I’ll answer the question in reference to domestic cats, if you don’t mind. The question was asked on Quora.com under ‘domestic cats’. The answer can take many different forms. These are my instant thoughts.
There are two stages in the evolution of the domestic cat (1) the wild cat phase when the North African wildcat evolved and (2) the 10,000 years of evolution of the domesticated wildcat.
The obvious thing that you could argue is a useless behaviour in domestic cats is hunting. Domestic cats don’t need to hunt for obvious reasons. They’ve become domesticated. They are cared for. That particular skill, in a strict sense, and on the basis that all homes where there are domestic cats are good, is redundant. In fact it can be an annoying feline trait. Who likes to chase mice around the home or clean up bits of mice after your cat has eaten the rest?
However, as you can expect, it is not as simple as that. Firstly, if you take away the domestic cat’s hunting skills you take away a huge part of the character of the domestic cat. You would no longer be living with a domestic cat but with another animal that looked like a domestic cat but was not quite what he or she looks like. Therefore, there is a strong argument which says that animal behaviours that are left over from evolution, even though they are no longer needed, are still useful. They are useful because they define the animal’s character and people adopt domestic cat both for their appearance and their character. Take playing with a cat. It is always based on hunting. Without their hunting skills there would be no playing with domestic cats as we know it.
Also there is the obvious problem of the many hundreds of thousands of bad households in terms of cat ownership. These cats might need their hunting skills to stay alive when neglected and abandoned.
You also have to ask the question in the title from the standpoint of the animal and not the person because you get a completely different answer. Obviously from the standpoint of the cat every behavioral trait is useful. None are useless because they make the cat what he/she is.
But from the standpoint of the human cat ‘owner’ certain behavioral traits are worse than useless. There are a lot of examples such as spraying urine to mark territory. For the domestic cat owner that is not a great trait. What about scratching the furniture? The cat does this for good reasons such as stretching, sharpening claws and marking territory. These are not really useful for the owner or worse. It leads to declawing in many instances. But if you try and take away cat scratching by removing the claws you take away an important part of the cat and you cause a lot of pain and discomfort.
My conclusion is that no feline behavior has become useless through evolution because every form of behavior, whether you like it or not makes the cat what he or she is.
I’m sure the same applies to all companion animals. There may be some exceptions. I guess the exceptions prove the rule. What about the bark of the domestic dog? Dogs need to bark to communicate. Perhaps the dog’s bark is so loud because it is needed to communicate over a long distance. The loud bark is a useless animal behaviour left over from evolution and I would argue it is unwanted in 99% of households. However, even that example is not 100% true because in many parts of the world domestic dogs are used as guard dogs as they have been for centuries. Under these circumstances a loud bark is essential.
I think that the basic rule that I stated at the beginning namely that even though a characteristic of a companion animal is no longer useful in a practical sense it cannot be described as useless because it is part of the animal’s character and behavioural traits.
The question in the title is open-ended and could apply to livestock but I don’t want to discuss livestock because I don’t know much about the topic.