A mother can tell a baby's health just by its poop. For penguins, a satellite in space has taken on that mother's role. By tracking penguin droppings piled up in Antarctica, a satellite showed that the nutritional value of their prey has steadily declined for decades due to warming.
A team led by Casey Youngflesh, a professor in the department of biological sciences at Clemson University, said that the Earth-observing Landsat satellite tracked droppings piled up in Adélie penguin (Pygoscelis adeliae) habitats in Antarctica over 30 years to identify the effects of climate change, according to a paper published on the 7th (local time) in the journal Current Biology.
Adélie penguins are a small-to-medium species that grow to about 70 centimeters tall and up to 6 kilograms. Along with emperor penguins, which are twice as tall, they inhabit the southernmost reaches of Earth. The team said this study is the first to investigate, across the entire Antarctic continent in 10-year increments, changes in Adélie penguins' diet and population.
◇ Tracking diet shifts with satellite images of poop
The team reconstructed penguin diets using guano images taken by Landsat from 1984 to 2013. Guano is bird droppings that have hardened like rock and are widely used as fertilizer. The team analyzed the same droppings samples in different ways to link the colors seen by satellites with the actual prey.
First, Youngflesh collected guano from Adélie penguin habitats in Antarctica and measured in the lab how much light it reflects by wavelength, confirming its spectral characteristics. Simply put, they quantified what color penguin droppings appear in satellite imagery.
At the same time, co-author Michael Polito, a professor in the department of ocean sciences at the University of California, Santa Cruz (UC Santa Cruz), measured ratios of isotopes such as carbon and nitrogen in the droppings to determine whether penguins were eating krill, a crustacean that looks like shrimp, or mainly fish. Isotopes have the same atomic number but different mass.
Based on the two datasets, the team built a computational analysis model that links the wavelengths of light reflected by droppings with diet. The analysis revealed a trend in which global warming is threatening penguin survival. Over 30 years, Adélie penguins in regions with more sea ice ate more fish, while those in areas with less sea ice relied more on krill. Sea ice is frozen seawater, serving as both a corridor and a base camp for penguins to reach hunting grounds. As warming erases sea ice, penguins' opportunities to hunt fish have plunged.
The team said penguins could face a survival threat if their diet changes. Colonies that rely on krill as a staple are more likely to decline in number than colonies that eat fish. According to a 2018 paper, juvenile penguins that ate more fish tended to be larger and had higher chances of survival than those that ate more krill.
◇ Warming-driven diet shifts cut population
Adélie penguins are the most numerous predators in Antarctica, but they depend on only a few prey species. During breeding season, their diet consists mainly of fish such as Antarctic silverfish and krill. If Antarctic sea ice keeps shrinking as it is now, Adélie penguins' diet will inevitably shift toward krill.
Even krill are becoming scarce. Polito said that not only is krill less nutritious than fish, but increased consumption by seals and whales recently has reduced krill numbers in parts of Antarctica. For penguins, it is a double blow.
Polito said, "Adélie penguins serve as a 'canary in a coal mine' because they are an iconic species that breed across the Antarctic continent." Just as miners once used canaries, with their fragile respiratory systems, to first detect harmful gases like methane or carbon monoxide, penguins can help first capture how warming affects the ecosystem.
But Antarctica is so vast and difficult to access that field research is hard. Scientists can visit penguin habitats to collect samples and track population changes, but it is practically impossible to collect samples regularly at every habitat across the continent over decades.
Satellites can help overcome limits in today's Antarctic research. Youngflesh said, "This study is the first case to use satellite observations to understand, at a continent scale and in decadal steps, the dynamics of the food web," adding, "Thanks to satellites, we accomplished what would have been impossible by other means."
◇ Declining penguin droppings signal an Antarctic ecosystem in crisis
This is not the first time penguin poop has revealed a crisis in Antarctica's ecosystem. In 2023, Spain's Institute of Marine Sciences of Andalusia reported in Nature Communications that chinstrap penguins (Pygoscelis antarcticus) are supplying far less iron to the Southern Ocean through their droppings, threatening the ecosystem. That is because iron feeds the phytoplankton at the base of the marine food web.
At the time, the team flew drones to analyze chinstrap penguin numbers and the volume of droppings, again estimating the amount using photos. Areas covered in penguin droppings turn bright orange. An analysis of collected droppings found 3 milligrams of iron per gram (1 mg is 1/1000 of a gram).
Based on drone photos and the droppings analysis, the team estimated that chinstrap penguins supply 521 tons of iron to the Southern Ocean every year through their droppings. Due to climate change, chinstrap penguin numbers have fallen 50% over the past 40 years. Iron inputs likewise are about half of 1980s levels. The marine ecosystem, accordingly, can only deteriorate. Their droppings both sustain the ecosystem and warn of its crisis, making them truly precious.
References
Current Biology (2026), DOI: https://doi.org/10.1016/j.cub.2026.06.028
Nature Communications (2023), DOI: https://doi.org/10.1038/s41467-023-37132-5
Marine Ecology Progress Series (2018), DOI: https://doi.org/10.3354/meps12687