Mote Marine Laboratory

Eyeing Hugh and Buffett

By: Kasey Williams

How well do manatees see underwater? Studies with Mote's resident manatees show the answer is "not so well"

In “Underwater Visual Acuity of Florida Manatees,” published in the International Journal of Comparative Psychology, the research team of Gordon B. Bauer, Debborah E. Colbert, Joseph C. Gaspard III, Brandie Littlefield, and Wendi Fellner report on how well resident manatees Hugh and Buffett can see detail — their visual resolution.

Human resolution is tested using a Snellen eye chart with an “E” at the top and descending rows of progressively smaller letters. Since animals can’t read, black and white striped targets are used to determine the same thing.

The manatees were shown two targets, one with very narrow black and white stripes that most likely appeared gray to them and one with wider stripes. The manatees were trained to select the wider striped target then, gradually the stripes on the wider target were narrowed until the manatees could no longer tell the difference between it and the gray-appearing target. When this occurred, Hugh and Buffett had reached a level called the “threshold of visual resolution.”

Scientists typically talk about that threshold in terms of cycles per degree or minutes of visual arc. This can be a little confusing, so those numbers can be converted to the familiar Snellen Acuity. For instance, 20/20 is a Snellen measure and in humans, we usually consider 20/20 vision “normal” and 20/200 vision as legally blind.

Buffett’s vision was about 20/420 and Hugh’s was more than 20/1200. This means that Buffett can see at 20 feet what a normal human could see at 420 feet. Clearly Buffett’s vision is not good by human standards, but it approximates the limits of manatee visual resolution imposed by their visual anatomy — in other words, it’s what we might expect from a typical manatee.

Hugh’s vision is dreadful, even by manatee standards. He can see at 20 feet what a human would see from four football fields away.

Allowing the manatees to approach the targets from a closer position did not improve their vision.

So, the question remains, if manatee vision is so poor, how do they find their way around out there? Additional studies with Hugh and Buffett are trying to discover just that.

Learn more about: Mote's Resident Manatees Hugh and Buffett

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