Fighting - or just posturing?

Question
My daughter has two new guinea pigs from the pet shop - we were told that they are both female, one being about 1 month old, the other about 3 months.  The younger, smaller one is a bit skittish and she is spraying urine on the walls and also at the face of the older one.  She also tries to "mount" the older female.  Is this normal behavour?  We've had them about 10 days and she's just begun this behaviour.  We have a 14 day "warranty"... I'm wondering if they are just working out their social issues or if we should be concerned?

Answer
Well, if there is no biting or nipping then they are not fighting.  Even if there was a little nipping, it's okay because they are just trying to establish who is more dominant.  As long as actual biting does not occur, I wouldn't worry too much.

What I would worry about is the mounting.  I have had many female guinea pigs, and I have also introduced them to each other later in life, not even as early as you have.  They never sprayed each other or mounted.  Do you know how to sex the guinea pigs?  To see if it is a boy or a girl, hold the guinea pigs so you can see their genital area.  Press down gently right above it, and if it is a boy then the penis will protrude slightly, and you should be able to see a little bulge, which are his testicles.  A female will not have anything protrude, and she will be flat down there.  I'm worried that the one that is mounting is a male.  Just try to make sure for yourself.  I work at a pet store, and I have even been wrong a few times when it came to sexing, especially younger guinea pigs.  

You can visit this website for more information on sexing.
www.cavyspirit.com/sexing.htm
There are A LOT of helpful pictures at this website.