Ronald Kröger from Lund University, Sweden, has been fascinated by eye optics ever since his PhD, but more recently he has begun investigating eyes that have an unexpected characteristic; their lenses have multiple focal points, each tuned to a different wavelength of light. Kröger explains that this remarkable characteristic compensates for the natural chromatic aberrations found in single focus lenses, producing a sharply focused colour image rather than the fuzzier images from conventional lenses. But on closer inspection, Kröger realised that round pupils could inadvertently block some colours from being sharply focused, while slit shaped pupils would not. Had creatures that evolved multifocal lenses also adopted slit pupils to make the most of their sharp vision? Curious to know how common multifocal lenses are in terrestrial animals and whether they are correlated with slit pupils,Kröger and his student Tim Malmström embarked on an eye testing odyssey (p. 18).
Fortunately, Malmström and Kröger didn't have to travel to the four corners of the earth to find the exotic creatures they wanted to test;most of the species they chose to investigate were available in local zoos,and the less exotic were often on their doorstep. Filming 14 species with slit pupils and 9 with round pupils using an infrared sensitive digital camera, the team was able to determine which animals had multifocal lenses and which did not. Fortunately the filming could be done from a distance of several metres when necessary, keeping the cameraman safe from the more carnivorous subjects. Malmström also took photographs of the animals' eyes during bright light conditions to get a clear view of the pupil's shape. 13 out of the 24 species that Malmström and Kröger investigated had mutlifocal lenses, and almost all of the creatures with multifocal lenses had slit pupils.`Multifocal lenses aren't a freak solution for a few species' says Kröger, and they seem to explain why some creatures have opted for a slit pupil.
But the team are still puzzled by which came first, the slit pupil or the multifocal lens? Kröger explains that they deliberately chose to investigate groups of closely related animals, having slit and round pupils,with the hope of finding intermediate eyes that didn't fall into either category. Fortunately, their strategy worked. Malmström and Kröger found two species with multifocal lenses that still had round pupils, the common house mouse and the Taiwan beauty snake, suggesting that the multifocal lens might have evolved first in some species. But when Malmström investigated cats ranging in size from small domestic cats to the big predators, the team were surprised to see that the mid-sized lynx has an oval pupil and its lens is only slightly multifocal; everything is intermediate. Kröger suspects that there are several distinct evolutionary pathways to solve this problem, and cats and snakes may have taken different ones.