What is my guinea pig doing?

Question
QUESTION: Hi!
I have 2 female guinea pigs, one of which I got last week to join my older pig. The have been getting along great and were sharing a cage after 3 days.
Tonight the young one was acting very strange. She would approach the older one making a rumbling sound and somewhat getting in her face. the older one would chase her away but she kept coming back and doing it (though when she ran away she would popcorn so I'm not sure if she was playing).
But the weird thing is she would stretch out when she walks and her genitals would open really wide (almost like she wanted to be breeded?).
What is she doing? I seperated them and hope they are back to normal tomorrow but I'm really concerned because up until tonight she's been very calm and somewhat shy


sexing babies  
sexing babies   ANSWER: I thought I'd already posted the answer but it obviously didn't go through.  I apologize for that.

The description you're giving me is one of a boar making advances to a sow. I think you should take a serious look at the new pig because my bet is that the younger one is not a girl at all. The stretching out, rumblestrutting, swaying and spreading the genitals is a typical boar behavior. Sows do not do this.

A sow in heat may make purring noises and try to mount another sow because they're driven by hormones. But the description of your pig is not that of a sow in heat, it's a perfect picture of a young boar making a serious pass at the sow.

Keep them separated. Double check the sex of the young pig. I will be extremely surprised if this is not a boar.

In case you're not sure how to tell the difference here's a couple of pictures to help you. A young boar will make these gestures by the time they're a month old, but if you look carefully you should be able to visualize the penis.

Softly pulling upward and pushing on the belly just above the genitals should make it pop out.

---------- FOLLOW-UP ----------

QUESTION: Oh wonderful...looks like she is a he....crap....
How can I tell if the female is pregnant? she has been acting very lazy lately...

Answer
I hope you don't mind my changing this to a public posting as it's not an uncommon thing and may help others who read it.

You actually can't tell if she's pregnant until you see her belly getting very large. The gestation is about 70 days. It's possible if he's young and small enough that conception did not take place. Unfortunately only time will tell.

She may be lazy because she's exhausted from his constant advances. I certainly would be! Now that you have them separated it's just a waiting game.  Keep me posted, I'm interested to know the outcome.