Why do fish look down when they swim? This is not a question linked to a riddle but a research question that has formed the basis of a recent scientific inquiry. The quirk sounds strange at first – fish swim forwards – whilst looking downwards. Yet, this makes sense once all of the data has been considered.
The answer is due to a reliance upon visual information and riverbeds provide more abundant and reliable visual cues. Hence, there is an evolutionary advantage in looking downwards when navigating water. In particular, through this behavior helps fish can estimate swimming direction and speed.
Scientists have recently discovered that fish look down when they swim. As to why this is the case, a new study by Northwestern researchers has explored the mystery behind this behavior. To do so required researchers to combine data from zebrafish’s brains, information from the native environment and fish behavior and out the output into one computational model.
The researchers believe this practice to be an adaptive behavior that has evolved in order to help the fish self-stabilize, especially when they are swimming against a current. Notably. as water moves, fish are constantly trying to self-stabilize in order to stay in place. Otherwise, the fish would be swept away in a moving stream.
If fish were to focus on other fish, plants or debris then this might give the fish a false sensation that it is moving. In contrast, the stable riverbed below provides fish with far more reliable information about their swimming direction and speed.
To show this, the researchers used zebrafish in their native environment in India. A series of experiments showed that fish respond to motion below them more strongly than motion above them. This simple outcome was based on video recordings.
From the video data, the researchers were able to model hypothetical scenarios where a simulated fish moved arbitrarily through a realistic environment.
Back in the laboratory the researchers showed that when patterns appeared on the bottom of the tank, the fish swam along with the moving patterns. The scientists combined these data with data from how motion signals get encoded into the fish’s brain.
The researcher then fed the datasets into two pre-existing algorithms used for studying optic flow (or the movement of the world across our eyes or camera lenses). This confirmed that zebrafish look down when swimming forward.
The research is not purely of academic interest and it may offer hints of the development of artificial vision systems and bio-inspired robots.
The research has been published in the journal Current Biology, titled “Optic flow in the natural habitats of zebrafish supports spatial biases in visual self-motion estimation.”
