Basics of Speculative Biology

So speculative biology is a fun field to study and it is particularly useful for designing monsters or races. I am going to discuss these zoological / evolutionary rules that you might find useful (or at least interesting) for creature or race designs in tabletop roleplaying games. Rather than use these rules as a guideline, you should just keep them in mind. When designing something that violates one of these rules, perhaps discuss why or use it to highlight a difference from the other sort of creatures in their environment.

(PS: The title of each of the “rules” are linked to additional reading/material on the subject)

Bergmann’s Rule

So the first rule states that the colder an environment is, the larger the creature will be. The inverse is also true; the warmer it is the smaller the creature. Hesse’s rule, an expansion on Bergmann’s rule, states that creatures in colder environments have a larger heart proportionally.

Allen’s Rule

This rule is actually proven by Bergmann’s Rule and states that creatures from warmer environments traditionally have a higher surface area to volume ratio and this allows them to bleed off heat faster while the opposite allows stocky creatures to retain warmth better. Creatures (humanoids in particular) from colder regions have thicker cores and shorter limbs while those found in warmer regions are often thinner and possess longer limbs..

Gloger’s Rule

In simple terms, “things in humid places are darker”. This relates mostly to birds but also applies to other creatures with less consistency. In specific, it refers to creatures found in equatorial or tropical regions.

Insular Dwarfism

Creatures whose range is limited to a small area will eventually grow smaller. This is prevalent in environments like caves, small islands, etc.


Basically, “big creatures can regulate their temperature better than small ones”. This is likely due to Allen’s rule.

Foster’s Rule (“The Island Rule”)

The root of this rule is, “things get bigger (or smaller) based on the availability of resources in their environment”. Another component of Foster’s rule is that small creatures get big when there are less predators.

Cope’s Rule

This one is a little contested but basically states that the longer a species has been around the larger it grows (“evolution tends to want to make things larger”). This may be a result of the fittest things growing larger or it may be that all evolution tends towards an increase in size without external pressures. Left unchecked, this can lead to the downfall of a species.

Drawing Conclusions

So, what broad generalizations can we draw from this?

  • Cold Environments: Short limbs, larger size, thick torsos, large hearts, low surface area to volume ratio.
  • Hot Environments: Long limbs, smaller size, thin torsos, small hearts, high surface area to volume ratio.
  • Moist Environments: Dark pigmentation
  • Lots of Food: Grow big
  • Less Food: Grow small
  • Lots of Space: Grow big
  • Less Space: Grow small
  • Ancient Creature: Grow bigger

Thus, speculatively, we can draw the following conclusions from those rules.

  • Small Creatures with Dark Pigmentation: Hot, moist, environment with one or more of the following traits: not a lot of food, a lot of predators, or not a lot of space.
  • Large Creature with Light Pigmentation: Cold, low-rainfall, environment with one or more of the following traits: abundance of food, not a lot of predators, lots of space, or a long lineage.

(Disclaimer: I actually wrote this for my game design column at our Studio, Little Red Goblin Games a while back but I thought you’d all enjoy it here)