If you've ever sat down and watched a nature documentary, you've probably heard the term 'big cat' bandied about. But this is not a defined scientific term and there is often some confusion with what the big cat species actually are. Generally speaking, people asked this question usually reply with these four species: lions, tigers, jaguars and leopards. All of these species belong to the genus Panthera (which are renowned for their ability of roaring) and make up the largest, heaviest families of cat.
Other people may give the same list as above, but also include pumas (cougars), cheetahs and the Eurasian lynx. This is fine as well since the term is ambiguous by nature and, as I've already mentioned, isn't scientifically defined. But what is incorrect is the inclusion of the panther. Obviously panthers are quite big and, upon spotting one, you would see a big, black cat. So why aren't they on the list? The answer is simple – panthers are not actually a species of cat.
Panthers are actually just leopards or jaguars (depending on whether they live in Asia or the Americas) that have an all-black coat. Their confusing and famous colouring—which is called melanism—is simply the result of their ‘dark-coding’ allele being defective. The allele is overactive and they produce so much of the black protein melanin in their fur that it masks the cat’s normal phenotypic colouring.
This is evident upon close inspection of a panther’s fur, where you will be able to see that it is not completely black. If you look very closely, you will be able to make out the normal colouring of a leopard or jaguar (although it will be very faint). Some have even gone as far as terming this phenomenon ‘ghost striping’!
Like all mutations, this defective allele was a fluke of nature. This means that it's rarer in jaguar and leopard populations than the normal protein (although the mutation is dominant in jaguars, which is fairly unusual), so most cats simply have their normal colouring. This is true as a basic rule, although research has shown that the all-black coat is actually selected for in certain environmental conditions so the panther phenotype is slightly more prevalent there than it is normally (although it never becomes more common than the normal phenotype). The main example of this is in areas that are very dark with densely packed foliage, where the panther’s darker colouring gives it an advantage in camouflage over the rest of its kin.
Obviously their uniform colour means that panthers are visually distinctive from their normally coloured family members, but this is the only difference between them. All other aspects of panthers, such as their size, diet and behaviour are exactly the same as normal! This knowledge—along with the fact that panthers are genetically viable with their normally coloured kin (so can breed with them successfully)—means that they aren't their own distinct species!