Sex differences in body condition and hematological parameters in Little Auk Alle alle during the incubation period

Abstract

Incubation may be an important component of reproductive costs, and birds can spend considerable amount of energy during this period. We investigated how the stress associ-ated with incubation affects hematological parameters (proportion of leukocytes) and size-adjusted body mass of males and females of Little Auk (Alle alle), a monogamous species. Body mass of both sexes increased significantly between the early and late phases of incubation; both sexes gained 34% of their initial mass on average. Stress level (heterophil/lymphocyte ratio) was higher in females than in males during the early incu-bation phase, which could result from physiological stress associated with egg formation and production, and/or allocation of resources from the immune system to reproduction. Female stress level decreased with the advance of incubation, whereas in males both stress level and body mass increased. The latter finding could be associated with the higher contribution of males in activities other than incubation. Our results show that body mass and hematological parameters respond independently to stressors. Thus, the assessment of the condition of an individual should be based on various parameters.