The association of higher fertility rates of bulls with better volume regulating spermatozoa [51] and higher vol-ume regulating ability of human spermatozoa from fathers compared with patients [52] suggests that volume regula-tion by spermatozoa is an important property.
However, the loss of the droplet at or around ejacu-lation seems to be important for fertility because its re-tention is associated with infertility in several domestic species [48]. In all the infertile transgenic mice men-tioned above, flagellar angulation to some extent has oc-curred in the epididymis, which is further accentuated upon release into medium, and might make loss of the droplet impossible with consequences for post-ejacula-tory function. In the boar this might be a result of ob-struction of oviductal binding sites [53]. That the im-proved binding of droplet-free spermatozoa to the ovi-duct is associated with spermatozoa displaying better volume regulation [54] suggests that the droplet per se is not required for volume regulation; a view consistent with its postulated osmolyte loading function within the epid-idymal canal.