Albert Barnes Commentary Job 39:2

"Canst thou number the months that they fulfil? Or knowest thou the time when they bring forth?" — Job 39:2 (ASV)

Canst thou number the months ... - That is, as they wander in the wilderness, as they live in inaccessible crags and cliffs of the rocks, it is impossible for man to be acquainted with their habits as he can with those of the domestic animals.