Differential Survival by Sex in Juvenile Sage Grouse and Gray Partridge

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Abstract
Studies of Capercaillie Tetrao urogallus and Black Grouse T. tetrix in Northern Europe have found that juvenile males of these sexually size dimorphic species suffer higher mortality than juvenile females during adverse conditions. This difference may be due to the more rapid growth rates among males. Differences in the juvenile survival of the dimorphic Sage Grouse Centrocercus urophasianus and the monomorphic Gray Partridge Perdix perdix were studied in western North America. Juvenile male Sage Grouse survive less well than juvenile females during years unfavorable for juvenile survival and in poorer habitats. Juvenile male Gray Partridge showed little or no such trend. These results are consistent with those obtained in the European studies. Pressures of sexual selection may have led to a growth rate in juvenile males of highly dimorphic grouse species which is near the upper limit of that which can be sustained by their ecological niche.

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