Catching food is hard enough, but animals that dive underwater in search of a meal face a more difficult challenge: how to get enough oxygen to their muscles to keep searching for food while being unable to breathe. Lots of deep diving animals have evolved ways around this issue. However, diving ducks needed to come up with a different way so that they could remain light enough to fly off after eating their fill. So, ducks power their dives using their energy-expensive leg muscles. But what allows ducks to dive under the water and forage for food, but be unconcerned about the fact that they can't get the oxygen they need from the air to make more energy? To solve this conundrum, Elizabeth Schell and Kevin McCracken of the University of Miami, USA, teamed up with Graham Scott of McMaster University, Canada, Neal Dawson of the University of Glasgow, UK, and Kevin Winker of the University of Alaska Fairbanks, USA, to see how the leg muscles of ducks with different diving abilities might hold the answer to how ducks cope with the lack of air while diving for their food.
The team collected samples from the breast and calf muscles of 17 different duck species in Alaska between March and May of 2019–2022. They then divided the duck species into three groups: a group containing long diving ducks (sea ducks), a group of ducks that dive for shorter amounts of time (pochards) and group of ducks that don't dive to find their food (dabblers). After bringing the samples back to the lab, Schell began the laborious task of staining the muscle cells to determine which muscle fibers were more reliant on oxygen to make energy. Afterwards, Schell then counted how many mitochondria – the cell's energy-producing machinery – were in each type of muscle cell to see whether the deep diving ducks could use these to supply energy to their muscles while diving. Unsurprisingly, the sea ducks’ calf muscle cells were packed with mitochondria whereas the dabblers had less mitochondria powering their cells. However, having more mitochondria isn't necessarily useful for powering underwater diving if the oxygen can't reach the mitochondria.
So, Schell and colleagues counted the number of capillaries providing blood to the muscle cells and how many mitochondria were close to the surface of the muscle cells rather than in the middle. They found that the more mitochondria the ducks had near the surface of their muscle cells, the more capillaries they had as well. Having mitochondria close to the surface of the muscle cells and closer to the capillaries could allow the mitochondria to receive oxygen from the blood more easily, allowing them to produce more energy, but will it really help a duck be a better diver?
In short, yes. When the researchers looked at how long each species could dive for and compared that with how full of mitochondria their muscles were, they found that the deeper diving sea ducks could dive for longer and had more mitochondria, in addition to other changes that could help them stay underwater for longer. Although there are other factors that that determine duck diving depth – such as how much oxygen is found in the blood and myoglobin (a protein in the muscles that stores oxygen) – having muscles loaded with energy-producing machinery helps fuel their dive even as they lose their oxygen stores. Even though they can't breathe while underwater, it seems that these ducks rely on oxygen from their blood to meet their energy demands while diving, unlike diving mammals, which use oxygen stored in their muscles to power their dives. So, when it comes to diving, ducks certainly do it differently.
