system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What biologically prevents interspecies breeding?

I remember learning in Biology that different species could not create offspring. That seems to be counteracted by mules with horses and donkeys. How does the reproductive system determine that there's another species involved and prevent insemination?
Ex: If that same horse from earlier got drunk at a barn party and went home with a cow, how does nature prevent them from making some weird hybrid horse-cow (a corse if you will) baby, and why didn't that same system prevent the mule from being born?
gpt
Two different obstacles to interspecies breeding.

One is physical, where the sperm dies before it can reach the egg, due to different chemical environments in the uterues, or simply cannot reach the egg.

The other is genetic, where either each animal has a different number of chromosones, so the genetic information from the father can't find a suitable match with the information from the mother.