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The perforate condition recorded in Table 1 may represent any of several stages in oestrus or pregnancy, but is regarded as a crude index of rate of breeding, since voles in the anoestrus stage lack the vaginal orifice.

Anoestrus females are imperforate, and a vaginal orifice is present only during an active oestral cycle or in pregnancy.

In anoestrus the orifice is sealed, the genitalia are reduced in size and the skin in the genital region is white.