By Jessica M. Castillo
A diving bird, the torrent duck is like a seal or penguin. Not in looks, of course, but in physiology. Like emperor penguins or Weddell seals, University of Miami researchers discovered, torrent ducks have among the highest concentrations of myoglobin, the protein that holds oxygen in muscle tissue.
That’s partly how this tiny waterbird species, averaging less than a pound, expertly dives in high-altitude rivers in the Peruvian Andes and has managed to survive for close to a million years in permanent states of hypoxia, or lack of oxygen. Travelers can see the ducks in the surging whitewater on the train ride to Machu Picchu.
“In most environments where you encounter hypoxia, it’s usually where animals will be able to escape it at some point,” said Neal Dawson, a postdoctoral biology researcher at the UM College of Arts and Sciences and McMaster University in Ontario, Canada, who with fellow UM biologists Kevin McCracken and Luis Alza discovered this adaptive trait on a July 2015 expedition. “In high altitudes, there’s no escaping it. These territorial ducks are stuck year-round with this hypoxia environment.”
The researchers’ expedition was part of a larger study, started in 2010, where scientists from UM, McMaster University, the University of British Columbia, the University of Alaska Fairbanks, and the CORBIDI institute in Peru set out to uncover what helps birds in the Andes thrive in such a high-altitude, low-oxygen environment.
For this latest study, the waterbird biologists focused on the torrent duck (Merganetta armata), a reluctant flier which, except when nesting, lives almost exclusively in the water. Working in remote villages north of Lima, the researchers learned how the ducks are able to hold their breath for a long time during repeated dives at high elevation by measuring their enzyme function across different metabolic pathways and the energy generation rates of their mitochondria, the cell’s powerhouse.
“As a diving species, the torrent duck will do everything it can to not fly,” noted Dawson, lead author of the study. “So, instead of looking at its ability to fly, we looked at its ability to dive. We looked at its leg muscles, enzymatic muscle activity, properties of the blood, and body measurements and noticed there are a whole host of changes that have really ameliorated its ability to dive at high altitudes.”
The bird has uniquely adapted to its resource-poor habitat. In addition to high levels of myoglobin, the researchers found that the torrent duck has high levels of hemoglobin, the oxygen-carrying protein in red blood cells. That enables it to “max out what their oxygen stores can be,” said McCracken, the James A. Kushlan Chair in Waterbird Biology and Conservation.
“If your hemoglobin levels were at 18 or 19 grams per deciliter, you’d be visiting your doctor to discuss it,” McCracken said. “The torrent ducks are going as high as 25. Double the concentration of yours. You would probably go into cardiac arrest if your hemoglobin was as high as a torrent duck’s.”
The scientists also discovered that the torrent duck’s hemoglobin has a high affinity for oxygen, which may affect its recovery time after a dive. The duck’s dive time is limited by the amount of oxygen it can hold in its lungs, air sacs, and intercellular oxygen stores.
“Once this oxygen runs out, the torrent duck has to resurface and recover quickly to be able to dive again,” said Dawson.
Recovery time is especially crucial for this species. “For the torrent duck, everything happens in the river,” said Alza, a doctoral student in the Department of Biology. Just as soon as they’re out of the water, the ducks are looking to dive back in for food or to escape external threats.
The oxygen-deprived torrent ducks also seem to be adapting to the high elevation by adjusting their energy consumption and fuel usage. Humans have access to energy from various fuel sources—including carbohydrates, fats and sugars. But most mammals in the wild are typically at the mercy of their environment and eat whatever energy sources are available; some even becoming specialists in digesting one source over another.
Digesting carbohydrates is more efficient per unit of oxygen, whereas digesting fats is more energy efficient per the actual amount of food that’s eaten.
The research team is currently analyzing the data but their preliminary results seem to contradict their initial hypothesis that this hypoxic species would tend to eat more carbs. The torrent ducks, it seems, tend to eat fats—mostly from aquatic insects—over carbs.
“Food might be more of a scarce resource than even oxygen at high altitudes,” said Dawson, who emphasized it’s still too early to say with certainty, but it looks like that’s what’s happening. “These ducks may be prioritizing getting their biggest bang for their buck every time they have a meal.”
The researchers also noted that, just over the last few years, they have seen a decline in the number of river ducks, which may be related to the impacts of the changing climate and increasing development near the duck’s habitat.
Poor development and infrastructure building pollutes the rivers with silt, choking aquatic insects and crashing the insect population. The ducks, with a staple diet of aquatic insects, must either move to another river or starve and die out.
Lima and other large cities along Peru’s Pacific coast depend on water from precipitation and glacial runoff. But the warming climate has melted many of the glaciers. In turn, rivers and artificial lagoons, built within the last 50 years for water retention, are drying up. Add in large and still growing urban populations and the pressures mount.
What does this mean for the ducks?
“No water, no ducks,” said Alza. “The duck is an easy way to monitor the river’s condition. If the ducks are still around, the river is quite healthy then.”
“The torrent ducks are a sentinel for conservation, particularly for protecting water resources used by local indigenous communities,” added McCracken, who also holds appointments as associate professor in the Department of Biology at the College of Arts and Sciences and in the Department of Marine Biology and Ecology at the Rosenstiel School for Marine and Atmospheric Science.
The researchers still have some more studies they’d like to conduct, such as on migration of juvenile ducks up and down the river, which would provide clues about the interplay between nature and nurture, or genetics and development in different environments. The scientists also would like to compare the torrent duck, a river duck with a vertical habitat, with the ruddy duck (Oxyura jamaicensis), another Andean diving bird species whose habitat is more isolated—in lakes and at either low or high altitude.
Their study “Mitochondrial physiology in the skeletal and cardiac muscles is altered in torrent ducks, Merganetta armata, from high altitudes in the Andes,” is published online in the Journal of Experimental Biology.