Question: why do animal mate within the same species?

Sam Tazzyman answered on 21 Jun 2011:

Animals have to mate within the same species to reproduce – this is actually one way that biologists define what “species” means, by saying that a species is a group of animals that can reproduce fertile offspring by mating together.

But some animals do mate across species. For example donkeys and horses sometimes mate even though their offspring – mules – are infertile. The reasons animals sometimes mate outside their species are poorly understood, but it might be because they evolved under conditions where it was a better strategy to mate first before checking whether what you were mating with was the right species.