Puppies often cry during a transition period, such as littermates leaving, to help them adjust to their environment.
Newsweek on MSN

Cavapoo howls for owners

A pet camera captured the moment a 6-month-old cavapoo began howling after he thought he was home alone.
The owner told Newsweek that her 6-month-old Cavapoo typically uses a frozen Kong or snuffle mat before sleeping when home alone.