Response to Wingfield's commentary on ''A continuing saga: The role of testosterone in aggression''

We thank Professor Wingfield for his commentary in thisissue and the interesting questions he raises about thechallenge effect and its functions in the control of aggressivebehavior. He suggests several intriguing ideas that are worthpursuing, particularly in relation to the temporal nature ofthe testosterone (T) changes that occur in response to acompetitive encounter. Testosterone changes can be shortterm, long term and the timing of the change can vary.Furthermore, some of these changes may be modulated bylearning processes.The idea of ‘‘persistence of aggression,’’ as described byWingfield (2005) and supported by research from hislaboratory, covers several possible scenarios for temporalpatterns of change in T. Testosterone implants alter T forlonger periods of time and provide an excellent tool formanipulatinghormonesunderfieldconditions.AsWingfielddescribes, the use of T implants nicely demonstrates how Textends the expression of aggression within an encounter,thereby increasing ‘‘persistence of aggression.’’ Repeated Tinjections are not generally feasible under field conditionsbecause of the difficulties in recapturing animals. Injectionsare, however, useful for testing how rapid transient increasesinTcaninfluencefuturebehavior.Thetransientnatureofthechanges in T after an encounter in California mice,Peromyscus californicus is shown in Fig. 1. These resultsalso suggest that there may be variation in the T pattern, asOyegbile and Marler (2005) did not find a difference in Tafter a single aggressive encounter (for variation in transientT profiles see Amstislavskaya and Popova, 2004). Thetemporal pattern of changes in T in Fig. 1 was mimicked byTrainor et al. (2004) via injections and demonstrated thattransient increases in Tcan also influence future aggression.Trainor et al. (2004) tested the idea developed in Oyegbileand Marler (2005) that link the ‘‘challenge’’ and ‘‘winner’’effects. Those results further supported the general idea of‘‘persistence of aggression’’ but, combined with the T-implant studies described by Wingfield, suggest that T caninfluence the persistence of aggression in different ways.One is to extend aggression in a current encounter; the otheris to increase aggression in future encounters.It remains to be seen whether T acts through similarmechanisms to influence different aspects of ‘‘persistence ofaggression.’’ Wingfield raises the issue of whether T isinfluencing persistence of aggression through estradiol orandrogen receptors. In the California mice, transientincreases in Tcombined with winning experiences influencefuture aggression via androgen and not estradiol receptorpathways (Trainor et al., 2004). However, baseline levels ofaggression are at least partially dependent on estradiol-mediated pathways. An aromatase inhibitor decreased thebaseline levels of aggression, but did not affect aggressionchanges observed in response to transient increases in Tandthe experience of winning. It is also possible that implants,and therefore possibly persistence of aggression after anencounter, may be acting through estrogen receptors. Thus,we agree with Wingfield that it will be very important to testhow T influences these different aspects of aggression.Furthermore, we also need to consider that neural mecha-nisms unrelated to steroid hormones may be activated inaggressive encounters and then influence and interact in the