A species of Kiwi bird is losing its sight. Madeleine Armstrong takes a look at why this might be.
While Darwinian evolution is a well-established fact, it is not without its oddities. A team of researchers have recently found that large numbers of the common brown kiwi (Apteryx rowi), native to the Okarito forest of New Zealand’s South Island have significant vision problems. Through a process known as regressive evolution, these animals appear to have lost their eyesight adapting to life in the unbroken darkness of caves.
“Through a process known as regressive evolution, these animals appear to have adapted to life in the unbroken darkness of caves.”
One third of the 160 kiwis studied were found to have eye lesions, while three were entirely blind. Despite this, the animals were found to be in perfect health. Kiwis are nocturnal, and their habitat offers plenty of food and few predators. For this reason, the blind kiwis seem able to survive just as well by using other senses such as touch, smell and hearing. For them, maintaining their eyesight is simply a waste of energy. This degradation of sight has been seen before in moles and cave-dwelling fish such as the Mexican tetra. However, the actual mechanisms of regressive evolution are poorly understood.
Researchers have speculated that a gene called sonic hedgehog could be responsible for the loss of vision. This gene is a critical component in nervous system development and has been known to affect sight loss in other animals, such as the aforementioned blind Mexican tetra cavefish. As such, the sonic hedgehog gene could potentially, at the expense of the kiwi’s vision, enhance the functioning of touch and smell sensors in the kiwis’ long beaks. This would enable them to more readily handle their surroundings.
“With about 400 left in the wild, these kiwis are classed as a critically endangered species.”
These blind flightless birds provide scientists an excellent opportunity to study how the visual system evolves and changes. Their survival is central to understanding this process, however. With about 400 left in the wild, these kiwis are classed as a critically endangered species.