If the female is ready to mate, she will hold her tail to the side (known as "flagging") and stand calmly. As the male mounts and penetrates, he clasps her around the loins with his forelegs and thrusts forward. At full penetration, he treads up and down instead of thrusting forward. (Dogs must "pump" their sperm directly from their testicles, as they do not have seminal vesicles to store sperm.) His bulbus glandis swells and is clasped by her vulva. This produces the tie and stimulates the male to ejaculate. After ejaculation, the male unclasps his forelegs and places both paws on the ground on one side of the female. He may lift his hind leg over the female's back and swing around so they stand rump to rump. The dogs usually remain joined for 15-30 minutes, occasionally longer. As their muscles relax, the male separates. Separating can be momentarily painful, and either dog may whine, cry or snap.