Testing different adaptive and non-adaptive explanations for this phenomenon.
In almost one fourth of hummingbird species, some females sport the same colorful feathers as males. This estimate is based on a recent study in the Proceedings of the Royal Society B in which the researchers inspected no less than 16,542 museum specimens. They suggested that this phenomenon – known as female-limited polymorphism – occurs in every major group of hummingbirds, suggesting that it arose independently at least 28 times (but see this paper for another perspective). These patterns raise the question why some females resemble males in so many hummingbird species. We have the reflex to immediately look for adaptive explanations. A trait that is so widespread should be beneficial, right? However, we should not forget about the possibility of non-adaptive origins. As evolutionary biologist Stephen Jay Gould nicely put it: “The primary flexibility of evolution may arise from non-adaptive by-products that occasionally permit organisms to strike out in new and unpredictable directions.” In the present study, Eleanor Diamant and her colleagues followed the advice of Gould and tested two non-adaptive explanations for the evolution of female-limited polymorphism in hummingbirds.
Genetic Conflict
The first non-adaptive explanation entails a shared genetic architecture for certain traits between the sexes. Strong selection on plumage patterns in males (and the underlying genes) might also affect the genetic make-up of females. Because males and females tend to have opposite evolutionary interests – males need to be colorful to attract mates while females need to be inconspicuous to avoid nest predation – a genetic conflict arises. Theoretically, this situation can be resolved if the genetic basis for particular plumage traits becomes sex-linked, resulting in clear sexual dichromatism (i.e. males and females always look different).
This scenario suggests that female-limited polymorphism is an intermediate stage between sexual monochromatism and sexual dichromatism. Following this reasoning, we can expect that the transitions from sexual monochromatism to female-limited polymorphism to sexual dichromatism are more likely than the other way around. When the researchers calculated transition rates between these different stages, they found that they were all similar. Hence, they concluded that “these results do not support the non-adaptive hypothesis that FLP [i.e. female-limited polymorphism] is an intermediate state.”
Bill Sizes
Moving on to the second non-adaptive explanation: perhaps the male-like traits in females are the outcome of selection on other traits with a shared genetic basis (i.e. pleiotropy). The researchers focused on bill length which often differs between the sexes. Possibly, selection for certain bill sizes indirectly causes male-like plumage to develop in females. An intriguing hypothesis, but the analyses did not support it. Overall, there were no clear relationships between relative bill length and the plumage-type of females across the tested species (see figure below). This result led the researchers to dismiss the pleiotropy hypothesis, although they mention that “correlations with other unmeasured morphological traits could exist.” Indeed, we should not throw out the baby with the bathwater and test other traits for pleiotropic effects. Bill length was an obvious first candidate, but more morphological features remain to be tested.
Social Selection
Given the limited support for non-adaptive explanations, the researchers turned to adaptive reasons for the evolution of female-limited polymorphism. Towards the end of the discussion, they noted that:
We found significant associations with migratory status, lower mean precipitation, and marginal association of FLP with social dominance, all of which are linked to interspecific interactions and competition over resources.
Although these patterns are certainly interesting, I am not convinced yet. It is tricky to draw general conclusions from such statistical associations. And indeed, the researchers remain careful and present these results as “preliminary support” for social selection. A promising starting point, but more analyses are warranted. Because female-limited polymorphism has evolved independently so many times, it is possible that different evolutionary mechanisms are operating in different species. For example, the Anna’s hummingbird (Calypte anna) did show a positive correlation between bill length and female plumage, suggesting that a pleiotropic effect might be present in this species.
It would worthwhile to explore each hummingbird species with female-limited polymorphism separately to determine which adaptive or non-adaptive explanation is most likely. A recent study in the journal Current Biology provides a nice example, showing how female White-necked Jacobins (Florisuga mellivora) have evolved male-like plumage to avoid social harassment. Combining such behavioral studies with genomic analyses to pinpoint the genetic basis of female-limited polymorphism across different hummingbird species will provide exciting insights into the evolution of this phenomenon.
References
Diamant, E. S., Falk, J. J., & Rubenstein, D. R. (2021). Male-like female morphs in hummingbirds: the evolution of a widespread sex-limited plumage polymorphism. Proceedings of the Royal Society B, 288(1945), 20203004.
Featured image: White-necked Jacobins (Florisuga mellivora, female, male-like female and male) © Jillian Ditner | Falk et al. (2021) Current Biology
Avian Hybrids 