Heart rate variability during high-intensity exercise

The aim of this paper is to describe and analyse the behaviour of heart rate variability (HRV) during constant-load, high-intensity exercise using a time frequency analysis (Wavelet Transform). Eleven elite cyclists took part in the study (age: 18.6±3.0 years; VO_2max: 4.88±0.61 litres·min^?1). Initially, all subjects performed an incremental cycloergometer test to determine load power in a constant load-test (379.55±36.02 W; 89.0%). HRV declined dramatically from the start of testing (p <0.05). The behaviour of power spectral density within the LF band mirrored that of total energy, recording a significant decrease from the outset LF peaks fell rapidly thereafter, remaining stable until the end of the test. HF-VHF fell sharply in the first 20 to 30 seconds. The relative weighting (%) of HF-VHF was inverted with the onset of fatigue, [1.6% at the start, 7.1 (p <0.05) at the end of the first phase, and 43.1% (p <0.05) at the end of the test]. HF-VHF_peak displayed three phases: a moderate initial increase, followed by a slight fall, thereafter increasing to the end of the test. The LF/HF-VHF ratio increased at the start, later falling progressively until the end of the first phase and remaining around minimal values until the end of the test.     

A diploid population ofPoa annua L. from Australia

Résumé On décrit une population diploide dePoa annua L. d'Australie. Elle se montre différente des «mphihaploides» qui ont été trouvées en Californie. La signification de cette population en rapport avec l'origine supposée deP. annua est considerée.

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Acknowledgement. This work was supported by a grant from the Nuffield Foundation.     

Pervasive density-dependent recruitment enhances seedling diversity in a tropical forest

Negative density-dependent recruitment of seedlings, that is, seeds of a given species are less likely to become established seedlings if the density of that species is high, has been proposed to be an important mechanism contributing to the extraordinary diversity of tropical tree communities because it can potentially prevent any particular species from usurping all available space, either in close proximity to seed sources or at relatively larger spatial scales. However, density-dependent recruitment does not necessarily enhance community diversity. Furthermore, although density-dependent effects have been found at some life stages in some species, no study has shown that density-dependent recruitment affects community diversity. Here we report the results of observations in a lowland, moist forest in the Republic of Panamá in which the species identities of 386,027 seeds that arrived at 200 seed traps were compared with the species identities of 13,068 seedlings that recruited into adjacent plots over a 4-year period. Across the 200 sites, recruit seedling diversity was significantly higher than seed diversity. Part of this difference was explained by interspecies differences in average recruitment success. Even after accounting for these differences, however, negative density-dependent recruitment contributes significantly to the increase in diversity from seeds to seedling recruits.     

Kelvin-Helmholtz instabilities as the source of inhomogeneous mixing in nova explosions

Classical novae are thermonuclear explosions in binary stellar systems containing a white dwarf accreting material from a close companion star. They repeatedly eject 10(-4)-10(-5) solar masses of nucleosynthetically enriched gas into the interstellar medium, recurring on intervals of decades to tens of millennia. They are probably the main sources of Galactic (15)N, (17)O and (13)C. The origin of the large enhancements and inhomogeneous distribution of these species observed in high-resolution spectra of ejected nova shells has, however, remained unexplained for almost half a century. Several mechanisms, including mixing by diffusion, shear or resonant gravity waves, have been proposed in the framework of one-dimensional or two-dimensional simulations, but none has hitherto proven successful because convective mixing can only be modelled accurately in three dimensions. Here we report the results of a three-dimensional nuclear-hydrodynamic simulation of mixing at the core-envelope interface during nova outbursts. We show that buoyant fingering drives vortices from the Kelvin-Helmholtz instability, which inevitably enriches the accreted envelope with material from the outer white-dwarf core. Such mixing also naturally produces large-scale chemical inhomogeneities. Both the metallicity enhancement and the intrinsic dispersions in the abundances are consistent with the observed values.     

The hummingbird's restraint: A natural model for weight control

Summary By day, a breeding male hummingbird gives priority to aerial performance over energy security, minimizing body weight gain to expedite flight activity. We observed behavior and monitored weight via perches on electronic balances in a territory with unlimited resources (artificial feeder). The male restricted his first feeding to less than 1/6 of crop capacity. In the ensuing 15 h, his body weight increased only 1% despite an unlimited resource of artificial nectar. This facilitated acceleration for pursuit and courtship flights and reduced energy cost. In 20 min at dusk, an intake of 34% of body weight provided adequate energy for temperature regulation all night. In July, cessation of territorial activity was accompanied by a regular upward drift of dawn weight, suggesting that weight had been regulated previously as a balance between expenditure and intake, perhaps without involving set-points.This research was supported by grants 3242-85 and 3513-87 from the National Geographic Society. Mr William Barber donated Perky-Pet hummingbird feeders. We thank Dana Bradley-Spencer, Ian Billick, Josephine Axt, and Sonda Eastlack for technical assistance. Federal banding permit #8081; Colorado #0063-87-89. Several colleagues and authorities and two anonymous reviewers contributed time and helpful advice following their thoughtful reading of various drafts of this paper. To the extent that their views diverged or our stubbornness persisted, it would unfair to implicate them by names.     

Electronically soft phases in manganites

The phenomenon of colossal magnetoresistance in manganites is generally agreed to be a result of competition between crystal phases with different electronic, magnetic and structural order; a competition which can be strong enough to cause phase separation between metallic ferromagnetic and insulating charge-modulated states. Nevertheless, closer inspection of phase diagrams in many manganites reveals complex phases where the two order parameters of magnetism and charge modulation unexpectedly coexist. Here we show that such experiments can be naturally explained within a phenomenological Ginzburg-Landau theory. In contrast to models where phase separation originates from disorder or as a strain-induced kinetic phenomenon, we argue that magnetic and charge modulation coexist in new thermodynamic phases. This leads to a rich diagram of equilibrium phases, qualitatively similar to those seen experimentally. The success of this model argues for a fundamental reinterpretation of the nature of charge modulation in these materials, from a localized to a more extended 'charge-density wave' picture. The same symmetry considerations that favour textured coexistence of charge and magnetic order may apply to many electronic systems with competing phases. The resulting 'electronically soft' phases of matter with incommensurate, inhomogeneous and mixed order may be general phenomena in correlated systems.     

Evaporative cooling in the zebra finch

Zusammenfassung O₂-Verbrauch und Gesamt-Wasserverdunstung wurden bei australischen Zebrafinken gemessen. Bei geringem Wärmeaustausch zwischen Körper und Umgebung durch Strahlung, Konvektion und Ableitung können diese Wüstenvögel den für die Wärmebilanz erforderlichen Wärmeabfluss einzig durch Verdunstung auf dem normalen Niveau halten. Eine solche Fähigkeit ist bei Passeres bisher nicht beobachtet worden.

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This investigation was supported in part by funds provided for medical and biological research by State of Washington Initiative Measure No. 171, and in part by a Public Health Service research career program award (No. AM-K3-18, 370) from the National Institute of Arthritis and Metabolic Diseases.     

Large changes in Pluto's atmosphere as revealed by recent stellar occultations

Pluto's tenuous nitrogen atmosphere was first detected by the imprint left on the light curve of a star that was occulted by the planet in 1985 (ref. 1), and studied more extensively during a second occultation event in 1988 (refs 2-6). These events are, however, quite rare and Pluto's atmosphere remains poorly understood, as in particular the planet has not yet been visited by a spacecraft. Here we report data from the first occultations by Pluto since 1988. We find that, during the intervening 14 years, there seems to have been a doubling of the atmospheric pressure, a probable seasonal effect on Pluto.     

The structure of ceroplastodiol,a new tricyclic sesterterpene isolated from insect wax

Summary The isolation and structure elucidation of cereoplastodiol, a new tricyclic sesterterpene isolated from wax of the insectCeroplastes albolineatus are reported.Contribution No. 550 from Instituto de Química de la Universidad Nacional Autónoma de México.