If you've ever wondered why some dogs have floppy ears, you're in pretty good company. After all, Charles Darwin was once curious about the very same thing.
In fact, this week marks the 150th anniversary of the publication of Darwin's The Variation of Animals and Plants under Domestication, in which the great scientist ruminated (at some length) upon a mysterious contradiction observed in nature.
Specifically, why do many domesticated animals (including dogs) have floppy ears, while others have ears that are upright and erect?
Darwin had first remarked upon the contrast in On The Origin Of Species, in which he wrote: "Not a single domestic animal can be named which has not in some country drooping ears," acknowledging the hypothesis that "the drooping is due to the disuse of the muscles of the ear, from the animals being seldom alarmed by danger, seems probable".
Almost a decade later, Darwin listed the extent of species in which this altered trait could be seen.
"Cats in China, horses in parts of Russia, sheep in Italy and elsewhere, the guinea-pig in Germany, goats and cattle in India, rabbits, pigs and dogs in all long-civilised countries," he wrote.
"The incapacity to erect the ears is certainly in some manner the result of domestication."
As NPR's Skunk Bear points out in the excellent video above, Darwin was definitely onto something with his observations.
In fact, domesticated or tame animals don't just have floppier ears than their wild counterparts, they also tend to have shorter snouts, and their fur is often paler, or bears coloured patches. Why?
Well, one explanation for this so-called 'domestication syndrome' lies in what are known as neural crest cells, first identified by Swiss anatomist Wilhelm His Sr. in 1868, which coincidentally was the same year Darwin published his almost 900-page tome on domestication.
The current thinking – per a 2014 study unifying neural crest cells and domestication syndrome – is that humans, over countless centuries of domesticating animals, have affected the level and quality of neural crest cells in animals.
We now know that neural crest cells help determine or shape a number of features of animal physiology, including facial, skeletal and connective tissues, teeth, pigmentation, and, importantly, adrenal glands (affecting animals' 'fight or flight' response).
Another thing they regulate is cartilage, which, you could understand, is the primary physiological component separating droopy ears from their upright, erect counterparts.
"The [paper authors] argue that the domestication process selects for pre-existing variants in a number of genes that affect neural crest development," Don Newgreen and Jeffrey Craig, two researchers from Murdoch Childrens Research Institute in Australia, who weren't involved with the 2014 study, explained at The Conversation.
"This causes a modest reduction in neural crest cell number or activity. This in turn affects the broad range of structures derived from the neural crest, giving rise to domestication syndrome."
So that, in a nutshell, could be why all these floppy-eared dogs are flapping and flopping their floppy ears about the place.
Basically, eons of domestication have altered their makeup at the cellular level… although it's worth noting that some scientists at least aren't totally on board with the hypothesis.
For more on the history, details, and science, be sure to check out Skunk Bear's awesome video explainer, and just remember: regardless of whether dogs have firm or floppy ears, be excellent to them.