© 1999 Macmillan Magazines Ltd High metabolic rates in running birds The ability to increase metabolic rate during locomotion has been important in the structural evolution and evolutionary suc- cess of both birds and mammals. Greater endurance capabilities are conferred directly by greater maximal metabolic rates, which vary between species. These maximal rates are known for many mammals1 but have not been determined for birds. We have mea- sured oxygen consumption in a large flight- less bird, the rhea, Rhea americana, while it was running on an inclined treadmill, and find an upper limit to aerobic metabolism that is 36 times greater than the minimum resting rate, a factorial increase exceeding that reported for nearly all mammals. We trained two female rheas (average mass 21.8 kilograms) for two years to run on a treadmill while wearing clear plastic hoods. We then measured linear increases in rates of oxygen uptake while the birds ran up a 16° incline. The maximum rate of oxygen uptake was 2.85 millilitres per kilo- gram of body mass per second, reached at a running speed of 4.0 metres per second. There were no further increases in oxygen uptake when running speeds were increased to 4.7 metres per second, and plasma lactate concentrations indicated an increasing reliance on anaerobic glycolysis to provide metabolic energy at these higher speeds. Rates of lactate accumulation (4.4 to 6.8 mM min11) and peak concentrations (24.8 mM) matched values reported for mam- mals running at speeds above their aerobic maximum. Remarkably, the aerobic scope (the maximum aerobic metabolic rate divided by the minimum resting rate2) of these relatively inactive birds is more than 3.5 times those of typical mammals1, and equals those of some of the most capable mammalian endurance runners (Fig. 1). Although the rheas’ factorial increase in anaerobic metabolism is 1.7 times greater than the highest previously reported avian values (Fig. 1), we believe that their aerobic scope is representative of birds in general. Previous avian values do not provide evi- dence of being upper limits; they are proba- bly below species’ maxima, but to unknown extents. Nonetheless, the minimum meta- bolic increase required for flight3 of 15 times the resting value exceeds the aerobic power of a typical mammal by a factor of 1.5, and most avian species can fly. Even birds with more limited endurance, such as the pen- guin and domestic fowl, can increase their aerobic metabolism by as much as a typical mammal4,5. We suggest that, on average, the aerobic scope of birds exceeds those of mammals by a factor of two or more. The structural basis of the rheas’ aero- bic power indicates that respiratory structure–function relationships, previously undetermined for birds, are quantitatively similar to those of mammals throughout most of the respiratory system, and are consistent with the concept of reasonable animal design6 (Table 1). Rheas, like aerobic mammals, achieve maximal high rates of oxygen uptake with existing cardiovascular and muscular structures, rather than by scientific correspondence NATURE | VOL 397 | 7 JANUARY 1999 | www.nature.com 31 humidity and climate variations related to the North Atlantic oscillation11,12, showed that none of them, taken individually, is a good explanation for the observed positive trend in cirrus occurrence and its regional distribution. The trends in cirrus ‘amount when pre- sent’ over the previously defined regions of high fuel consumption are 11.9% and 14.2% for land and ocean, respectively (Table 1). The combination of a large posi- tive trend in cirrus occurrence associated with a negative trend in the cirrus amount when present is consistent with an increas- ing amount of condensation-trail cirrus covering a small portion of an otherwise clear sky at the cirrus cloud level. This results in a trend in cirrus coverage (cirrus occurrence 2 cirrus amount when pre- sent) of 0.0% averaged over land, rising to 2.9% over the continental regions with most air traffic. Over ocean, these values are 2.3% and 1.0%, respectively, because of the aforementioned contribution of the North Pacific Ocean to the global increase in cirrus occurrence. Both radiative trans- fer calculations and Earth Radiation Bud- get Experiment measurements of the net change in radiation associated with high, thin clouds13 suggest a 0.2 W m12 cloud radiative forcing per 1% of condensation trail or high-level cloudiness. I therefore estimate that the trend in cirrus amount over the continental regions of high air traffic (2.9%, as discussed above) would correspond locally to an additional cloud radiative forcing of about 0.7 W m12 between 1982 and 1991. A larger forcing would be expected if calculated from the beginning of commercial jet traffic until the present time. However, we cannot rule out the possibility that the observed change in cirrus amount is not due solely to avia- tion, or that other climate processes have not masked an even larger aviation impact on cirrus cloudiness. For instance, a previ- ous analysis14 showed that other regions experienced large positive trends in cirrus amount between 1952 and 1981. Olivier Boucher Laboratoire d’Optique Atmosphérique, U.F.R. de Physique, Université de Lille–I, 59655 Villeneuve d’Ascq Cedex, France e-mail: boucher@loa.univ-lille1.fr 1. Seinfeld, J. H. Nature 391, 837–838 (1998). 2. Minnis, P. et al. Geophys. Res. Lett. 25, 1157–1160 (1998). 3. Schumann, U. Meteorol. Z. 5, 4–23 (1996). 4. Jensen, E. J. & Toon, O. B. Geophys. Res. Lett. 4, 249–252 (1997). 5. Hahn, C. J., Warren, S. G. & London, J. Edited Synoptic Cloud Reports from Ships and Land Stations Over the Globe, 1982–1991, NDP026B (Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Tennessee, 1996). 6. Baughcum, S. L., Henderson, S. C., Tritz, T. G. & Pickett, D. C. Scheduled Civil Aircraft Emission Inventories for 1992: Database Development and Analysis (NASA CR-4700, 1996). 7. Mortlock, A. M. & van Alstyne, R. Military, Charter, Unreported Domestic Traffic and General Aviation: 1976, 1984, 1992, and 2015 Emission Scenarios (NASA CR-207639, 1998). 8. Energy Statistics and Balances of Non-OECD Countries, 1994–1995 124–125 (International Energy Agency, Organization for Economic Cooperation and Development, Paris, 1997). FFiigguurree 11 Factorial increases in metabolic rates above resting minimum for rheas and athletic mammals at the aerobic maximum and for other avian species at the highest rates available. Triangles, flying birds; squares, walking and running birds; circles, swim- ming birds; and diamonds, running mammals. 9. Minnis, P., Ayers, J. K. & Weaver, S. P. Surface-Based Observations of Contrail Occurrence Frequency over the U. S., April 1983–April 1994 (NASA Reference Publication 1404, 1997). 10. Jensen, E. J. & Toon, O. B. Geophys. Res. Lett. 19, 1759–1762 (1992). 11. Hurrell, J. W. Science 269, 676–679 (1995). 12. Mächel, H., Kapala, A. & Flohn, H. Int. J. Climatol. 18, 1–22 (1998). 13. Hartmann, D. L., Ockert-Bell, M. E. & Michelsen, M. L. J. Clim. 5, 1281–1304 (1992). 14. Warren, S. G., Hahn, C. J., London, J., Chervin, R. M. & Jenne, R. L. Global Distribution of Total Cloud Cover and Cloud Type Amounts over the Ocean (NCAR Technical Note TN-317 + STR, Boulder, Colorado, 1988). 15. Hahn, C. J., Warren, S. G. & London, J. J. Clim. 8, 1429–1446 (1995). Supplementary information is available on Nature’s World-Wide Web site (http://www.nature.com) or as paper copy from the London editorial office of Nature. Humming- bird Budgerigar Raven Fox Coyote Rhea Wolf Pony Ostrich Emu TurkeyPigeonMost mammals Penguin Stork Penguin 40 30 20 10 0 0.001 0.01 0.1 1 10 100 1,000 Body mass (kg) · E · E lo co / st d Table 1 Mitochondrial volume densities and capillary densities of rhea muscles Muscle Bird A Bird B Vv (%) Na (mm –2) Vv (%) Na (mm –2) Iliotibialis lateral 10.1 803 6.5 787 Gastrocnemius 11.8 864 7.9 642 Pectoralis 4.2 411 4.2 469 Humerotriceps 4.3 619 4.9 508 Mean mitochondrial volume densities (Vv) and capillary densities (Na) for leg muscles (iliotibialis and gastrocnemius) were 9.04% and 774 mm–2, respectively; values for wing muscles (pectoralis and humerotriceps) were 4.37% and 502 mm–2, respectively, for the rheas. Values for equivalent leg muscles of similarly sized mammals: for a 28-kg dog, Vv 4 9.1% and Na 4 884 mm –2; for a 26-kg goat, Vv 4 3.4% and Na 4 719 mm –2. © 1999 Macmillan Magazines Ltd The Thomas Jefferson paternity case The DNA analysis of Y-chromosome haplo- types used by Foster et al.1 to evaluate Thomas Jefferson’s alleged paternity of Eston Hemings Jefferson, the last child of his slave Sally Hemings, is impressive. How- ever, the authors did not consider all the data at hand in interpreting their results. No mention was made of Thomas Jeffer- son’s brother Randolph (1757–1815), or of his five sons2,3. Sons of Sally Hemings con- ceived by Randolph (or by one of his sons) would produce a Y-chromosome analysis identical to that described by Foster et al. Further collaborative data (for example, the whereabouts of any of those who might have been involved at conception) are needed to confirm that Jefferson did indeed father his slave’s last child, as claimed in the title. We know Thomas Jefferson was there, but how about Randolph Jefferson and his sons? David M. Abbey Health Science Center, University of Colorado, 1512 Teakwood Court, Fort Collins, Colorado 80525, USA e-mail: dabbey1000@aol.com If the data of Foster et al. are accurate, then any male ancestor in Thomas Jefferson’s line, white or black, could have fathered Eston Hemings. Plantations were inbred communities, and the mixing of racial types was probably common. As slave families were passed as property to the owner’s off- spring along with land and other property, it is possible that Thomas Jefferson’s father, grandfather or paternal uncles fathered a male slave whose line later impregnated another slave, in this case Sally Hemings. Sally herself was a light mulatto, known even at that time to be Thomas Jefferson’s wife’s half sister3,4. Gary Davis Evanston Hospital, 2650 Ridge Avenue, Evanston, Illinois 60201, USA Foster et al. reply — It is true that men of Randolph Jefferson’s family could have fathered Sally Hemings’ later children. Space constraints prevented us from expanding on alternative interpretations of our DNA analysis, including the interesting one proposed by Davis. The title assigned to our study was misleading in that it repre- sented only the simplest explanation of our molecular findings: namely, that Thomas Jefferson, rather than one of the Carr broth- ers, was likely to have been the father of Eston Hemings Jefferson. It had been suggested to us earlier (by Herbert Barger, who also helped to recruit the descendants of Field Jefferson who par- ticipated in our study) that Isham Jefferson, son of Thomas Jefferson’s brother Ran- dolph, might have been the father of one or more of Sally Hemings’ children. Barger’s proposal was based on a statement5 that Isham was reared by Thomas Jefferson; he suggested that Isham could have been at Monticello or at Snowden (Snowden was across the James River from Scottsville, which is about 20 miles from Monticello) when Eston Hemings was conceived. But it is not known for certain that Isham was at Monticello at that time, whereas it is docu- mented that Thomas Jefferson was. From the historical knowledge we have, we can- not conclude that Isham, or any other member of the Jefferson family, was as likely as Thomas Jefferson to have fathered Eston Hemings. We know from the historical and the DNA data that Thomas Jefferson can neither be definitely excluded nor solely implicated in the paternity of illegitimate children with his slave Sally Hemings. When we embarked on this study, we knew that the results could not be conclusive, but we hoped to obtain some objective data that would tilt the weight of evidence in one direction or another. We think we have provided such data and that the modest, probabilistic interpretations we have made are tenable at present. E. A. Foster*, M. A. Jobling†, P. G. Taylor†, P. Donnelly‡, P. de Knijff§, R. Mieremet§, T. Zerjal¶, C. Tyler-Smith¶ *6 Gildersleeve Wood, Charlottesville, Virginia 22903, USA e-mail: eafoster@aol.com †Department of Genetics, University of Leicester, Adrian Building, University Road, Leicester LE1 7RH, UK ‡Department of Statistics, University of Oxford, South Parks Road, Oxford OX1 3TG, UK §MGC Department of Human Genetics, Leiden University, PO Box 9503, 2300 RA Leiden, The Netherlands ¶Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, UK 1. Foster, E. A. et al. Nature 396, 27–28 (1998). 2. Mayo, B. & Bear, J. A. Jr Thomas Jefferson and his Unknown Brother (Univ. Press of Virginia, Charlottesville, 1981). 3. Brodie, F. M. Thomas Jefferson: An Intimate History (Norton, New York, 1974). 4. Randall, W. S. Thomas Jefferson: A Life (Holt, New York, 1993). 5. History of Todd County, Kentucky (1884). scientific correspondence 32 NATURE | VOL 397 | 7 JANUARY 1999 | www.nature.com qualitative adjustment. Rhea heart mass (267 grams) conforms to that predicted for a mammal of the same body size and aero- bic power. Volume densities of mitochon- dria in leg muscles are similar to those of mammals of the same size and aerobic capacity, whereas those in the relatively inactive flight muscles are only half as large and correspond to values reported for less aerobic mammals (Table 1). Given that 30% of the rhea’s body mass consists of leg musculature, rates of oxygen uptake by mitochondria per millilitre dur- ing locomotion at the aerobic maximum appear to fall within the range reported for mammals. Capillary densities in the rhea leg and flight muscles, like the mitochon- drial volume densities, also parallel values reported for the muscles of athletic and less mobile mammals. In contrast to the apparent conservation of structure–function relationships in most of the respiratory system, our results sug- gest that there are basic differences in the performance of the lungs of birds and mammals. We could not measure the lung volumes of the rheas directly, but in birds these are normally slightly more than half of those of mammals of the same mass7,8. The rheas therefore achieved maximum oxygen flux rates, equal to those of the most aerobic mammals of their size, using lungs that are probably only half as large. This supports the general belief that avian lungs provide relatively more function per unit volume than mammalian lungs8,9. Although the aerobic limits of rheas and athletic mammals are similar, the metabolic power available in practice, and their func- tional needs, are not. Unlike dogs, horses and other athletic mammals that sustain high metabolic rates for hours during pre- dation and migration, rheas do little or no sustained running and are poor at dissipat- ing metabolic heat loads10. Rheas have apparently not been under strong selective pressures like those that promoted the aero- bic power of extant running mammals. Large flightless birds lead fairly inactive lives, and may have lost the ability to fly primarily because of a lack of predation. Why rheas have an aerobic power that greatly exceeds their apparent functional needs remains a puzzle. Matthew W. Bundle*, Hans Hoppeler†, Ruth Vock†, June M. Tester*, Peter G. Weyand* *Museum of Comparative Zoology, Harvard University, Bedford, Massachusetts 01730, USA e-mail: pweyand@oeb.harvard.edu †Department of Anatomy, University of Bern, Buhlstrasse 26, CH 3000 Bern 9, Switzerland 1. Seeherman, H. J., Taylor, C. R., Maloiy, G. M. O. & Armstrong, R. B. Respir. Physiol. 44, 11–23 (1981). 2. Lasiewski, R. C. & Dawson, W. R. Condor 69, 13–23 (1967). 3. Ellington, C. P. J. Exp. Biol. 160, 71–91 (1991). 4. Brackenbury, J. H. & El-Sayed, M. S. J. Exp. Biol. 117, 349–355 (1985). 5. Kooyman, G. L. & Ponganis, P. J. J. Exp. Biol. 195, 199–209 (1994). 6. Weibel, E. R., Taylor, C. R. & Hoppeler, H. Proc. Natl Acad. Sci. USA 88, 10357–10361, (1991). 7. Maina, J. N. & King, S. A. J. Anat. 163, 67–73 (1989). 8. Dubach, M. Respir. Physiol. 46, 43–60 (1981). 9. Piiper, J. & Scheid, P. in Comparative Physiology (eds Bolis, L., Schmidt-Nielsen, K. & Maddrell, S. H. P.) 161–185 (North- Holland, 1973). 10. Taylor, C. R., Dmi’el, R., Fedak, M. & Schmidt-Neilsen, K. Am. J. Physiol. 51, 772–776 (1981).