Wednesday 26 October 2016

The mystery of the pregnant guinea pig - solved by Pete the vet

When Maria Cassoni bought two female guinea pigs, she didn't expect a pregnancy. Vet Pete Wedderburn comes to the rescue with the reason.


Maria has kept guinea pigs for over 10 years, but to date, she has always managed to avoid an accidental pregnancy amongst her pets. She knows about the need for guinea pigs to have companions (they get lonely on their own), but she has kept them in pairs of the same sex.

Her first two pets were females, who lived together until they died of old age. She then got a pair of two boys, and they have also been living contentedly together. In fact, Maria wondered if they might be gay, because she has seen them mounting each other. I assured her that guinea pigs often play together in this way, and that it doesn't necessarily mean anything else.

A month ago, Maria bought two more guinea pigs, and this time, she chose another pair of females to live separately from her two males. The man in the pet shop told her that they were both female, and when she saw them jumping on each other (like her two males), she didn't think anything of it. Perhaps it was just more guinea pig playing behaviour.

In the past two weeks, Maria noticed that Omelette's stomach was swelling up. At first she thought she might just be eating a lot (which she does), but every day, her abdomen seemed to be bigger again. Soon, Omelette was waddling around with an enlarged body that was definitely not normal.

She also noticed that Omelette was drinking a lot more than usual: Maria had to fill the water bottle several times daily. She suspected that she knew what might be happening and she brought Omelette to see me for a pregnancy test.

In guinea pigs, there's no need for complicated blood or urine tests. Simple palpation (feeling) of the abdomen tells you everything you need to know.

I could feel several hard lumps in Omelette's abdomen, like small sections of solid sausages. There is only one thing that can cause this: pregnancy. But how could a female guinea pig fall pregnant when she was living with just another female?

The pregnancy seemed quite advanced, so my first theory was that she could just have been mated with a different guinea pig at the pet shop, before Maria had bought her.

Just to be sure, however, I examined Grapes as well, to double check that she was definitely female. To my surprise (and to Maria's astonishment) Grapes turned out to be a male.


It can be difficult to tell the difference between young guinea pigs, and clearly, the pet shop had made a mistake. Maria had accidentally bought a male and female pair of guinea pigs.

The "play" behaviour that she had witnessed was obviously something much more significant than she had thought. It was no wonder that the female was now pregnant.

Maria is not too upset: it will be interesting to have a litter of baby guinea pigs later in the summer, and she knows that she'll be able to find good homes for them. She does, however, have a problem as to what to do with Grapes. If he stays with Omelette, she'll just get pregnant again.

Her first plan is to introduce Grapes to her other two male guinea pigs: if they get on well together, she'll then get another female to live with Omelette. If the male guinea pigs fight (as they sometimes do), she'll get Grapes castrated, and allow him to live with Omelette for the long term.

It might take a bit of time to reorganise the long-term living arrangements for Maria's guinea pigs, but one way or another, she will find a way to make it work out for them all.

Just as for many human families, life for guinea pig families can be unexpectedly complicated.

> Guinea pigs are prolific breeders

> It can be difficult to tell the sex of young guinea pigs

> If in doubt, it's best to separate them.

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