Male mate choice selects for female coloration in a fish

Abstract

Although sexual selection theory has proved successful in explaining a wide array of male ornaments, the function of ornaments occurring in females is largely unknown. Traditionally, female ornaments have been considered nonfunctional, being merely a genetically correlated response to selection for male ornamentation. However, this hypothesis is only relevant to species in which the ornament is basically the same in the two sexes. Alternatively, female ornaments may be influenced by selection acting directly on the females, either through female-female competition or male choice. We tested the latter hypothesis in mate-choice experiments with two-spotted gobies (Gobiusculus flavescens). In this small marine fish, females have bright yellow-orange bellies during the breeding season, a conspicuous trait that is not present in males. We conducted two aquarium experiments to test whether males preferred to mate with more colorful females. In the first experiment, males had a choice between two females that varied in natural coloration (and belly roundness). In the second experiment, we manipulated belly coloration and kept roundness constant. Males spent more time with colorful than with drab females in both experiments and also performed far more courtship displays toward colorful females. Our study provides experimental evidence that males prefer ornamented females in a fish that is not sex-role reversed, supporting the hypothesis that female ornamentation is sexually selected.

Figure 1

Mutual courtship display of two-spotted gobies Gobiusculus flavescens. The female (Upper) displays her colorful belly by bending the body toward the male (photo by E.F.).

Figure 2

Characteristics of females participating in experiments aimed to test male preference for female coloration in two-spotted gobies. (a) Natural color variation: males were exposed to two females of the same total length but varying in natural coloration. Because female color is related to roundness, colorful and drab females differed somewhat in wet-body mass in this experiment. (b) Color manipulation: males were exposed to two females that were very similar in total length and wet-body mass; their belly coloration had been manipulated to become drab and colorful, respectively. (Bars = means +1 SE.)

Figure 3

Preferences of male two-spotted gobies for females varying in natural coloration. (a) Percentage of time spent in association with colorful and drab females (n = 15). (b) Percentage of displays directed toward colorful and drab females (n = 10). (Bars = means +1 SE.)

Figure 4

Preferences of male two-spotted gobies for females varying in experimentally manipulated coloration. (a) Percentage of time spent in association with colorful and drab females (n = 16). (b) Percentage of displays directed toward colorful and drab females (n = 8). (Bars = means +1 SE.)