ected, all megabats had high densities of rod photoreceptors, the prerequisite for nocturnal visual orientation. In addition, all species could be shown to possess cone photoreceptors, comprising about 0.5 percent of the photoreceptors.
"This share of cones appears small, but from studies of other night-active mammals we know that it allows daylight vision", says lead author Brigitte Mller. 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", says Muller.
The studied flying fox species (genus Pteropus) were shown to have two spectral cone types, the so-called blue cones that detect short-wave light, and the so-called green cones that detect middle-to-long-wave light. With these two cone types, flying foxes have the prerequisite for dichromatic colour vision, the common mammalian condition.
Curiously, the retinas of the three other studied genera Rousettus (rousette fruit bat), Eidolon (straw coloured fruit bat), and Epomophorus (epauleted fruit bat) completely lack blue cones, they possess only green cones. "With just one cone type, spectral discriminations are not possible, so these species must be colour blind", says Leo Peichl. "A loss of blue cones is a rare event in evolution, it has been found in only a few mammals." The scientists conclude that for the three affected fruit bat genera colour vision is less crucial than for the flying foxes.
Flying foxes (Pteropus) have their daytime roosts in large open treetops, where they are exposed to birds of prey (Fig. 2). Here, a visual early warning helps survival. "Furthermore, flying foxes dont sleep all day; they often change their positions in the tree and interact with their neighbours. Young flying foxes also make training flights durPage: 1 2 3 Related medicine news :1
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