This week we celebrate the twentieth anniversary of Blade II, the movie that asked viewers to imagine what vampires are afraid of. The answer? Reapers, mutated vampires with jaws that split horizontally down the middle and who feed on their own kind.

Raising the stakes for a sequel can be a challenge and creating a vampire that is even more vampiric is certainly a solution, but does it hold up to scientific scrutiny? Let's find out.

DOES IT PAY TO EAT YOUR OWN?

Cannibalism is one of humanity's major taboos and for good reason. For one thing, it's just not a very nice thing to do. Most of us, we assume, don't want to be eaten, so it stands to reason we should avoid taking a bite out of our same-species neighbors. Nature, however, does not agree. In fact, sometimes it's the right thing to do, evolutionarily speaking. There are a number of scenarios in which animals regularly eat members of their own species, often members of their own family.

Sometimes animals eat their own offspring, an act known as filial cannibalism, when the need arises. Mammals, birds, fish, and spiders have all been observed eating their own young, but scientists aren't totally clear on why it happens. It could be that consuming young provides the same adaptive pressure as an outside predator. Offspring who develop more quickly are less likely to be consumed, pushing the species toward independence more quickly.

Killing and consuming young can also be a response to resource constraints or illness. Many mammals have been observed killing and eating their offspring shortly after birth if they are ill or malformed. If a mother struggles to acquire the resources needed to nurse and raise young, they may shift from an evolutionary advantage to a burden. Then they become a resource. It's cold, and we don't like it, but that's the calculus nature sometimes makes.

Parents aren't the only familial cannibals in the animal world. Young are also guilty of eating both their parents and their siblings. In the case of the crab spider, when a mother reproduces, she provides unfertilized eggs for her babies to eat. They eat those, but when they're done, they eat her too. Over the course of a few weeks, they completely consume the mother before venturing out into the world. There seems to be a significant advantage to this behavior. Mother-eating spiders have higher weights and rates of survival.

Perhaps the most hardcore example of animal cannibalism involves sharks who get down to the business of eating their siblings before they're even born. In some species, embryonic sharks eat their siblings while still in the womb.

ALL THE BETTER TO EAT YOU WITH

Blade II's reapers weren't terrifying only because they ate other vampires. What really set them apart were the morphological adaptations they'd acquired to more effectively hunt. Becoming a reaper involved a whole host of body changes including a bony casing around their organs and a modified jaw better suited to latching onto necks.

Animals in nature also sometimes undergo physical changes in order to more effectively eat the ones they once called friends.

Spadefoot toad tadpoles are perfect for this sort of transformation because their bodies are already undergoing a metamorphosis into their adult form. Observations of spadefoot tadpoles has revealed that they have two potential fates depending on their preferred diet early in life.

Omnivorous tadpoles that feed on plankton and other organic matter suspended in the water remain comparatively docile. They get along with their peers and continue feeding until they grow into adult toads. Tadpoles that feed on fairy shrimp undergo a rapid growth spurt becoming several times larger than the other tadpoles and that's when the pond-bound bloodbath ensues.

In addition to growing larger overall, the carnivorous tadpole also develop stronger jaw muscles and a row of sharp teeth. Then they turn those jaws on the other tadpoles. Research has shown that they recognize their siblings and avoid them, at least initially. As they swim through the pond they nip on other tadpoles and, if they find them to be strangers, consume them. If they nip onto a sibling, they'll release them uneaten as long as there's still plenty of food to go around.

That preferential selection goes out the window, however, when hunger wins out over table manners. When it comes down to it, they won't turn a meal away even if that meal is family.

Suddenly, the Reapers don't look so supernatural. A quick look at nature shows us that, in fact, we're the weird ones for not eating one another. If we'd evolved from another species, even a closely related one, or if our species and societal development had gone another, way we might very well be on the menu.