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A team of marine biologists at the University of North Carolina and Duke University has found humpback whales have poorer eyesight than previously assumed. In their study, in the journal Proceedings of the Royal Society B, the group dissected and tested the left eye of a humpback whale.

After cutting open the eye of the juvenile whale, the research team found that the whites of its eyes were thicker than expected in the back. That meant that its focal length (the distance between the retina and lens) was shorter than expected. Prior research has shown that the longer the focal length, the sharper the vision. Such a short focal distance suggests poor acuity.

Further investigation showed that the number of retinal ganglion cells in the back of the eye was much lower than expected. The researchers compared them to pixels on a computer screen—the more of them, the sharper the picture. As a comparison, they note that humans have up to 40,000 of them per square millimeter—for the whale, it was just 180.

Additional testing suggested the whale was capable of seeing at 3.95 cycles per degree, which is a standard way of measuring vision as determined by the number of pairs of black and white lines that can be made out in one degree of space. Humans, in sharp contrast, vary between 60 and 100 CPD.

To better understand the visual capabilities of the whale, the researchers created a simulation using the data they had collected. It showed that the whale was likely capable of making out large objects in the distance, such as a school of fish, but details would only come into focus when the whale was within three to four body lengths of a given object, which they note would typically translate to 45 to 60 meters.

The research team suggests that the poor eyesight of the humpback whale helps explain why they so often become entangled in fishing nets. They cannot see them, despite their large eyes, until they are too close to alter their course.

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