Sudden dislike advice

Hi, I have a male who turned a year at the end of last month and a female who is now around 4.5 months. When I first got her, I did the slow introduction thing and everything was fine up until a few weeks ago and now my male growls and hisses at her whenever he comes back in.

They used to play together and seemed to be getting along. He has been neutered and I got him checked out for any health issues (just in case) but he is absolutely fine. The only thing I can think of is that she is starting to mature now so maybe her scent has changed in that way?

She will be getting spayed but my vet said they won't do it until she is at least five months (?) and she also needs to weigh a bit more (she was the runt of the litter).

Does anybody have any ideas or suggestions as to what might be going on here? They used to get along really well and this change seems like it has come out of the blue.

There might be something going on with the female. Some cats become more aggressive towards another who might have health issues. 

They were both recently vet checked and no issues came up in either of them.

A rather odd situation ... It could be physiological changes in one or both, or, something is happening outside the home to upset your male cat possibly? Does he wander and interact with other cats, or stays close to home? The other possibility is that if the vet visits were recent, there could be some smell that is lingering on one or both of them from the clinic that the male cat is picking up on. What you can do is take a dry wash cloth and gently rub this along the cheek area of the female cat, and then take this and rub your other cat all over. This is known as scent transfer of pheromones and is usually effective at correcting situations having to do with vet visits or when you encounter this kind of behavior between once friendly cats.

Thank you for the suggestion, I will try that. He has been closer to home lately as we have been having something of a heatwave (I'm in the UK) with humidity not dropping at night either. There are a number of other cats in the area also so it could be likely that there are some that are intact and there are at least 2 I know of who sometimes hang around what would be his territory.

Would a plug-in diffuser help at all? I have never used them before, so it would be good to know if anybody has used them (hopefully with success).

I should also add that he does let her groom him, but growls more when she trys to play with him. He has shown some signs of wanting to play and will initiate it but then growls/hisses at her again only a short time after.

They do also sleep together but not like they used to, although she has started sleeping in her own spots now, whereas before it was either with him or with me.

That was one of the reasons I thought it could be where she is getting more mature, because apart from the heatwave, nothing else here has changed, unless it is something outside, as suggested.

You're welcome. I hope it can work for your kitties. I have used feliway diffusers in the past, but don't use them much. Certainly worth a try in your situation.

As heat can have an impact on humans, so too can it impact our furbabies. It could be a combination of things. Now that your male is neutered, you may be seeing a change in his personality? As your female is maturing, you will see a change in hers, and then again possibly when she is spayed? All very possible contributors. I think the fact that he lets her groom him, they sleep together and are able to play together at times, is a very good sign that they still accept each other but that this may be a passing phase they are going through as the result of a change in dynmaics as the result of the changes that have and are taking place in their bodies.

What might also help you to figure this out, is to start a journal and make some notes of their interactions from day to day. Its amazing what we can learn when we put it in writing which can help to provide some clues as to what might be going on when you see patterns of things in relation to different events. Include things like time of day when you introduce things, like doing the scent transfer, installing feliway diffusers and where, food served and when, and the responses your cats exhibit as the result.