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 to face away from his mate, with his penis still stuck inside her.
It’s called the copulatory tie, and the dog really doesn’t 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 for up to 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 glans. The penis-stiffening type of tissue doesn’t inflate much in dogs, but it does make the penis stiff enough to let the male enter the vagina when he first mounts the female (a long bone embedded in the dog’s glans tissue may also help). Only after he’s inside her does the glans tissue expand, and expand, and expand.
The most impressive expansion occurs in an area at the base of the glans called the bulbus glandis. When the dog’s penis is flaccid, the bulbus glandis doesn’t 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 of tissue, it effectively anchors the male in place. He’s not going anywhere until her muscles relax and his tissues deflate.
But instead of resting atop her back during this stage of mating, he typically dismounts and swings around until the pair looks like the pushmi-pullyu from Doctor Doolittle. The behavior isn’t 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 doesn’t seem to be required 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 veins shut, 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 should happen butt-to-butt rather than belly-to-back is still a mystery.
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