The neuroendocrine basis of paternal behaviour is hypothesised to be homologous to that of maternal behaviour, with the expression of paternal behaviour resulting from the activation of estrogen-sensitive pathways in the brain. However, despite the fact that Phodopus campbelli fathers are highly paternal, previous studies have failed to identify an activational role for estradiol in the expression of this behaviour. The goal of this thesis was to exclude or define a role for estradiol in P. campbelli paternal behaviour. In the first study, estrogen receptor alpha (ERα) expression was examined in three brain regions relevant to maternal behaviour (medial preoptic area, bed nucleus of the stria terminalis, and medial amygdala). Male and female P. campbelli and its close relative P. sungorus were sampled as sexually naïve adults, following mating to satiety, or as new parents. As predicted, new fathers of either species did not upregulate ERα expression in these brain regions. Mating also had no effect on ERα expression in males or females from either species. However, ERα expression was also not upregulated in any of the three brain regions in new mothers. Thus, it is possible that estradiol does not play a primary role in activating maternal behaviour in Phodopus females. In the second study, P. campbelli males were administered the aromatase inhibitor letrozole at three different developmental stages (independence from the mother, testicular maturation, and sexually naïve adult) and tested for adult sexual and paternal behaviour. Aromatase inhibition during juvenile development resulted in diminished expression of adult paternal and sexual behaviour, suggesting that estradiol acts at critical periods in development to organize the paternal brain for subsequent behavioural responses. Treatment of sexually naïve adults did not alter male behaviour, which further supports previous evidence that estradiol does not activate paternal behaviour in P. campbelli males, although sexual behaviour in females is completely estradiol-dependent in this species. These findings suggest that the activational roles of estradiol in maternal and male sexual behaviour identified in the rat do not generalize to Phodopus, and imply greater species to species variation in the neuroendocrine control of these behaviours than previously suspected. / Thesis (Ph.D, Biology) -- Queen's University, 2008-09-15 10:52:00.161