Torrent ducks inhabit fast-flowing rivers in the Andes from sea level to altitudes up to 4500 m. We examined the mitochondrial physiology that facilitates performance over this altitudinal cline by comparing the respiratory capacities of permeabilized fibers, the activities of 16 key metabolic enzymes and the myoglobin content in muscles between high-and low-altitude populations of this species. Mitochondrial respiratory capacities (assessed using substrates of mitochondrial complexes I, II and/or IV) were higher in highland ducks in the gastrocnemius muscle -the primary muscle used to support swimming and diving -but were similar between populations in the pectoralis muscle and the left ventricle. The heightened respiratory capacity in the gastrocnemius of highland ducks was associated with elevated activities of cytochrome oxidase, phosphofructokinase, pyruvate kinase and malate dehydrogenase (MDH). Although respiratory capacities were similar between populations in the other muscles, highland ducks had elevated activities of ATP synthase, lactate dehydrogenase, MDH, hydroxyacyl CoA dehydrogenase and creatine kinase in the left ventricle, and elevated MDH activity and myoglobin content in the pectoralis. Thus, although there was a significant increase in the oxidative capacity of the gastrocnemius in highland ducks, which correlates with improved performance at high altitudes, the variation in metabolic enzyme activities in other muscles not correlated to respiratory capacity, such as the consistent upregulation of MDH activity, may serve other functions that contribute to success at high altitudes.

The metabolic cost of breathing at rest has never been successfully measured in birds, but has been hypothesized to be higher than in mammals of a similar size because of the rocking motion of the avian sternum being encumbered by the pectoral flight muscles. To measure the cost and work of breathing, and to investigate whether species resident at high altitude exhibit morphological or mechanical changes that alter the work of breathing, we studied 11 species of waterfowl: five from high altitudes (>3000 m) in Peru, and six from low altitudes in Oregon, USA. Birds were anesthetized and mechanically ventilated in sternal recumbency with known tidal volumes and breathing frequencies. The work done by the ventilator was measured, and these values were applied to the combinations of tidal volumes and breathing frequencies used by the birds to breathe at rest. We found the respiratory system of high-altitude species to be of a similar size, but consistently more compliant than that of lowaltitude sister taxa, although this did not translate to a significantly reduced work of breathing. The metabolic cost of breathing was estimated to be between 1 and 3% of basal metabolic rate, as low or lower than estimates for other groups of tetrapods.

We examined the control of breathing and respiratory gas exchange in six species of high-altitude duck that independently colonized the high Andes. We compared ducks from high-altitude populations in Peru (Lake Titicaca at ∼3800 m above sea level; Chancay River at ∼3000-4100 m) with closely related populations or species from low altitude. Hypoxic ventilatory responses were measured shortly after capture at the native altitude. In general, ducks responded to acute hypoxia with robust increases in total ventilation and pulmonary O 2 extraction. O 2 consumption rates were maintained or increased slightly in acute hypoxia, despite ∼1-2°C reductions in body temperature in most species. Two high-altitude taxayellow-billed pintail and torrent duckexhibited higher total ventilation than their low-altitude counterparts, and yellow-billed pintail exhibited greater increases in pulmonary O 2 extraction in severe hypoxia. In contrast, three other high-altitude taxa-Andean ruddy duck, Andean cinnamon teal and speckled tealhad similar or slightly reduced total ventilation and pulmonary O 2 extraction compared with low-altitude relatives. Arterial O 2 saturation (S aO2) was elevated in yellow-billed pintails at moderate levels of hypoxia, but there were no differences in S aO2 in other high-altitude taxa compared with their close relatives. This finding suggests that improvements in S aO2 in hypoxia can require increases in both breathing and haemoglobin-O 2 affinity, because the yellow-billed pintail was the only high-altitude duck with concurrent increases in both traits compared with its low-altitude relative. Overall, our results suggest that distinct physiological strategies for coping with hypoxia can exist across different high-altitude lineages, even among those inhabiting very similar high-altitude habitats.

Feathers are one of the defining characteristics of birds and serve a critical role in thermal insulation and physical protection against the environment. Feather structure is known to vary among individuals, and it has been suggested that populations exposed to different environmental conditions may exhibit different patterns in feather structure. We examined both down and contour feathers from two populations of male Torrent Ducks (Merganetta armata) from Lima, Peru, including one high-altitude population from the Chancay-Huaral River at approximately 3500 meters (m) elevation and one low-altitude population from the Chillón River at approximately 1500 m. Down feather structure differed significantly between the two populations. Ducks from the high-altitude population had longer, denser down compared with low-altitude individuals. Contour feather structure varied greatly among individuals but showed no significant difference between populations. These results suggest that the innermost, insulative layer of plumage (the down), may have developed in response to lower ambient temperatures at high elevations. The lack of observable differences in the contour feathers may be due to the general constraints of the waterproofing capability of this outer plumage layer.ResumenEl plumaje es una característica que define a las aves y cumple roles críticos en el aislamiento térmico y protección física del ambiente. Se sabe que la estructura de las plumas varía ente individuos, y se ha sugerido que poblaciones expuestas a diferentes condiciones ambientales pueden exhibir diferentes patrones en la estructura de las plumas. En este estudio se examinaron tanto el plumón como las plumas de contorno de machos adultos del Pato de los Torrentes (Merganetta armata) de dos poblaciones, una en el río Chancay-Huaral a 3,500 msnm y otra en el río Chillón a 1,500 msnm, ubicadas en Lima, Perú. La estructura de los plumones difiere significativamente entre las dos poblaciones. Los patos de la población a grandes elevaciones tienen plumones largos, y densos comparados con los individuos de las partes bajas. La estructura de las plumas de contorno varía ampliamente entre individuos pero no muestra diferencias significativas entre poblaciones. Estos resultados sugieren que las diferencias entre las capas interiores de aislamiento del plumaje (plumón), haberse desarrollado como respuesta en ambientes de bajas temperaturas a grandes elevaciones. En cambio la falta de detectables diferencias en las plumas de contorno puede ser debido a la constante selección en la capacidad impermeable de la capa de plumas exteriores.

The cardiovascular system is critical for delivering O 2 to tissues. Here, we examined the cardiovascular responses to progressive hypoxia in four high-altitude Andean duck species compared with four related low-altitude populations in North America, tested at their native altitude. Ducks were exposed to stepwise decreases in inspired partial pressure of O 2 while we monitored heart rate, O 2 consumption rate, blood O 2 saturation, haematocrit (Hct) and blood haemoglobin (Hb) concentration. We calculated O 2 pulse (the product of stroke volume and the arterial-venous O 2 content difference), blood O 2 concentration and heart rate variability. Regardless of altitude, all eight populations maintained O 2 consumption rate with minimal change in heart rate or O 2 pulse, indicating that O 2 consumption was maintained by either a constant arterial-venous O 2 content difference (an increase in the relative O 2 extracted from arterial blood) or by a combination of changes in stroke volume and the arterial-venous O 2 content difference. Three high-altitude taxa (yellow-billed pintails, cinnamon teal and speckled teal) had higher Hct and Hb concentration, increasing the O 2 content of arterial blood, and potentially providing a greater reserve for enhancing O 2 delivery during hypoxia. Hct and Hb concentration between low-and highaltitude populations of ruddy duck were similar, representing a potential adaptation to diving life. Heart rate variability was generally lower in high-altitude ducks, concurrent with similar or lower heart rates than low-altitude ducks, suggesting a reduction in vagal and sympathetic tone. These unique features of the Andean ducks differ from previous observations in both Andean geese and bar-headed geese, neither of which exhibit significant elevations in Hct or Hb concentration compared with their low-altitude relatives, revealing yet another avian strategy for coping with high altitude.

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∙ Feathers are one of the defining characteristics of birds and serve a critical role in thermal insulation and physical protection against the environment. Feather structure is known to vary among individuals, and it has been suggested that populations exposed to different environmental conditions may exhibit different patterns in feather structure. We examined both down and contour feathers from two populations of male Torrent Ducks (Merganetta armata) from Lima, Peru, including one high‐altitude population from the Chancay‐Huaral River at approximately 3500 m a.s.l. and one low‐altitude population from the Chillón River at approximately 1500 m a.s.l.. Down feather structure differed significantly between the two populations. Ducks from the high‐altitude population had longer, denser down compared with low‐altitude individuals. Contour feather structure varied greatly among individuals but showed no consistent difference between populations. These results suggest that the innermost, insulative layer of plumage (the down), may have developed in response to lower ambient temperatures at high elevations. The lack of observable differences in the contour feathers may be due to the fact that this outer plumage layer is more important as waterproofing than insulation.Resumen ∙ La estructura del plumón del Pato de los Torrentes (Merganetta armata) varia entre bajas y grandes altitudes en los Andes El plumaje es una característica que define a las aves y cumple roles críticos en el aislamiento térmico y protección física del ambiente. Se sabe que la estructura de las plumas varía entre individuos, y se ha sugerido que poblaciones expuestas a diferentes condiciones ambientales pueden exhibir diferentes patrones en la estructura de las plumas. En este estudio se examinaron tanto el plumón como las plumas de contorno de machos adultos del Pato de los Torrentes (Merganetta armata) de dos poblaciones, una en el río Chancay‐Huaral a 3500 m s.n.m. y otra en el río Chillón a 1500 m s.n.m., ubicadas en Lima, Perú. La estructura de plumón difiere significativamente entre las dos poblaciones. Los patos de la población a mayor altitud presentan un plumón más largo, y denso que sus congéneres de menor altitud. La estructura de las plumas de contorno varía ampliamente entre individuos, pero no muestra diferencias significativas entre poblaciones. Estos resultados sugieren que las diferencias entre las capas interiores de aislamiento del plumaje (plumón), pueden haberse desarrollado como respuesta en ambientes de bajas temperaturas a grandes elevaciones. En cambio, la ausencia de diferencias detectables en las plumas de contorno puede deberse al hecho de que esta capa externa es más importante en la impermeabilización que en el aislamiento.

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