Potential for behavioral reproductive isolation between greater prairie-chickens and sharp-tailed grouse in west-central Minnesota

Abstract

When ecological or behavioral species-isolating mechanisms are relaxed or incomplete, hybrid zones form. Greater prairie-chickens (Tympanuchus cupido) hybridize with sharp-tailed grouse (T. phasianellus) wherever their ranges overlap. The objective of this study was to document the potential for reproductive isolation between greater prairie-chickens and sharp-tailed grouse in west-central Minnesota, a newly established sympatric and hybridizing population which developed following the translocation of greater prairie-chickens to the area. We describe and evaluate several potential mechanisms of behavioral isolation (intensity of display and aggressive behaviors, and vocal and non-vocal sounds). In addition, we monitored copulation success to determine the patterns of interspecific mating. Individuals of putative mixed genetic makeup (based on morphology) comprised 8 % of the population, a rate higher than previously reported for most areas of the hybrid zone between these two species. Apparent hybrid individuals stomped their feet faster than parental species during courtship displays, and their vocalizations were intermediate between the parental species. Intensity of display and aggressive behaviors were similar for parental species and hybrids. All copulations observed involved conspecific pairs; hybrid males were not observed to mate. Taken together, this study documents behaviors that might reinforce reproductive isolation (foot stomping and vocalization) and other behaviors that are similar among the parental species and hybrids that would hinder reproductive isolation (display and aggressive behaviors). We suggest that the small population sizes of both greater prairie-chickens and sharp-tailed grouse in this newly established population contribute to the higher rate of hybridization than is observed elsewhere in this hybrid zone.