These days, the amazing world of animals and their complex lives and emotions has been opened up to us by the work of scientists and others working with animals, and stories abound which make it hard to argue that animals are anything but truly sentient beings.
There are some wonderful books about animals and their emotions; some you might like to check out are: When Elephants Weep: the Emotional Lives of Animals by J. Moussaieff Masson and Susan McCarthy, The Emotional Lives of Animals by Marc Bekoff and Jane Goodall and The Pig Who Sang to the Moon: The Emotional World of Farm Animals by Jeffrey Moussaieff Masson.
As a taster, here are some remarkable true stories and examples which show that animals really are complex, emotional beings, and more like us than we may realise!