Any female who mates with more than one male in rapid succession and then releases more than one egg can host babies with different fathers in her uterus simultaneously. It happens when one male’s sperm fertilizes one egg and another male’s sperm fertilizes a different egg. This is true for all mammals, including cats, dogs and humans.

Female cats are, shall we say, quite promiscuous. If given the chance, most female cats will mate with several males when they are in heat. Therefore it is quite common for feline littermates to have different fathers.

After dogs mate, they become “tied together” for several minutes. This is an evolutionary adaptation that prevents another male from immediately mating with the female. Canine littermates therefore are more likely to share a father. However, the evolutionary adaptation is far from perfect, and it is not uncommon for puppies in a litter to have different fathers.