Vernal changes in the behavioral and endocrine responses to GnRH application in male European ground squirrels

This field study was aimed at examining hypothalamic involvement in the behavioral changes of male European ground squirrels (Spermophilus citellus) before, during, and after the mating season. The effects of exogenous gonadotropin-releasing hormone (GnRH) application on androgen secretion and behavioral patterns were investigated. Animals were captured, bled, and injected intramuscularly with 40 ng/100 g of GnRH. A second plasma sample was collected 40 min after the treatment to document changes in testosterone secretion. Behavioral parameters such as intra-sexual aggression, scent marking, and home range size were compared on the days before and after the stimulation. In the first two phases, before female emergence and during mating, GnRH-injection caused increases in plasma testosterone. In the post-mating phase, initial plasma testosterone levels had decreased and no elevation could be induced. Sham treatment of controls had no effect in any phase. Conditional parameters like emergence body mass and testicular size covaried with androgen increases only in the pre-mating period. Behavioral changes after GnRH administration occurred during the pre-mating period. Intra-sexual aggression, scent marking, and home range size increased significantly in experimental individuals. Later, during mating and post-mating, we found no behavioral changes associated with the GnRH treatment or the testosterone increase. The results demonstrate changes in the endocrine and behavioral sensitivity to GnRH application, according to the phases of the active season. An exogenous pulse of GnRH can apparently release behavior in male European ground squirrels, which is normally context dependent with the emergence of females.