Saturday, July 13, 2013

Snake pattern evolution

Color patterns of snakes have been the subject of many studies and even more speculation. Why is it that some species are uniform in color, while others are striped, banded, or some other combination of patterns. Allen and colleagues (2013) note that an understanding of the diversity of color patterns found in snakes requires detailed pattern measurements. But, that the most common approach to snake pattern quantification is often subjective classifications based on researchers’ observations of whether, for example, a snake is blotched, uniform, longitudinal or transverse striped or an apparent mimic or nonmimic. Categorical classification may be appropriate for answering specific questions, but it masks considerable variation within categories and reduces the ability to detect evolutionary patterns.

Alan et al  built a molecular phylogeny using up to 4 genes for each taxon: 2 mitochondrial (cytochrome b and ND4) and 2 nuclear (c-mos and RAG1). They used 171 taxa from Australia and North America. They then collected colored photographs of each species of snake, excluding captive bread specimens - using 828 photos in all.  The third step was to gather ecological information on each species using the literature. They then used an R-D model, and recruited observers to classify the patterns. They then used a phylogenetic generalized least squares (PGLS) analyses in the caper package  for R (R Development Core Team) to determine how snake patterns are related to  ecology and behavior.

They found a uniform  pattern was associated with an active hunting strategy. Species with longitudinal stripes were generally small, fast, and often exposed to visually hunting predators. Species frequently classified as having regular spotted patterns were more common in North America, found frequently near cover, and predators of birds. Transverse stripes were seldom on species with grasslands or arboreal lifestyle. When they removed coral snake mimics from the sample, transverse stripes are predicted by erratic movement, habitat specialism, and egg consumption. Blotched patterns  associated with an ambush hunting strategy, slow movement, large body size, and pungent cloacal defense. The model of complex patterning showed a positive association toward species where females grow longer than males, those which live in North America and those which are more terrestrial.

High contrast between the colors and tones of pattern elements was observed on small terrestrial snakes and those that can move rapidly away from threats. Habitat generalists generally had patterns with smaller elements in absolute terms, but none of the predictors were associated with the size of pattern elements relative to snout-vent length.

When mimics were included in the analysis, no predictors were related to the cryptic color score. However, when mimics were removed, the minimal adequate color model included main effects of escape speed, mammalian predation, and cloacal defense, with snakes that are slow, predate on mammals, and described as having highly pungent cloacal defenses generally having more cryptic colors.

Transverse striped snakes did not show a clear overall pattern to  eco-behavioral associations; the idea that transverse stripes achieve flicker-fusion during escape is plausible but requires focused empirical testing. Though analysis of snake color was coarse, the expectation that bright colors would be common on well-defended species to function as an aposematic signal was not supported; the factors that lead to the evolution of warning coloration in snakes await further investigation.

Allen WL, Baddeley R, Scott-Samuel NE, Cuthill IC. 2013, The evolution and function of pattern diversity in snakes. Behavioral Ecology 10.1093/beheco/art058

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