Why do some dogs become locked together during sexual intercourse?

First of all we should be clear that in dogs this locking together during mating is not an exceptional feature; it occurs in almost all instances, and it is those individuals in which it does not take place (especially those of certain breeds, such as the Chow Chow) that should be regarded as unusual.

After mounting the bitch (during which he clasps her in front of the hips with his front legs) the male dog introduce~ his penis into her vagina and commences a series of pelvic thrusts. During this behavior the end of the penis, the bulb, becomes considerably enlarged, and the contraction of the constrictor muscles of the vagina around the penis holds the male firmly in position, preventing withdrawal, in the so-called ‘tie’ or ‘lock’.

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