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"Why do so many African cichlids have egg dummies on their bodies? Why are the egg dummies on different body parts in different species?"
David Barnes, Camden, NJ
Close-up of the anal fin in a male Astatotilapia burtoni.
About 80% of all East African cichlid species are members of a single tribe, the so-called haplochromines. Several behavioral and/or morphological traits are considered to have facilitated this enormous diversification: the special maternal mouthbrooding behavior, the wealth of color morphs, sexual color dimorphism, and the occurrence of egg-spots. Egg-spots are yellow to reddish ovoid markings on male anal fins of many haplochromine cichlids. The more descriptive name egg dummies goes back to Wickler (1962), referring to their suspected function in mouthbrooding behavior by mimicking real eggs. Accordingly, females are attracted by males displaying egg-spots and follow them to the pit to lay their eggs. The female immediately takes up these eggs into her mouth and snaps at the male’s egg-spots, enabling the male to fertilize the eggs in the female’s mouth due to the close position of egg-spots to the genital papilla. Egg-spots therefore have two hypothesized functions: attracting females and maximizing fertilization rate.
These hypotheses were tested in several studies throughout the past decades. Number or size of egg-spots did not influence the fertilization rate, but it had an effect on female choice. Interestingly females of different species showed different preferences, e.g. Astatotilapia elegans females preferred males with many egg-spots compared to males with one egg-spot (Hert 1989), whereas Pseudotropheus lombardoi females chose males with one big egg-spot over one small or two egg-spots (Couldridge 2002). Using Astatotilapia burtoni, we found that females randomly chose between males with many or few egg-spots. They even had a tendency to choose males without egg-spots compared to males with egg-spots (Theis et al. 2012). Note that the fertilization process was successful even if the males’ egg-spots had been removed artificially. But why does A. burtoni still have egg-spots if they are not important in female choice? As a carotenoid based ornament, egg-spots are likely to be an honest signal of male quality, which could also play an important role in male-male competition. A male aggression experiment revealed that egg-spots in fact have an influence in interactions between males: males without egg-spots received more attacks than males with egg-spots. Therefore, only males with many egg-spots could pay the costs of owning a territory to gain access to females. Our findings are in contrast to findings of other studies about the function of anal fin egg-spots. It seems that anal fin egg-spots have evolved multiple functions in different haplochromine cichlid species even tough they share common ancestry.
Another type of egg dummies are yellow colored tips on the ventral fin of ectodine males (e.g. Cyathopharynx foae, Ophtalmotilapia ventralis, Ophtalmotilapia nasuta). If they are stretched out to the back, these tips lay in close proximity to the genital papilla. This suggests that they have a similar function as anal fin egg-spots in haplochromine cichlids, representing a potential case of functional convergent evolution. However, to my knowledge, the function of egg-spots has not yet been tested experimentally in these species.
|Egg spots are well visible on the anal fin of these A. burtoni males. On the left is a freshly collected male from Zambia. On the right, a male presents his egg spots during an aggressive interaction with another male in the aquarium.|
- Couldridge, V.C.K. (2002): Experimental manipulation of male eggspots demonstrates female preference for one large spot in Pseudotropheus lombardoi. Journal of Fish Biology 60: 726-730.
- Hert, E. (1989): The function of egg-spots in an African mouth-brooding cichlid fish. Animal Behavior 37: 726-732.
- Theis, A., W. Salzburger & B. Egger (2012): The function of anal fin egg-spots in the cichlid fish Astatotilapia burtoni. PLoS ONE 7: e29878.
- Wickler, W. (1962): "Egg-dummies" as natural releasers in mouth-breeding cichlids. Nature 194: 1092-1093.