Below-freezing temperatures this week have left many Philadelphians thinking twice before heading outside. That includes some of the animals at the city's zoo.

 

When the mercury dips into the single digits, some species just can't hack it and opt to keep warm indoors.

"Many of the animals, physiologically, aren't equipped to handle this cold weather," said Tammy Schmidt, The Philadelphia Zoo's curator of carnivores. "As an example, our hippos, they have hair, but they have more exposed skin than they do covering them."

Giraffes and rhinoceros are also on the list of animals not built to hang outside when the ground is frozen.

But others, such as the polar bears and snow leopards, can't get enough of the cold.

"It's their temperature range," said Schmidt. "They can handle any weather, but they're made for this. They have fur between their toes and they have nice insulated fur coats."

The Philadelphia Zoo is home to approximately 1,300 animals. Only a small percentage of the zoo's outdoor animals have stayed inside this week.