Many cichlid species have striking spots on their anal fin. This is especially the case among the males of the mouth-breeding African species, while they are absent in South-American mouthbreeders. Haplochromine males uniquely possess such conspicuous yellowish to orange spots, that are often called egg-spots or egg-dummies, due to their relation with courtship and spawning behaviour of the haplochromines. The true function of these spots is yet under dispute.
Wickler (1962) suggested that the egg-spots of male haplochromines function as egg-dummies, attracting females and enhancing fertilization of the eggs. He was convinced that the spots mimicked the eggs of the particular species in colour, size and form. He based his opinion on thorough observations:
They are displayed to the female during courtship and during fertilization and elicit her reaction of snapping up eggs. During courtship they make the male more attractive to the female. In spawning they assure fertilization of the eggs, which are taken into the female’s mouth immediately after they have been laid and before the male is able to fertilize them. In snapping at the egg-spots with her lips, the female inhales the sperm, thus fertilizing the eggs within her mouth
For several reasons, Wickler’s hypothesis was not undisputed. The resemblance of egg-spots to the ova, their role in fertilisation success and the role of natural selection in the evolution of eggspots were questioned.
In a comparative investigation of the egg-spots in haplochromines from Lake Victoria, Goldschmidt demonstrated that interspecific variation in egg-dummy size correlated negatively with light intensity in the habitat (Goldschmidt, 1991). Still, Goldschmidt remained a declared supporter of the hypothesis that sexual selection on egg-dummy morphology could lead to speciation (Goldschmidt & de Visser, 1990) as he stated:
One glance at an egg-spot was certain to dispel all doubt, because how else, other than through sexual selection, could these egg-dummies on the anal fins of the males have originated? It must have been the work of females who showed a preference for males whose egg-dummies most closely resembled their own eggs.
He repeated his earlier suggestion that conspicuousness may benefit successful courtship with conspecific females and on the other hand attract visual predators (Goldschmidt, 1990). Due to the resulting compromise, in surface-dwellers the egg-spots were smaller than the eggs and in species from turbid and murkier habitats the egg-spots were relatively large. Thus, natural selection for example through predators may have modified the outcome of sexually selected traits (Tobler, 2006).
Goldschmidt also observed an intraspecific variation of egg-spot numbers: large males had more spots than smaller ones. Recent investigations in males of Astatotilapia burtoni reveals that the size of egg-spots does not depend on age, but rather on the condition in which the fish had been raised (Heule & Salzburger, 2011). The fact that number, size and colouration of the egg-spots seemed to be more correlated to male individual traits and habitat, made it less plausible that they played a prominent role in species recognition.
There are some indications that egg-spots may attribute to the attractiveness of males in a sense that females prefer large and numerous egg-spots (Couldridge, 2002; Hert, 1989; 1991) as if they were a key-signal in courtship behavior. However, experiments showed that fertilization success was independent of the presence of the egg-spots, as their removal had no effect on the fertilization rate (Hert, 1991). Their significance for female preference indicated that egg-spots may have evolved through sexual selection rather than natural selection.
According to the sensory exploitation hypothesis (Endler & McLellan, 1988), male signals can evolve in response to pre-existing sensory biases in females. Experiments using animated photographs of the egg-spotless haplochromine Pseudocrenilabrus multicolor revealed that females prefer images of males with virtual egg-spots over images showing unaltered males (Egger et al., 2011). The observed preference for yellow to reddish spots suggest that the evolution of egg-dummies should be considered an adaptive process as it is related to high-quality (carotenoids-enriched) food uptake. A similar correlation between colour preference and diet was found in guppies (Grether et al., 2005).
If egg-spots were to play a role in sympatric speciation, it is of vital importance that they are significant in both intra- as well as intersexual selection (Van Doorn et al., 2004). The observed intraspecific variation of egg-spots may appear to be counterintuitive since selection might be expected to stabilize traits that are correlated with fitness measures. Experimental studies on a Malawian cichlid revealed that the number of egg-spots was related to male age, body condition and dominance status (Lehtonen & Meyer, 2011)and that the egg-spot number also had a high heritable component. This, indeed, suggests that the function of egg-spots might have less to do with fertilization success or species recognition, but rather relate to mate choice and/or male–male competition, helping to explain the high variability in this important trait. The latter is supported by recent experiments with Astatotilapia burtoni. Males with fewer egg-spots received significantly more attacks, suggesting that egg-spots are an important signal in intrasexual communication (Theis et al., 2012).