247 Interindividual Variability in Body Composition and Resting Oxygen Consumption Rate in Breeding Tree Swallows, Tachycineta bicolor Gary P. Burness 1 ism at rest in its thermoneutral zone (Brody 1945) during its period of daily inactivity (Aschoff and Pohl 1970). A large Ronald C. Ydenberg 2 Peter W. Hochachka 1 comparative data set exists relating BMR to body mass across a wide variety of taxa. Studies investigating BMR have often 1 Department of Zoology, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada; been concerned with accurate determination of the slopes of these allometric relationships (e.g., mammals [Elgar and Har- 2 Behavioural Ecology Research Group, Department of Biological Sciences, Simon Fraser University, Burnaby, vey 1987], birds [Bennett and Harvey 1987]). These studies and others (e.g., Koteja and Weiner 1993) have also been inter- British Columbia V5A 1S6, Canada ested in species that deviate from the regression lines. For a given body mass, two species can vary considerably in their Accepted by G.K.S. 10/31/97 BMRs. As an example, the Virginia opossum (Didelphis virgin- iana) has a BMR that is 30% lower than that predicted for a similar-sized eutherian mammal (Fournier and Weber 1994). ABSTRACT Such deviations also exist in birds. For example, island species have much lower BMRs than mainland species of the same Basal metabolic rate is one of the most widely measured physio- logical traits. Previous studies on lab mice and field-caught liz- body size (McNab 1994). The mechanistic basis underlying variability in BMR among ards suggest that individuals with relatively high basal metabolic rates or standard metabolic rates have relatively large masses of similar-sized species is gradually being determined. McNab (1994) found a positive correlation between BMR and pectoral metabolically active tissues (e.g., heart, kidney, liver). As these are energetically expensive organs, there may be variability be- muscle mass in many flightless birds. Kersten and Piersma (1987) speculated that interspecific differences in BMRs among tween breeding seasons dependent on, for example, availability of prey and capacity for energy intake. We present data from birds reflect differences in the size of a species’ ‘‘metabolic machinery.’’ Daan et al. (1990) subsequently demonstrated that breeding tree swallows (Tachycineta bicolor) collected over two successive seasons. There was no difference between years in those species of birds with relatively high BMRs for their body size have relatively large masses of hearts and kidneys. In fact, resting oxygen consumption rates, although there were signifi- cant interannual differences in the masses of all organs and in an analysis of 22 avian species, these two organs, which contribute only 0.61% of body mass, explained 50% of the tissues except the pectoralis. Interindividual differences in the masses of the kidney and small intestine explained 21% of the variation in BMR (Daan et al. 1990). Both of these organs have exceptionally high oxygen consumption rates in tissue-slice variation in oxygen consumption rates. Although individuals with relatively high resting oxygen consumption rates had rela- preparations (Krebs 1950). Within a species, the relationships between BMR and organ tively large, metabolically active kidneys, they had relatively small intestines and pectoral muscles. This is in contrast to all previous masses are unclear. As the slopes from regressions of BMR on mass vary with taxonomic level (see, e.g., Bennett and Harvey studies on mammals and to the single interspecific study of birds. Oxygen consumption rate also correlated positively with 1987), mechanisms may be acting within species that are differ- ent from those acting between species. The only research on hematocrit. Our results suggest that assumptions of consistent positive relationships between resting oxygen consumption rate this question to date has considered small mammals (Konar- zewski and Diamond 1994, 1995; Koteja 1996; Speakman and and organ masses cannot be extended intraspecifically for birds. McQueenie 1996; Meerlo et al. 1997) and lizards (Garland 1984; Garland and Else 1987). As an estimate of BMR, many studies measure resting meta- Introduction bolic rate (RMR; e.g., Konarzewski and Diamond 1994, 1995). Basal metabolic rate (BMR) is defined as the minimum rate In comparisons among and within strains of inbred mice, Ko- of energy expenditure in a nongrowing, postabsorptive organ- narzewski and Diamond (1994, 1995) demonstrated that indi- viduals with high RMRs have relatively large kidneys, livers, hearts, and intestines. In contrast, relationships between organ Physiological Zoology 71(3):247–256. 1998.  1998 by The University of Chicago. All rights reserved. 0031-935X/98/7103-9752$03.00 mass and BMR are weak or absent in Peromyscus maniculatus 9g13$$my03 05-07-98 05:13:28 pza UC: PHYS ZOO