Adélie penguins living in Antarctica switched from eating fish to krill (shrimp-like marine invertebrate animals) around the time that humans began hunting seals and whales, ancient eggshell fragments reveal.
According to Steven Emslie of the University of North Carolina in Wilmington, and William Patterson of the University of Saskatchewan in Saskatoon, it suggests that when humans removed krill-eating predators, the penguins exploited the resulting shrimp surplus.
Krill is an attractive food for penguins because it is high in protein and tends to travel in swarms. "The birds can capture lots of high-energy prey in a short time," says Emslie.
Ernslie and Patterson analysed over 220 fossilised eggshells, ranging from 100 to 38,000 years old, and compared them with samples from modern nests.
By comparing the proportion of certain forms of carbon and nitrogen in the shells with the proportions found in fish and krill, the researchers could tell what the birds had been eating.
According to Emslie, the penguin menu remained biased towards fish until about 200 years ago. The switch to krill took place thereafter.
Both Ernslie and Patterson also say that global warming and the rise in krill fisheries has reduced krill stocks and could be contributing to the decline in Adélie penguin populations on the Antarctic Peninsula.
Their study, which is published in the Proceedings of the National Academy of Sciences, further says that between 1793 and 1807, an estimated 3.2 million seals were killed or captured from the Southern Ocean, reducing the seal population.
Whaling, on the other hand, took took off in the 1800s and continued until the mid-twentieth century, eventually depleting baleen whale populations by more than 90 percent. It's estimated that the combined harvest of seals and whales resulted in more than 150 million tonnes of extra krill each year.
Their dietary flexibility demonstrates the penguins' ability to adjust to large ecological changes, but that doesn't mean they'll survive the changes to come, other researchers opined.