Run-ins with humans are the leading cause of death for humpback whales — and new research may explain why these gentle giants are so vulnerable to collisions with boats and entanglement in fishing nets.
The softball-size eyes of humpbacks offer shockingly poor vision, according to a study published Wednesday in the journal Proceedings of the Royal Society B. Unfortunately, this isn’t the kind of vision issue that can be corrected with a prescription (as much as we’d all like to see a whale in glasses).
Simulations indicate whales see most of their surroundings in shadowy silhouettes and struggle to resolve fine-scale details until they’re extremely close, said Jacob Bolin, who conducted the study while completing his marine biology degree at the University of North Carolina Wilmington. That means the spidery lines of fishing nets may be virtually invisible to humpbacks until it’s too late to avoid them.
Better understanding how humpbacks and other whales see could inspire “strategies for making fishing gear more visible” and help the whales avoid deadly encounters with humans, said Lorian Schweikert, a sensory biologist also at the University of North Carolina-Wilmington and an author of the paper.
To understand humpback vision, Bolin and his team dissected the eye of a juvenile humpback whale that died on a North Carolina beach in 2011. While most whales that wash ashore have been dead for weeks and are badly decomposed, this individual was euthanized on land. That afforded scientists an opportunity to study humpback anatomy.
Having such large eyes should, based on optical principles, be an asset: the longer the focal distance, or the length between the lens and the retina, the sharper the image, Bolin explained. But the researchers found that more than a third of the eye’s depth was taken up by meaty sclera, the white of the eye that’s not involved in vision.
But the greatest limiting factor for the whale’s vision is the low density of retinal ganglion cells, which Bolin compared to “the pixels of the eye.” The cells capture the image reflected on the retina and convert it into electrical signals the brain can understand. Denser retinal ganglia mean a higher-resolution picture arrives to the brain.
The highest density Bolin tallied in the whale’s eye was about 180 retinal ganglia per square millimeter. Humans can have about 35,000 to 40,000 per square millimeter, while keen-eyed birds of prey can have up to 70,000 in the same area.
To see how these anatomical features work together, the researchers used software to simulate how underwater scenes might look to humpbacks, which scientists already knew see only in black and white. They modeled two images that are important to whales navigating the modern seas — a school of small fish, and a commercial fishing net — at various distances.
The results suggest a significant drop-off in perception of objects at a distance of around 150 feet to 200 feet — not so far considering adult humpbacks are about 45 feet long. While the school of fish was reduced to a hazy blob, the gill net became almost invisible.
It’s not just that humpbacks will struggle to identify a more distant object — they might not even realize it’s there, Schweikert said.
These findings crystallize just how poor the humpback’s vision is, said Dr. Elena Vecino Cordero, an ophthalmologist at the University of the Basque Country in Spain who has studied whale eyes and wasn’t involved in the study.
In fact, Bolin’s estimates of whale vision might even be too rosy, Vecino said — her analysis of three other baleen whale species found even lower densities of retinal ganglia. Vecino and Bolin agree that the North Carolina whale’s eyeball might have shrunk slightly in the 12 years it spent preserved in a jar before Bolin dissected it, which would have led him to overestimate their sight.
But how do humpbacks get along with such poor vision? Frankly, before the seas were filled with human infrastructure such as boats and nets, they never needed it, Schweikert said. Adult humpback whales don’t really have predators to look out for, and researchers believe they rely on other senses, including smell, to detect their prey. Unlike their cousins the toothed whales, humpbacks can’t echolocate.
“Our justifiable fascination for whale song and sonar has left us with few clear answers on what those huge eyes are for,” said Sönke Johnsen, a biology professor at Duke University and an author of the study.
