“…The basic tenet of this proposal is that caudal spermatozoa are immobilized by the mechanical constraint imposed by the high viscoelasticity of plasma from the cauda epididymidis, the dilution of which permits motility (Usselman and Cone, 1983). However, the present study and earlier work (Armstrong et al, 1994;Murdoch et al, 1999) has clearly demonstrated that activation of sperm motility can occur in the absence of dilution. Moreover, only plasma from the cauda epididymidis of rats, hamsters and to a much lesser extent guinea pigs, contain immobilin, yet spermatozoa from other species including mice, dogs, elephants and tammar wallabies, are quiescent in situ in the absence of immobilin (Jones, 1973(Jones, , 1978Carr et al, 1985;Clulow et al, 1992;Jones and Murdoch, 1996).…”