile range. Outlines are shown as open circles. (b) Boxplots showing the proportion of time spent (I_time) by the male in each of the chamber (E2 set). The test males spent statistically greater proportion of time in M patch compared to the other three patches. H patch also had greater proportion of time spent compared to N patch. Similar alphabets placed above the plots indicate no statistical difference whereas dissimilar alphabets indicate significant statistical difference (p < 0.05). The lower and upper ends of the box plots represent the 1st and 3rd quartiles, the horizontal line within each boxplot is the median, and the upper and lower whiskers are the ×1.5 the interquartile range. Outlines are shown as open circles.