There are three principal Ecographic Rules for terrestrial environments: Allen’s Rule, Gloger’s Rule and Bergmann’s Rule. Ecographic Rules are concerned with variations in traits (mainly morphological) of organisms over goegraphical gradients. Allen’s Rule was discussed previously. (see “Gloger’s Rule” 10-10-16)
Allen’s rule states that endothermic vertebrates (animals that maintain their body heat at metabolically favorable temperatures) from colder climates have shorter appendages than closely related species from warmer climates. This is due to higher surface area on longer appendages. All mammals give off approximately the same amount of heat per unit surface. So longer appendages would cool the animal more in hotter climates because of the greater surface area while shorter appendages would help conserve body heat where temperatures are colder. Although there are exceptions, Allen’s Rule seems to apply in most cases.
Allen’s Rule is especially evident in Lagomorphs (pika, hares, cottontails and rabbits). These mammals have longer ears in warmer climates and shorter ears at higher latitudes and elevations. Another study showed that horned larks (Eremophila alpestris) in hot climates have longer legs than those in colder habitats.
This generalization also seems to apply to humans (tall, slender African Masai with long limbs compared to Inuits with stocky bodies and shorter arms and legs) as well as ectotherms (cold-blooded animals).
Some ecologists believe that animals’ adaptation to colder climates has more to do with their vascularization, insulation and tissue adaptation rather than appendage size. Another explanation for longer ears in Lagomorphs states that sound does not carry as well in hot, dry air as it does in cool, humid air. Therefore longer ears in hotter climates were developed to hear enemies before they come too close better.
Allen’s Rule was formulated in 1877 by Joel Asaph Allen (1838 – 1921), an American zoologist.
The horned lark was photographed near Pilot Butte Parking Area in Ash Creek Wildlife Area (Modoc County CA). The black-tailed jackrabbit (Lepus californicus) was running along Salt Creek Trail in Redding CA (Shasta County).
Bergmann’s Rule follows in the next post.