WEATHER, CLIMATE, AND THE EXTERNAL MORPHOLOGY OF PACIFIC TREE TOADS.

Abstract

Morphological variation between different populations of a species can be interpreted as a response to their differing environments. Some of this variation has led to several generalizations collectively referred to as rules. Bergmann's rule states that in homeothermal species the larger sized races will be found in the cooler regions of the range. Allen's rule deals with extremities and generalizes that in cooler parts of its range a species will have shorter protruding body parts. Scholander (1955) asserts that there is no proof that the minor and erratic subspecies trends expressed by Bergmann's and Allen's rules reflect physiological adaptation to heat-conservation. He believes that heat-conservation is accomplished through insulation and vascular control. Mayr (1956) points out that rules are empirical generalizations describing parallelism between morphological variation and physiogeographic features. Scholander (1956) however, says that the clines expressed by ecogeographical rules are based on measurements inadequate as a way of determining the thermal adjustment of the animal. Mayr (1963) has emphasized that the validity of this parallelism is independent of the interpretation. He claims that thermal conservation has no validity in the interpretation of ecotherms and suggests three situations which can lead to variation in size. (1) If sexual maturity is reached in one year, the largest animals will be found in the warmest regions because they will have longer growing periods. (2) If sexual maturity is reached in more than one year because more growing seasons are required to reach that size, the largest animals will be found in cool regions. (3) The size of the animal will be related to the environmental conditions which exist in each locality. In the Pacific tree toad Hyla regilla the largest animals are found in the mountains and in the mild Pacific northwest while the smallest are found in the interior valleys and in the deserts. This toad reaches sexual maturity in a single year. In various localities and years a few individuals that reach sexual maturity live none, one or two years. Thus none, of Mayr's suggestions apply to these animals. Allee et al. (1949) state that poikilotherms tend to have their largest species in warmer rather than colder climates, but these authors examine variation between, rather than within taxa. Martof and Humphries (1959) suggest that in colder regions a small frog has the advantage of rapid exchange of heat with itself and its environment, while in warmer climates large size is an advantage because the smaller surface area to volume ratio brings about a slower rate of exchange. Ray (1960) surveyed the literature on poikilotherms and found that 80% (of 40 species) conform to Bergmann's Rule and 81% (of 16 species) conform to Allen's Rule. In contrast to the size cline reported for Rana sylvatica by Martof and Humphries (1959), newly-metamorphosed juveniles of this species reared at 16.4C were 10% longer than those reared at 20.6C (Ray, 1960). Ray states that the untested assumption that Bergmann's and Allen's rules describe an adaptation for heat conservation has confused the issue of their validity.