ENVIRONMENTAL-INFLUENCES ON MASS DYNAMICS OF THE COTTON RAT (SIGMODONHISPIDUS) THYMUS GLAND

The influence of season on thymus gland mass was examined relative to captivity, gender, and age in 921 cotton rats (Sigmodon hispidus) from free-ranging and laboratory populations. Age-related involution of th e thymus gland was evident in free-ranging males and females and capti ve females. A distinct seasonal cycle in thymus mass dynamics was appa rent among adult cotton rats. Mass of the thymus gland was greatest fr om late fall to early winter before declining 2-4 fold during spring. Thymus gland mass remained low through spring and summer in adult cott on rats when reproductive activity was maximum. No seasonal cycle in t hymus mass was apparent among juveniles. Possible involvement of sex h ormones in regulating thymus size is discussed.