Leucocyte counts variation and measures of male fitness in the lekking Black Grouse

Abstract

Differences in the ability to sustain chronic stress can underlie the variation in individuals’ fitness directly or indirectly by impairing the expression of condition-dependent signals used in female mate choice. Therefore, understanding the effect of the stress response on individuals’ fitness is key to identifying the mechanism of sexual selection. We tested the hypothesis that an indicator of stress—the heterophil/lymphocyte ratio (H/L ratio)—is related to several major components of male fitness in the lekking Black Grouse (Tetrao tetrix), a species with intense male–male competition. The H/L ratio accurately measured the stress response, as it increased within a few hours of trapping and during the months prior to the mating season. The H/L ratio was positively related to the expression of morphological traits, suggesting that males of higher “quality” might be able to express elaborated ornaments and better cope with stressful conditions or with changes in health status. However, in Black Grouse, ornaments are (partly) independent of behavioural display and are under only weak selective pressure. Therefore, the H/L ratio was not significantly related to male mating success and survival to the following year. Under natural conditions, observed levels of the H/L ratio might not be associated with strongly deleterious effects because of behavioural adjustments. Mechanisms other than the stress response may mediate the observed variance in male mating success.