If you’ve taken an introductory biology course, it’s likely you’ve heard of Allen’s Rule. According to this rule, endotherms in warmer climates usually have longer appendages than those in colder climates. The concept is simple: a higher surface-area-to-volume ratio is more conductive to heat exchange. Therefore animals from colder regions will theoretically find it easier to regulate their temperature with shorter limbs and appendages, while animals from warmer regions will benefit from longer limbs. General trends in accordance with Allen’s rule have been found in the legs of seabirds and toucan beak lengths, among others.
Allen’s Rule is very nice and neat. Animals adapt to their environment! Evolution is fun, kids! Rainbows and unicorns! Unfortunately, it doesn’t capture the full complexity of appendage length determination.
First of all, Allen’s Rule does not apply in all cases. Studies on American rabbits and hares show that appendage lengths do not necessarily follow the rule, even though hares' ears are often given as an example of how a higher surface-area-to-volume ratio has a beneficial cooling effect.
Limb length is also partially determined by environmental factors, and does not necessarily result from purely genetic changes. In fact, temperature changes on their own have been shown to modulate cartilage growth, one of the main factors determining bone growth. Not only do mice raised in colder environments show marked decrease in tail length, but metatarsal bone cultures also show expansion independent of “mechanical, dietary, vascular and systemic hormonal influences” – that is, under the influence of temperature alone.
Though all this may seem somewhat 'anti-evolution' for a blog specifically devoted to evolution, it emphasizes the dangers of quick solutions to patterns we see in nature, and of automatically assuming evolution without a genetic basis.
It also makes me wonder about whether it is possible to still read evolution in this tale of appendage length development. The experiment above only suggests that Allen’s Rule could largely be explained by purely environmental factors, not that genetic factors are necessarily non-existent. Could phenotypic plasticity play any role? When would a hypothetical new gene regulating appendage length persist? After all, it seems that any gene that works to perform something the environment does “for free” would incur some sort of fitness cost to the organism. What about the cases where genetic regulation of appendage length would be beneficial, perhaps when limbs are used for flying or swimming?
Just to add to the confusion, here’s one final study. Researchers looked at appendage length in subterranean South American rodents. These rodents are not exposed to the surface temperatures much, if at all. Accordingly, the rodents did not experience the longitudinal appendage length variation that Allen’s Rule predicts. However, they did experience variation among burrows at different altitudes – even though the authors suggest that “all species maintain similar temperature conditions within their burrows independently of latitude and altitude”. Factors unrelated to temperature and thermoregulation may be needed to explain these results.
All in all, however, questioning Allen’s Rule is cause for celebration. Simplified biological trends are hardly ever as interesting as the complexity of the real thing.
Bidau, Claudio J., Martí, Dardo A., Medina, Alonso I. A test of Allen's rule in subterranean mammals: the genus Ctenomys (Caviomorpha, Ctenomyidae). Mammalia: International Journal of the Systematics, Biology & Ecology of Mammals, Vol. 75 Issue 4 (2011).
Serrat MA; King D; Lovejoy CO. Temperature regulates limb length in homeotherms by directly modulating cartilage growth. Proceedings Of The National Academy Of Sciences, Vol. 105, No. 49 (2008 Dec 9).
Nudds, R. L., S. A. Oswald. An Interspecific Test of Allen's Rule: Evolutionary Implications for Endothermic Species. Evolution, Vol. 61, Issue 12 (Dec., 2007).
Stevenson, R. D. Allen's Rule in North American rabbits (Sylvilagus) and hares (Lepus) is an exception, not a rule. Journal of Mammalogy, Vol. 67, No. 2 (May, 1986).
Symonds, M., Tattersall, G. Geographical variation in bill size across bird species provides evidence for Allen’s Rule. The American Naturalist, Vol. 176, No. 2. (August 2010).