The neuropeptide arginine vasotocin (AVT) and its mammalian homologue arginine 
vasopressin (AVP) are neuromodulators known to be steroid sensitive and 
associated with social behaviors in a number of vertebrate taxa. However, the 
role of AVT/P in the regulation of aggression remains unclear and contrasting 
effects of this peptide on aggression are seen in differing species and 
contexts. In this study, we used immunohistochemistry to examine the effects of 
testosterone on the AVT system in male and female tree lizards, Urosaurus 
ornatus, and to determine whether AVT is related to territorial aggression in 
this species. Tree lizards are a free-living species that exhibit natural 
hormonal fluctuations across breeding seasons. We detected a male-biased sexual 
dimorphism in centrally projecting AVT fibers within the limbic system. 
Furthermore, changes with season, reproductive state, and hormonal treatment 
suggest that testosterone regulates AVT immunoreactivity in limbic brain 
regions, especially in the bed nucleus of the stria terminalis. Testosterone 
also affects AVT immunoreactivity in peripherally projecting cell clusters, as 
well as the size of AVT cell bodies in the paraventricular nucleus. Although 
higher testosterone levels alter AVT immunoreactivity, and are known to increase 
the frequency and intensity of male-male aggression in this species, no 
individual correlations between AVT immunoreactivity and aggression were 
detected.