i have taking a pair of eggs and kept them in a egg box in the bottom of the fridge for 12 weeks. floated them hatched 1 the other clear. still got the youngster.
there is loads of ways to keep rats away but some of these need doing before the loft is built. ie the footings and base should be filled with broken glass. rats and mice find it almost impossible to dig through it.
i have cctv in my sheds the 1 in my corridoor is a pir type and mice set this off. got some great footage of mice getting there necks snapped with the little nipper (in my opinion the best trap). ive never had rats here but when i lived in wales i had a big aviary full of cage birds, finches and quails etc. a rat dug 3ft under the footings and killed the hen quails for there eggs even the unlayed 1's. i found spreading a bit of ferret poo around kept them away.
never tried them but what about the ultrasonic chasers ?