Why is my cat yowling?

[catmamaof2]

My cat has started yowling in the middle of the night all of a sudden and I can't for the life of me figure out why.
Background:
I've had him for just about 3 years now. He is pretty chatty and likes to have conversations with me, but his meow is very soft and quiet. At night he has always been practically silent.
Three weeks ago I adopted a senior cat. She has been very scared and rarely comes out of hiding. My boy cat desperately wants to be friends with her, but she is having none of it. Ever since I brought her home, Boy has been yowling as loud as he can in the middle of the night. It's obnoxious, and I'm sure my apartment neighbors can hear him. My only thought is that bringing home Girl has disrupted his routine but I don't know why that would lead to singing the song of his people in the midnight hours.
Any thoughts or advice on how to get him to cut it out?

[Mollycat]

Maybe he isn't happy about a new cat being imposed on him. Did you follow any kind of advice about introducing two cats successfully? Is the new cat in her own secure room so she can get used to her new home before being expected to make friends with the resident cat? It sounds like she is absolutely terrified trying to hide away, your boy is upset, you seem to be getting frustrated and angry ... can you talk us through a bit more detail so we can help unravel this?

[catmamaof2]

Yes, I followed all the advice to the letter. She has her own space away from him and I've been slowly introducing them to each other. We are at the point where she is allowed to roam freely throughout the house and she is getting more comfortable by the day.
My Boy seems to be totally fine and content during the day. There's been so fighting, no hissing or growling, he just seems politely interested in her. If she expresses discomfort at him getting close to her, he backs off. And then night comes around and he wanders the house yowling as loud as he can. I can't make sense of it.

[fjm]

I have known one cat who yowled for company - he belonged to a neighbour and he fell madly in love with my Rosie-cat. He would sit outside the cat flap and howl until she came out to play (or I meanly woke her up and persuaded her out through the flap for a bit of peace). Both were neutered long before they met - he just wanted her company. It sounds as if you may have a similar situation on your hands. If that is the case then it should improve if and when Girl gains enough confidence to interact with him, but in the meantime I would try keeping him as far as possible from her room at night and see if that helps.

[Mollycat]

Molly did that, fjm. She would yowl when I first adopted her and eventually I started shoving poor Boo off the bed to go to her. She continued after Boo stopped going to her and she started coming to sleep on the bed with us. I thought perhaps the hyperthyroidism treatment might end it, it certainly made a difference but didn't stop it completely. After 8 years I'm so used to it OH says I actually call out to her in my sleep.

Three weeks is quite quick for successful introductions, I'm sorry to be push this one but if the newcomer is still hiding all the time how are you so sure she's ok with your resident cat and not hiding from him? It's just that I've been through that, the resident being keen to make friends and the new one being a nervous wreck, in fact in our case it was RC who brought her out of her shell and not me, but it is more unusual.