Studies on other grouses have shown that much variation in comb size and colour exists between the species, and that the comb is used in courtship display and aggressive interactions between males. Many studies have shown that there is a strong correlation between the comb size and the level of testosterone in males; one report from 1981 showed that the amount of testosterone is correlated to aggressiveness against other males. The male's comb has been the focus of studies regarding sexual selection. Studies of a population of male rock ptarmigans in Scarpa Lake, Nunavut, have shown that during the first year, mating success among males was influenced by comb size and condition, and bigamous males had larger combs than monogamous males. The correlation to size disappeared after the first year, but the correlation to comb condition remained. This is consistent with another study of the same population of L. muta that showed that mating success overall is correlated to comb condition. Exceptions were first-time breeders, in which the size of the comb influenced mating success. The rock ptarmigan becomes sexually mature at six months of age and commonly has up to six chicks. Because of this high breeding rate, the size of the population is affected very little by factors such as hunting. Ecophysiology Energy storage and assimilation Rock ptarmigan have a limited capacity for fat storage, which requires overwintering birds to forage frequently. Most of the minuscule mass gained over winter is to the ovary, oviduct and hypertrophy, in preparation for the spring breeding season. Rock ptarmigan maximize assimilation of nutrient poor foods with their elongated ceca. Metabolic requirements can be partially supplemented by fermentation, the energy gain from fermentation alone, however, is not independently significant. The Svalbard subspecies of rock ptarmigan is the only subspecies that exhibits a significant seasonal mass gain. Larger fat deposits can help them survive during periods of low food availability. However, this alone is not an adequate source of energy to survive during winter. Locomotion appears to have no energetic cost in these birds. This adaptation is key for a species that must move frequently to forage. Fat assimilation in these birds is correlated to changes in liver weight. Most rock ptarmigan have no more than 20 grams of adipose tissue year round. Without food, these reserves can supplement energy for 2 days. The Svalbard rock ptarmigan, however, gains about 100 grams of