Habitat and climate profoundly influence woodpecker plumage, but many species show remarkable convergence that cannot be explained by these factors alone.
There are a couple of rules in biology, generalized principles that describe patterns observed in living organisms. Gloger’s Rule, for example, states that “more heavily pigmented forms tend to be found in more humid environments.” A recent review paper in Biological Reviews indicated that there are actually two versions of Gloger’s Rule. A simple version that predicts that animals are darker in warm and humid areas due to more deposition of eumelanin (i.e. the pigment responsible for dark colors). The complex version, however, makes a distinction between temperature and humidity which have different effects on the deposition of pheomelanin, the pigment that leads to red coloration. It is deposited more in warm climates, but less in humid ones. The figure below gives a good overview of the simple and complex versions of Gloger’s Rule.
Both versions of Gloger’s Rule are used in the scientific literature and it is important to know which version is being tested. A recent paper in Nature Communications, for example, tested the simple version in woodpeckers. Eliot Miller and his colleagues used the online database eBird to gather information on 230 woodpecker species.
The analyses revealed a clear correlation between plumage color and humidity. Darker species tend to occur in regions with higher annual precipitation, supporting Gloger’s Rule. The exact mechanism behind this pattern, however, remains unknown. There are several hypotheses, such as improved background matching because of higher predation pressure in humid areas or defense against feather-degrading parasites.
Another interesting pattern emerged from the analyses: woodpeckers from the same area looked more alike than predicted by climate and habitat alone. The authors argue that this plumage convergence is due to another process, namely mimicry. This phenomenon occurs when multiple species evolve similarly in response to a shared signal receiver (e.g., a predator). A classic example concerns butterflies. Some butterfly species don’t taste good and they advertise this with bright warning colors. Predators learn quickly and tend to avoid these species. Other edible butterfly species adopt the same colors and fool predators into thinking that they are distasteful. Over time the edible and distasteful species converge on similar color patterns.
What drives plumage mimicry in woodpeckers remains to be investigated. Perhaps smaller species use particular plumage patterns to fool other species into thinking that they are a bigger, more dominant species. This has been proposed as an explanation for resemblance between the small Downy Woodpecker (Picoides pubescens) and the more dominant Hairy Woodpecker (Leuconotopicus villosus).
A Role for Hybridization?
An intriguing question that arises from these plumage patterns is how convergence is achieved genetically. Are multiple mutations needed? Does selection choose between a subset of genetic modules (similar to head patterns in wagtails)? Are genes related to plumage patterning occasionally exchanged when species hybridize? The latter mechanism seems plausible given the relatively high incidence of hybridization in woodpeckers…
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Miller, E.T., Leighton, G.M., Freeman, B.G., Lees, A.C., & Ligon, R.A. (2019). Ecological and geographical overlap drive plumage evolution and mimicry in woodpeckers. Nature communications, 10(1), 1602.