Acute and chronic effects of an aromatase inhibitor on pair-maintenance behavior of water-restricted zebra finch pairs

• Acute oral fadrozole decreases brain and plasma estradiol levels in male zebra finches.
• Acute oral fadrozole increases brain and plasma testosterone levels in males and females.
• Acute fadrozole treatment increases proximity time in zebra finch pairs.
• Chronic fadrozole has no effect of pair maintenance.

Zebra finches are highly social songbirds that maintain life-long monogamous pair-bonds. They rely heavily upon these pair-bonds to survive their ever-changing and unpredictable habitat in the Australian desert. These pair-bonds are maintained via a large repertoire of affiliative behaviors that for most of an individual’s life are predominately associated with pair maintenance. Water restriction reduces circulating testosterone levels in male zebra finches and the size of the ovary and oviduct in female zebra finches, but water restriction has little or no effects on pair-maintenance behaviors and local levels of testosterone and estradiol in behaviorally-relevant brain regions. These data suggest that in water-restricted zebra finches, local synthesis of testosterone and estradiol in the brain may support the expression of pair-maintenance behaviors. Here, we directly test whether pair-maintenance behaviors are regulated by estradiol, acting via non-genomic or genomic mechanisms, in water-restricted (i.e., non-breeding) zebra finches. In two experiments, subjects were treated with an aromatase inhibitor (fadrozole) either acutely or chronically, and a variety of pair-maintenance behaviors were quantified. Additionally, we quantified the effect of acute fadrozole treatment on brain and circulating estradiol and testosterone levels. Acute fadrozole administration rapidly decreased estradiol levels in the circulation and brain of males and also rapidly increased testosterone levels in the circulation and brain of both males and females. However, neither the acute nor chronic fadrozole treatment decreased pair-maintenance behaviors. In one case, acute fadrozole treatment promoted affiliation. These data suggest that pair-maintenance behavior in non-breeding zebra finches is not promoted by estradiol acting via either non-genomic or genomic mechanisms.