What would cause two cats (living together) to lose weight (enough weight to notice)

When was their last visit to the Vet? Sounds like it is time for a professional visit. Could be be dozen different things. Have the Vet give them the once over.

It has been a few years for the male. The female, it has been 4, I think.

Could be worms or worst case it could be feline leukemia. But take them to a Vet.

Are they new to each other? Maybe they are playing hard and exercising more.

Could be parasites, change in diet habits, as @Judi said an increased in activity….the list goes on.

@Judi They have been together for six months. The female doesn’t get along well with other cats, and has finally started to warm up to him. The male is my boy and the female is my partner’s. My partner is taking care of both of them. The cats are inside together while my partner is at work, and are let outside when she gets home. This past month or so, my partner has been coming to stay with me on the weekends and the cats are left home at her apartment. Perhaps, they’re not liking the fact that they have to be with each other so much?

Also, they were exposed to a kitten who had round worms. We gave our two cats de-wormer. One round of it, though. Perhaps, they need another round? Although, like I said, they’re bowel movements are fine. Firm stools. Not gasey.

My partner told me last week that she thought that maybe her female had lost some weight. I was waiting on my passport to be renewed, and went I finally received a few days ago, I went to my partner’s (in Michigan). First thing that I noticed was how thin my boy looked. His “cheeks” were drawn in. He is losing his little kitty pounch (belly). Her female was thin to begin with, but now, she looks thin and sickly looking.

I am going to take him in to the vet. I’m wondering if I should have blood work done, or just have him tested for Feline Aids and Leukemia and receive his shots.

I would guess worms especially if they seem healthy otherwise and are eating normally. Take a sample of the stool into the vet and have it checked. That would be my first step before blood work or the expensive stuff.

He might be depressed because he misses you and she might be pissed because he’s there.

It could be anything from intestinal parasites to F.I.P., F.L.V. Take them to the vet asap.
Both of them manifesting the same symptoms together is major cause for concern.

If they’re both 13 yrs old, they are senior citizens and should definitely have bloodwork done to check for the variety of conditions to which elderly cats are prone which really benefit from early detection.