On the Spot

Have you ever looked at a leopard and wondered why it has spots and not stripes or why it is patterned and not plain? A new study in the Proceedings of the Royal Society B looks at just that question.

The patterns themselves come in a large variety even within the wild cats, and previous studies have suggested that they are for camouflage in these predominantly ambush predators. It is known that hunts are more successful when an attack is initiated from shorter distances. Makes sense. And smaller cats are probably camouflaged for both a hunting advantage and protection from predators. Other studies have also found spots to be significantly associated with arboreality (the presence of trees or a forest or movement within the trees) with dark spots, in particular, associated with closed habitats and predators that prey on ungulates (hooved animals). The conclusions about stripes have been a little less than clear.

The researchers collected images of 35 species of Felidae from the Internet from various wildlife photography resources. They then picked 6 of the best images from each species, images where the animal was shown in profile, full view, free of distortions and occlusions, in focus, in dry weather, and in natural lighting. Then they took rectangular crops of the images using the base of the neck and tail. They then had to classify the images according to the type of pattern they were seeing.

So how do you go about measuring the influence of spots? I mean, consider the jaguar vs. the clouded leopard vs. the serval. All of them have spots, but none of them have the same kinds of spots. So in order to study the evolution of pattern you need to come up with a better way than just calling something spotted. The method used in this study was inspired by reaction–diffusion theories of biological pattern formation. I'm not so much up on those kinds of theories, but here is how the paper describes it:

"Human observers classified standard examples of felid flank patterns to the closest matching comparison pattern in this multidimensional space. The values of variables in the underlying equation that generated the chosen pattern and distribution of observers' classification decisions parameterize the important properties of each standard image. The five dimensions can be conceptualized as: (i) patterned versus plain, (ii) pattern irregularity, (iii) pattern complexity, (iv) pattern element size and (v) the anisotropy (directionality) of pattern elements."

One the measures were characterized they were tested against ecological variables that have been proposed to drive the evolution of pattern phenotypes. These variables include habitat, locomotion, activity time, social systems, prey size, body size, and weight.

After lots of images and lots and lots of stats what did they find? Basically that flank patterns function as camouflage. Okay, not really a new finding, just the same finding with a different method (but the more support for a hypothesis the better right?). They also found that evolution has generally paired plain (unpatterned) cats with relatively uniformly colored, textured, and illuminated environments. Patterned cats were paired with environments that are "full of trees and bushes and stripy, speckly, patchy-blatchy shadows." It is likely that the pattern on the cat resembles the background pattern of the habitat in which it lives or hunts. The cat species living in closed environments and who move around in the trees are more likely to have complex patterns than those who live in open environments and move around on the ground. Felids that have especially irregular patterns live in tropical areas and tend to be nocturnal hunters, preferring to hunt in the trees.

On the topic of stripes, as with previous studies, these authors found no evidence to support the proposition that vertical stripes are associated with grasslands. Considering that the tiger was the only one to be classified as having vertical stripes and its favored habitat is not a grassland, that seems pretty plausible to me.

It wouldn't be a scientific study if you didn't have some outliers to speculate upon. For example, the cheetah (Acinonyx jubatus). This cat has a pattern and yet lives and hunts in a grassland/savanna habitat. Similar outliers include servals (Caracal serval) and black-footed cats (Felis nigripes). On the other side of the coin there are those species who have plain coats but live in closed environments, such as the bay cat (Pardofelis badia) and flat-headed cat (Prionailurus planiceps). Its possible that these outliers could be utilizing different microhabitats within the categories used by this study. Perhaps plain and patterned cats have instances in which the (non-)pattern works and some where it doesn't. Or maybe they are constrained genetically or developmentally. Its difficult to say.

Here's the paper:
Allen, William L., Innes C. Cuthill, Nicholas E. Scott-Samuel and Roland Baddeley (2010) Why the leopard got its spots: relating pattern development to ecology in felids. Proceedings of the Royal Society B: published online. ( DOI: 10.1098/rspb.2010.1734)

If you want to take a look at some of these cats and here patterns then look here:
http://www.zooinstitutes.com/Zoology/family.asp?name=Felidae

And here are some story links:
http://www.bris.ac.uk/news/2010/7264.html
http://www.sciencedaily.com/releases/2010/10/101019212914.htm

(image from webshots.com)