When a Guinea Pig Bond Breaks Down: Dominance, Real Fighting and the Medical Causes

Quick answer

Guinea pigs that have lived together happily for months can suddenly start fighting, and the reason is rarely that they have fallen out. It is usually a change in the space, a change in hormones, or a change in one animal's health.

Most of what looks alarming is normal. Rumbling, mounting, chin-lifting, teeth chattering and hair raising are how guinea pigs settle rank without injury. The line that matters is blood. Once one animal has bitten another hard enough to break skin, the situation has stopped being a negotiation.

Most conflict is decided by layout. Watch where each animal sits and which routes it can and cannot use.

• Separate immediately if there is blood, and do not put boars straight back together after a blood fight.
• Weigh both animals daily during any conflict. The one losing weight is the one being suppressed.
• Sudden aggression from a sow that was previously calm is often ovarian cysts.
• Sudden aggression towards a cagemate can also mean the aggressor is in pain.
• Every hide needs two exits, and every resource needs a duplicate placed well away from the first.

Decide in 60 seconds

Normal dominance and where it ends

Guinea pigs live in groups with a stable hierarchy, and they maintain it constantly rather than settling it once. That maintenance looks dramatic to people and means very little to them.

Rumbling and rumblestrutting. A low vibrating call with a swaying, stiff-legged walk. This is a rank and courtship display and it happens daily in stable groups.

Mounting. Performed by both sexes and in all combinations. It is about rank, not reproduction.

Chin-lifting and nose-offs. Two animals face each other and each pushes its chin higher. The one that lowers first has conceded. It is a formal contest with a formal ending.

Teeth chattering. A rapid clicking. This is the genuine warning. Occasional chattering is normal; loud, sustained chattering with hair raised means the situation is escalating.

Yawning at each other. Showing the teeth. A display, not a bite.

Hair raising along the back. Escalation. Watch closely from here.

Butt-dragging. Scent marking, and a normal territorial behaviour.

What is not normal: sustained chasing where one animal cannot escape; lunging with a snapping head movement; wounds anywhere, particularly on the ears, rump, genitals and face; an animal that cannot reach food, water or a hide; screaming as a repeated event rather than a single startled squeal.

Why an established bond breaks

Duplicated resources placed apart, and hides with two exits, remove most of the reasons guinea pigs fight.

The layout, more often than anything else.

One hide with one door is a trap. A dominant animal sits in the entrance and the subordinate cannot get out or cannot get in. One hay pile, one water bottle and one food bowl are three separate flashpoints. Too little floor length means there is no room to break off a contest and walk away, which is how guinea pigs normally end a dispute.

Fixing this is often the entire treatment. One more hide than there are animals, every hide with two openings, two water sources, two hay stations, and as much uninterrupted floor as you can provide.

Adolescence.

A young boar reaching maturity will challenge an older companion, and a pairing that worked at eight weeks can come apart months later. This is predictable rather than a failure, and it is best handled by having got the space and resources right in advance.

A sow in scent range.

Boars will fight over a female they cannot see. Housing boars in a room where sows also live, or where sows have recently been, is a common and invisible cause of sudden serious fighting. Physical separation is not enough; scent separation is what matters.

Medical causes in the aggressor.

Pain makes any animal short-tempered. Dental disease, bladder stones, arthritis, sore feet and abscesses all cause a previously tolerant guinea pig to lash out. In sows specifically, ovarian cysts produce a hormonal picture that includes mounting, aggression, symmetrical hair loss on the flanks and crusty nipples, and it appears in middle age in unspayed animals.

Medical causes in the victim.

Guinea pigs displace a group member that is unwell. An animal that has become slow, that smells different because of infection, or that is not defending its position, gets pushed out. If one animal is suddenly being attacked, examine that animal as well as the attacker.

A change in group composition.

The death of one animal reorganises the hierarchy of the rest, and instability for several weeks afterwards is normal. Adding a new guinea pig to an established group without a proper introduction on neutral ground almost guarantees conflict.

Environmental stress.

Heat, noise, a cage moved next to a television, a dog or cat with access to the room, building work, or a cage that has been thoroughly scrubbed so that all familiar scent is gone at once.

What to do when it is serious

Separate physically but not socially.

A grid divider down the middle of the enclosure, or two enclosures side by side, lets the animals see, hear and smell each other without contact. Complete isolation is its own welfare problem for an obligate social species, and it makes rebonding harder.

Do not put your bare hands into a fight.

Guinea pig bites are deep. Use a dustpan, a piece of card, a towel dropped over one animal, or drop food in two widely separated places to break the focus.

Check both animals thoroughly.

Look at the ears, the muzzle, the rump, the genitals and the feet. Part the coat. Wounds in this species are often small punctures that seal and abscess later.

Weigh both, daily, and write it down.

This is the objective measure of who is coping. A stable weight in both animals means the situation, however noisy, is not doing harm. A falling weight in either means it is.

Get both examined.

Not just the injured one. The aggressor is as likely to have a medical reason as the victim.

A guinea pig that sits tucked, alert and immobile in one place all day is not settled. It is avoiding something.

Putting them back together

Only attempt this once both animals have been examined and any medical cause has been treated, and never within days of a fight that drew blood between boars.

Use genuinely neutral ground.

A space neither animal has used, large enough that they can move away from each other, with a non-slip surface. A bathroom floor, a run set up in a different room, or a pen on a fresh mat.

Strip the scent from everything.

Clean the enclosure completely, replace or wash all fabric, and change the hides. Returning a pair to a space that one of them owns restarts the dispute.

Put in more of everything than you think is needed.

Piles of hay in several places, several hides all with two exits, multiple water sources, vegetables scattered rather than placed in one bowl. Animals that are eating are not fighting.

Give it hours, not minutes.

Expect a full round of rumbling, mounting and chin-lifting. Supervise continuously. Interrupt only if there is sustained chasing or a bite.

Know when to stop.

Blood means stop. Boars that have had a blood fight are generally not re-paired with each other, and continuing to try is unkind to both. Sows are more often recoverable.

Think about the combination.

Sow groups tend to be the most stable. A neutered boar living with one or more sows is a very stable arrangement. Boar pairs are the most fragile and need the most space and the most duplicated resources. Three boars together is a demanding arrangement that frequently fails.

Be clear about what neutering does.

Neutering a boar allows him to live with sows without breeding. It does not reliably stop an established adult boar fighting other boars, and it is not a treatment for aggression. Spaying a sow is a bigger procedure and is usually done to treat ovarian cysts rather than to change behaviour, although behaviour often improves as a result.

What never to do

Do not reach into a fight with bare hands.

Do not house a guinea pig alone as the first solution. Separate with a divider so contact continues.

Do not reintroduce boars after a blood fight.

Do not add a new guinea pig straight into an occupied enclosure.

Do not keep boars where they can smell sows.

Do not assume the aggressor is healthy. Pain and hormones are common causes.

Do not clean the entire enclosure to remove scent as a way of resetting a conflict, unless you are doing a full neutral-ground reintroduction, because a scent-free territory is contested from scratch.

Do not house guinea pigs with rabbits and expect it to substitute for guinea pig company.

When to get help

Book a vet appointment for both animals after any fight that broke skin, for any guinea pig losing weight during a period of conflict, and for a previously tolerant animal that has become aggressive.

Ask specifically about ovarian cysts for a middle-aged sow with new aggression, hair loss along the flanks, or a swollen abdomen.

Go the same day for a guinea pig that has stopped eating, has an infected or swelling wound, is hunched and grinding its teeth, or has been unable to reach food and water for a prolonged period.

A settled group is one where every animal is eating, resting in the open and holding its weight, not one that is silent.

Bring the daily weights for every animal, the age and sex of each, when the conflict started and what changed around that time, a description or video of the behaviour, the enclosure dimensions and the number and type of hides and feeding stations, and whether any sows are within scent range of any boars. Expect both animals to be examined for wounds, dental disease, abdominal masses and pain.