The intrinsic rate of natural increase, r, was calculated for 44 mammalian species using the Cole (1954) equation and life history data from the literature. Values of r so calculated were plotted as log r versus log body mass revealing a linear relationship with a slope of-0.2622. The equation of the regression line fitting these data was then used to correct r for body mass so that interspecific comparisons with respect to r and basal metabolic rate could be made to determine if a positive relationship exists between these two parameters. Basal metabolic rate correlates positively with r , and apparently is one of many factors operating in the evolution of r. Implications of these conclusions with respect to food habits, resource limitations, and the possible existence of a trade-off between maintenance and reproduction in certain environments is discussed.If one assumes that all mammals face environmental limits on the amount of energy available for maintenance, growth, and reproduction, it follows that any reduction in maintenance costs should provide more energy for growth and/or reproduction. The proposed existence of such a trade-off between maintenance and reproduction was a major premise upon which MacArthur and Wilson (1967) based their concept of "r- and K-selection". Recently, however, McNab (1980) has suggested that for mammals that reproduce when food is not limiting, an increase in one maintenace cost, i.e. basal metabolic rate, may not detract from but may actually increase the intrinsic rate of natural increase, r . Although this idea may seem counterintuitive, if one assumes an unlimited amount of energy, the factor limiting growth and reproduction will be the rate at which the energy can be used; a higher metabolic rate will mean a higher rate of biosynthesis, a faster growth rate, s shorter generation time, and hence a higher r . Since some animal species appear not to be food-limited during their reproductive seasons (Armitage and Downhower 1974; Millar 1977; Rabenold 1979), natural selection may favor maximization of metabolic rate as a means of maximizing r in these species. If these hypotheses concerning the correlation between basal metabolic rate and r are correct, a comparison of mammalian species with respect to basal metabolic rate and r should reveal a positive relationship between these two parameters.

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Condor 8491-96 0 The Cooper Ornithological Society 1982 * M, = basal metabolic rate, T, = body temperature, T,, = lower critical temperature, C, = minimal thermal conductance. ** M, predicted by the equatmn of Aschoff and Pohl (1970). *** C, predicted by the equation of Lasiewski et al. (1967).

Previous studies have indicated that some cardiac control systems, such as autonomic innervation, are incomplete in the newborn and undergo postnatal maturation. However, relatively little is known of maturational changes which will affect the ability of the coronary circulation to regulate blood flow during various interventions. We tested the hypothesis that the coronary circulation of the rabbit develops a progressive, age related increase in responsiveness to exogenous adenosine during the first 16 weeks of life. We examined the response of coronary circulation to exogenous adenosine in four age groups (6-10 days, 3-4 weeks, 7-8 weeks, 16 weeks) of rabbits by using an isolated heart preparation. Coronary flow was measured during an control period, during reactive hyperemia (following release of a 2 min coronary flow interruption), and during infusions of exogenous adenosine. All age groups had similar levels of control coronary blood flow, with a large increase in flow observed during reactive hyperemia. The volume of coronary flow was higher during reactive hyperemia (P less than 0.001) in the 6-10-day-old animals than in the other age groups. However, the 6-10-day-old age group showed a smaller (P less than 0.01) flow response to exogenous adenosine than the other age groups. In addition, the 6-10-day-old animals required higher adenosine concentrations to produce an initial detectable coronary flow increase and to produce the maximal flow response. Additional studies demonstrated a significant reduction in percentage flow debt repayment after theophylline administration in adult rabbit hearts. However, neonatal rabbit hearts showed no change in flow debt repayment following theophylline.(ABSTRACT TRUNCATED AT 250 WORDS)