This might be a stupid question but do you mean 25% by population (i.e. out of every 100 mammals alive on the planet, 25 are bats) or 25% by number of species (i.e. for every 100 different species of mammals, 25 of them are bat species.) Not sure if my wording makes sense, but I hope you know what I mean.
If it's the latter, to me, that just means more specialization, or more mutation within the... Genus? I didn't do that well in my high school biology class. If it's the former, how many bats would that be? What % are humans?