Circannual Rhythms of Plasma Testosterone and Luteinizing Hormone Levels in Golden-Mantled Ground Squirrels (Spermophilus lateralis)

Abstract

Male squirrels, born approximately June 1 and kept under constant conditions of photoperiod (14 h of light per day) and temperature (23 ± 2°C) from birth, manifested circannual rhythms in plasma testosterone (T) and luteinizing hormone (LH) levels. Blood samples were collected once per month for 22 months and hormone levels determined by radioimmunoassay. Testosterone levels were low or undetectable (≤0.05 ng/ml) between October and January of the first year, and began increasing in February and peaked during April and May (approx. 4.5 ng/ml). By August of the second year T had declined to low levels which were maintained through January (0.05 ng/ml). The cycle then repeated, with T titers elevated during April and May of the second year. LH levels were undetectable (≤0.7 ng/ml) throughout most of the first 8 months of life; LH peaks were reached by individual animals between March and July. There was a close correspondence between seasonal peaks and troughs of testosterone and those in LH. The circannual pattern of plasma T and LH also was documented for a group of field-trapped males (≥11 months old) maintained under similar conditions and bled at monthly intervals for 14 consecutive months. Rhythms of plasma T and LH persisted with a period of 11 to 12 months under constant conditions and thus qualify as circannual cycles.