This is Poppy, a Jack Russell / Cairn terrier cross and not much like the wolf her ancestors were only a few thousand years ago.
Sometimes her heritage shows through – when a hot air balloon floats across the Sussex skies above her, she’ll raise her hackles and growl. But generally Poppy resembles a wolf cub more than a wolf; she’s kept her floppy ears well into adulthood, begs for attention and will happily play for hours on end.
My cat Storm may look and act a bit more like his wild ancestors, but his grey and white fur looks frankly ridiculous when he tries to conceal himself during a hunting session. He’s yet to realise that the only way he’d be hidden from view would be if he lay on his back in the snow (which itself isn’t massively conducive to mousing). I wonder how he manages to catch anything at all. I also wonder why he’s so adorable and smooshy.
How did these wild beasts end up in our laps, what effects has it had on their bodies and why?
What’s interesting about domesticated mammals is not only how artificial selection has changed them, but how similar these changes are between quite distantly related species. From little mice to big shire horses, a number of traits tend to keep cropping up.
Were these features actively picked out during domestication thousands of years ago or are they by-products of selection? To find out, Russian scientist Dmitry Belyaev set up a bold experiment – to domesticate the undomesticated by selecting only for tameness without training. He chose the silver fox (vulpes vulpes), which had been bred for fur in captivity for about 60 years but without artificial selection they were barely distinguishable from wild animals.
Pups were tested for behaviour towards humans periodically and when pups reached sexual maturity they were classified into three groups:
– Class III foxes which run away or attempt to bite experimenters
– Class II which don’t mind being touched but show no sign of pleasure from it
– Class I foxes which wag their tails and whine happily when petted.
Only the friendliest foxes in each generation were allowed to breed and no other trait was selected for, with the caveat that inbreeding was to be avoided.
It didn’t take long for some astonishing results to show through. By the sixth generation they had to introduce Class IE for the “domesticated elite” who actively sought out human contact by scampering up, whimpering and licking them. At the start of the experiment, 90% of foxes were the hostile Class III. By just the twentieth generation, 20% of all foxes were IE.
The changes in innate behaviour of new pups were accompanied by some striking physiological changes, and amazingly they followed the same patterns as animals which were domesticated thousands of years ago. Curly tails? Check. Floppy ears? Check. Solid grey and white piebald patterning? Definitely check.
These changes started showing up after just ten generations, an eye-blink for evolution, so it was clear these alterations were not the result of random mutations alone. What’s more, if the changes were random, we would not see the same features popping up again and again. Something deeper is going on. By selecting for changes in neurohormonal systems (after all, what is behaviour but hormones and nerve signals?!), domestication has repercussions which reverberate through many aspects of development, especially those dependent on timing.
For example, eyes opened earlier but stress hormone responses were both delayed and muted. This gave some chilled-out foxes which had longer to get used to humans and enjoy their company before the natural fear response kicked in. Domestication also altered the foxes’ normally strict reproductive cycle.
Ever wondered why piebald pets almost always have coloured backs with white tummies rather than the reverse? It’s also a matter of timing. The cells which will produce dark pigment begin their life in a stretch of cells down the back and neck. During development these cells migrate and spread through the fetus down the flanks of the body and around the front of the head. They only have a set time to do this before they settle down and become pigment-producing melanocytes. In my cat Storm and some of these pups, this journey was delayed by a couple of days, so they don’t spread quite so far before settling down. The places the cells don’t reach (usually the belly, legs and tail-tips) stay white and unpigmented.
The reason why similar effects have been seen in so many domesticated animals is that patterns of development in mammalian embryos are very similar. The details may differ but the groundrules are the same, and this is what artificial selection for tameness affects. Knowing this, it’s not too surprising that putting this shared and interconnected system under similar selection pressures will give similar outcomes in different species.
Belyaev proved that the animals which share our lives were not necessarily chosen for their cute ears or pretty colour scheme – tameness alone was all that was needed to bring the beasts over the threshold of our homes as working animals, companions or future food. While these traits might have arisen spontaneously, it’s certainly not hard to believe early humans would have been really attracted to these exotic new features and worked to keep these traits until they became the fabulous friends we enjoy today.
Sources and Resources
Great description of the tame fox experiment and discussion of results: (here – web) or (here – PDF)
Colours of Domestication paper: here
Dmitry Belyaev’s paper: here
Animal evolution – Foxy friends: here