Exodus | Death of the Firstborn | Why weren't the Hebrews' animals slain?

So I've been reading through Exodus a lot lately, and one of the things that stood out to me was the death of the firstborn of the animals during the final plague / strike. Now, the firstborn of the Hebrew people were redeemed by the blood of the year-old sheep & goats that was placed upon the door frame of their houses. However, there is no instruction to either take their animals into their homes or else to place blood along the enclosures for their animals (stables / barns / etc). So then, by what sign did the Destroyer know not to destroy them? What is the spiritual significance of this, if any? Let me know your thoughts :)