There are many reasons why a guinea pig can act agressive when meeting a new one - fear and insecurity are most often at the core of it.
At your sows' age, they are currently in the prime of their lives.
Older and bereaved sows well past ideal pup bearing age can be tricky to re-bond, but it is not necessarily impossible. I have had success several times with eventually bonding two live-alongside single sows (bereaved or fallen-out).
Sometimes a baby or two is also much easier accepted as they cannot upset the hierarchy.
Neutered boars of all ages are also something you can consider. My Tesni was 5 years old when she fell head over heels for 5 months old gentle Gethin and happily lived with him until her death at 7 years.
Now 3 year old Gethin is currently living with Tesni's 8 years old sister Ffwlbri (who it took several months and three bonding attempts to join Gethin and Tesni after Ffwlbri's last companion died) plus a 6 months old girl, which I adopted at 5 weeks old and which has given Ffwlbri a new zest of life.
Tesni is giving her new 'toy boy' a piggy kiss in 2016
Gethin with Tesni's sister Ffwlbri (joined in 2017) and newly bonded baby Carys in 2018
It has taken my two bereaved divas 8 years old Calli and 6 years old Tegan 10 months and several tester bondings/shared lawn time until they were ready to move in with each other although they always had a special bond even through the bars. Calli died soon after her 9th birthday, but both her previous owner and I are convinced that she wouldn't have lived as long without Tegan.
And I have just managed to get my human-orientated once beloved ex-single Beryn, who was found straying on the grounds of a cricket club and who was (literally!) scared stiff of other piggies as she had no idea of handling them, to make friends with a bereaved non-aggressive under-sow, who is the last survivor from a large group (long term owner doesn't want to continue with guinea pigs). It has taken me countless intros from single piggies (both sows and neutered boars, laid-back groups, 5 weeks old babies (they actually scared her most) over the course of one and a half years of living next to plenty of other guinea pigs. Beryn did interact with piggies as long as they were not in her space; that is why I have persevered.
Bonding her and Betsan has taken me about 5 weeks of living alongside and several bonding sessions in the undivided run after each clean right to the point where Beryn went on overload. It had it moments but it was never full-on aggressive. Being able to read piggy body language and being able to understand the dynamics between them has helped enormously.
My spayed Nesta never rebonded after the death of her 8 year old lookalike 'auntie' Nerys (they were not related), but she died as a next door neighbour Nosgan, who was as similar to her looks and outlook and his - after bonding failed and Nosgan became rather withdrawn - little sidekick Nye. She enjoyed interacting with them through the bars.
Nosgan and Nesta in their divided pen
The best way to find a new mate of a bereaved piggy's liking is by rescue dating under expert supervision at a good rescue so you come home with a new (healthy/quarantined, properly sexed/guaranteed not pregnant companion unless acceptance has happened.
A few rescues offer residential bonding while boarding a piggy to allow time to introduce several candidates and to see through the full bonding.
Age and gender come a long way behind mutual liking. It can sometimes take more than one trip and take patience and persistence, but most piggies can be rebonded, and they most certainly profit from companionship through the bars. If you can't rescue date, then finding another single for living side by side is your best alternative.
PS: I have written a two-part in depth article about single guinea pigs in various situations, solution, challenges and responsibilities for Guinea Pig Magazine. The first part is in the current issue #48 and the second part will come out in the next issue in two weeks' time, if you are interested in that topic.
Guinea Pig Mag
