Large eyes come at a high cost

Researchers have long associated the presence of a well-developed brain with major energy consumption. This means that animals that develop advanced nervous systems require environments where this is possible. There has to be good access to nutrients, and every investment in an organ comes at a cost to some other organ system that is less essential in that particular environment. Up to now, there have been few concrete measurements of how high the cost of a nervous system actually is.

A research team at Lund University has conducted measurements on a vertebrate and for the first time calculated the actual cost of well-developed vision in these animals. The researchers studied the Mexican cavefish, a fish that lost its visual system through regression. This fish is clearly different to the surface-dwelling variant, known as a morph, of the same species. The surface-dwelling morph has large eyes, but also far greater access to food, which the cave-dwelling morph lacks. The cavefish lives in a very dark, nutrient-poor environment and has no use for eyes.

"Our measurements in the Mexican cavefish show that the visual system requires between 5% and 15% of the animal's total energy budget, depending on the age of the fish. This is a tremendously high cost! Over evolution, this morph lost both eyes and visual cortex, without a doubt because of the unsustainable energy cost of maintaining a sensory system that no longer had any significance," says Damian Moran, one of the researchers behind the study.

"Animals with large and well-developed eyes, necessary for their survival, pay a high price for them. As all animals have a strictly limited energy budget, a major investment in the visual system only occurs at a cost to other organ systems," says Eric Warrant, researcher in Functional Zoology at Lund University.

The researchers were surprised that the visual system of Mexican cavefish required such a large proportion of the fishes' total energy budget; the cost was much higher than expected.

The new findings also lead to a better understanding of selective pressure in evolution, i.e. what causes the same species to develop in different ways depending on their environment.