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Thursday, 15 January 2015

Female mate choice and feather-produced sounds in male Greater Sage-Grouse (Centrocercus urophasianus). Koch et al 2015

Investigating female mate choice for mechanical sounds in the male Greater Sage-Grouse

The Auk 132(2):349-358. 2015 

doi: http://dx.doi.org/10.1642/AUK-14-63.1

LINK

Authors
Rebecca E. Koch 1,2*, Alan H. Krakauer 1, and Gail L. Patricelli 1
1- Department of Evolution and Ecology, University of California, Davis, California, USA
2- Department of Biological Sciences, Auburn University, Auburn, Alabama, USA
*Corresponding author: rek0005@auburn.edu

ABSTRACT
Although birds are generally known for their vocally produced songs and calls, some species have evolved alternate means of acoustic communication that do not require the syrinx. While many of these mechanical sounds are used in a courtship context, the importance of among- and within-individual variation in these sounds is almost entirely unknown. We investigated feather-produced sounds in male Greater Sage-Grouse (Centrocercus urophasianus), which congregate on leks during the spring breeding season and perform elaborate displays to attract females. Despite decades of research on the vocal components of the display, the frequency-modulated and mechanically generated “swish” sounds remain poorly studied. We used 2 years of acoustic data to evaluate the relationship between the time and frequency characteristics of the swish display and male mating success. Although characteristics of the swish sounds showed individual-specific patterns of variation, neither univariate nor multivariate analyses revealed direct effects of the acoustic qualities of these mechanical sounds on number of copulations. However, we did find that the frequency range of individual notes was correlated with note duration, and that males who successfully copulated showed a larger frequency range for a given duration than unsuccessful males. Furthermore, successful males increased this frequency change more strongly with the approach of a female than did unsuccessful males. These results parallel previous findings that successful and unsuccessful males show different patterns of adjustment with changing courtship conditions. Our results emphasize the importance of considering the interaction among multiple components of displays in analyses of mate choice, and help to broaden our understanding of the function of mechanical sounds in this and other species of birds.

Keywords
Mechanical sound, sexual selection, Centrocercus urophasianus, display plasticity, mate choice, sonation

DISCUSSION
To investigate whether female Greater Sage-Grouse use mechanical swish sounds as criteria for choosing mates, we tested whether male mating success varied according to the spectral and temporal characteristics of his sonations. We found that swishes tended to vary more among males than within males. Our analyses did not indicate any direct relationship between the acoustic components of swish displays and male mating success in our population. However, we found evidence that successful males had a larger frequency range per unit duration in at least one component of their display—the second swish downslope. Further, we found that males may have adjusted this rate according to changes in their social environment (in this case, the distance to the female they courted), with successful males adjusting their mechanical sound production more than the unsuccessful males.

Our finding that successful males may have adjust their displays to increase the frequency range of the second swish downslope when females were near echoes the findings of a concurrent 2007 study of display behaviors and vocalizations on this lek. Patricelli and Krakauer (2010) found that successful males adjusted their display rate more strongly with female proximity, displaying at a high rate only when females were close. These successful males were able to increase their display quantity without a decline in the quality of vocalizations. In contrast, unsuccessful males adjusted their display rates less and showed a quantity–quality tradeoff. These results suggested that males tactically allocated their signaling effort to produce high-performance displays during close courtship, when they were most important in influencing female choice (Gibson 1996). Consequently, successful males may have displayed both at a faster rate and with larger frequency ranges for at least one component of the swish display when close to females. It is possible that the kinematics of producing the swish sounds could drive a performance tradeoff such that only some males could both accelerate display rate and optimize frequency range for a given duration. To test this possibility, further work is needed to understand how frequency modulation in the swish is achieved, such as whether modulation is due to changes in the rate of wing movement across the chest or by stimulating feathers with a broader range of resonant frequencies (K.S. Bostwick and L. Louis, personal communication). Interestingly, the closely related Gunnison Sage-Grouse, a sister species to the Greater Sage-Grouse with a similar strut display and habitat, has acoustically simple swishes with no frequency modulation (Young et al. 1994). Frequency sweeps are therefore not a necessary consequence of strumming feathers to produce swish sounds in these taxa and instead appear to be unique characteristics of the Greater Sage-Grouse sonations.

Another potential explanation for the increase in frequency range we detected is that successful males could adjust swish amplitude depending on distance to the female, and that the increased frequency range during close courtship was a byproduct of detecting a broader range of frequencies from a louder display. We used variation in distance between the male and closest array microphone to examine this possibility: for the second swish there was no relationship between distance and FR, and the trend went in the opposite direction as predicted for the second swish upslope, with slightly higher frequency ranges at greater distances from the microphone rather than when closer to the microphone as we would expect if received amplitude were influencing detectability. We are currently working to calculate amplitude of the acoustic displays directly from our array recordings to test this possibility more rigorously, but results to date suggest that amplitude was not a significant determinant of frequency range in this study.

We did not find evidence for selection acting on any one feature of swishes, but rather a combination of multiple features—the rate of change in frequency and the adjustment of frequency range with proximity to females. Females might have assessed these traits directly, or judged how these traits covaried with display rate or another trait. Swishes could also function as signals in male–male competition or as amplifiers to attract attention to a different trait, such as the bright yellow vocal sacs prominently displayed during the swish motions, the pop-whistle display that the swishes precede, or even the strummed breast feathers themselves (Hasson 1989, Boyce 1990, Saether et al. 2005, Ord and Stamps 2008, Takahashi et al. 2008). Experimental manipulation or ablation of the swish display would help to test these alternative functions. Unfortunately, such manipulation would prohibitively interfere with other aspects of male sage-grouse display and was not possible during our study. We are currently investigating seasonal patterns in swish acoustics, and whether the swishes may interact with other components of the sage-grouse display.

To our knowledge, this study is the first to describe the acoustic components of the swish sonation in male Greater Sage-Grouse and investigate the fitness consequences of individual variation in a mechanically produced sound in birds. Additional studies among the widespread populations of Greater Sage-Grouse and in other species with prominent sonations will be necessary to enhance our understanding of the functions of these behaviors.

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