Bulls become fertile at about seven months of age. Their fertility is closely related to the size of their testicles, and one simple test of fertility is to measure the circumference of the scrotum: a young bull is likely to be fertile once this reaches 28 centimetres (11 in); that of a fully adult bull may be over 40 centimetres (16 in).
Bulls have a fibro-elastic penis. Given the small amount of erectile tissue, there is little enlargement after erection. The penis is quite rigid when non-erect, and becomes even more rigid during erection. Protrusion is not affected much by erection, but more by relaxation of the retractor penis muscle and straightening of the sigmoid flexure.
Bulls are occasionally affected by a condition known as "corkscrew penis"