title: No evidence for melatonin-linked immunoenhancement over the annual cycle of an avian species.
abstract: The winter immunoenhancement hypothesis associates long nights and increased exposure to melatonin with enhanced immune function in winter when resource availability is low and the chances of becoming ill are high. Thus, increased exposure to melatonin in the winter could be adaptive for species facing difficult winter conditions. This idea has found some support in studies of resident mammals. In birds, the link between day length and melatonin over the annual cycle is weaker, and contributions of melatonin to seasonal timing are unclear. Furthermore, many species, especially migrants, do not experience the most difficult conditions of their annual cycle in winter. In this study, we tested whether the winter immunoenhancement hypothesis holds in an avian species, the red knot Calidris canutus. We found that melatonin duration and amplitude varied significantly over the annual cycle with the highest values occurring in winter. However, peaks did not correspond to the winter solstice or with annual variation in immune function. Our findings do not support the winter immunoenhancement hypothesis in knots and question whether the idea that immune function should be bolstered in winter can be generalized to systems where winter is not the most difficult time of the year.
authors.affiliation: Animal Ecology Group, Centre for Ecological and Evolutionary Studies, University of Groningen, P.O. Box 14, 9750 AA, Haren, The Netherlands. d.m.buehler@rug.nl
authors.forename: Deborah M
authors.initials: DM
authors.lastname: Buehler
authors.forename: Anita
authors.initials: A
authors.lastname: Koolhaas
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Van't Hof
authors.forename: Ingrid
authors.initials: I
authors.lastname: Schwabl
authors.forename: Anne
authors.initials: A
authors.lastname: Dekinga
authors.forename: Theunis
authors.initials: T
authors.lastname: Piersma
authors.forename: B Irene
authors.initials: BI
authors.lastname: Tieleman
mesh_terms: D000818:Animals; D001770:Blood Bactericidal Activity; D002176:Candida albicans; D046091:Charadriiformes / Q000276:immunology / Q000502:physiology*; D002940:Circadian Rhythm / Q000502:physiology*; D005241:Feathers / Q000502:physiology; D008550:Melatonin / Q000097:blood / Q000502:physiology*; D010587:Phagocytosis; D017440:Photoperiod; D012621:Seasons; D013211:Staphylococcus aureus
chemical_list: D008550:Melatonin