2 Comparative Animal Physiology 



respect to the environment; these characters will not be discussed in this book. 



The Organism and the Environment. Foremost among general principles 

 which emerge from a study of comparative physiology is the functional adapta- 

 tion of organisms to their environment. The distribution of a species is deter- 

 mined through natural selection by its hmits of tolerance. Every species can 

 live within certain limits of variation of each environmental factor. One 

 environmental factor may limit the distribution of one group, and another 

 may limit another group. Salinity of an aquatic habitat limits some animals, 

 oxygen tension limits some, and temperature extremes hmit others. Explana- 

 tions of both restricted distribution and widely diversified distribution can 

 be obtained by examining physiological reactions to environmental stress. 

 The extent to which adaptation to a changed environment can occur and 

 the mechanism by which such adaptation occurs provide a basis on which 

 to account for existing distribution and to predict the possibilities of future 

 migrations. Animals can respond to environmental stresses either by alter- 

 ing themselves (adjusting) to correspond with the environment, or by regu- 

 lating themselves by means of protective mechanisms and thus maintaining 

 a certain internal constancy. Each type of response permits survival and 

 is, therefore, adaptive. For example, some aquatic animals follow osmotic 

 changes in the medium by proportionate internal changes, whereas others 

 maintain a constant internal osmolar concentration. Most animals are at the 

 temperature of the external environment (adjustment), whereas a few show 

 temperature regulation. When an environmental stress is removed an animal 

 tends to return to its previous mean state; that is, recovery stops at a point of 

 balance of body functions. 



Different groups of animals solve a given problem of environmental stress 

 by different means. For example, methods of obtaining oxygen (external 

 respiration) and methods of oxygen transport show wide variation within 

 single groups, as in insects. Conversely there are examples of physiological 

 convergence. One respiratory pigment, hemoglobin, has arisen a number of 

 times in totally unrelated groups. Animals never react to a complex environ- 

 mental situation with a single organ system. It has often been stated that the 

 organism as a whole is more than the sum of its parts, and out of the whole 

 organism emerge characters not present in any of the parts. Animals react as 

 integrated wholes, many organ systems combining in single responses. 



Adaptive reactions to environmental stress may be of two types, physiological 

 (within the individual) or genetic. Physiological adaptations are those respon- 

 ses of an individual which occur within the genie limits of lability. The 

 genetic limits for physiological response are much wider than is sometimes 

 recognized. Modifying effects of the environment on morphological develop- 

 ment are well known, as in the effects of oxygen tension on gill size in tad- 

 poles, or of salinity on anal gills of mosquito larvae, but environmental modifi- 

 cation is much better seen in physiological characters. When there are environ- 

 mental changes in oxygen tension, salinity, or temperature, a variety of com- 

 pensating or restorative reactions may occur, some even involving changes in 

 enzymatic pattern. For example, lethal temperatures depend on acclimatiza- 

 tion temperature; the nature of nitrogen excretion depends in some species on 

 the amount of available water. It is one task of comparative physiology to 

 learn the extent and kinds of lability permitted by a given genotype. 



