Blind but perfectly healthy.
Okarito brown kiwis (Apteryx rowi) is the New Zealand's pride. Known to be flightless and nocturnal, these endangered adorable birds also have their surprise: sightlessness.
This was first found during a study of 160 Kiwi birds in the Okarito forest on New Zealand’s South Island. “We found a very high prevalence of birds with eye lesions,” says Alan Tennyson at the Museum of New Zealand Te Papa Tongarewa in Wellington. “A third of them had eye problems.” Three of them were even completely sightless!
“The finding of completely blind birds in good physical condition was absolutely stunning,” says team member Christopher Murphy at the University of California, Davis. “No other birds are known to have a free-living population of blind [individuals],” says Tennyson. “Vision is not essential for survival in all animals.”
Researchers say this could be explained by what's called regressive evolution. Kiwis could've not needed their vision anymore due to where and how they live: they thrive in the dark, and their habitat provides ample food and has no predators.
“Kiwis are flightless and generally nocturnally active, and have very good senses of smell, hearing and touch, so it seems that vision is not essential for their survival, at least for some individuals,” says Tennyson.
See? With all these useful senses, maintaining good eyesight might not be so practical and even be a waste of energy.
Other researchers suggest a gene called Sonic hedgehog could be the culprit. This (potentially) enhances their beaks' touch and smell sensors but at the expense of their vision.
“Eye degeneration can be seen as ‘collateral damage’,” as the birds adapt to their “nocturnal, lightless niche in which normal, functioning eyes are not necessary”, says Stanley Sessions at Hartwick College in Oneonta, New York. “That’s our best guess as to what’s going on in these birds."
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