Darwin’s Postulates & Firemouth Cichlids

Why cichlids?

Cichlid fish are one of the most biologically diverse groups of vertebrates on the planet. This diversity (as well as population size, reproduction rate, etc) allows scientists to study the cichlid evolution, and the role of natural selection, more closely than other populations. Many aspects of cichlid characteristics have undergone (and are currently undergoing) selective pressures. Cichlid evolution can be found at multiple levels, including behavioral changes, molecular adaptations, size and coloration variation.

Screen Shot 2014-07-24 at 10.42.50 AMThorichthys meeki is a species of cichlid commonly known as the “firemouth”, because of its bright red-orange coloration on the jaw. This specific coloration is unique to the firemouth, and is used in mating, competition and defense, and is therefore strongly affected by selective  pressures. In general, male firemouth cichlids have prominent  jawline coloration (10). The female Firemouth cichlids have specific molecular adaptations that that allow them to see the male coloration very clearly (6).

In the case of the firemouth cichlids, jawline coloration in males (and the subsequent female response) plays an important role in the evolution of the species due to the pressures of sexual selection. Sexual selection has been shown to be a mechanism of significant biological change for firemouth cichlids, even if the environment stays very stable (9), and research confirms that male coloration corresponds to sexual selection by the females (8). The females tend to mate with the more dominant males, so any characteristic that improves one’s chance of winning in a male-male competition scenario will be selected for. In any cichlid population, males who display the same color will compete more intensely. In firemouth cichlid populations, all of the males have a distinguishing red color, so male-male competition is very strong (4).

firemouthIt has been shown that smaller, less dominant males have physiological and behavioral changes during any social interaction with a more dominant cichlid. (3).  Many times, cichlids will engage in “pre-fight behaviors” (7) and  whichever male is smaller and duller will back down (5).    The coloration allows the more dominant males to emerge victorious from a fight with other males, without even having to engage in actual physical contact. The firemouth cichlids open their mouths and expose more of the red jawline. This behavior enlarges the head considerably when seen from the front and the side. In the event that there is no definitive pre-fight winner, they will engage in the more dangerous physical form of fighting. (1) In many cases, coloration is directly correlated to body size, which also plays a role in determining the winner of the conflict. (2) (more…)