A female protection of her litter, or even toys or possessions during false pregnancy. This can appear to be competitive aggression however it is hormonal based. This aggression only lasts two months during the false pregnancy stage or longer if there is an actual litter produced. It can frequently occur without a threat display.
However, some hormonal imbalances can cause abnormal maternal behavior that requires treatment with medication, and is particularly important to seek help from your vet if the mother dog injures her pups. Spaying reduces maternal aggression.
**Aggression categories compiled from:
Clinical Behavioral Medicine for Small Animals, Karen L. Overall, M.A., V.M.D., Ph.D. Diplomate, American College of Veterinary Behavior, Department of Clinical Studies, School of Veterinary Medicine, University of Pennsylvania, Philadelphia. Pennsylvania, Mosby, Inc. 1997
The Dog Who Loved Too Much: Tales, Treatments, and the Psychology of Dogs, Dr. Nicolas Dodman, BVMS, MRCVS, Bantom Books, 1997