Adaptive Significance of Annual Variation in Immune Parameters and Endogenous Hormones (Melatonin and Thyroxine) of a Tropical Rodent Funambulus pennanti

The significance of annual variation in metabolic (thyroxine) and chronobiotic (melatonin) hormones in relation to immune adaptation of the Indian palm squirrel was investigated. The squirrel is summer breeder and, hence, remains healthy during reproductively active phase (Feb - Aug) when melatonin level is low and thyroxine level is high. Our data show that the circulating levels of melatonin (MEL) and thyroxine (T₄) have their respective annual rhythms, which are inversely related. Thyroxine level is low during the winter months (Sep-Jan) when metabolic activity of this rodent is also low. During this period melatonin level is high because of the short photoperiod and the squirrels indicate poor reproductive activity, with low gonadal steroids, which could be due to high circulating melatonin. The immune parameters also present an annual rhythmicity, which is parallel with circulating melatonin level, but inversely related to thyroxine level. This suggests that the metabolic hormone, thyroxine, and the chronobiotic hormone, melatonin, might be responsible for maintenance of the immune system to adapt the rodent for the rigors of the seasonal environmental changes.