Day length affects immune cell numbers in deer mice: The influence of age, sex, and in utero photoperiodic history

The extent to which day length affects immune function was examined in the present study. Three goals were pursued: 1) to confirm and extend the observation that the immune systems of adult deer mice (Peromyscus maniculatus) are responsive to changes in photoperiod, 2) to examine the development of the photoperiod-associated changes in immune function, and 3) to discover whether photoperiodic information transmitted to the young during gestation influences immune function. In experiment 1, adult mice housed in short days had higher white blood cell and lymphocyte numbers than their long-day cohorts. Red blood cell and differential cell counts did not differ between long- and short-day animals. No sex differences were observed in the pattern of immune responses to photoperiod. The effect of photoperiod on immune cells in prepubertal animals was examined in experiment 2; a similar pattern of results was obtained as that for experiment 1, suggesting that the photoperiodic effect on the immune system is not mediated by sex steroid hormones. Prenatal and postnatal photoperiodic effects on immune cells were examined in experiment 3; pups gestated in one day length were cross-fostered to mothers in the same day length conditions or to mothers maintained in the alternative day length. The results of experiment 3 suggested that photoperiodic information transmitted from the mother to the young in utero subsequently affected immune systems of the pups. Animals gestated in short day lengths displayed higher immune status throughout life than mice gestated in long days. These results are discussed from an adaptive functional perspective.