Biomechanics of the Peafowl's Crest Reveals Frequencies Tuned to Social Displays

Haverford College Haverford Scholarship Faculty Publications Physics 2018 Biomechanics of the peafowl’s crest reveals frequencies tuned to social displays Suzanne Amador Kane Haverford College, [email protected] Daniel Van Beveren '20 Class of 2020, Haverford College Roslyn Dakin Follow this and additional works at: https://scholarship.haverford.edu/physics_facpubs Repository Citation Kane, S.A.; Beveren, V.D.; Dakin, R. (2018). "Biomechanics of the peafowl’s crest reveals frequencies tuned to social displays," PLoS ONE, 13(11). This Journal Article is brought to you for free and open access by the Physics at Haverford Scholarship. It has been accepted for inclusion in Faculty Publications by an authorized administrator of Haverford Scholarship. For more information, please contact [email protected]. RESEARCH ARTICLE Biomechanics of the peafowl's crest reveals frequencies tuned to social displays 1 1 2,3 Suzanne Amador KaneID *, Daniel Van Beveren , Roslyn Dakin 1 Physics Department, Haverford College, Haverford, PA United States of America, 2 Department of Zoology, University of British Columbia, Vancouver, BC, Canada, 3 Migratory Bird Center, Smithsonian Conservation Biology Institute, Washington, DC, United States of America * [email protected] a1111111111 a1111111111 Abstract a1111111111 a1111111111 Feathers act as vibrotactile sensors that can detect mechanical stimuli during avian flight a1111111111 and tactile navigation, suggesting that they may also detect stimuli during social displays. In this study, we present the first measurements of the biomechanical properties of the feather crests found on the heads of birds, with an emphasis on those from the Indian peafowl (Pavo cristatus). We show that in peafowl these crest feathers are coupled to filoplumes, OPEN ACCESS small feathers known to function as mechanosensors. We also determined that airborne sti- Citation: Kane SA, Van Beveren D, Dakin R (2018) muli with the frequencies used during peafowl courtship and social displays couple effi- Biomechanics of the peafowl's crest reveals ciently via resonance to the vibrational response of their feather crests. Specifically, frequencies tuned to social displays. PLoS ONE 13 vibrational measurements showed that although different types of feathers have a wide (11): e0207247. https://doi.org/10.1371/journal. pone.0207247 range of fundamental resonant frequencies, peafowl crests are driven near-optimally by the shaking frequencies used by peacocks performing train-rattling displays. Peafowl crests Editor: Matthew Shawkey, University of Akron, UNITED STATES were also driven to vibrate near resonance in a playback experiment that mimicked the effect of these mechanical sounds in the acoustic very near-field, reproducing the way pea- Received: June 8, 2018 fowl displays are experienced at distances � 1.5m in vivo. When peacock wing-shaking Accepted: October 26, 2018 courtship behaviour was simulated in the laboratory, the resulting airflow excited measur- Published: November 28, 2018 able vibrations of crest feathers. These results demonstrate that peafowl crests have Copyright: © 2018 Kane et al. This is an open mechanical properties that allow them to respond to airborne stimuli at the frequencies typi- access article distributed under the terms of the cal of this species' social displays. This suggests a new hypothesis that mechanosensory Creative Commons Attribution License, which stimuli could complement acoustic and visual perception and/or proprioception of social dis- permits unrestricted use, distribution, and reproduction in any medium, provided the original plays in peafowl and other bird species. We suggest behavioral studies to explore these author and source are credited. ideas and their functional implications. Data Availability Statement: All data and code necessary to reproduce the results of this study are available at: https://doi.org/10.6084/m9.figshare. 5451379.v5. Funding: Funded by Haverford College. www. Introduction haverford.edu. National Sciences and Engineering Research Council of Canada (NSERC) Postdoctoral Bird feathers are known to act as mechanosensors that allow birds to detect and respond to a Fellowship to Roslyn Dakin. www.nserc-crsng.gc. variety of mechanical stimuli [1±3]. For example, flight, contour, and facial bristle feathers can ca. act as sensors that provide important information during flight and prey capture [3±7]. Competing interests: The authors have declared Indeed, feathers have been suggested to have evolved originally to serve sensory functions, that no competing interests exist. because even isolated protofeathers could have played a sensory role before the evolution of PLOS ONE | https://doi.org/10.1371/journal.pone.0207247 November 28, 2018 1 / 27 Biomechanics of peafowl feather crests specialized arrays of feathers that enabled flight or thermoregulation [8]. Feather head crests have been found in fossils of some dinosaurs and early birds as well as a wide variety of living species of birds [9,10]. While feather crests have usually been studied for their roles as possible visual signals [11±14], recent behavioral studies of two auklet species have shown that their erect head crest feathers can play a mechanosensory role during tactile navigation similar to that of mammalian whiskers and arthropod antennae [14,15]. These findings suggest that feather crests in other birds also might play a previously unrecognized mechanosensory functional role. Crest feathers and other types of feathers found on the heads of birds have received little attention in the literature, especially in comparison to the significant body of literature on the morphology and mechanical properties of wing, tail and train covert feathers [16]. In addi- tion, no research has considered whether birds, like some arthropods, might detect air-borne stimuli generated during social displays via mechanoreception, or what influence this may have on their social interactions. Here we report on the first biomechanical study of bird crest feathers, with an emphasis on understanding how their physical properties might relate to their various possible functions. This work focused on the large crest of the Indian Peafowl (Pavo cristatus), which is found on both sexes [17]. The male of this species (ªpeacockº) performs elaborate multimodal courtship displays accompanied by mechanical sound experienced by nearby females. During their displays, male Indian peafowl (ªpeacocksº) attract mates by spreading and erecting their train (a fan-like array of long, colorful feathers) and performing two dynamic courtship behaviors. First, during ªwing-shaking, the male flaps his partially-unfurled wings at approximately 5.4 Hz with his backside facing the female (ªpeahenº). Next, during ªtrain-rattlingº, the male vibrates his tail and train at 25±28 Hz (mean 25.6 Hz) while facing toward the female at close range (1 to 1.5 m) (Fig 1A, S1 Movie), causing the train to shimmer iridescently and emit a prolonged ªrattlingº sound [18±20]. Train-rattling performance by peacocks is obligatory for mating success [18], and eye-tracking experiments have shown that both wing-shaking and train-rattling displays are effective at attracting and holding the peahen's gaze [21]. Peahens also perform a tail-rattling display at 25±29 Hz in a variety of contexts [20], suggesting that feather vibrations might serve other communicative functions as well. Three studies have found that birds respond behaviorally to playbacks of low frequency sound generated by social displays: peafowl detect the infrasound (< 20 Hz) component of train-rattling and wing-shaking recordings [19]; male houbara bustards (Chlamydotis undulata undulata) respond to low frequency (40±54 Hz) boom vocalizations [22]; and male ruffed grouse (Bonasa umbellus) respond to 45 ± 6 Hz wing beating ªdrummingº displays [23]. Several other studies have also measured the behavioral response of birds to amplitude-modulated low repetition rate broad- band pulses, which are similar to the mechanical sounds associated with peacock train-rattling; the results of these studies showed that birds from three different families can detect such sounds with repetition rates < 40 Hz [24±26]. However, no studies have considered whether peafowl or other birds might detect such mechanical sounds via vibrotactile perception (i.e., sensing sound air particle velocity or airflow impulses via feather vibrations or deflections) as well as by hearing (sound pressure wave reception). One possible means by which peafowl might sense sound by vibrotactile perception is the fan-like crest (Fig 1A and 1B), a planar array of feathers oriented in the sagittal plane that is found on the heads of both sexes [17]. Each crest feather has a spatulate ªflagº of pennaceous vanes at the distal end and a long shaft that is mostly bare except for short, sparse barbs along its proximal end (Fig 1C). The pennaceous flag of peafowl crest feathers might couple to oscil- lations in surrounding air via drag forces induced by flow of the surrounding medium, as well as via forces exerted on the flag's face by incident pressure waves. PLOS ONE | https://doi.org/10.1371/journal.pone.0207247 November 28, 2018 2 / 27 Biomechanics of peafowl feather crests Fig 1. Morphology of peafowl crests and crest feathers. (A) A peahen (foreground) with the plane of her crest oriented towards the displaying peacock (background) as he performs train-rattling vibrations. (B) Both sexes have a crest with an inverted pendulum shape made

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