Humpback whales have shockingly poor eyesight, which can cause them to bump into objects, claims a new study.
The gentle giants lack the ability to make out fine details until they are right up close, potentially explaining why the animals have a tragic tendency to get caught up in fishing nets and crash into small boats.
In an effort to understand more about how humpback whales see the world, a team of marine biologists dissected the left eye of a whale which had died after becoming stranded.
In the study, the team revealed the back of the whale's eyeball was much thicker than expected, meaning the distance between the retina and lens – the eye's focal length – was much shorter than assumed.
A shorter focal length means the whales' ability to resolve fine details, known as acuity, is much lower than previously thought.
This idea was backed up when the team counted how many retinal ganglion cells – neurons which transfer visual information to the brain – the eye had.
While the typical human eye has around 40,000 retinal ganglion cells per square millimeter, the humpback eye had just 180.
Such poor eyesight isn’t normally a problem for the whales, however, Professor Lori Schweikert from The University of North Carolina Wilmington told BBC Science Focus.
“Besides birds of prey, humans, and a few other groups, most animals have rather low visual acuity,” says Schweikert.
“That visual acuity, however, is often suited to what they need to survive in their natural environment. For humpback whales, they may only need to resolve large silhouettes, such as that of a large school of prey or another whale swimming by.”
The humpbacks' vision would allow them to see larger objects at a distance, but finer details would only come into focus when they were 45 to 60 metres, or three to four whale lengths, away.
This would mean that the whales aren't able to make out the thin ropes of a fishing net until they are right on top of one.
“At close range, they should be able to resolve such structures," says Schweikert, “but that leaves little time for them to manoeuvre away.”
About our expert
Professor Lori Schweikert is an Assistant Professor of Biology and Marine Biology at The University of North Carolina Wilmington. Her focus is on the physiological mechanisms that govern photoreception in the marine environment.
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