Small rat problem...

[Angelica]

I'm going to try to put this the most polite way, but I have a problem with my female rats. They keep, erm, humping each other! They are definitely both females and they seem to take it in turns, one day Saffron will be all over Nutmeg and vice versa the next day. Is this normal? Are female rats in heat every other day? I have never had female rats before so I am not sure what to expect! It is normally just sniffing each other but sometimes they will really go for it! Maybe they're just very friendly?

[MyBirdIsEvil]

This is a dominance thing. Rats will establish a heirarchy similar to the way dogs and other animals do.
It has nothing to do with anything sexual, though it is influenced by hormones.

Just watch them to make sure that the behavior doesn't become aggressive, but yeah, them humping each other is completely normal.

Sometimes if there's a lack of other rats they'll actual pick an inanimate object to perform this behavior with. My female rabbit has a thing for plastic milk jugs

[Riluke]

Yup, I have two male rats and they do the very same. It's not so much 'humping' in their case as just trying to climb on each other's back to establish who is dominant. Then five minutes later they are curled up on each other sleeping. They can be funny little critters those rats.