I adore these two things: magic and biology. Honestly, the two go hand in hand. After all, Biology is magic to me. However, sometimes people in fantasy explain their monsters with ‘it’s just magic!’ without really looking into the biological aspects of it, because you can do pretty much anything in fantasy without thinking too much about the science. I love books that do that, personally, because not everything needs an explanation.
Still, some writers really like to go into an explanation of things, so either style can be a ton of fun. I’ve done both, where I’ve chosen to let the reader use their imagination or say ‘it happens this way because magic’. However, I lean more into explaining my monsters just because it’s how I enjoy to do things. There is no right or wrong answer! It’s up to the writer, and can be different based on the author’s background as well.
In that sense, it’s a ‘yay’ to do so if that’s what you want to do, simply put. What exactly do I mean by ‘explaining the monster biology’, however? Well, if you’re going to stray from using the ‘it’s magic’ idea from your monsters, you would talk about how they came to be. Honestly, I like melding the two ideas myself – anything not explained by science is therefore covered by magic.
Let’s take a look at my Living Vampires for an example. I go into detail regarding how they came to be – sister species of humans, branching off of a common ancestor similar to dogs and wolves – along with their healing capabilities requiring massive amounts of energy for the cellular division. I enjoy diving deep with those, but there’s some key points that just do not work in science. While the bacteria in their gut can digest matter from blood, the fat content is so small in a blood-diet that they would need to non-stop be eating. I avoid this by explaining they substitute the remainder of their energy requirements with ‘mana’, the fuel for magic in my universe.
A healthy blend, therefore, is what makes a really interesting sci-fi fantasy. It may not seem like a genre blend, but in reality, it is! Fantasy is the primary flavor with a nice side of sci-fi in my works, typically. I don’t make my things strictly sci-fi, given the presence of magic and breaking of physics (via magic, no doubt). Though technically you could explain your magic away by ‘science’ and ‘new discoveries that break the rules of science as we know it’ because we’re always discovering new things in science. If you pick that route, have fun as well! I think that is also very valid.
When it comes down to it, monster biology is super interesting whether you explain it with strictly magic, a blend of science and magic, or just science. You can go into as much detail as possible, or leave out little details to leave the reader to guess. In fantasy, your imagination is the limit!
Interested in a Norse Mythology Urban Fantasy? Check out my book here, Claws of Midgard!
Des M. Astor 
Interesting post. I like how you blend science and magic here and have a solid explanation for your living vampires. That’s an interesting thought too, by the way. XD I’m partial to having an explanation myself and also lean a bit into the biology side of things to help bolster my magical creatures. Lore and origins and having a reason for stuff happening is very much an element I love to delve into. It’s harder with a science fiction setting, like the one I’m tackling now, since biology is much more important than allowing for ‘magic’ but still quite fun. ^_^ And as for your book, Claws of Midgard, I might well have to check that out. I started out as a Greek mythology buff but quickly slipped into Norse mythology when I discovered the Valkyries (and the video game Valkyrie Profile). haha
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