Wag the dog: When left vs. right matters

The asymmetry in a dog wag could mean more than you think

Dogs can tell a left wag from a right wag, a new study finds.

M. Siniscalchi et al/Current Biology 2013

Most of us see a wagging dog’s tail and think it’s got to be a good sign. Wagging = welcome, right? Especially if it’s the kind of wag that’s knocking over small items.  But it turns out that not all wags are equal, and some are a lot more welcoming than others.  

When I walked into my college biology course freshman year, we started out with a discussion of symmetry. Most animal are built with some symmetry, either radial or bilateral — radial like a starfish, bilateral like a human. Symmetry means things, like health or attractiveness. But it turns out that asymmetry can mean things too. And an asymmetrical behavior might mean some important things for dogs.

Marcello Siniscalchi of the University of Bari Aldo Moro in Italy and colleagues decided to look at asymmetry in dog wags. They noticed that sometimes, dogs wag more to the right, usually when seeing their owner or something else happy. They wag more to the left when they see something like a dominant or unfamiliar dog. So the wag itself could represent the emotional state of the dog doing the wagging.

But can the dogs seeing the wagging (the wagees) tell the difference? In a paper published October 31 in Current Biology, the authors found that they can. They used videos of a real dog or the silhouette of a dog wagging to the right (the wagging dog’s right, by the way) or to the left, and examined 43 other dogs as they watched (OK, they started with 56, but 13 didn’t pay attention), to see how the wagee reacted. The observing dogs wore a vest to monitor their heart rate, and were videotaped so behaviorists could look at their behaviors afterward.

When shown a right wag or no wag, the viewing dogs showed more relaxed behaviors and appeared low stress. But when shown a left wag, the viewing dogs showed higher levels of stress, with tails tucked down, legs braced, whining, all the way up to running away. Other dogs showed increase alertness, scanning the area, tail up, and ears forward. They all showed higher heart rates as well, and many salivated, a sign of stress. A left wag is not a welcoming wag!

The question left now is … what do dogs do with this information? They show more signs of stress, but how do they then behave? Do they back off? Approach with caution? Get ready to attack? Also, we’ve now seen what this means for the dog observing the wag, the wagee, to use the technical term. But what does this mean for the wag-ER? What makes a dog exhibit a left or a right wag? Is it a real reflection of emotional state as hypothesized? And is it an honest signal? Can a dog FAKE a wag, to, say, look less or more dominant?

In addition, are there other aspects of the wag that are different? Is it more than just left vs. right? For example, is the right wag more “open” and relaxed, with bigger movements (characteristic of a relaxed dog), than the left wag, which might be more “tense,” and have smaller movements? This may not be the case in the video demonstration, but in real life, is there a difference, and does that contribute to how other dogs respond?

While the wagee’s were all different breeds (from Jack Russell to Rotweiler), the stimulus dog shown in the video was only a single dog. A bigger dog, it’s a little Shepherd-like. I wonder how the reactions might change if the dog were smaller, or how they change when two dogs are radically different sizes?

What’s interesting is that the findings from the paper are completely opposite to previous findings when scientists used a robotic dog! Makes me wonder if the dogs in that study knew a fake when they saw one.

The authors suggest that the difference in tail wag is a left brain/right brain issue. They back this up with neuroanatomy, referring to the ruprospinal tract, a tract of nerves that controls voluntary movements. In both humans and dogs, this tract “decussates” or crosses on its way from the brain to the spine. The authors hypothesize that this cross means a lefty-wag would be “right brain,” while a righty-wag would be “left brain.” Which fits with a previous behavior hypothesis that the “left brain” in dogs is associated with approach behavior while the “right brain” is associated with withdrawal behavior.

But I feel like this might be more complicated. First off, it doesn’t just take one side of your muscles to wag (if you are the type of animal that wags). If you’re contracting the muscles at the base of the tail on the right (so you’ll wag right), you relax the ones on the left. If course, you can get a right LEANING wag with more muscle action on the right, so that doesn’t invalidate the hypothesis, but it does mean you’re dealing with something more than “right brain” and “left brain.” But there’s also the issue that fear-related behaviors and reward-related behaviors are associated with activation on BOTH sides of the brain. You don’t just get activity in one amygdala. Finally, dogs are clearly capable of complex social interactions, something that requires a lot of higher processing. It takes a lot of thinking to figure out which signal you want to pass on to other dogs, and to “decode” that signal. The two “sides” of the brain are always going to work together to a greater or lesser extent. It would be interesting to see the processing going on while a dog makes a left or a right wag.

Regardless of how it happens, which way the dog wags could be important for both how we approach dogs, and how we work with dogs when trying to do things like socialize them with other dogs. When you see a dog next time, look closer. Is that tail going to the right or left? If left, you could be in for a less friendly reception than you anticipated. 

Paper: M. Siniscalchi et al. Seeing Left- or Right-Asymmetric Tail Wagging Produces Different Emotional Responses in Dogs. Current Biology. Published online October 31, 2013. doi: 10.1016/j.cub.2013.09.027

Bethany was previously the staff writer at Science News for Students. She has a Ph.D. in physiology and pharmacology from Wake Forest University School of Medicine.

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