Asked by: Jonny Thompson, Glasgow
Big cats mostly hunt alone and rely on a short burst of speed to catch their prey, and swipes from their claws to bring it down. Wild dogs hunt differently, using teamwork to chase prey over long distances until it collapses from exhaustion. A larger body is a liability in this sort of endurance contest – it requires more energy to haul around and doesn’t improve the chances of a kill.
We colloquially group cats and dogs together because we keep domestic versions of both of them as pets, but they are different animals that have evolved to fit different niches. Cats and dogs are both in the Carnivora order, but their last common ancestor lived about 42 million years ago.
Since then they have diverged into feliforms (cats, hyenas and mongooses) and caniforms – a more diverse group that includes raccoons and walruses, as well as dogs. Bears are caniforms too, and are more closely related to dogs than cats are. So you could argue that big dogs do exist, and the equivalent of the tiger in the dog world is a grizzly bear!