Short-sighted Cetaceans 

Humpback whales have big, grapefruit-sized eyes. It would make sense to think that they have excellent eyesight, except that these smart animals keep getting entangled in fishing gear.  

In a new study, scientists dissected the left eye of a juvenile of this species (Megaptera novaeangliae) and found that humpback whales’ vision is much weaker than previously thought. 

“This work helps fill a major gap in our understanding of the sensory ecology of large whales, how humpbacks experience their world,” study author Lorian Schweikert said in a statement. 

When the researchers cut into a stranded whale’s eyeball, they noticed the white (sclera) of the whale’s eye to be particularly thick at the back, explained Smithsonian Magazine. 

This means that the whale’s focal length – meaning the distance between the eye’s lens, near the center of the eye, and the retina, near the back – was shorter than expected. 

Large eyes have longer focal lengths, which usually correspond to better vision. The white of the whale’s eye being so thick near the back of the eye shortened the focal length, providing backing for the idea that humpbacks might not see very well. 

Another clue for the scientists was the count of retinal ganglion cells. These neurons are found in the back of the eye, and their role is to transmit information to the brain. They function like pixels on a camera: The more retinal ganglion cells, the higher the resolution. 

In the eyeball analyzed, researchers found a low density of these neurons, a maximum of 180 cells per square millimeter. In comparison, humans have between 12,000 and 38,000. 

Humpback whales’ vision was measured at just 3.95 cycles per degree (CPD), meaning that they can detect large, simple shapes from a distance but need to be within three or four body lengths to see details. 

With anatomical and perceptual models, the study simulated how the animals perceive their surroundings.  

“(A low CPD) is bad for a human, but not bad for a whale at all,” Thomas Cronin, a visual ecologist not involved in the study, told bioGraphic, adding that whales don’t really need sharp vision to hunt, so if it weren’t for humans’ boats and nets, they wouldn’t have any problems getting by.