If you’ve ever watched a pair of dogs have sex, you’ve probably noticed something odd happen partway through the act. At first, the male grips the female’s back and humps away-nothing unusual about it. But then he dismounts and turns away from his mate, with his penis still stuck inside her.
It’s called the copulatory tie, and the dog really does not have much of a choice about it. His penis is literally locked inside his mate’s vagina. Also, he’s still ejaculating. The pair may stay that way for 5 minutes, or they may be tied up for 40 minutes.
The lock is the result of a combination of penile inflation and vaginal squeeze. Like all mammals, the dog’s penis has two distinct types of erectile tissue, one that stiffens the penis, and another that fills the shine. The penis-stiffening type of tissue does not inflate much in dogs, but it does not have the penis in the middle of the penis when he first mounts the female (a long bone embedded in the dog’s shine tissue may also help) . Only after he’s inside he does the shine tissue expand, and expand, and expand.
The most impressive expansion occurs in the area of ? the gloss called the bulbus glandis. When the dog’s penis is flaccid, the bulbus glandis does not look any different from the rest of the penis. But when it inflates it grows twice as thick and three times wider than normal, and very, very stiff. When the female’s vaginal muscles squeeze down on this fat mass or tissue, it actually stays in place. He’s not going anywhere until his muscles relax and his tissues deflate.
But instead of resting atop during this stage or mating, he typically dismounts and swings around until the pair looks like the pushmi-pullyu from Doctor Doolittle. The behavior is not unique to domestic dogs, it’s found in most canids, including wolves, foxes, and jackals. But no one really knows why they do it. It does not seem necessary to get the female pregnant: sperm-rich fluids get ejaculated before the tie begins. Best guess so far? The turn seems to squeeze some of the penile shots, keeping the blood inside the glans and holding him in place while he ejaculates an additional 30 mL of prostate fluids that can push his sperm further into the female.
But why that is supposed to be butt-to-butt rather than belly-to-back is still a mystery.