I know that shedding is different across breeds, but taking an average, which pet sheds more hair?
As already stated, it really depends on the breed of the cat or dog. Different breeds have different characteristics (for example, longer hair or a double coat) that will make their shedding amounts differ:
Some dogs do shed a lot more than others, especially those that have a double coat. Imagine wearing two of your winter coats in the summer! Your dog cannot just take off its coat(s) like we can, so it sheds them. Dogs lose their topcoat in larger quantities and more often then the coat underneath (the undercoat).
There is also a great article on the science of why cats, dogs and rabbits shed. It discusses the skin composition including layers (epidermis, ceramides, dermis, basal cell, hypodermis) and function; the hair itself including composition, structure, layers, follicle pattern, and different hair growth phases, and hair types (including guard hairs, under-hairs and tactile hairs).
Coat types for dogs, cats and rabbits are also discussed:
Based on hair length, dog breeds can be divided into three types: hairless, short-haired and long-haired breeds. Short hair is dominant to long hair. The length of the hair is variable within individuals and across the breeds.
Based on hair length, cat breeds can be divided into three groups: hairless, short-haired and long-haired. Just like in dogs, hair length in cats is managed by the FGF5 gene. Variation of coat texture is often determined by color of the individual cat.
Finally, it discusses factors influencing shedding, which includes:
- Temperature (seasonal)
- Lifestyle (indoor vs. outdoor animals)
- Nutrition (hair loss due to poor nutrition)
- Gender (spayed/neutered dogs can have more pronounced undercoats, so shedding can be more noticeable)
- Hormonal status (determine hair growth phases)
Topic 21 Dogs that shed the most: Akita, Alaskan Malamute, American Eskimo, Belgian Sheepdog, Belgian Tervuren, Bernese Mountain Dog, Boston Terrier, Chow Chow, Finnish Lapphund, Goldador, Great Pyrenees, Keeshond, Lakeland Terrier, Leonberger, Maltese Shih Tzu, Newfoundland, Pekingese, Pomeranian, Samoyed, Shetland Sheepdog, and the Sussex Spaniel.
Top 12 Cats that shed the most fur: American Bobtail, American Curl, Chartreux, Cymric, Maine Coon, Nebelung, Norwegian Forest, Ragamuffin, Ragdoll, Russian Blue, Selkirk Rex, and Siberian.
As you said, it really depends on the breed how much hair an animal sheds. It also depends on the size of the animal (bigger animals have more hair so the average amount they shed is higher, obviously)
I can imagine it can also depend on how you feed your animal and how healthy it is. I know it is like that with horses.
And last but not least it also depends on the animal itself. You could have two equally sized cats from the same breed and one could shed, say, twice as much hair as the other one.
So all in all I think you cannot say that one species sheds more or less hair than another one. You also have to consider if the animal can go outside, because then it will shed hair outside too and you have less of it in your home.
Of course, long haired breeds shed more hair than short hair breeds, but I think that's obvious ;-)