Lopsided jaw helps fish to eat.

Thomas Stewart and Craig Albertson from Syracuse University in New York have been looking at lopsided fish from Lake Tanganyika in Africa and trying to understand how this asymmetry comes about.

These cichlid fish have a very specific diet. They eat scales from other fish. To facilitate this, they have asymmetrical jaw and head shapes, as this allows them to graze scales off one side of theirprey more easily.

The principle of natural selection dictates that the fish cannot all pick the same side, as their prey would learn to avoid swimming on the side that makes them vulnerable to grazing. Thus, the population of adult scale-eating cichlids can be divided generally into 'righty' and 'lefty' individuals. The
researchers modelled the mechanics of the jaws and predicted that the side that curves closer to the prey is faster and possibly more efficient at snipping scales off.

The researchers wanted to find out how genetic and development factors contribute to jaw asymmetry - is it nature or nurture? To achieve this, they collected over a hundred larval fish whose head and facial
skeleton is still in development. They found that instead of having only distinct leftys and rightys, larval fish with symmetric or nearly symmetric jaws were also found. However, as these larval fish are good at grazing from neither left or right, they are more likely to starve; natural selection acts against them.

Even though previous work have indicated that leftys are genetically dominant, the combined processes of development and selection means that most of the fish that make it to adulthood are rather lopsided, half on the right and half on the left.