BASAL METABOLIC-RATE IN HOMEOTHERMIC ANIM ALS .2. EVOLUTIONARY ORIGIN, ENERGETIC AND ECOLOGICAL EFFECTS

The data on bioenergetical profile of many species Passeriformes and N on-Passeriformes show that the basal metabolic rate (BM) is strongly c orrelated with potential energy (MFE) and potential productive energy (PPE) (MFE is about 4 BM in all birds). BM is minimal power of an anim al and is strongly correlated with the daily external work, which may be determined by measuring total animal activity. Hence, BM is the fun damental scale of power which determines the intensity of the actual i nteraction of an individual with the environment. The increase in BM o f a particular bird should highten the potential energy (MFE), potenti al productive energy (PPE), and the level of daily work output. BM in Passerines birds is 1.3-1.5 fold higher than that in Non-Passerines an d Mammalia. Origin of endothermy in the course of evolution should be associated rather with needs of general activity than with the require ments of thermoregulation.Analysis of allometric regressions for MFE, PPE, MFE/BM, PPE/BM shows that Passerines have an energetic advantage at body mass in range 5-150 g, while at range 150-600 g the energetic capacity of Passerines and Non-Passerines are approximately equal. An advantage of Non-Passerines is shown for body mass higher that 600-800 g. Body mass range of 5-150 g in forest habitats is almost completely occupied by Passerines birds that can be related to large energetic c apacity. The 1.3-1.5 times increase in BM level of Passerines results in proportional increase of MFE and maximal aerobic metabolism (Fig. 5 ). The advantage of BM increase, is therefore. not an unmixed blessing since they entail so great a cost. If a passerine bird attempted to i ncrease its food intake by 1.3-1.5 fold or more, it would at the same time increase its exposure to danger. It is difficult to conceive of t hese metabolic increment occuring for strictly thermoregularory purpos es because origin Passeriformes in Mesozoic, the most thermally stabil ity period in the history of the Earth. The different exponents in the equations, describing the dependence of MFE, PPE, and BM of body mass , show greater working capacity of small birds, especially in Passerin es. This energetic feature allows small Passerines to survive better i n the regions with very Low temperatures, while the food is abundant. An animal with greater work output has an advantage that is readily co mprehsible in selective terms. It can sustain greater levels of pursui t of flight in gathering food or avoiding predators. It will be more s uccessful in territorial defense, in courtship and mating. Therefore t he enhanced capacities give their possessor the ability to increase en ergy intake to meet new energy demands.