A continuing saga: the role of testosterone in aggression.

We thank Professor Wingfield for his commentary in this issue and the interesting questions he raises about the challenge effect and its functions in the control of aggressive behavior. He suggests several intriguing ideas that are worth pursuing, particularly in relation to the temporal nature of the testosterone (T) changes that occur in response to a competitive encounter. Testosterone changes can be short term, long term and the timing of the change can vary. Furthermore, some of these changes may be modulated by learning processes. The idea of ‘‘persistence of aggression,’’ as described by Wingfield (2005) and supported by research from his laboratory, covers several possible scenarios for temporal patterns of change in T. Testosterone implants alter T for longer periods of time and provide an excellent tool for manipulating hormones under field conditions. As Wingfield describes, the use of T implants nicely demonstrates how T extends the expression of aggression within an encounter, thereby increasing ‘‘persistence of aggression.’’ Repeated T injections are not generally feasible under field conditions because of the difficulties in recapturing animals. Injections are, however, useful for testing how rapid transient increases in T can influence future behavior. The transient nature of the changes in T after an encounter in California mice, Peromyscus californicus is shown in Fig. 1. These results also suggest that there may be variation in the T pattern, as Oyegbile and Marler (2005) did not find a difference in T after a single aggressive encounter (for variation in transient