The first chapter contains a lot of references to animals - pigs, roosters, doves, goats, and also the scorpion which becomes the emblem of evil when it attacks the baby Coyotito. The effect of this is to emphasise the natural surroundings of Kino and his family, and indeed the whole Mexican Indian community.
In fact, Kino's whole race is regarded as being animal-like by the Spanish who have colonised and oppressed them for hundreds of years.
He could kill the doctor more easily than he could talk to him, for all of the doctor's race spoke to all of Kino's race as though they were simple animals. (chapter 1)
This quote serves to highlight the vast gulf between the races; there is simply no meaningful communication, far less any understanding and sympathy, between them. The Spanish view the Indians as being primitive and devoid of reason, the Indians resent the Spanish imposition.
On a more individual note, there is a striking use of animal imagery in relation to Juanita after the baby has been bitten:
She looked up at him, her eyes as cold as the eyes of a lioness. (chapter 1)
This comparison shows the strength of Juanita's maternal instinct. To save her baby, it is implied, she will do anything; she can turn dangerous if required.