How do you balance the evolutionary demands of being extremely colorful and attractive to potential mates while being camouflaged from predators?
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Lories and lorikeets are a group of small to medium-sized nectar parrots comprising over 100 species. They are native to New Guinea, eastern Indonesia, Australia and many archipelagos in Oceania. These parrots are coated in a range of truly dazzling colors: from bright lime green and lemon yellow to sunset orange and eye-watering scarlet to bright blue and iridescent black – and even colors that humans cannot see. Which begs the question: how do these parrots manage to be so amazingly colorful without becoming the meal of a hungry predator?
Previous research has shown that feather colors evolve to serve a wide variety of functions, including thermoregulation and climatic adaptation, but understanding that the subtle balance between sexual signaling and camouflage is what has most interested a team of evolutionary biologists from the American Museum of Natural History. They studied loris and loriinae (loriinae) museum specimens because many of these species are adept at both staying hidden from hungry lizards and hawks while being colorful enough to attract mates.
Unlike many birds, where males are colorful and have garish feather ornamentation while females are cryptic, lorises are monomorphic: males and females are brightly colored and can rarely be visually distinguished by color patterns alone . But interestingly, many of the color patterns of these parrots follow the same basic pattern: colorful faces and heads, breasts and bellies with green backs and upper wing surfaces. Could these patches of plumage colors be the result of mosaic evolution?
The term “mosaic evolution” was coined following the discovery of fossilized body parts of Archeopteryx, the iconic early bird, which has fascinated researchers with its combination of ancestral “reptilian” and avian-derived characteristics. Mosaic evolution has since become a central motif for understanding avian origins and diversification.
Basically, mosaic evolution describes the process by which traits evolve at different rates or in different ways. Traits that are strongly correlated with each other should show a coordinated response to selection, while dissociated traits evolve independently and at different rates. These trait relationships are governed by genetic, developmental, and functional associations, and these can give rise to distinct groups of traits that evolve semi-independently.
Knowing the strong relationships between color and mate selection in lorises, the researchers predicted that color patches involved in sexual signaling would be less constrained evolutionarily than plumage patches on the back and wings, because camouflage would be compromise if a new color suddenly appeared in these areas. Additionally, since environmental adaptation can drive evolution to or from particular color states, scientists have also proposed that the colors of specific plumage regions in lorises vary with climate.
The researchers tested these hypotheses by photographing 98 historic museum specimens of lorises and Australasian lories under visible light – and because lorises can see colors in the UV range, they also photographed them under UV light.
“The range of colors exhibited by lorikeets is one-third of the colors birds can theoretically observe,” study lead author Jon Merwin said in a statement. “We were able to capture variations in this study that are not even visible to the human eye.”
The researchers collected data on 35 plumage patches on each specimen’s face, head, back, wings, chest, and lower abdomen and fed it to a special computer program that translates the color data into “bird’s eye view”. This color data was modeled on the tree of life for lorises to test whether different color patches on birds are more likely to evolve in certain scenarios.
The study found that, as expected, plumage colors do not evolve as a single trait in lorises: modeling plumage regions either independently, in functional groups, or all together showed that evolution unfolded more quickly for the color spots grouped into distinct regions (head, belly) than in others (wings, back). The researchers also found that wing spots shifted more strongly with climate than plumage regions used in signaling (e.g., the head), which branched out into a rapid burst.
These results support the hypothesis that the extraordinary color diversity of lorises was generated by a mosaic of evolutionary processes acting differently on distinct subsets of plumage regions. Mosaic evolution is a scheme by which regions of plumage in different parts of the body are partitioned, thus providing an evolutionary mechanism to evolve bright colors for signaling while preserving the ability to remain camouflaged or adapt to local climatic conditions.
Jon T. Merwin, Glenn F. Seeholzer and Brian Tilston Smith (2020). Macroevolutionary bursts and stresses generate a rainbow in a clade of tropical birds, BMC Evolutionary Biology 20:32 | doi:10.1186/s12862-020-1577-y
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