פירוש על עבודה זרה 47:4

R. Eliezer’s response to the sages implies that he is concerned about bestiality. That’s why they can buy the heifer if it had been guarded by Jews from the time of its birth.

In the end, R. Eliezer is suspicious of bestiality. But he also has another reason why—the midrash on the word “and you shall take” as taught by Shila.