Polar Bear and a Chow have in common that is not true of other mammals except one very tall mammal?

This is a tricky one! Here's a possible answer, considering the information given:

* They both have a relatively short gestation period compared to their size.

Here's why:

* Polar bears have a gestation period of about 195-260 days, but the embryos actually implant in the uterus only a few days before birth. This is due to delayed implantation, a strategy to time births for optimal conditions.

* Chow Chows have a gestation period of about 55-65 days. This is considered relatively short for a dog breed of their size.

* The very tall mammal that likely shares this trait is the Giraffe. Giraffes have a gestation period of around 14-15 months, which is relatively short for their size.

Why other mammals don't fit:

Most mammals, especially larger ones, have longer gestation periods relative to their size. This allows for sufficient development of the offspring before birth.

Let me know if you'd like to explore other possibilities!