One cats suddenly aggresive towards other cat

Hi forumpeople.

We have two cats, a 6 year old and a 3 year old, both females.
Usually they are inside all the time and has never strayed away
from our balcony. But a day ago, the older cat ran away - she is the one that always sleeps with us and is always in the same room as us.

Now she has returned (after a day) and the younger cat sniffs her behind all the time and hisses at the older. They have always ignored eachother until now. The older cat just walks away, still ignoring the younger.

We wonder why the younger cat suddenly shows aggresive behavior towards the older?

We thought about the older cat might being pregnant after her short trip to the outside. We feed the cats pills as birthcontrol and may have forgotten the last pill..:-/ Could that explain the sniffing and hissing of the younger cat?

Thanks in advance

Moedeus and girlfriend

PS Not being a native speaker of english, I still hope I was clear about my question.

PPS If anyone wonder - we are from Denmark

Hello and welcome! I dont know if I have much advice.. but logically speaking it could be possible the older cat got in a fight with another cat, or even a dog or something, just enough to get their scent on him and maybe that's why the younger cat is upset. It would seem odd that it would be able to know the older cat got pregnant. lol. Cats have a great sense of smell so the older cat could have gotten into something that they younger cat doesnt approve of.

btw please spay and neuter your pets even if they are inside cats! It will save them some grief and discomfort in the future!

Thanks for the reply and welcome - didnt think of that one with the fight
Another funny thing - the older cat now examines every inch of the house like she has never seen it before..strange kitty

What do you mean with "spay and neuter your pets"?

Sometimes interactions with other pets, fights or whatever else will not show visible marks or scents to humans but cats can sense them a mile away!

Yes cats are strange.. if she was away for a whole day and has never been outside you should consider yourself lucky that she came back!

Here are links on Spaying!

http://www.thecatsite.com/care/spay.html

http://www.thecatsite.com/forums/sho...&threadid=5149

Spaying is desexing a female and neutering is desexing a male. Desexing your cats will significantly increase their life-spans and will reduce behavior problems. Also, spaying will ensure that no unwanted kittens are born. There are millions of kittens and cats put to death each year because of pets becoming pregnant.

Regarding the odd behavior of your cats. The one who got outside brought in many new smells. In addition, as Sicy said, it is possible that your cat is pregnant and smells different because of the hormonal changes. Try putting a dab of vanilla extract under each cats' chin and at the base of each cat's tail (on their backs). This will drown-out the outside smells and will make both cats smell the same.