Cichlid fish are an evolutionary biologist’s dream. There are thousands of species of cichlids, and more seem to crop up every day. Evolution never truly stands still in any species, but if you want to see it in action, cichlids are a good place to start.
Cichlid fish live, and evolve, in the East African Great Lakes. Not only are they fun to study, they’re also extremely beautiful. The many different colors and patterns found in these fish are, at least in part, the result of sexual selection. Sexual selection can be driven by the preferences of the opposite sex. For example, peacocks have ridiculous (but pretty) tails because peahens think they look nice. On the other hand, sexual selection could be driven by the same sex: ornaments or pigmentation might be a signal to rival males that it’s time to fight.
The largest group of cichlid species is the haplochromine cichlids (roughly 1500 species). A feature of many haplochromines are the colorful “egg spots” on the anal fin of males. Tons of studies have been done trying to pin down exactly what these spots are used for. Some studies have suggested that females prefer males with more egg spots, for various potential reasons (including this now largely disproved one), but this is still an unsettled issue.
Now, a paper by Anya Theis, Walter Salzburger, and Bernd Egger from the Unviersity of Basel (published this month in PLoS One) argues that egg spots can be used for male-male competition. The jist: the fewer egg spots a male has, the more likely it is he’s going to be attacked by other males.
The authors performed two separate experiments to first test whether females preferred more or less egg spots on their guy. These experiments used a new method for testing preference. Usually, these experiments involve measuring how much time a female will spend in the vicinity of each male presented to her. In this paper, the authors instead counted how many eggs a female laid in front of each male (when given a choice of two males with different numbers of egg spots). You can check out Figure 2 of the paper to see how this was done.
In a separate experiment, females were able to move freely between 4 chambers, each of which held a different male, with different numbers of egg spots, and lay their eggs. Females of this species scoop up the eggs into their mouths and let the male fertilize them there. Once females were at this “mouth brooding” stage, the researchers removed them and did paternity tests to find out who the daddy was.
The results of these two experiments were a little confusing and the graphs not extremely helpful (at least to me), but the take-away message was this: in this species, females didn’t seem to give a hoot how many egg spots a male had. At least, not in any consistent or predictable way. (Though, keep in mind, this may not be the case for other cichlid species).
So far, the experiments seemed to be a bust. They still had no idea what those egg spots were for. On to the final experiment: let the boys fight!
The set up for experiment 3 was simple enough: put a focal male in the middle of the tank and flank him by two “stimulus” males on either side, in transparent cylinders. The middle male was always territorial (so they knew he’d fight to protect his love-shack). One of the stimulus males had normal, intact egg spots, and the other male sadly had his egg spots removed by “freeze-branding” with liquid nitrogen.
The researchers then presumably grabbed a bowl of popcorn and watched the battle of the fishes (no fish were harmed in the collection of these data). The total number of aggressive behaviors (“bites, butts, or quivers”) of the focal male toward the two stimulus males was recorded for 10 minutes.
In this case, the result was clear: males without egg spots induce rage in their rivals. This is the first time cichlid egg spots have been implicated in male-male competition. Perhaps lack of egg spots indicates that a male is weak and easy to fend off. This could be possible, since those pretty dots are costly to make. The authors point out that this may also explain the apparent lack of female preference for spots in this species. Females are only going to be courted by males with a territory. If all the egg-spotless males have been scared off by the spottier boys, a female doesn’t need to choose. The best males are the only ones left to shake a tail fin her way.
Theis, A., Salzburger, W., & Egger, B. (2012). The Function of Anal Fin Egg-Spots in the Cichlid Fish Astatotilapia burtoni PLoS ONE, 7 (1) DOI: 10.1371/journal.pone.0029878