All animals thrive in their own natural habitats. In the wild, wild animals are able to fulfill all of their physical and psychological needs by relying on their own instincts. They are free to roam and find food, fresh water, exercise, interact and play with other animals in their group. They are able to lead naturally happy and healthy lives.
However, throughout human civilization, people have also needed the help of animals in our lives. Horses are used as a work animal in many societies, dogs help humans with hunting and protecting property, cats have long been used to keep rats away from the fields, some farm animals like chickens and cows are consumed by humans for their protein, as well as the captive wild animals kept in places like zoos for what some groups claim to be essential for public education purposes.
If we have been bold enough to separate these animals from their natural wild habitats, then we have a responsibility to ensure that their physical and psychological wellbeing are well cared for. Because however way you look at it, all animals are sentient beings that have the capacity to feel pain (rasa) and desire (karsa). Although each species carries a different degree of rasa and karsa, we still owe them the same equal consideration, because after all even for every individual human being the degree of rasa and karsa varies. There is no living being exactly quite like another, and therefore it is irrational to differentiate treatment to other living creatures based on this rationale. The responsibility that we bear is none other than the responsibility to respect the welfare of animals.