Female cuckoo colors provide evolutionary advantages

Sexual dimorphism, which is the observable difference in appearance between male and female members of a species, occurs widely across the animal kingdom, including in humans. An interesting variation of this is sex-limited polymorphism, where one sex exhibits more variation in a certain trait compared to the other. 

An international team of scientists has explored this concept by examining the genetic basis for color polymorphism in adult female cuckoos of the genus Cuculus, which are known for their brood parasitic behavior. The research provides insights into the evolutionary development and practical implications of this trait variation.

Genetic basis of female cuckoo colors

The scientists examined why adult female cuckoos, unlike their uniformly gray male counterparts, may exhibit either gray or rufous (reddish-brown) plumage. This variability is not only evident across different cuckoo species but also within populations of the same species.

Using genome-wide association analysis on nuclear DNA from both Common and Oriental cuckoos, the researchers aimed to pinpoint the genetic factors behind the females’ diverse plumage in an attempt to understand the underlying pigmentation mechanisms and the evolutionary history of these traits.

In birds, unlike in mammals, it is the females that are heterogametic (ZW), differing from the homogametic males (ZZ). This led researchers to speculate that the observed color variation in female cuckoos might be linked to genes on the W chromosome. 

“We found that, instead of a single gene or set of genes, nearly all variations in the gray or rufous coloration were associated with the full length of the female-limited W chromosome,” said co-author Mark Haber, the executive director of the CUNY Advanced Science Research Center (CUNY ASRC) and a psychology professor at the CUNY Graduate Center.

Biochemical processes of cuckoo colors

The scientists also examined the biochemical processes of coloration, revealing that both the Common and Oriental cuckoos use two forms of melanin-based pigmentation – similar to that in human skin – to produce their color variations. 

Further genomic analysis of these species traced the origin of the color polymorphism in female cuckoos to a mutation that occurred over one million years ago, long before these species diverged around 140,000 years ago.

Potential advantages of female cuckoo coloration 

The implications of these color variations are profound. For instance, female cuckoos with the rarer rufous plumage might benefit from mimicking juvenile cuckoos to avoid male harassment during mating seasons. 

Moreover, as brood parasites who lay their eggs in the nests of other bird species, rufous females could potentially escape detection by host species better than their gray counterparts, which might reduce the risk of being attacked by host parents.

“The fascinating idea that many other sex-specific traits, which are harder to study, might also be genetically determined in the matrilinear genome, similar to the color polymorphism in female cuckoos,” concluded senior author Jochen Wolf, an expert in evolutionary biology at LMU Munich.

More about cuckoo birds

Cuckoo birds are known for their distinctive calls and unique breeding behavior, particularly their practice of brood parasitism. This involves laying their eggs in the nests of other bird species, leaving the unsuspecting host birds to raise the cuckoo’s offspring. 

As a result, the host species may inadvertently neglect their own young due to the demanding nature of the cuckoo chick, which often grows larger than its foster siblings.

Cuckoos are found across the world and are highly adaptable to various environments. The family Cuculidae, to which they belong, includes both brood-parasitic cuckoos as well as species that raise their own young. Their diet varies from insects to caterpillars, and some species also consume fruits and seeds.

The cuckoo’s call, which gives them their name, is emblematic in culture and often signals the arrival of spring. The most commonly recognized call is that of the common cuckoo in Europe, famously captured in the phrase “cuck-oo.”

This sound has not only embedded them in folklore but also inspired the creation of the cuckoo clock, where the bird’s call marks the passing hours.

The study is published in the journal Science Advances.

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