Plasma Leptin in Relation to Breeding Stage, Season, Estrogen and Photoperiod in Female European Starlings(Sturnus Vulgaris)

Leptin, a protein hormone secreted by adipocytes, has wide-ranging physiological functions, including regulation of feeding behavior and body weight, and effects on reproduction and immune function. Our first study demonstrated seasonal and breedingstage patterns of variation in plasma leptin-like immunoreactivity in free-living female starlings (Sturnus vulgaris), which did not parallel changes in body mass and composition. Plasma leptin-like immunoreactivity was elevated from egg-laying through clutch completion, decreased during incubation and chick-rearing, and was elevated in non-breeders in November. Next, we manipulated wild-caught females to determine whether elevated plasma leptin-like immunoreactivity is associated with a) seasonal, photoperiodic (long day length-dependent) reproductive development, or b) elevated estrogen required for egg production. Plasma leptin-like immunoreactivity was unaffected by photoperiod or estradiol treatment; however, because estradiol treatment on 18L:6D only incompletely stimulated the reproductive axis, we cannot unequivocally exclude a link between elevated plasma estradiol and plasma leptin-like immunoreactivity during egg-production in free-living birds.