Abdu'l Baha tells us:
"Then, O ye friends of God! Ye must not only have kind and merciful feelings for mankind, but ye should also exercise the utmost kindness towards every living creature. The physical sensibilities and instincts are common to animal and man. Man is, however, negligent of this reality and imagines that sensibility is 374 peculiar to mankind, therefore he practices cruelty to the animal. In reality what difference is there in physical sensations! Sensibility is the same whether you harm man or animal: there is no difference. Nay, rather, cruelty to the animal is more painful because man has a tongue and he sighs, complains and groans when he receives an injury and complains to the government and the government protects him from cruelty; but the poor animal cannot speak, it can neither show its suffering nor is it able to appeal to the government. If it is harmed a thousand times by man it is not able to defend itself in words nor can it seek justice or retaliate. Therefore one must be very considerate towards animals and show greater kindness to them than to man. Educate the children in their infancy in such a way that they may become exceedingly kind and merciful to the animals. If an animal is sick they should endeavor to cure it; if it is hungry, they should feed it; if it is thirsty, they should satisfy its thirst; if it is tired, they should give it rest."
(Abdu'l-Baha, Baha'i World Faith - Abdu'l-Baha Section, p. 373)