Guinea pig sows live in groups and need a strict hierarchy to keep order. Dominance Sorting out the dominance is therefore vitally important for guinea pigs of the same gender. Typical behaviour includes rumbling while shifting the weight from one back leg to the other ("rumblestrutting" ), teeth chattering, headbutting and nipping (with the other girl protesting loudly) and bullying practices like taking over the hut, food bowl, hay etc. She who rumbles last is first lady! In most cases, dominance is usually decided very quickly, but it can take days and even weeks until a balance has been struck between how much the dominant girl can push her companion and how far the undergirl allows herself to be pushed. After they have sorted out the terms of their relationship, girls will usually become best friends. When sows are pretty evenly matched, there can be a dispute. Teeth chattering (with the other party making an answering defiant clucking noise), yawning, going chin to chin (pushing their heads up facing each other), chasing, nipping and little scuffles can result. It can look pretty rough to us humans. Don't separate until there are serious, bloody fights; the girls NEED to sort out their differences without our interference! Only in rare cases will a girl attack others on introduction or not bond. If blood has been drawn, the sows should be separated. Sows in season Girls come into season about every 15-17 days. Often you won't notice, but sometimes, they can be very hormonal. I have observed that this happens more often with adolescent girls, freshly bonded or introduced girls or after an operation that interfered with the estrus cycle. The girl coming into season can be grumpy or temperamental for a few days (especially if she is the alpha sow). Over the perhaps one and a half day of her season, she will sniff bottoms, rumble, chase and mount her companion as if she were a male. Her companion will either kick her off straight away or allow her to hump until she's fed up, all accompanied by lots of squeaking and often a fair bit of kerfuffle! However, things should stay well below the level where fights would threaten. The humped sow will make it clear when she's fed up. Well bonded girls will often reaffirm their bond with tender cuddling on the following day. Coming into season can spark a reopening of the dominance dispute, especially when the undersow is not happy with the way things are. What we think of as sexual behaviour is very often used as a dominance tool to sort out and redefine the relationship. Permanent fall-outs In rare cases, a sow that has been happily living with her sister or a companion for years can suddenly and for no apparent reason decide that she will not tolerate her companion any longer. It can also happen after a medically necessary separation that the operated girl may not be accepted back, even if she has been kept next door and had interaction through the mesh/the bars. You can try to reintroduce the girls on neutral ground (if necessary after a bath), but if they don't get on - especially after blood has been drawn - you will have to keep them separated permanently. Sadly, once a guinea pig has decided that another piggy is no longer part of "us", it will rarely change its mind.