Depth perception in small, flying animals – Introduction

Just recently the last article from my PhD thesis was published by Frontiers in Integrative Neurophysiology! I want to explain to you over the next few posts what my thesis was all about – several posts because I don’t like long posts :P. So, today I start with this brief introduction:

My collaborators and I want to know how small animals can see things during fast flight. Well, it is actually not only about just seeing things. Small animals, especially fast moving ones must be able to quickly realize where all the things around them are and which of those they might collide with if they don’t maneuver around them in time. Imagine you are a little zebra finch, just 12 grams body weight, and you are sitting at the water pond home in Australia minding your own business, taking a sip, washing the dust off your feathers or hopping around looking for grains to eat. And then all of the sudden a warning call! One of your peers has seen a bird of prey approaching! What follows is total chaos – well at least from the perspective of an outsider – everybody, maybe hundreds of fellow zebra finches, rush from the water pond into the bushes and tree tops near by. Flying buddies everywhere, leafs, twigs, branches and you have to be quick! And this is not a made up story, just watch this video!

The question is

Why do zebra finches not crash into each other and into branches?

What you might not appreciate at this point is that the zebra finches do this while being virtually incapable of perceiving depth the way we are used to think about it. They have very bad ‘stereopsis’. This is when you look at something with your two eyes and because the two eyes see that thing from different angles your brain can get information about how far away it is. 3D movies work this way – the glasses you wear ensure you are looking at a different image with each eye. People are somewhat good at stereopsis and they can use it to see depth for a few meters or so.

For the tiny zebra finches the situation is different. Their eyes are so close together that they don’t look at things from very different angles. In fact, their heads are so small that the eyeballs of a zebra finch almost touch in the skull. The bone that separates the eyeballs is paper thin (actually thinner)! So, because the eyes of the birds are so close together the range in which zebra finches can estimate depth with stereopsis is probably just a few centimeters. It is so small that at a mediocre flight speed they wouldn’t be able to realize they are about to crash before it is too late.

So… what now? Well, there are other animals that face exactly the same problem: flying insects. The eyes of an insect as small as a blowfly or a bee are virtually only one eye. In terms of stereopsis, of course. They do have two separated compound eyes, but how different can the viewing angles be when the whole head is smaller than one eye of a zebra finch?

We think that these insects solve this problem by flying in ‘bee lines’. Flies and bees are very capable to fly smooth curves but they choose not to do so. This was demonstrated in blowflies. When a male blowfly wants to mate, it pursuits a female until it catches up with it. In this case the blowfly chooses a smooth intersection course that involves very round curves in the flight. But when the fly is just flying around on its own it will go in straight lines as much as possible. When a fly changes flight direction it will do so in a very fast turn instead of a smooth curve.

How does a straight flight path help a small flying animals see depth? There are many examples that show, that small insects make full-body movements in order to see the distance to an object. Mantids for example will stare at the prey and move their upper body left and right before they catch it (want to see?). When you look at bees or wasps visiting you at your breakfast in the garden in summer, you can watch them hover in front of probably interesting things such as your orange juice glass and go left and right, too.

When the insects are moving left and right like this they make use of the so called ‘motion parallax’ which is a great source of information about the position of objects. This is because the images of the objects in your environment that are projected on your retina move across the retina with different speeds. In my PhD we proposed that zebra finches also use motion parallax to see depth cues during fast flight.

To explain the reasoning for the experiments we did, in the next post I want to explain what motion parallax is, what optic flow is and why flying in straight lines helps insects see distances!

Can’t wait to read more about depth perception through self-motion? Try this review by my collaborator!