A study published in the September issue of Ecology looks at how human activities can diminish the usefulness of an ornamental trait, such as colorful feathers, as a signal of fitness. Cardinals, for example, need carotenoids in their diet to produce their red plumage; brilliant red plumage can signal an individual's health and fitness. Researcher Amanda Rodewald (Ohio State University) and colleagues looked at the socially monogamous Northern Cardinal (Cardinalis cardinalis) in 14 forests in Ohio between 2006-2008, measuring plumage color, reproduction, and quantifying habitat.
They found that the non-native Amur honeysuckle (Lonicera maackii) altered the selective environments for coloration by creating an evolutionary trap for the cardinals in rural landscapes and possibly relaxing selection in cities. Evolutionary traps occur when behavior that was once beneficial is a drawback in an altered environment.
The non-native honeysuckle is appealing to cardinals because it provides dense vegetation for nesting. Honeysuckle fruits are also a source of carotenoid pigments the birds need for their red plumage. Previous studies suggest that plumage brightness or hue signal a bird that is in good condition, has a good territory, and will put energy into raising its offspring. But the non-native honeysuckle's appeal to cardinals comes with a price: a nest in this shrub is more vulnerable to predators. Rodewald and colleagues found that in rural areas the mostly brightly colored male cardinals were in best condition, bred earliest in the season, and secured the more preferred territories that included the non-native shrub. But their annual reproductive success was lower than that of duller males. The authors did not see these results in urban forests, where color was not related to any reproductive indicators, likely because the abundant honeysuckle and birdseed reduce the usefulness of color as a signal of quality. This scenario might lead to relaxed selection for bright color in urban forests and selection against bright color in rural forests.
"Our study provides evidence that human -induced changes to ecosystems can both create evolutionary traps that alter relationships between sexual and natural selection (i.e., via exotic shrubs in rural landscapes) and facilitate escape from evolutionary traps (i.e., via anthropogenic resources in urban landscapes)," write the authors.
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