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Guinea pig dominance behavior can be surprising and startling to the first-time owner. You may have carefully chosen a pair of females to avoid breeding and the potential dangers pregnancy can cause, only to witness one of your guinea pigs chasing and mounting the other. This is a normal guinea pig behavior, and part of their social dynamic, although if it begins to happen frequently, it may be a sign of a health problem.
Any time two guinea pigs have to coexist in the same location, they have to work out which one is the boss. In some cases, this happens naturally and without much strife. In other cases, neither cavy wants to give up the dominant role, so they must have a contest to work out who is the leader. This usually starts with verbal confrontations, face-offs where the loser is the one who drops her head first, and "rumblestrutting," a low, sinuous dance accompanied by a rumbling or chuttering noise. If neither guinea pig backs down, it may lead to a physical confrontation.
Chasing and Mounting
When dominance displays escalate to mounting behavior, the more aggressive guinea pig typically chases the other around the cage, nipping at her behind and rumbling. Once she corners her opponent, she will mount and hump her adversary for a few seconds, before dropping back down and rumbling some more. This may repeat itself several times in a row until the other pig offers the proper submissive response, accepting her new role in the cage. Fortunately, once guinea pigs establish their pecking order, the strife in the cage usually ends quickly.
In some cases, however, you may see a long-term female pair exhibit this behavior more than once. A hormonal imbalance can cause one guinea pig to feel it necessary to reestablish her dominance, or an illness may impair the dominant cavy and encourage a submissive animal to try to take over. In most cases, these phases are infrequent, and should only last a day or two before order returns to the cage. If you find one pig chasing and mounting her cage mate frequently, however, it could be a sign of a potentially dangerous problem.
A severe hormonal imbalance can cause a guinea pig to develop ovarian cysts, tumors that grow on her ovaries and cause pain and aggression. A guinea pig suffering from cysts can become extremely dominant toward her companion, chasing and mounting her frequently. In addition to the sexually aggressive behavior, a cavy with cysts typically begins to lose hair on either side of her abdomen, resulting in bald patches. Spaying a guinea pig suffering from cysts will resolve the problem immediately, and should restore harmony in the cage.
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