Two Sage-grouse Species
Two Sage-grouse Species
Male Gunnison sage-grouse displaying (© Helen Richardson) and
female Gunnison sage-grouse (© Gerrit Vyn)
A species is one of the basic biological units used to understand patterns of biodiversity. There are several factors that biologists take into consideration when determining how to apply the designation of species to a group. While there are wide-ranging opinions on how to define a species, most species definitions or concepts consider physical, behavioral and genetic traits. The most common species concepts used to define vertebrates are the biological species concept or the evolutionary species concept. A review of the Gunnison sage-grouse's physical, behavioral, and genetic characteristics supports species designation under the biological species concept. When examined through the lens of the evolutionary species concept, it appears that the Gunnison sage-grouse are reproductively isolated because traits important to reproduction are significantly different from other grouse, which arose through sexual selection. Molecular divergence in alleles not related to secondary sexual characteristics is proceeding at a slower pace (Young et al. 2000).
The traits that distinguish the Gunnison sage-grouse include the following: they are about 2/3 the size of the greater sage-grouse (Hupp and Braun 1991), have differences in their plumage, and produce different mating vocalizations. For example, note the whiter and more distinct tail feathers in the Gunnison male and the filoplume feathers being tossed over his head (see photos above), in comparison to the duller colors and simpler filoplumes of the greater sage-grouse male (above). They use these traits in a highly elaborate strutting display that begins with a male taking a few steps forward as the wings are raised and lowered and brushed twice against the stiff feathers of his white pouch, producing loud swishing noises. In addition to the mechanical noises produced by the wing movements, males utter a distinctive series of sounds by vocalizing and popping two air sacs within their pouches. The entire display for both types of grouse lasts approximately three seconds. Grouse in Gunnison differ in their display by performing fewer displays per minute; however, they pop their yellow air sacs nine times instead of two and create different sounds (Young et al. 1994). Young (1994) found that females in the Gunnison Basin and northern Colorado avoided playbacks of male courtship vocalizations of the other species. In addition, they have different visual displays, such as throwing their filoplumes over their head and often wagging their tail at the end of their display (Young, unpublished).
Genetic evidence has shown that Gunnison sage-grouse are genetically distinct as well. At the species level, Gunnison sage-grouse have low levels of genetic diversity in both the nuclear and mitochondrial genomes when compared to greater sage-grouse (Kahn et al. 1999, Oyler-McCance et al. 1999, Oyler-McCance et al. 2005). Dr. Oyler-McCance also identified that there is a significant amount of genetic structuring among Gunnison sage-grouse populations, suggesting little gene flow occurs among existing populations.
Females have similar plumage in both species; however, Gunnison females are about 2/3 the size of the greater sage-grouse females. In contrast to the difference seen in the male displays, ecological traits of females are quite similar in both species. For example, nesting, brood-rearing, and non-brood site selection by Gunnison sage-grouse hens are similar to those chosen by greater sage-grouse females throughout their geographical range. In addition, despite the smaller body size of Gunnison sage-grouse, egg and clutch sizes are in the same range as those observed in other populations (Schroeder et al. 1999). Juvenile coloration matches adults, but can be distinguished by pointed, mottled and tattered outermost primary wing feathers (Northern Prairie Wildlife Research Center 2006).