Leopards have but tigers have stripes.  Why the difference?   British Nobel laureate Rudyard Kipling, author of "The Jungle Book" and other stories, suggested the difference was because the leopard moved to an environment "full of trees and bushes and stripy, speckly, patchy-blatchy shadows".   Was he right or was that a just-so story?

Experimental psychologists (nee behavioral ecologists(!)) from the University of Bristol wanted to know and they investigated the flank markings of 35 species of wild cats to understand what drives the evolution of such variation. They captured detailed differences in the visual appearance of the cats by linking them to a mathematical model of pattern development.

They found that cats living in dense habitats, in the trees, and that were active at low light levels, were the most likely to be patterned, especially with particularly irregular or complex patterns, which suggests to them that detailed aspects of patterning evolved for camouflage. Their numerical model of the evolutionary history of the patterns says they can evolve and disappear relatively quickly.

They say their hypothesis would also explains why black leopards are common but black cheetahs are unknown. Unlike cheetahs, leopards live in a wide range of habitats and have varied behavioral patterns. Having several environmental niches that different individuals of the species can exploit allows atypical colors and patterns to become stable within a population.

They say a clear link between environment and patterning was established but the study also highlighted some anomalies - cheetahs have evolved or retained spotted patterns despite a strong preference for open habitats, while a number of cats, such as the bay cat and the flat-headed cat, have plain coats despite a preference for closed environments.   

So dense habitat may not be the whole story.

The study also highlighted how few species of cats have vertical stripes - of the 35 species examined, only tigers always had vertically elongated patterns and these patterns were not associated with a grassland habitat as might be expected under the density hypothesis.

But tigers seem to be very well camouflaged so why aren't vertical stripes more common in cats and other mammals?

Experimental psychology still has some work to do if they are going to solve this mystery of biology.