Yoreh Deah Siman 16 Seif 5

 

ה אִם הוּא כָּרוּךְ אַחֲרֶיהָ, חֲזָקָה שֶׁהִיא אִמּוֹ.

Seif 5

If the child is following a female animal in a way that demonstrates that it is tied to her emotionally, we can assume she is his or her mother.