Dogs as a species are intelligent, adaptable, and opportunistic. When we domesticated the dog all those millennia ago we changed nature's rules - and dogs quickly learned new skills in order to thrive in a human-dominated environment.
I would imagine that the feral dogs ExMilExPat encountered had learned that well-developed hunting skills and pack behavior were necessary in order to survive in those harsh conditions. Dogs are first and foremost adaptable.
By contrast, I spent some time observing ferals in a very different environment (rural China, and Lantau Island in HK), one in which food was more plentiful, whose behavior towards each other and towards humans was quite different.
A pack of semi-feral dogs on Lantau - most of whom were offspring of abandoned domestic dogs, with some of those once-pet dogs still living in the pack, lived on the outskirts of our neighborhood. Their primary food source was scavenging; food was fairly accessible and the best pickings came from wherever humans lived. The semi-ferals quickly adopted more domestic dog-like behavior when an opportunity arose to live with, or in close proximity to, humans. In the pack seemed far more fluid, only loosely organized, and there was less of the working co-ordination to be seen. Individuals begging for hand-outs quickly replaced pack scavenging behavior when opportunities arose. These dogs had adapted behaviors that were most likely to succeed in their particular environment.
In fact, I'd call these dogs semi-domesticated rather than semi-feral.
I also had regular encounters with another pack who to my layman's eye seemed to indeed be wild(er) dogs, or at least those who had been many more generations removed from domesticated strays. Perhaps because these dogs found scavenging around human habitation less consistently rewarding hunting small game provide more of their food. The wilder pack remained warier of interaction with people, and would not initiate direct contact with humans, mostly retreating when humans came close. Their basic behavior towards humans remained appeasement rather than conflict. These dogs were still quite opportunistic scavengers - given the choice between game and scavenged food, they tended to go for the easier option. Again, they had adapted to the resources available in their environment.
Fascinating critters.
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Niranjan, the 'spiritless' behavior you see is certainly anything but. It's not lack of spirit - it's good training. A 'spirited' dog - for which read: untrained and out of control, will not thrive in Swiss society. We demand obedience to the owner and calm behavior - these intelligent and adaptable dogs quickly learned that displaying those qualities earns rewards, while the opposite has negative consequences..
Yes, it is still survival of the fittest, but in this environment we have simply redefined fitness. The 'fittest' dog is no longer the best hunter, but rather the one who has trained his humans to open the biscuit cupboard.
Take those same 'spiritless' dogs to an open field where free running and play is allowed, or better yet, take them to an Agility or Flyball or other dog sport course - you'll see 'spirit' in abundance.
As others have said, the presence or lack of reproductive organs has less influence on behavior in a well-trained dog. That a dog becomes lethargic and fat after spaying/neutering is largely a myth fueled by owner's behaviors. Most pet dogs are neutered after the frenetic puppy and teenage stage is past; of course a young adult is calmer than a pup, and an older adult is calmer still. That's largely maturity, one will see it with or without reproductive organs. That a dog is allowed to gain weight is solely the fault of the owner. Stop feeding too much, get off the couch and give the dog appropriate exercise!
Neutering/spaying is indeed 'unnatural', but so is the human-dominated environment in which most dogs - pets, working dogs, and ferals - live. And that environment simply can not sustain the ever increasing numbers of dogs born - hence the need to limit reproduction.
In many places where large populations of strays, semi-feral, and feral dogs live in close proximity to humans, welfare groups often practice 'catch, castrate and release' as one of the preferred management options. Catch, castrate and release is a far more humane option to limit dog populations than the usual practices - allowing the dogs to starve to death, to die in agony from poisoned bait, or mass culls where the dogs are clubbed to death. There is nothing 'natural' about the way many feral and semi-feral dogs die.
Far better in my eyes to prevent the numbers from spiraling out of control in the first place.
(For what it's worth - in my wholly anecdotal observations of a limited sample size... I've never seen a spayed/neutered feral become lazy and lethargic. Living by their wits, as they do, they tend to stay lean and keen.)
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More or less off topic, but for an interesting fictional look at the human/wolf interaction (with liberal doses of philosophy, psychology, and fast-paced entertainment) I highly recommend the novel 'Wolf Totem' by Jiang Rong.