Like many people who know things, I often find inaccuracies in movies frustrating. I’ve learned to let this go in most instances, but I still have a weakness around behavioral ecology, possibly because I spent so long studying it and use that knowledge so little now. This week’s victim is How to Train Your Dragon (1 and 2). The questions I want to answer: how/why are they so many different types of dragons, and what is their social structure? The ecosystem as described by characters makes so little sense I’ll never come up with a plausible system that makes their statements true, so I’m going to focus on generating a system that could generate their observations without being constrained to make them actually true. For example: real species always have variation between individuals, even if they’re all clones (because of environmental variation). Vikings indicate all dragons of a given type are identical. So I will design dragons that have little enough variation that Vikings could plausibly mis-measure them as having none, but not actually none, because that would be dumb.
I’m also just going to ignore the fact that that ecosystem couldn’t support that many predators of that size, and the whole fire breathing thing, because those are just things you accept when you watch a movie.
Here are the observations I need to explain (enormous spoilers for both movies, although mostly not the parts anyone else cares about):
- There are a lot of different types of dragons that live together and appear to work cooperatively.
- Nests with lots of dragons tend to be controlled/led by a single enormous dragon.
- Movie 1 had a queen dragon and Movie 2 had a king dragon. Valka says that kings outrank queens.
- The nest with the queen (a Red Death) was in a volcano, lesser dragons hunted for her, and she ate them if they failed.
- The nest with the king (a Bewilderbeast) was located in an iceberg, and he fed the other dragons.
- There was a second collection of dragons led by a different Bewilderbeast under the control of a human that we don’t know much about.
- Dragons react to human speech at a level that indicates understanding, not just keyword matching. They pick this up without formal instruction.
- The kings are shown giving sophisticated commands to lesser dragons. The queen did not do so on screen, but that may be how she compelled the lesser dragons to hunt for her.
- The bewilderbeasts breathe ice and can stay underwater indefinitely. They can survive on land but do not fly.
- A number of underwater dragons are mentioned.
- The Vikings believe very strongly that dragons are groupable into distinct species where members have identical stats. They refer to these as species, although it’s never mentioned how Vikings define that.
- Dragons vary enormously in size. Bewilderbeasts are 520′ x 160′. Terrible Terrors are 1.5 feet tall, and judging by their behavior, adults.
- For all their morphoological diversity, we see dragons doing mostly the same things. This could be an artifact of their interaction with humans.
- Dragons appear to congregate in massive nests.
- There is some variation in morphology even among juveniles
The mass nesting and single head sound a lot like bees/ants/termites, where thousands of sterile workers coordinate to support a single breeding individual. The technical term for this is eusocial. Eusociality was something of a puzzle to evolutionary biologists for a while because wait, individuals are not breeding? How could that ever be selected for?
The answer is kin selection: workers bees are able to pass more of their genes onto the next generation by caring for eggs the queen lays than they would by raising their own offspring. In the particular case of Hymenoptera (bees, wasps, ants), this may be helped by the fact that females are more related to full sisters than they are to their own offspring (I’ll explain this tomorrow), but this is neither necessary nor sufficient to generate eusociality. Eusocial animals have some variation in their breeding structure: ant queens mate once and store sperm for life, males die shortly after mating. Termites, certain shrimp, and Damaraland mole rats have a single breeding pair. Naked mole rats and some bees have a queen and several breeding males. Bees and wasps can have either either the first or third structure (and not all bees or wasps are eusocial). However, and this is important, there’s no known (non-fictional) eusocial species with one male and multiple breeding females. There are animals with a harem structure (e.g. gorillas, lions), and females may have some cooperative care (e.g. lionesses will nurse others’ cubs), but children are cared for primarily by their mother and immature siblings, and there are no permanently non-breeding females.
The sheer number of dragons, the size differential, the fact that they cooperate to feed the alpha (sometimes), and that they can be compelled to take orders points to eusociality. However eusocial animals are usually pretty dumb. Dragons are smart, and have relationships outside that with their alpha (e.g. Toothless trying so hard to impress Valka’s dragon). This suggests they have a more complex social structure than a beehive. Moreover, they actively work to understand what humans want and to do it: this suggests they’re something like wolves, where there is a complex hierarchy beyond a simple alpha, and that like wolves dragons have applied their ability to interpret intentions and respond to them to us.* This doesn’t necessarily mean they view humans as alpha, they may view us as a senior pack member to be listened to.
If they are eusocial, they’re going to be more like naked mole rats (where any individual can theoretically become alpha) than insects (where alpha-ness is designated at birth). I believe this because we see three different dragon types as alpha (Red Death in the first movie, two Bewilderbeasts and Night Fury in the third), and because alphas obviously retain the capacity to be ordered around (by both humans and other dragons).
That’s social structure. What about speciation? We see an enormous variation in dragon morphology and behavior in the movies. There’s a few ways to generate that:
- There are many closely related species, like whales. This does not gel with the fact that dragons are clearly cooperative. While different species sometimes cooperate (e.g. humans and dogs), there’s nothing approaching interspecies eusociality.
- There are very different species with tight, possibly obligate, mutualisms (humans and dogs were probably this once, and gibbons and gazelles have some mutualism as well). But those species tend to be good at very different things (dogs and gazelles have smell and we and gibbons have eyesight), while the dragons seem to mostly do the same thing. This could be sufficient to explain them nesting together, but not following the alpha.
- They could all be the same species (meaning they can interbreed), but have diverged into semi-distinct genetic pools, like dogs. Rare types could be hybrids. If hybrids are frequent and distinct enough to be recognized as their own type, it suggests morphology is controlled by only a few genes, or that all the genes are located on the same chromosome. Otherwise you would just have a bunch of dragons that were intermediate between their two parents.
- Dragons are born having the ability to take many forms, and move through multiple forms in their lifespan or settle on one form based on the environment. This would be a good fit with eusociality, where drones often specialize in a single task.
- Dragons vary a lot and the Vikings more or less made up categories to shove them into
I suspect there’s more than one base form, because the babies in HTTYD2 were already bigger than the smallest adult dragons we’ve seen. But the different types must be able to interbreed, because different species don’t cooperate that extensively. This points to 3 and possibly 4.
I also have to explain the feeding. Eusocial alphas get fed, they do not feed their drones, which doesn’t match the behavior in movie 2. The easy answer is that it depends on environmental conditions: when food is plentiful the alpha feeds their minions, when it’s not their minions feed them. Since the sex making the larger investment in offspring is usually the one fed by the other, this suggests alphas are male in the water and female in volcanos. Normally that would make no sense, but I’m about to get to a really satisfying explanation.
Let’s talk about reproduction. Given the babies in movie 2, alphas must reproduce with members of their own nest, not an alpha from another nest. All the examples I can think of with a single alpha that have that level of control (as opposed to a dominance structure with one member at the top) are eusocial and have a female leader. This gels with the minions feeding the alpha in movie 1. Or they could be like gorillas, with a single male keeping a harem of females. That matches well with the fighting between the two alphas and the loyalty shifts, plus Valka would probably notice if her king laid hundreds of eggs. It seems to me to be incredibly unlikely a species would have the ability to have alphas of either sex, so we have to choose one or choose hermaphroditism. There’s only one reptile that could be called hermaphroditic, and that’s really more like being intersex than a true hermaphrodite.
But… hermaphroditism is really common in fish. Fish also feature more morphological variation through their life than reptiles, and have more and more complex social/pack behavior. And we know there are underwater dragons. What if those came first, and they moved out of the sea later? The intermediate step would look a lot like flying fish, which totally exist. I’m not sure about the fire breathing thing, but it’s not like assuming they’re reptiles makes that so much easier to explain. Dragons eat fish a lot, even the terrestrial ones. While I said I was going to ignore the inability of that environment to sustain that many predators, the ocean comes closer to meeting their requirements than land. And it explains the variation in feeding behavior.
- Dragons are descended from fish, and have a highly cooperative harem structure when food is plentiful and a eusocial one when it is not.
- The alpha is a hermaphrodite who leans male when food is plentiful and female when it is not. Either minion dragons are also hermaphroditic, or male and female forms exist and whichever complements the alphas goals gets to reproduce.
- There may be other dragons that get an occasional shot at reproduction, who may or may not be considered part of the pack (a la the side-blotched lizard or marine isopods).
- There are probably a lot of non-breeders of either sex, or dragons stay sexually immature until promoted by an alpha.
- A given baby dragon can turn into many (but not all) kinds of dragons, and may be more than one over its lifetime. Minion dragons are extremely intelligent and can understand complex instructions, but have limited ability to talk back. They can be compelled to follow these against their will but also have a strong desire to follow them without compulsion.
- Not all dragon types can be alphas, but more than one can.
As for the movie plots… yeah, they were pretty good.
*Wolves are obviously not dogs, but they’re much more trainable than other animals.