Boar introduction: initial success followed by conflict

[AviN4]

I have a senior (7+ year old) male guinea pig, Moon, who has been alone for several years after the death of his male friend Midnight. My wife and I decided to adopt a new 3-year old male friend for him, Paulie. We adopted Paulie on Jan 4, 2025, and initially placed Paulie to a separate cage, near Moon. There were no obvious signs of aggression, though Paulie seemed very timid and somewhat afraid of Moon. Given Paulie's timid behavior, as well as Moon's history of dominance over his previous friend, we expected Moon to be the dominant of the two.

On Jan 8, 2025, we attempted an introduction in a neutral area, mostly following the introductions page on Guinea Lynx. To our surprise, as soon as Paulie greeted Moon on the floor, he established his dominance, repeatedly mounting him. Moon clearly wasn't happy about it, but made only a half-hearted effort to escape from the mounting. During the introduction, I don't recall aggressive behavior such as teeth chattering, nose offs, etc. (In retrospect, Paulie's dominance makes more sense given how fast and strong he is, which is evident each time I need to remove him from the cage.)

After about an hour, we decided that they had both undergone enough stress, and decided to prepare Moon's cage for the both of them. We removed the bedding, washed the cage floor with vinegar, and put in new bedding. For the next few days, Moon and Paulie were best friends. Paulie would sometimes mount Moon, who wasn't happy about that, but would either accept it or run away without responding aggressively. But more recently, perhaps starting Jan 11, 2025, we noticed some conflict. It seems that Moon had reduced his tolerance for mounting. Then on Jan 12, 2025, we noticed that Paulie was blocking Moon from accessing food, responding with teeth chattering and nose offs. We added additional food on the other side of the cage, but Moon still seems to be under stress, staying in his hideout most of the time, while Paulie eats from both food areas. We've also noticed continued teeth chattering and sometimes nose-offs, and I did witness at least one scuffle. With that said, I don't see evidence of physical injuries. Other times, they seem to get along okay, sometimes sharing the same hideout in apparent peace.

In addition to adding more food, we've tried some other environmental changes, though it's not clear which (if any) have helped:

• Rearranging hideouts. This has been mixed. Some changes made things better, some worse. Right now they have 3 hideouts. They normally are in separate hideouts, though sometimes they stay in the same hideout peacefully.
• Cleaning the cage again in an effort to reduce scent.
• Applying some imitation vanilla to their noses and behinds.
• Cutting their nails to reduce risk of physical injuries.

Because of the food blocking and potential stress, I've also checking Moon's weight to ensure he's eating enough. He's lost some weight with age, and has mostly varied between 1100 to 1200 grams over the past few months. On Dec 29 (before the adoption), he was on the low side, at 1103 grams. Then the day after the introduction (Jan 9) he was 1071 grams, and today (Jan 13) he was 1052 grams. I plan to continue weighing him daily to see if the trend continues.

At this point, I'm not sure how to proceed. Some possibilities that come to mind are:

• Attempt the introduction again, perhaps including a buddy bath and a 2+ hours of introduction.
• Separate them to help them cool down before the introduction, and also to help Moon resume normal eating behavior.
• Just wait it out on the hope that they sort it out.
• Give up and either return Paulie to the shelter or keep them in separate cages indefinitely. We don't want either of these, but if Moon is worse off with Paulie as a cage mate, then we'd prefer to let Moon live alone.

We'd be grateful for any advice.

[AviN4]

One follow-up: The boars are in a Large Cage (10.5 square feet) from Guinea Pig Cage Company.

[Lynx]

It sounds like you are doing what you can to make sure each is safe and to enable them to work it out. I haven't done introductions for some time.One thing I remember is to break up the cage visually with things like a "fleece forest". That you have not seen dangerous aggression is a good thing.

[ItsaZoo]

I’d be concerned about Moon’s weight loss each week. He may have something else going on and this new cage mate is adding stress to the situation. The problem with older guinea pigs is once they lose weight, they rarely gain it back.

Was Moon showing signs of loneliness before this? If not, I’d divide the cage, or place the second cage next to Moon but keep the separated so Moon knows he’s safe.

That’s only my opinion. I tried to bring a buddy home for my single girl. The newbie was dominant and my girl just hid and cried every time they were together. So now they’re in separate cages. My original girl ended up developing some behavioral changes due to the stress of them being together. She hides more, she’s quick with a sneak attack to rush out, bite and then hide, and she started spraying to try and protect herself.

[Lynx]

Thanks for your opinion, ItsaZoo!!

I kinda felt out of my depth (no recent experience, getting old!) and missed the weight loss. A real consideration.

[AviN4]

Thanks ItsaZoo and Lynx. I greatly appreciate the advice.

I checked Moon's weight again today. It declined again from 1052 to 1014 grams. So in the context of ItsaZoo's advice, I decided to separate them. I'll continue to monitor Moon's behavior and to check his weight daily. We could consider another introduction if Moon's weight rebounds or at least stabilizes, though in context of ItsaZoo's advice, I don't know if it would be a good idea.

ItsaZoo, to answer your question, Moon seemed OK from a behavioral perspective. I got the impression that he was bored much of the time, but I don't think I have good evidence to support that. It's mostly that he would spend more time begging for treats, which made me think he had nothing better to do.

With regard to Moon's recent weight history, here's a chart of Moon's weight in grams over the past 3 months. The point on 2025-01-09, with a value of 1071 grams, was a day after the introduction, when they were still getting along well. That last point, with a value of 1014 grams, is from today. I often find it difficult to distinguish changes in weight from normal variation vs worrying trends. But it's now clear to me that the current trend is not good, and that last decline of 57 grams is particularly worrying.

[Lynx]

You are so thorough! Glad you had this data. Does not, as you note, initially seem worrying. The recent drop might be a fluke.

[ItsaZoo]

Good detail and history to have in a situation like this. There does seem to be a trend of weight loss, but so gradual it would only be noticeable in a chart like this.

I think your reason for getting Moon a buddy was pretty similar to mine at the time. Millie was single from a rescue. She was previously kept by a teenage girl in her bedroom. So I’m assuming there was little enrichment and she was alone all day. She sits around a lot, loves treats, not into zoomies, and I thought she must be pretty bored. Turned out, she preferred her solitary life.

I find that even if they’re not together, having 2 is still more interesting for them because they hear each other and seem more alert to what goes on around them. Every few weeks I do a thorough cage clean and then swap them so they have new scenery from a different location. They both get pretty excited about that.

[AviN4]

Moon's condition unfortunately took a downturn yesterday morning (Jan 21). Since these are more clearly medical issues, I've posted the updates on Moon's Medical Topic.

[Renonvsparky]

I generally avoid making any major changes to the environment, living space, diet (unless it's for health reasons), etc for my seniors. That's largely because I don't want to cause them any undue stress. Guinea pigs live for 5-7 years with an occasional case of one living longer. Being in the "window" , as I call it earns my seniors extra time and attention to ensure that everything is as nice and peaceful as I possibly can make it, especially if I could get one to make it to the age of 7 years. The closest I've ever been is with Dean, who is currently 6 years, 6 months and 12 days old. He's not doing well and is not likely to make it more than another week or so. I guess my point is that trying to pair Moon is something I wouldn't do.

[Sef]

Same here. We currently have one (Jeremy) who just turned 7. That is a record for one that we have raised from a baby. We just adopted an 8-year old male, but I cant take any credit for his longevity.

I'm so sorry to hear about Dean, Renonvsparky.

[ItsaZoo]

I agree. If they’re mature adults and they’re doing okay, I would leave well enough alone. There are plenty of stressors for older pets, and sharing cage space shouldn’t be one of them. Just my opinion.