Intelligent and experienced cat owners invariably state that the domestic cat’s ‘belly flap’ aka ‘primordial pouch’ serves a dual purpose. This bit of floppy belly near the cat’s hind legs (a) allows the cat to run faster as it provides freedom of movement and (b) protects the belly.
I’d like to challenge this. Someone, online, many years ago stated that the loose skin on the belly served these purposes and it has been replayed, unthinkingly ever since by countless thousands of cat ‘experts’ without challenging it. I reinforced the theory once. I have changed my mind.
Best reference books
It is interesting to me that none of the highly regarded books on domestic cat anatomy and behavior refer to the ‘primordial pouch’ or ‘belly flap’. I have searched Dr Bradshaw’s Cat Sense, Turner’s The Domestic Cat and The Welfare of Cats edited by Irene Rochlitz and nada, nothing. I have checked Dr Morris’s Cat World A Feline Encyclopedia and Jackson Galaxy’s Total Cat Mojo; likewise nada, nothing.
The best, popular book on cat anatomy is Dr Bruce Fogle’s The Encyclopedia of the Cat and there is no mention of the primordial pouch in the index or in the book proper.
One knowledgeable person on Quora.com said that the primordial pouch was so well-known that it is referred to in the breed standard of the Bengal cat. There is no reference to this anatomical feature in the CFA Bengal cat breed standard. The photos of two Bengal cats on this page show no sign of these feature. The Egyptian Mau is meant to be famous for the belly flap but the picture on this page of a show cat shows no belly flap.
I have real doubts about the theory behind the ‘primordial pouch’ which isn’t a pouch. It’s simply some loose skin with, sometimes, added fur which is longer than normal. My cat has a very pronounced belly flap but he won’t let me photograph it!
Vestigial reason for existence more likely
Anyway, what’s it’s purpose? Does it have to have a purpose? It is more obvious in older cats. That may give a clue as to why it exists. It may be that this is simply belly fat and a slack belly in older cats. In younger, fit cats it is certainly a part of the anatomy as people state but it does not need to serve a particular modern day purpose.
There may be a vestigial reason for its existence as in the cat’s dewclaw. In the domestic cat the dewclaw barely serves a purpose. I am not even sure what it’s original purpose was. Perhaps it aided capture of prey in the early African wildcats (the wild ancestor of the domestic cat).
We see belly flaps or slightly slack and enlarged bellies in the large wild cats such as the captive tiger and lion. This applies particularly to older cats. Does it serve a purpose? I have my doubts. In human terms, males develop belly fat as part of growing old but it serves no purpose.
How does it work?
Let’s think about it. How does this bit of belly make the cat more flexible and run faster? I don’t see it myself. The skin is naturally elastic. The cat does not need this extra bit of skin to make her more flexible. As for protection that does not sound correct to me either. How is more fat and skin on a small area of the cat’s belly going to protect the cat? It may protect to a small extent that small area of the belly. However, the belly is already protected by virtue of its position under the cat.
For the lion, the mane provides protection. This looks more obvious. I can’t see any obvious way that the primordial pouch protects the domestic cat. I’d suggest, as stated, that it is a vestigial part of the cat’s anatomy no longer serving a purpose. I should be noted too that not all domestic cats have it.