Fruit bats are not ‘blind as bats’ in daylight

June 16 : Fruit bats have cells in their retinas that enable them to see in the daytime and that too in colour, a joint research by American and German scientists has revealed.

The retinas of most mammals contain two types of photoreceptor cells, the cones for daylight vision and colour vision, and the more sensitive rods for night vision.

Previously nocturnal fruit bats (flying foxes) were believed to possess only rods.

Now scientists at the Max Planck Institute for Brain Research in Frankfurt and The Field Museum for Natural History in Chicago have discovered that these bats possess cones in addition to rods.

The scientists believe the cone photoreceptors might be useful for spotting predators and for social interactions at periods of roosting during the day. Flying foxes often use exposed treetops as daytime roosts, where they assemble in large colonies.

Fruit bats do not echolocate like microbats. They rely solely on vision and their sense of smell when foraging at night for fruit and nectar.

During the flight to the foraging grounds at dusk and the return to the daytime roost at dawn, the animals navigate solely by vision. On moonless nights, fruit bats cannot fly and stay hungry.

According to Brigitte Müller and Leo Peichl of the Max Planck Institute for Brain Research in Frankfurt/Main and Steven Goodman from The Field Museum for Natural History in Chicago, visual navigation at twilight and sometimes also during the daytime did not fit the older view that fruit bats only possessed rods, the photoreceptors for night vision.

This prompted the team to study the photoreceptors of fruit bats with modern histological methods.

To identify the different photoreceptor types, the researchers stained the retinas of various fruit bat species with visual pigment-specific antibodies.

Findings revealed while the fruit bats had high densities of rod photoreceptors, the prerequisite for nocturnal visual orientation, they also possessed cone photoreceptors, comprising about 0.5 percent of the photoreceptors.

While this share of the cones appeared very small, it did enable the animals to see in the daytime as well.

“For example, cats and dogs only have two to four percent cones, and even the diurnal human retina contains an average of only five percent cones. The retina of flying foxes is no ‘evolutionary quirk’, but conforms to the general mammalian blueprint that comprises rods and cones”, said lead author of the study Brigitte Müller.

Findings further revealed that the fruit bats had two spectral cone types, the so-called blue cones for detecting short-wave light, and the so-called green cones for detect middle-to-long-wave light.

With these two cone types, flying foxes have the prerequisite for dichromatic colour vision, the common mammalian condition, said Müller.

The study, ‘Cone photoreceptor diversity in the retinas of fruit bats (Megachiroptera), appears in the journal ‘Brain, Behavior and Evolution’. (ANI)