Tail wagging hypothesis

Posted in Society at 12:49 by RjZ

Not that this blog has much of a theme, but this thought is so off-topic I thought it deserved a warning. Still, I’d like to hear any comments or if someone could suggest other research done on this.

Recent research has shown that the behavior of domesticated dogs has evolved to match and be dependent upon humans. The relationship between the species has been a profitable one for dogs who have, as a species, exploded in significance on the planet. The same can be argued for nearly every species that has adapted to and been adapted by humans, from sheep to corn.

I’d like here to posit two adaptive behaviors and suggest some evidence that their prevalence is thanks to their relationship with humans.

Dogs wag their tales and cats purr. In both cases, these behaviors indicate pleasure to the creature in a way that is easy for a human to interpret, satisfying and thus gives the human a signal of how to please the animal further. Other pets, either do not exhibit such characteristics or they are not easily read or acted upon by humans, perhaps, because they have not been domesticated as long or as intensely as dogs and cats.

Easy to read/Hard to miss

Even without prior association, it’s easy to guess what wagging means. The behavior is similar enough to an excited child, barely able to contain his own movement. I propose that early humans could easily guess the meaning of this behavior among their new found camp guards. Animals may show signs of joy, pleasure or contentment, but few are as easy to read, even with little prior experience, as the tail wag. Purring of cats is both more difficult to evoke and less immediately obvious, yet purring is consistently, and strongly associated with cat contentment in a way that other animal behaviors may not be. When purring , cats close their eyes and relax their bodies in such a way that it would be difficult to misread even without instruction.

Do other animals show their feelings?

Parrots grind their beaks, often muttering to themselves when content, but this behavior is difficult for humans to invoke, happening usually not in response to interaction, but rather before going to sleep when the animal feels safe. Rats brux, or grind their teeth, often bulging their eyes simultaneously in an expression that is both odd, and humorous and certainly something a rat owner can identify and enjoy, but unfortunately not as easy to elicit as tail wagging or purring, and, well, unless you have a pet rat, not all that attractive either. Rabbits grunt, but rarely in response to human interaction, and many other exotic pets have equally exotic signals, but, and here is the big question for you the reader, nearly all cases these expressions are either:

  • Difficult for humans to elicit
  • Difficult to read or unappealing
  • Inconsistent (applied both to fear/aggression and happiness such as tail-wagging in rats)

Wagging and purring are adaptations that have evolved from existing animal behaviors but have been consistently selected for because they ensure that the animal gets what it wants from its human hosts. Dogs and cats, having spent the most time of any animals as human companions have evolved the most noticeable behaviors that are easy for humans to see, understand, and act upon.

So what’s up?

Can you suggest other animals with such obvious behaviors? What about horses? They’ve spent a long time with humans. Is this connection causal, that is, is wagging so darn obvious because of the dog-human relationship? Certainly wolves and other wild dogs wag their tails, but, I understand this behavior is not expressed the same as it is in dogs. Is this true? Perhaps readers could add their thoughts or links in the comments.


  1. Dave K. said,

    October 20, 2011 at 15:20

    This is an interesting question. I often think about evolutionary stuff like this – why did this trait evolve in humans, animals, etc? I am not sure about the other animals behaviors you mention, but are also known to purr when in distress. Syl and I have experienced this first hand at the vet when one of our cats was freaked out and purring. The vet had a hard time listening to the heartbeat over the purrs. There is also scientific speculation out there that purring acts as a homing beacon from mother to kitten when the kittens are young and blind.

  2. RjZ said,

    October 20, 2011 at 16:55

    I figured my argument is significantly more supported for dogs than cats. We more readily understand the inherent behavior, it is more correlated to the desired behavior (they don’t wag when they’re unhappy), and it’s easy to ellicit.

    Cats may meet many of these qualities a bit more weakly, but then, cats are hardly as interested as dogs are in, well, being domesticated. They enjoy the food and the petting, but they seem to want to be more independant. It’s not for nothing that people describe difficult actvities as “herding cats”

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