The mating behaviour of mammals is, unsurprisingly, highly varied across different groups and species. We would hardly expect the behaviour of dolphins to resemble that of reindeer, for example, or hedgehogs to resemble cheetahs. Reproductive information is one of the key biological factors we tend to look at when describing a species, even if it isn't quite as easy to evaluate as, say, diet or habitat. (Or anatomy, of course, which doesn't even necessarily require the animal to be alive). Look at almost any write-up of a mammal species in an encyclopedia, and there'll be something on reproduction - assuming we know it.
But, for a great many mammal species, we don't. This may be because it's rare, or difficult to observe in the wild, or perhaps that it's a newly discovered species that we can reasonably assume isn't that different from close relatives we already knew about. But, at least when it comes to reproductive behaviour, one of the biggest gaps in our knowledge concerns the bats.
