The Alberta government estimates the offspring of a single pair of Norway rats can multiply to 15, 000 in one year.

WSJ: In Alberta, Citizens Smell a Rat After Long-Banned Rodents Appear

Genes are switched back and forth between each pair, so that each chromosome received by the offspring is a mosaic of the parent's pair.

FORBES: DeCode's Genetic Breakthrough Barely Registers

Severino had previously mated with a female parrot, but the pair never produced any offspring.

BBC: Brazil hunts for unique lost bird

As cats are harder to round up than dogs, and breed prolifically a pair can produce ten offspring a year large colonies of 80 or so cats hiding out in disused buildings are increasingly common.

ECONOMIST: Cats and dogs fight it out for prime pet position