The application of Bergmann's and Allen's rules to the poikilotherms

The relationship between environmental conditions and the morphologic characteristics of a species is basic to the study of evolution. Similar adaptations shown by widespread species which live in similar conditions are often expressed in terms of ecological rules. Two of the most important of these are Bergmann’s Rule and Allen’s Rule, which have been briefly stated by May (’42) as follows: Bergmann’s Rule : “The smaller-sized geographic races of a species are found in the warmer parts of the range, the largersized races in the cooler districts. ” Allen’s Rule: “Protruding body parts, such as tail, ears, bill, extremities, and so forth, are relatively shorter in the cooler parts of the range of a species than in the warmer parts. ” These rules may also occasionally be applied to closely related species as well as to races which live in differing environments. In the classic view, both Bergmann’s and Allen’s Rules express the relationship between body surface and enviTonmenta1 temperature. Theoretically they are founded on mathematid grounds: as an object becomes larger, its surface becomes proportionately less and as protruding parts become relatively shorter, surface is reduced still further. Since the surface of an object is the site of its heat radiation, the assumption logically follows that reduction of body surface is associated with conservation of body heat. But, as will be pointed out, these rules may or may not be concerned with heat conservation. Rensch (’29, ’36, ’38) has brought the greatest quantity of data to bear on these and other ecological rules. The most distinct interest of Rensch, Mayr, and others has been to determine in what percentage of warm-blooded animals these rilles operate. It has certainly been demonstrated that the overwhelming majority of birds and mammals studied show size and body proportion clines in conformity to these rules. A second distinct, but related, interest is represented by the work of Scholander (’55). He does not necessarily question the observation that warm-blooded species become larger and their protruding parts shorter in increasingly cold environments, but he strongly doubts the rule’s sign&cance in terms of heat conservation. He says that increased size and change of body proportion are hopeIessly inadequate for the conservation of a significant quantity of body heat. Instead, he relegates the role of heat conservation entirely to insulation and vascular control. Though Mayr (’56) and the present writer do not entirely agree with Scholander’s views, there is no question that these rules have never been put to a critical test; that is, never has the conservation of heat by means of surface reduction been actually measured. Rensch established the validity of these rules for birds and mammals. It is the purpose of the present study to see to what extent Bergmann’s and Allen’s Rules operate in the poikilotherms. If the rules do operate in these animals, the conclusion is apparent that increase in size correlated with decreased environmental temperatures is aphenomenon basic to the animal kingdom. Similarly, if there are changes in body proportion correlated with different environmental temperatures, it might be concluded that this phenomenon is also basic in animals. There have been a number of isolated observations on temperature, size, and body proportion in poikilotherms, but no widely accepted conclusion has been rcached. In general, three sorts of state