Scientists are calling for a shake-up of the penguin kingdom, saying the gentoo penguin is four species, not one.
According to new evidence, the birds are slightly different in shape and size, and can be told apart by their DNA.
Counting them as separate species will help in conservation, they argue, making it easier to monitor declines.
The change would raise the tally of penguin species from 18 to 21.
Dr Jane Younger of the Milner Centre for Evolution at the University of Bath told BBC News: “Superficially, these species look very similar to each other; it’s very hard to tell them apart just with your eyes.
“But if we sequence their genomes we can see very clearly that they are different. We also can tell based on different measurements.”
Penguins face a number of threats in the wild, including plastic pollution, over-fishing and climate change. When it comes to climate change, gentoos are faring relatively well compared with other penguin species, but scientists say some populations have not been monitored for decades.
Numbers are increasing on the Antarctic Peninsula but have fallen on some surrounding islands.
“Currently gentoo penguins are fairly stable in numbers, however there is some evidence of the northern populations moving further south as the climate gets warmer, so we need to watch them closely,” said Dr Younger.
Over the course of hundreds of thousands of years, gentoo penguins have become isolated from each other to the point where they don’t interbreed, even though they could easily swim the distance that separates them.
They should thus be considered four species living in different latitudes in the southern hemisphere, on the Antarctic continent, and further north, where conditions are milder, the scientists argue.
The study, published in the journal, Ecology and Evolution, looked at the genetic make-up of gentoos living in the Falkland Islands and South Georgia in the southern Atlantic Ocean, the South Shetland Islands in the Antarctic and Kerguelen Islands in the Indian Ocean.
The data was combined with measurements of museum specimens from each of the four populations.
Gentoo penguins, which go by the Latin name Pygoscelis papua, are currently split into two subspecies. Any changes to the classification will need to be reviewed by an international committee of scientists before being formally accepted.