Bergmann & Allen Rules

Bergmann's Rule

"Body Size & Latitude"

This rule correlates body mass with latitude in animals (especially vertebrates).

The Rule: Adult body mass increases towards higher (colder) latitudes.
Scope: Applies to both inter-specific (between species) and intra-specific (within species) variations.
Example: White-tailed deer are larger in Michigan (Cold) compared to Nicaragua (Warm).

Why it happens?

Thermal Independence: Larger animals have larger reserves and can withstand harsher climate conditions better than small animals.

Competition Hypothesis: In the tropics, high diversity favors small bodies. At high latitudes, low diversity favors large bodies.

Allen's Rule

"Appendage Length"

Focuses on the extremities (ears, tails, limbs) of endothermic vertebrates.

The Rule: Animals from colder climates have shorter appendages than closely related species from warmer climates.
Visual: Think of the short ears of an Arctic Fox versus the massive ears of a Desert Fennec Fox.

Why it happens?

Thermoregulation: Shorter appendages conserve heat (less surface area exposed), while longer appendages are more effective at dissipating heat in hot climates.

Marine Variations

Ecogeographic rules also apply to life underwater in marine biomes.

Jordan's Rule

The number of vertebrae in marine fish increases as you move from the tropics to cooler waters at higher latitudes.
Likely related to body size gradients similar to Bergmann's rule.

Thorson's Rule

Concerns development modes. Marine invertebrates at high latitudes are more likely to have Direct Development rather than Pelagic (larval) development.

Important Note

Exceptions Exist: The generality of these rules is often questioned. All have exceptions and mechanisms are mostly hypothesized.