The truth about flying fox vision

Article and images by Jesse Holt, natural areas field leader, Sunshine Coast Council

Blind as a what? The truth about flying fox vision might surprise you!

You may have heard someone with poor eyesight referred to as being “as blind as a bat”. This saying is based on a common belief that bats can't see very well and instead rely heavily on other senses.

Some bats are able make small noises and carefully listen to the way the sounds echo back at them. They can use these echoes to determine the location of objects and navigate their environment.

This technique is called echolocation and it works in much the way a submarine uses sonar. Whales and dolphins can do this too and it is even possible for humans to learn to use echolocation.

But did you know that some bats can’t do it at all? In fact, some species of bat rely on vision that is in some ways even better than ours!

Bats (which make up the taxonomic order Chiroptera) are traditionally classified into 2 groups. There are the small microbats (Microchiroptera) and the much larger megabats (Megachiroptera). We have both types on the Sunshine Coast, but the familiar grey headed, black and little red flying foxes are all megabats.

Along with their size, there is another big difference between the two. Only the microbats have the ability to use echolocation. Megabats do not. They rely on their keen sense of smell and good eyesight to navigate instead.

All animals have 2 types of cells in their eyes that allow them to see. The first are ‘rods’, which help us see in low light and detect fast moving objects. The second are ‘cones’, which work allow us to tell the difference between colours and see more detail.

There are 3 types of cones, and each allows us to see either red, green, or blue wavelengths of visible light. Humans have all 3 of these and they allow us to see the full spectrum of colours that we do. This is called trichromatic vision.

We are very lucky in this regard as almost all other mammals only have 2 types of cones. They see a much less diverse combination of colours than us (dichromatic vision).

Flying foxes lack the type of cone that allows them to see red light wavelengths. This is the same condition that causes people to become red-green colour-blind (also called protanopia). The result is that everything appears to them in a blue or yellow tone.

To top it off, flying foxes have about 10 times less cones than people. So not only do they see less colours, they see them less intensely.

See image below. If you would like to try seeing for yourself how a flying-fox, (or someone you know who is red-green colour blind!) views the world, you can download the free Chromatic Vision Simulator app for Android or Apple. When you open it, select ‘P’ for Protanopia and you’ll be able to compare what you see side-by-side, just like the image above.

Flying foxes don’t lose out completely on their vision though, there is a trade-off for the lack of colour. Having a smaller percentage of cones means that they have a much higher amount of rods instead. This gives them far better low-light vision than people. It allows them to navigate very well at night by using landmarks like rivers, roads and streetlights.

Good night vision is not only important for flying foxes, but for the broader ecosystem too. This is when flying foxes forage for food and pollinate trees in the process.

There is strong evidence that many trees in the myrtaceae family (which includes the eucalypts and melaleucas) have co-evolved with nocturnal foragers like flying foxes and use them to maintain genetic diversity over wide ranges. These trees tend to flower and produce their nectar at night. They have bright, light coloured blooms on the edge of the canopy that are easy for flying foxes to see and smell.

Some of these species play another important role on the Sunshine Coast: they are key habitat species and provide homes for koalas.

Without flying foxes, it would be harder for these trees to maintain their genetic diversity over long distances. Without a strong population of these trees, koala populations would be even more at risk.

So next time you think someone is “as blind as a bat”, remember that flying foxes can actually see even better than us in the dark and how important this ability is for the health of the bush.

References

• Birt P., Mutualistic interactions between the nectar-feeding little red flying-fox Pteropus scapulatus (Chiroptera: Pteropodidae) and flowering eucalypts (Myrtaceae): habitat utilisation and pollination. 2004, PhD Thesis, University of Queensland
• Blackwood, S et al., Ocular Parameters in a captive colony of fruit bats. Veterinary Opthamology, 2010-09, Vol.13 (1), p.72-79
• Muller, B et al., Cone Photoreceptor Diversity in the Retinas of Fruit Bats (Megachiroptera). Brain, Behavior and Evolution, 2007-08, Vol.70 (2), p.90-104.