Rare morph Lake Malawi mbuna cichlids benefit from reduced aggression from con‐ and hetero‐specifics. (25th September 2021) - Tyers, Alexandra M.
Abstract: Balancing selection is important for the maintenance of polymorphism as it can prevent either fixation of one morph through directional selection or genetic drift, or speciation by disruptive selection. Polychromatism, the presence of multiple genetically determined colour phenotypes, can be maintained if the fitness of alternative morphs depends on the relative frequency in a population. In aggressive species, negative frequency‐dependent antagonism can prevent an increase in the frequency of rare morphs as they would only benefit from increased fitness while they are rare. Heterospecific aggression is common in nature and has the potential to contribute to rare morph advantage. Here we carry out field observations and laboratory aggression experiments with mbuna cichlids from Lake Malawi, to investigate the role of con‐ and heterospecific aggression in the maintenance of polychromatism and identify benefits to rare morphs which are likely to result from reduced aggression. We hypothesize that rare morph individuals receive less aggression than common morph individuals and therefore have an ecological advantage. Within species we found that males and females bias aggression towards their own morph, adding to the evidence that inherent own‐morph aggression biases can contribute to balancing selection. Over‐representation of rare morph territory owners may be influenced by two factors; higher tolerance of different morph individuals as neighbours, and the ability of rare morphs to spend more time feeding. Reduced aggression to rare morph individuals by heterospecifics may also contribute to rare morph advantage. Abstract : The rocky‐shore habitat of Lake Malawi (top) is home to densely‐packed multi‐species communities of highly‐aggressive "mbuna" cichlids (middle). Maylandia callainos is a colour polymorphic species comprised of common blue and rare white morph individuals. Using field observations we identified advantages to being rare: white morph individuals are over‐representation among territory owners (bottom‐left), able to hold territories closer to blue morph males (bottom‐middle), and spend more time feeding (bottom‐right). Laboratory aggression experiments showed an own‐morph aggression bias, which indicates that negative frequency‐dependent antagonism is likely contributing to the balancing selection necessary for the maintenance of polychromatism.