The Strut Display of Male Sage Grouse: a "Fixed" Action Pattern

Abstract

AbstractThe Strut display of male Sage Grouse, Centrocercus urophasianus, exemplifies a complex "fixed" action pattern. The display results in the inflation of the male's large esophageal sac, which is then used to produce a strange sound that includes two sharp snaps 0.19 seconds apart. Measurements of the durations of an individual's displays usually yielded a coefficient of variation (C.V.) less than 4%; the most accurate measurements had C.V.'s less than 1%. The intervals between Struts, in contrast, had much higher variability. No geographic differences were detected among three populations in Wyoming and Montana. Nor did the slight differences in the displays of individual adult males have a consistent relation to their successes in mating. First-year males performed their Struts with faster tempo but not with greater variability. The temporal stereotypy in the Strut display approximates or exceeds the stereotypy in passerine songs. Stereotyped motor patterns in vertebrates often subserve communication at long range or between individuals in brief, but biologically crucial, contact with each other. The Strut display exemplifies both of these functions in communication. Stereotyped, complex signals incorporate more redundancy and/or more efficient encoding; they thus enhance the receiver's chances of detecting and correctly interpreting the signal. Reafference that depends on the harmonic properties of moving structures might help to explain both the developmental and physiological stability of this stereotyped motor coordination. Because year-old males' displays show no greater variability than those of older males, the peculiarities of young males' performances probably are not the result of incompletely crystallized neural mechanisms.