Pregnant rats

Question
Hi,
I brought my first rat one and a half years ago, the pet shop owner identified the rat as a female.  Recently (one month ago) I brought another female rat.  After 1 week at home this rat gave birth to 11 babies.  I spoke to the pet shop and they appoligized, as other females rats at the shop had also given birth recently (the females and males were in the one cage). The baby rats are now two weeks old, and both older rats are taking care of them.  The mother seemed to loose her baby weight in the first week after giving birth, but now two weeks on seems to be back to pregnant weight!!! I am now wondering whether my first rat is a female? How long does it take for a mother to loose her baby weight after giving birth?

Answer
 If your other rat is not a female it should be fairly obvious at this point as their testicles are very prominent as early as 6 weeks of age.  If you compare the bottoms of your two adult rats and they are the same, they are both female.  In very very rare cases some male rats can have undescended testes, or be a hermaphrodite but like I said it's pretty rare.  Here are some websites that have some pictures of male and female rats for comparison:
http://www.thefunmouse.com/info/sexing.cfm
http://www.ratz.co.uk/sexingrats.html
http://www.afrma.org/sexing101.htm

 It's fairly common for the female rat to gain weight after the babies are born.  Part of it is that she looks very skinny once the babies are born and she is just getting back to normal and part of it is that she is less active while she is on the nest and gains a bit of weight that way.  The only way you can tell for sure is to wait and see.  If she gives birth again, separate your two rats as the other one must be a male or hermaphrodite.  Make sure also that you separate the male babies from the mother between 5 and 6 weeks or her sons may impregnate her.

 Other signs of pregnancy include nest building, hair loss around the nipples, and behavioural changes.  If it is a second pregnancy the gestation period will be around 25-28 days instead of the normal 23, as hormones due to lactation will slow the growth of the babies so she has time to wean the first litter before giving birth again.

 Hopefully she is just getting chubby!  Good luck with your rats!