1 2 3 4 5 6 7 8 9 10 11 The distinction between adult and immature individuals is further complicated by an autumnal moult when the dark pelage of immature individuals is replaced by the lighter one characteristic of adult animals, and a substantial error may occur in allocating males taken from hibernation into categories of sexually mature or immature. These characteristics, which are the result of detailed study of pipistrelles and noctules, are not so clear in some other species such as long-eared bats. 5.2.4 Reproductive status of females Oestrus Oestrus is the time when females will allow males to mate with them, and the best criteria of its occurrence are behavioural, particularly in females soliciting copulation. Copulation A male mounts a female dorsally and often holds on by biting the neck of the female while curling its tail and penis underneath. Copulation is generally accompanied by much vocalisation, and some vespertilionids such as the serotine may remain in copula for several hours, without moving, giving the impression of one bat roosting on top of another. Pregnancy Early pregnancy is difficult to diagnose in bats. The nipples of nulliparous females (those that have never given birth or reached an advanced stage of pregnancy) remain tiny until around the time of first implantation of the egg in the wall of the uterus and so maybe used as a criterion of nulliparity. Palpation can diagnose pregnancies that are between one-half and two-thirds progressed.