Paedomorphism is the retention of juvenile features or characteristics into adulthood, typically caused by neoteny, the delayed development of an animal. The animal goes through puberty to reach sexual maturity, but retains certain juvenile physical and/or psychological characteristics.
If the genes that determine maturity present mutations, then the animal will mature to, say, one year old, and that will be considered normal adult development for that line. The creation of all the dog breeds we see today is thanks to mutations to and selection for these genes.
Mutation and selection never involve just one trait, but many others as well, desirable or not. The most prominent example of this can be seen in Belyaev’s foxes. In the ’50s, Dr. Belyaev, a Russian physiologist and geneticist, set out to see how long it would take him to completely domesticate a species ‘from scratch’. Since the beautiful dark charcoal fur of the silver fox (a melanistic version of the more well-known red fox) was so prized in Russia, Dr. Belyaev and his team had very easy access to large numbers of these animals. But docility towards humans wasn’t the only trait that distinguished these foxes from their undomesticated counterparts. After several generations, the foxes started to present dog-like physical features such as a curled (spitz) tail, pendant, floppy ears, a smaller skull and ‘unnatural’ coat colours and patterns such as light silver, chocolate and piebald (white with patches of colour).
They also barked and whined for attention, a behaviour limited to puppyhood in wild canines, contrary to our domestic dogs, who retain the ability to bark for the entirety of their lives (a trait humans appreciated when they realized that these animals could alert them to an intruder or other danger). The foxes’ dentition also changed, just like with our domestic dogs, they had the same number of teeth as their wild ancestors, but with a smaller jaw, some of the teeth (the second and third premolars) have to sit on an angle with respect to the others. Possibly the most curious trait that showed up in these domesticated foxes was a second yearly heat cycle, just like in dogs (with exception of the Basenji).
Since the selection process (both in dogs and foxes) focused on docility and trainability, and certainly not aesthetics, what does coat colour have to do with these traits? It turns out that genes that code for melanin are also involved in other processes, such as the metabolism of neurotransmitters, namely adrenaline and dopamine; the first being a chemical released in response to stress, responsible for the fight or flight response, and the second being the major neurotransmitter involved in reward-motivated behaviours. A mutation somewhere in these genes caused a decreased adrenergic response, increased sensitivity to or production of dopamine as well as an alteration of melanin production, resulting in the variegated coat patterns seen in both domestic dogs and foxes.
Even though dogs and wolves belong to the same species, it is paedomorphism that makes dogs so domesticable and wolves not. In the domestic dog, limbic system development is diminished, resulting in a 20% decrease in size in dogs compared to their lupine relatives. A smaller limbic system means an attenuation of aggressive and fearful responses. Docility and the absence of fear towards humans are traits that play a large role in the mutual success of the human-dog relationship.
Almost all the traits we find desirable in our pet dogs are essentially juvenile wolf characteristics: being friendly, extroverted, dependent animals is what makes dogs so easy to form a bond with. In a way, dogs can be considered adolescent wolves in their behaviour (barking, whining, dependence on parental figures for essential resources and affection… ) as well as some physical features, such as pendant or semi-erect ears and a shorter, broader snout, as seen in many dog breeds.
Coppinger and Coppinger were the first to attempt to group dog breeds based on their behavioural maturity with respect to wolf development, hypothesizing that certain facial features that liken adult dogs to wolf pups of specific ages were telling of their maturity level. The younger the pup the dog resembles, the more paedomorphic he is. And while the above statement is a little too simplistic, it is true that very many breeds do resemble (as far as snout shape and ear carriage) a wolf pup at their same level of developmental maturity. For instance, mountain dogs or mastiffs, such as the Rottweiler, with a relatively short, broad snout and pendant ears have a similar profile to that of a two to three week old wolf pup. They are among the more playful and jovial breeds, considered “perennial puppies”. Not too far off, we have many retrievers, such as the Golden Retriever and the Labrador Retriever, along with many hounds, such as the Beagle. The slightly finer snout and pendant ears would seem to suggest the resemblance to a slightly older wolf pup; these dogs are typically more orally fixated than other breeds and enjoy chewing and playing with objects, in fact at three to four weeks of age is when wolf pups start to venture out of the den and explore. Progressing further, we have the Collies, with their semi-erect ears and more gracile snout, they most resemble wolf pups of about three to four weeks of age, with their strong desire to chase. Finally, we have many Nordic breeds, shepherds and sighthounds, all possessing decidedly more lupine features: erect ears and long, pointed snout. They most resemble wolf pups between six and ten weeks of age, with a gradually increasing prey drive.
Despite a definite correlation between appearance and maturity, the association isn’t perfect. There are many breeds that have traits not seen in wolf pups, due to the growing popularity of selecting for physical traits rather than temperamental ones and a predilection for hypertypes, in other words animals possessing exaggerated physical features that are not only useless, but typically harmful (short and/or deviated limbs, bulging eyes, brachycephaly, etc… ), the above proposed model is becoming increasingly unreliable, and attempting to deduce an individual’s temperament by analyzing snout allometry (the relationship between size and shape) and ear carriage is a guess at best.
Although our domestic dogs are a far cry from their wolf relatives, certain wild adult behaviours still linger in their ethogram, but with pieces missing or without any precise end. The most evident example of this can be seen in herding dogs. These animals go through all the initial phases of hunting: they stalk and chase their prey, some even bite at their heels, but there is no killing or consumption of the livestock they are herding. While this motor pattern is very useful for the humans who work with these dogs, it is absolutely useless to the dogs themselves; it is simply self-remunerative: the reward lies in the behaviour itself (herding), rather than reaching a specific end (satiating hunger).
Learning and play behaviours are juvenile traits that diminish as an individual ages, therefore it has been hypothesized that even humans are paedomorphs, retaining an elevated ability to learn past adolescence and well into adulthood. And not only humans, it has been hypothesized that all domestic animals are paedomorphic versions of their wild ancestors. It stands to reason, then, that selection for juvenile traits (friendliness, dependence on parental figures… ) would yield an individual with an increased ability to learn even as an adult, a trait humans would definitely search for in the animals that work alongside them.