Until 6 November the Natural History Museum is running an exhibit called Vision and Colour, which explores the presence of colour and the many forms it takes in the animal kingdom.
All of life is actively evolving to be able to better sense and respond to its environment, and for many species vision plays a huge role in this interaction. To celebrate this, here at Sea Watch we wanted to take a closer look at the ways in which dolphins see and interact with their world to try to gain a better understanding of their experience under the sea.
The marine environment presents many challenges to the animals that call it home. It’s relatively constant cold temperature, lack of light and immense pressure all inform the morphology of marine creatures and force them to evolve mechanisms to maximize their chances at survival. Modern mammals evolved from what was once a group of nocturnal rodents, so the ability to operate in low-light situations is already written into our DNA. This focus on black/white contrast (useful in darker environments) over colour determination (only useful in the presence of light) is the reason why our colour vision is primitive compared to other branches of the animal kingdom. Marine mammals have evolved many additional adaptations to be able to sense their unique underwater environment. Below we will explore the adaptations of dolphins that enable them to live and play in their watery home.
If you have ever seen a dolphin looking up at you from just under the surface, you will have noticed that dolphin eyes are located laterally, with one on either side of the head. This placement allows them to see forward, backward and directly to either side of their bodies, helping them to avoid predators and catch prey more effectively. Their eyes are capable of functioning independently, allowing their brains to either process both images together to form a 3D image, or processing them as two separate images at one time. This versatility gives them a leg up in a world where prey and predators can approach or disappear in any direction, above, below or from any side.
Spy hopping is a behaviour exhibited by many dolphin species where they pop their head out of the water vertically, presumably to get a better look at their surroundings. They are able to use this behaviour because dolphin eye lenses are able to change shape depending on whether the dolphin is under or above water, meaning that they can see almost as well above water as below. Their high sensitivity to blue light also means they have great vision at depth and in low-light conditions when they dive to find their prey. Mirror cells behind the retina reflect light back out of the eye, causing it to shine like a cat’s in the dark, and increasing its ability to function in low-light. However, dolphins lack the cones that enable colour vision, so they are likely to be colour blind.
In situations where their vision isn’t accurate enough, like at great depth or distance, dolphins are equipped to use sound to sense their environment. In a process called echolocation dolphins are able to emit high frequency clicks and interpret the echoes as 3D images of their surroundings. Their phonic lips vibrate to form these clicks, which are amplified and directed out of the body and to their target by the melon, a large fatty organ that occupies the bulge on top of their heads. When the clicks reach their target, such as a fish, they will bounce back to the dolphin, where they are received by fatty deposits in the jawbone, or panbone, and directed up to the inner ear and to the auditory centre of the brain. The dolphin is then able to interpret these echoes as a 3D model of their surroundings, enabling them to “see” their prey and any obstacles or threats around them.
Though a dolphin’s eye may look very similar to ours when it gazes at us from beneath the waves, the way they sense their environment is not. The unique challenges presented by a life underwater have caused dolphins and other marine mammals to evolve to use different techniques to survive. Next time you see a dolphin breaking the surface to breathe, don’t forget to think about all the amazing things they do once they disappear back under the sea.
Written by Liz Allyn, Sea Watch Foundation Intern 2016