Why are some species of kittehs plain, while others have spots, stripes, or more elaborate patterns? A provisional answer comes from a new paper by William Allen et al., “Why the leopard got his spots: relating pattern development to ecology in felids”, in the Proceedings of the Royal Society. The paper’s title, of course, comes from Rudyard Kipling’s Just So Stories. And the short answer is this: the coats of wild cats help camouflage them, and what pattern evolves depends on where the species lives.
The simple answer comes from a rather elaborate analysis. The authors set up the paper with what I think is a good specimen of clear scientific writing. It’s not Joyce, of course, but these guys know how to write. I love the alliteration of “flanks of felids” and the breeziness of “pounce or quick rush.”
The patterns displayed on the flanks of felids are intriguing in their variety. Previous studies of the adaptive function of cat coat patterns have indicated that they are likely to be for camouflage rather than communication or physiological reasons [1,2]. The primary hunting strategy of felids is to stalk prey until they are close enough to capture them with a pounce or quick rush [3,4]. As hunts are more successful when an attack is initiated from shorter distances [5,6], cats benefit from remaining undetected for as long as possible and camouflage helps achieve this. Many smaller cats are also likely to be camouflaged for protection from predation .
The authors first note that others before them have suggested—and supported with some data—the idea that spotted or stripey cats live in forested habitats, and plain cats in open habitats. But they quantify this “complexity” by doing a developmental analysis of coat patterns on pictures taken from the internet. I won’t go into the details, but they match the photographs with patterns generated from a mathematical model in which pattern results from the interaction of two diffusible chemicals along gradients of the body. Given a model that matches an existing pattern (they used 35 species of felids), they could then encompass “pattern” in the mathematical constants involved in generating it. They could then correlate these constants with various aspect of cat ecology: where they live, preferred times of activity, how big they are, what they eat, and how social they are.
Here’s an example of a cat that came out “plain” in their analysis: the caracal (Caracal caracal), from Africa and the Middle East:
Nine of the 35 species were considered “plain.” Here’s a cat considered “patterned and complex”: the gorgeous clouded leopard (Neofelis nebulosa), from southeast Asia:
Sixteen species were considered patterned, with four of these, including the clouded leopard, as “always complex.” The other ten were considered “variable”,” since there was polymorphism: individuals within a species can look quite different.
- Pattern itself, whether complex or not, was significantly associated with habitat, with more patterned cats in more “closed” habitats (forest, jungle, etc.). Plain cats are found in open habitats like grasslands, deserts, and mountains.
- More irregular patterns, like the cloud leopard, are significantly associated with tropical forests and other “closed” environments.
- “The more time cats spent in trees, the more likely they were to be patterned.”
- Pattern polymorphism, as in the melanism of “black panthers,” was significantly associated with living in temperate forests that vary seasonally and also with habitat generalism. This supports the idea that “disruptive selection,” that is, selection for different patterns in different places, maintains the intra-specific variation in coat color.
- There were a few “outliers,” or exceptions—cats that had patterns not fitting into the habitat correlations given above. One is the very rare bay cat (Catopuma badia; I’ve posted on it before), which is plain though it lives in tropical rainforest:
And another outlier is the black footed cat of Africa (Felis nigripes), which is patterned though it lives in open habitat (savannah, grassland, and semi-desert):
The authors note that the tiger is the only wild cat with vertical stripes, and the common notion that this camouflages them in grassland is unfounded: tigers don’t live in grasslands.
The conclusion, then, is that the patterns of cat coats reflect, in large degree, selection for camouflage in their natural habitats. This camouflage almost certainly evolved to hide them from prey, and, in smaller cats, predators as well.
I love the inclusion of a Kipling quote in their conclusion (reference “45″ is to the Just So Stories):
These findings support the hypothesis that felid flank patterns function as background matching camouflage. Evolution has generally paired plain cats with relatively uniformly coloured, textured and illuminated environments, and patterned cats with environments ‘full of trees and bushes and stripy, speckly, patchy-blatchy shadows’ .
Now the sample size—35 species—is not large, and some of the associations were barely significant from a statistical standpoint. This could reflect the low power of tests in small samples. Nevertheless, the study offers a good working hypothesis for the evolution of pattern not just in cats, but other species that “need” to be cryptic. What remains to understand are those outliers like the bay cat, and also the existence of developmental change of pattern, in which some species are patterned when young and lose the patterns when they get older. Lions, which are spotted as cubs, are a good example of this:
This change might not be adaptive per se, but simply be an atavism: a holdover from an ancestral spotted pattern that still persists in the young.
Allen, W. L., I. C. Cuthill, N. E. Scott-Samuel and R. Baddeley. 2010. Why the leopard got his spots: relating pattern development to ecology in felids. Proc. Roy. Soc. B online: doi: 10.1098/rspb.2010.1734