The tail of dogs and allies (Canidae) is important for intraspecific communication. We used a life-sized dog model and varied the tail length and motion as an experimental method of examining effects of tail-docking on intraspecific signaling in domestic dogs, Canis familiaris. We videotaped interactions of 492 off-leash dogs and quantified size and behaviour of approaching dogs to the model's four tail conditions (short/still, short/wagging, long/still, long/wagging). Larger dogs were less cautious and more likely to approach a long/wagging tail rather than a long/still tail, but did not differ in their approach to a short/still and a short/wagging tail. Using discriminant analyses of behavioural variables, dogs responded with an elevated head and tail to a long/wagging tail model relative to the long/still tail model, but did not show any differences in response to tail motion when the model's tail was short. Our study provides evidence that a longer tail is more effective at conveying different intraspecific cues, such as those provided by tail motion, than a shorter tail and demonstrates the usefulness of robotic models when investigating complex behavioural interactions.