traits may appear as late as PND 55. Other reviews have suggested similar broad classifications defined by the appearance of mature hormonal cycling (PND 28–30) and the subsequent appearance of reproductive maturity which can occur as late as PND 60 in males (Ojeda and Urbanski, 1994; Smith, 2003). Within this broad range, some researchers have identified the categories of early- (PND 21–34), mid- (PND 34–46) and late- (PND 46–59) adolescence centered around the appearance of puberty (between PND 33–44) and encompassed by weaning on PND 21 and reproductive maturity on PND 59 (Tirelli et al., 2003). The beginning of preadolescence in the rat, in terms of sexual maturation begins at approximately PND 20 for females and PND 30 for males (Odell, 1990) with significant changes continuing between PND 28–55 (Ojeda & Urbanski, 1994).
