Available at: http://digitalcommons.calpoly.edu/theses/1691
Date of Award
MS in Biological Sciences
Multicomponent displays are composed of traits, such as coloration, structural ornaments, and behavior, that become integrated and signal information to conspecifics. Estimation of multicomponent displays in fishes often involves measurement of color traits. Fish color measurements are often obtained following immobilization via chemical anesthesia; however, the anesthetics may alter the resulting measurements, for example by darkening the skin. Girardinus metallicus, a poeciliid fish endemic to Cuba, has a multicomponent courtship and aggressive display. Black morph males exhibit black ventral coloration including the gonopodium (copulatory organ) and yellow in the non-black areas of their bodies. I investigated the effects of common anesthetics on coloration measurements of G. metallicus. I measured the hue, saturation, and brightness of the anterior dorsal, posterior dorsal, posterior ventral, and caudal body regions, from digital images of the same males obtained without using anesthetic and anesthetized using tricaine methane sulfonate (MS222) and eugenol (clove oil). Because multicomponent displays are intriguing with respect to sexual selection, I investigated the importance of size and coloration traits in sexual selection via female choice and male-male competition in G. metallicus.
I found that saturation and hue did not differ significantly across treatments (anesthetization using MS222, anesthetization using clove oil, and without anesthetic in a small glass chamber containing water). However, brightness was greater under the anesthetics, possibly due to photographing the fish behind water and glass in the Non-anesthetic treatment or due to reflectivity differences of the iridophores. The body regions varied in hue, saturation, and brightness. Most importantly, I found differences in the responses of different body regions to the anesthetic treatments, suggesting that anesthetics may affect coloration in unpredictable ways, and that multiple regions of fish should be measured when assessing overall coloration. My results suggest that photographing fish in a glass chamber without anesthetic may be an effective way to obtain digital images for color analysis without using anesthetics that may influence coloration.
Having determined a good method for color measurement, I then investigated the role of the multicomponent display in sexual selection. Through direct interaction tests, I found that dominant males had brighter and more saturated yellow coloration than subordinate males, and that dominant males courted more than subordinate males. Within high yellow males, dominant males attempted more copulations than subordinate males. Interestingly, low yellow, subordinate males attempted more copulations than low yellow, dominant males, suggesting that subordinate males invested time into attempting copulations rather than engaging in potentially risky aggressive behavior. I observed a greater difference in body size between the males in pairs to which I could assign dominance status than pairs to which I could not assign dominance status, suggesting the importance of standard length in aggression in this species. I found that yellow saturation may serve to signal status without the males resorting to aggressive interactions due to only half the pairs exhibiting aggression. Because aggression is key to mating success in G. metallicus, my findings that yellow coloration is correlated with aggression, in concert with previous studies showing the importance of ventral black area and body size for aggression, reinforce the idea that these males exhibit a multicomponent signal to conspecifics in the context of sexual selection.
Available for download on Friday, December 13, 2019