Stories of interspecies friendships are always adorable, but they’re truly heartwarming when they involve animals adopting young outside their species and caring for them as their own.

One Big Happy Family bookLisa Rogak collected more than 40 such tales in her new book, "One Big Happy Family: Heartwarming Stories of Animals Caring for One Another."

From the Labrador that nurtured a baby pygmy hippo to the orangutan that helped raise a couple of lion cubs, the animal kingdom is full of such adoption stories.

"It's not unusual for animals to be nurturing toward any species," says John C. Wright, Ph.D., author of "Ain't Misbehavin': The Groundbreaking Program For Happy, Well-Behaved Pets And Their People."

"The instinct to care for another animal can be hormonal, or simply related to age. If they're young, their behavior is malleable, and they’re open to just about any experience, opportunity or companion. Like humans, animals, for the most part, yearn for company."

For a sneak peek at some of the sweet stories and adorable pictures from Rogak's book, check out the photos below.

dog and fennec foxes

Photo: Helen Neafsey/Hearst Connecticut Media Group

Although foxhounds are bred to hunt foxes, this rescue dog nursed a litter of abandoned fennec foxes.

goat and wolf

Photo: Quirky China/Rex Features Ltd.

When residents of a Chinese village found an orphaned wolf cub, they brought it back and were surprised when a goat adopted it and raised it like its own kid.

cat with cub

Photo: SWNS

Zara the lion cub had to be hand-reared by zookeepers, but Arnie the ginger tomcat did his part by cuddling her and teaching her to socialize with other cats.

chicken and ducklings

Photo: Bournemouth News/Rex Features

Hilda the hen accidentally sat on the wrong nest and ended up hatching ducklings who, thanks to avian imprinting, assumed Hilda was their mother.

dog and baby kangaroo

Photo: Newspix/Rex USA

Rex pulled this joey from the pouch of its mother who'd been killed by a car, and the two animals bonded quickly.

kitten and fox

Photo: John Drysdale

Rescued as an orphaned cub, April the fox adapted to farm life and cared for numerous kittens when farm cats had litters.

dog and baby squirrel

Photo: John Drysdale

Leslie Claws took a chance when he laid three baby squirrels next to his bulldog, Susie, who'd recently had puppies, but Susie didn't only allow the squirrels to nurse, she also became very protective of them.

dog and baby goat

Photo: Richard Austin/Rex USA

Billy the boxer adopted Lilly, who was the runt of her litter, in 2008, and today the two are still inseparable.

lamb and pig

Photo: Alex Coppel/Newspix/Rex USA

\Edgar Allen Pig took a liking to 1-week-old lamb Arnie when the critter arrived at his sanctuary.

llama and dog

Photo: John Drysdale

Rosie has cared for several animals on her farm, but she developed a special relationship with a baby llama named Mahogany, whom she nursed, took walks with and slept beside.

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Laura Moss writes about a variety of topics with a focus on animals, science, language and culture. But she mostly writes about cats.

Interspecies love: New book explores true tales of unlikely animal adoptions
'One Big Happy Family' is full of heartwarming true stories of animals raising other species' young as their own.