AsianScientist (Dec. 16, 2015) – Behavioral scientists have observed mutual courtship displays between male and female monogamous blue-capped cordon-bleus (Uraeginthus cyanocephalus), a finding that challenges classical sexual selection theory. In their study published in Scientific Reports, they found that the birds used visual, tactile and acoustic cues in their display, suggesting that such complex behavior may be a form of intersexual communication.
The study of sexual selection has come a long way since Majerus’ 1986 seminal paper first reviewed the increasing corroboration for Darwin’s idea that male ornaments were favoured by female choice. Evidence for sexual selection has since been established in a wide range of taxa, most notably fish and birds.
Nevertheless, the traditional sexual selection theory usually presents the evolutionary explanation of males competing to signal to females that they possess good genes and can subsequently produce healthier offspring than their counterpart.
Thus, female choice is thought to function as the driving factor behind male courtship display and ornamentation, with majority of the studies focusing on polygamous males. However, socially-monogamous birds are known to exhibit directed displays, whether male-to-female or female-to-male, in the form of dance duets with the evolutionary function of these displays thought to increase pair-bonding and mate-guarding.
In a new turn of events, scientists from Hokkaido University and the Max Planck Institute have observed a sexual-display behaviour that is both multi-component—producing vocal and non-vocal sounds—as well as multi-modal—displaying acoustic, visual and tactile cues.
This unprecedented behavior was observed in blue-capped cordon-bleus, a species of finch which inhabits grassland, shrub and desert in subtropical and tropical East Africa. These brightly-colored passerines—so named for their sky-blue heads and caps—are omnivorous, feeding on seeds and small insects.
Professor Masayo Soma—corresponding author and an associate professor with the University of Hokkaido—chanced upon this discovery when they observed that “the bobbing made produced conspicuous sounds” and the birds’ left and right feet showed different movements.
Soma and her team then filmed eight pairs of blue-capped cordon-bleus with high-speed video cameras in two-hour sessions. They recorded 102 sessions in total and found that the behaviors could be categorized into occurrence of dance displays, number of bobs per second and the number of steps in on bobbing action.
The display of the blue-capped cordon-bleu consists of simultaneous up-and-down bobbing, singing as well as rapid-step dancing, the latter invisible to the naked eye and only observable when the high-speed video recording was played back at slower speeds. On average, the birds performed 3.17 steps per bobbing action. These were always done on the perches provided.
Investigating the differences between sexes, males were generally found to dance more, bob at higher speeds and take more steps. However, both males and females had a higher bobbing tempo and took more steps when their partners were present on the same perch. When the birds sang however, individuals had higher bobbing tempo but a decreased number of steps.
So what do these dub-step trends mean for the blue-capped cordon-bleu and classical sexual selection theory? It is a notable step in our understanding of the evolution of courtship displays: females still remained the choosier sex even when in a monogamous context. However, as high-motor performance individuals did not appear to necessarily impress their partners more, it is interesting that perhaps a more exaggerated display is not all there is in courtship moves.
Soma says that her team’s findings are contrasted with the classical view that males are the more ornamented sex and that “songbirds use songs but not non-vocal sounds.” Their findings also suggest “complex sexual signalling can evolve through mutual mate choice,” though the question as to why blue-capped cordon-bleus are special compared to other Estrildid finches remains to be answered.
This study is another step in the long history of songbird research, and the authors hope the findings will eventually contribute to our understanding of human language acquisition as well as how we use gestures for communication.
Copyright: Asian Scientist Magazine; Photo: Ornitologia Lodato/Flickr/CC.
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