Dogs will exhibit active or passive submissive body language based on the circumstances.
Active submissive postures are used when the situation is nonthreatening but when the dog is being confronted by a more dominant dog (or person). The ears are held back against the head and the mouth appears to be a long grin. The dog will look away from the other dog’s eyes and lower the front of its body or lie down slowly on the ground with the tail tucked or held low and wagging nervously. There may be some greetings exchanged, such as licking the muzzle of the dominant dog. This behavior should not be confused with “kissing.” This gesture conveys submissiveness and an awareness of its lower position to the dominant dog. Think of it as the human equivalent to groveling. The situation is generally a friendly one, as the dominant dog is not behaving in a threatening manner.
Passive submission takes on a more extreme manner that is meant to convey susceptibility and weakness. There is little friendliness or enthusiasm in passive submission. Passive submission is usually displayed by a subordinate dog that is being confronted in a threatening manner by a more dominant dog. The passively submissive posture includes lying down on the ground and rolling over to expose the genitals and tummy, with the front legs bent at the elbows. The dog either does not wag its tail or does so minimally, as the tail is usually positioned tightly between the hind legs (even when lying on the back). The ears are held firmly against the head and the dog will avoid all eye contact so as not to be seen as challenging the more dominant dog. This posture conveys complete submission and respect for the more dominant dog, as the submissive dog makes itself as small in size as it can.
Image: Todd / via Flickr