Background

Greater sage-grouse (hereafter sage-grouse) are a good model for studying variance in reproductive success and mating patterns in lekking species because they have highly skewed observed mating success among males, are well studied, and are easy to sample. Researchers have found that only a few males perform the majority of copulations on individual leks (e.g., Wiley 1973). Females visit one or more leks on several consecutive mornings and copulate only once with a single male (Wiley 1973; Gibson et al. 1991). Females then lay eggs, incubate, and rear the young on their own. However, males around the edges of leks also display to females and can follow females off-lek (Gibson 1996). Males are reported to display to females away from leks (Dunn and Braun 1985) and yearling males (males hatched the previous spring that are physiologically capable of reproducing, but are assumed not to breed based on reduced testis size; Eng 1963) can walk or fly onto leks accompanying one or more females (K. L. Bush, unpubl. data). Furthermore, most visiting females are never observed to mate even on intensively monitored leks (Semple et al. 2001). Therefore, the breeding system in sage-grouse may be more complex than previously thought. Consistent with this idea, a small-scale paternity study on sage-grouse in California found only 40% of broods were fathered by territorial males from focal leks, while 40% were fathered by males from other leks or males off-lek, and 20% of broods exhibited multiple paternity (Semple et al. 2001). This study by Semple and colleagues examined only 10 broods, making it necessary to assess the generality of these results with a larger study conducted in a different geographic region.

 

We used polymorphic microsatellites to study the mating system and parentage of sage-grouse in Alberta and address two main questions. First, do a limited number of sage-grouse males father the majority of offspring in a given year as predicted by most behavioral studies (e.g. Wiley 1973) or is paternity more evenly distributed across the population? Second, do sage-grouse broods exhibit multiple paternity? We anticipated some variance in male mating success, as sage-grouse are a highly sexually dimorphic species where copulation rates on leks are known to be highly skewed among males (Wiley 1973). However, Semple et al. (2001) found a greater spread in paternity than suggested by previous behavioural studies. We also expected to document cases of multiple paternity within broods because the pattern has been reported in most lekking species studied using genetic methods (Lanctot et al. 1997; Semple et al. 2001; Lank et al. 2002; Lebigre et al. 2007).

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