Patterns of progesterone secretion in free-living California ground squirrels (Spermophilus beecheyi)

During a long-term field study of a free-living population of California ground squirrels (Spermophilus beecheyi), blood samples were drawn at regular intervals from marked females via femoral venipuncture, and plasma progesterone (P) and prolactin (PRL) were measured by radioimmunoassay. Marked fluctuations with season and reproductive condition occurred in circulating levels of both hormones, with peak levels occurring during the spring breeding season. Two peaks in P concentrations were observed each spring, the first occurring during pregnancy, and the second during lactation. Peak PRL levels in females were also reached during the lactation interval, midway between the two P peaks. Analysis of repeated measures from individual females showed a marked decline in circulating P around the time of parturition. Juveniles had lower mean P levels than adults, and yearlings had lower peak levels during their initial reproductive episodes than older females did. The observed pattern of P secretion in S. beecheyi differs from that known for most mammals, but resembles those reported for other ground-dwelling sciurid rodents.