This is the #1 most disgusting thing dogs do. Every dog owner knows this, and I’m sure we all wonder why. There are a ton of theories and reasons at play here. When puppies are born, mother dogs clean their puppies and den to keep it clean and scent free so as to not attract predators, so they will often eat her puppies’ poop for this reason. Also, some think that since back before they were domesticated, dogs had to hunt and scavenge for any and all food, so eating poop might just be a part of their history of eating other animals’ poop to survive. Sometimes bored or lonely dogs play with and eat poop, especially if it gets their owner’s attention. Even bad attention is still attention!
Always make sure that if your dog is eating poop frequently that it’s not due to a lack of nutrition and not getting enough nutrients,or that they have parasites or a pancreas issue. Taking your dog to the vet is an important first step into stopping this behaviour and making sure it’s not borne of illness.
This behavior is such a solid part of dog-culture, even people who have never owned a dog know they do it. This may seem weird and gross to us, but it’s no mystery to science! Dogs’ noses are somewhere between 10,000 to 100,000 more acute than ours, and on top of that, they have a secondary olfactory system called Jacobson’s Organ, which has nerves that communicate straight to the brain, and specializes in chemical communication. This means that the smell of the other dog’s feces doesn’t overpower their incredible sense of smell, but that sniffing each others' butts actually helps dogs learn about each others' emotional state, sex, and diet. This is because the anal sacs that reside there communicate the dogs’ genetics, and the state of their immune system through secretions amines and acids. Basically, they learn a bunch from sniffing butts.
I’m sure every dog owner can relate to the story of walking one’s dog and recoiling in horror as they decide to roll in and rub their entire bodies in something awful that smells incredibly bad. People have come up with a plethora of theories on why they might do this, however, evolutionarily speaking the explanation that makes the most sense is that the dog is attempting to mask their own smell. It’s theorized that this behaviour is left over from when dogs still hunted in packs like wolves, and whatever prey they were stalking was much less likely to be able to detect the wolves if they smelled like their own droppings, or other smelly gross or dead things commonly found in the wild. A rabbit or deer is much less likely to spook at the smell of that than at the smell of the wolf itself.
Speaking of weird dog behaviours around feet, some dog just love licking them. Theories on why this is varies from the dog trying to get your love, attention, and approval, to them show subservience, to them recognizing who is family. The scientific reasoning, however is actually quite interesting. Not unlike dogs, humans also have sweat glands in our feet. Since dogs have a highly acute sense of smell and taste they are able to gain a lot of complex information which helps them recognize you and your other family members.