Dogs and cats might both be furry four-legged companions, but inside they are surprisingly different animals. Understanding these differences helps explain why they need different foods, medications, and care.
Dogs and cats share a lot on the outside. But anatomically they diverged significantly in their evolution, and those differences have real practical implications for their health and care.
Dogs have 42 permanent teeth and are omnivores -- their teeth include molars for grinding plant material alongside carnivore-style canines. Cats have 30 permanent teeth and are obligate carnivores -- every tooth is designed to shear meat. Cats have no flat grinding molars at all. This is why cats cannot survive on a vegetarian diet.
Both have four-chambered hearts. Cat hearts beat faster (140-220 bpm). Cats are prone to hypertrophic cardiomyopathy (thickened heart wall). Dogs are more prone to dilated cardiomyopathy and valvular disease.
Cats have a significantly reduced ability to metabolize many drugs and toxins because their liver lacks certain enzymes (particularly glucuronyl transferase). This is why acetaminophen (Tylenol) is deadly to cats -- even one tablet can be fatal. Aspirin must be dosed very differently. Many essential oils safe for dogs are toxic to cats. Permethrin in dog flea treatments is highly toxic to cats.
Rule of thumb: Never give a cat any medication designed for dogs or humans without explicit veterinary guidance.
Dogs can digest carbohydrates reasonably well and produce amylase in saliva. Cats have a shorter digestive tract optimized for protein and fat, produce very little amylase, and are poorly equipped for high-carbohydrate diets.
The single most important thing to remember: cats are not small dogs. Treatments, medications, foods, and care protocols that work for dogs can be dangerous -- sometimes fatal -- for cats.