Animals don’t break the Laws of Nature, so no question of incurring Sin

The tiger does not eat rice and wheat or drink cow’s milk, because he has been given food in the shape of animal flesh. Among the many animals and birds, some are vegetarian and others are carnivorous, but none of them transgress the laws of nature, which have been ordained by the will of the Lord. Animals, birds, reptiles and other lower life forms strictly adhere to the laws of nature; therefore there is no question of sin for them, nor are the Vedic instructions meant for them.

– Srila Prabhupada, Sri Isopanishad Mantra 1