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One of the smallest of the rodents, moreover, one of the smallest African mammals, the African pygmy mouse grows to about 5 g in weight and an adult length of about 11 cm, (with a tail making up about half that length). These mice mainly eat grass seeds and small insects, forage nocturnally, and live in individual family units in burrows that they make usually under fallen logs or in piles of debris. They live about 2 years, and have a prolific breeding cycle in which first mating occurs at 4-6 weeks of age and they can produce a litter of 1-6 mice every 20 days. Although in mammals the Y chromosome usually confers male sex determination, studies have found that up to 75% of female pygmy mice in Southern and Eastern Africa carry both a Y and an X chromosome, indicating that this species may have a different mechanism for determining gender. The X chromosome of XY females has been examined and found to be morphologically different from the X chromosomes found in XX females and XY males, and is hypothesized to contain an as yet undetermined mutation which prevents masculinization of XY individuals, so that they develop as females.
(Monadjem 2008; Mpushini wildlife; Veyrunes et al. 2010; Veyrunes et al. 2006; Wikipedia 2011)