These pictures recently made the rounds on the internet. According to the story, this mother tiger's babies died shortly after birth. She became listless and the zookeepers thought she was depressed at losing them so they devised a plan to cheer her up. They dressed these piglets in tiger skin coats and put them in the pen with the tiger. How easily they could have become her lunch! As shown by her face, she not only accepted them, but she cheered right up, too.
The strength of animal instincts always astounds me. I've observed many kinds of dogs on my daily walks with Winston and the behavioural traits of the various breeds are fascinating. The dogs that were once bred for herding still herd their owners, running back and forth at their heels, keeping them on the straight and narrow. The working dogs proudly carry a stick or a prized tennis ball. Winston, whose ancestors were hunting dogs, runs in wide circles on either side of the trail, flushing out imaginary birds. Terriers 'tear about' after rodents. No one taught them these behaviours, they were just born with them. (And don't get me started on migratory birds. Those instincts are too mind-boggling to comprehend.)
But mother love (and probably father love too though I can only speak for mothers) must be the strongest instinct of them all, and it must be the same for mothers of all species. Mother Nature built this trait into our characters in order to preserve each species. There's nothing a mother wouldn't do to protect her young.
The look on the tiger's face says it all. She doesn't care what her babies look like or where they came from. She loves them unconditionally. It's as simple as that.