Biological significance of daily variation in immunity of Perdicula asiatica: role of melatonin and testosterone

Daily variation of plasma melatonin affects daily activity pattern of many birds, but daily immunoprotective activity of melatonin in any seasonally breeding avian species is lacking. We report the influence of endogenous melatonin and testosterone on the daily variation in immunity of the Indian tropical bird Perdicula asiatica during reproductively active (RAP) and inactive (RIP) periods when the level of melatonin was high and in the former case. Daily variation in levels of melatonin, testosterone and immune parameters was noted during RAP and RIP. Maximum immune activity was noted at 2:00 hrs during RAP and at 14:00 hrs during RIP. During RAP, high testosterone in the circulation suppressed melatonin levels and immune parameters. A high basal level of melatonin during RIP was responsible for the suppression of testosterone resulting in high immune activity. Therefore, along with testosterone, melatonin acts like a major temporal synchronizer to maintain not only the reproductive rhythm but also daily immune adaptability of this avian species.