Penguins are incredibly cute and fluffy, and hilarious when they walk, but they also break records in diving, and have an amazing ability to survive in the cold.
Today, a comprehensive genetic analysis of penguins helps explain why, and sheds light on their future under a warming climate.
The study, published in Nature Communications, identifies genes that help penguins hunt under water — allowing them to tune into blue hues in the dim light, keep well insulated from the cold, and cope with low oxygen levels.
Another key finding was that over the aeons, when temperatures rose, penguins slowed their rate of evolution.
In fact, along with their close relatives, such as albatross and petrels, penguins have the slowest rate of evolution among birds, something that doesn't bode well for adapting to a warming climate.
Penguins lost their wings
Over 60 million years ago, a group of birds related to albatrosses, living in what is now New Zealand, shrunk their wings and developed a whole suite of features that turned them into water-loving penguins.
This included losing their ability to fly in the air, said the University of Copenhagen's Theresa Cole, a lead author of the study.
In their analysis of penguin genomes, she and her colleagues found genes related to making the bird's forelimb bones shorter, denser and more rigid to form flippers.
Dense bones are also important to prevent buoyancy when penguins dive, Dr Cole said.
Other genes may help widen blood vessels to decrease blood pressure during deep dives.
"They are the deepest divers of all birds."
The team also found genes that enable penguins to survive low-oxygen levels, helpful for when they are on a long dive.
And penguins have genes not found in any other birds that are key to their survival for long periods in the cold.
These include genes for white fat, which acts as insulation, especially handy when penguins moult every year, and as an energy reserve, which is useful for long-distance migration.
Blue, blue, my world is blue ...
The international and multidisciplinary team of researchers included Christopher Emerling, an evolutionary biologist at Reedley College in California.
He identified genes linked to penguins' ability to pick out tasty fish or squid for dinner when they're diving.
"When you're under water you'll notice that colours can become very muted as you get deeper and deeper," Dr Emerling said.
Red, orange and yellow light is filtered out and surroundings become more monochromatic.
"It becomes more and more blue," he said.
Looking at genes that code for proteins involved in colour vision, he found evidence penguins don't waste energy trying to pick up colours that aren't there.
Instead, they have reduced their number of colour-detecting cells (called cones), leaving room for more rods, which are useful for seeing in low light, and have tuned the remaining cones to blue.
"We found evidence that the penguin eye has accumulated mutations that allow them to be better suited for blue light," Dr Emerling said.
No taste for umami ...
Dr Emerling also confirmed that, like whales, seals, sea lions and walrus, which adapted to a life tied to the ocean, penguins have lost the ability to taste a range of flavours.
They do not appear to have taste receptors for detecting sweetness, bitterness and umami.
"It's really strange," Dr Emerling said.
He speculates it might be because their diet is so narrow or because of the way they eat — they swallow their prey whole — which could mean taste is a little redundant.
It is not certain yet whether penguins can enjoy salt or sour flavours, even if they have genes that suggest they do.
"The loss of the umami and bitter taste receptor genes may be the result of a wholesale loss of taste in penguins," Dr Emerling said.
So-called "taste" receptors can have other functions in the body (salt taste receptors, for example, produce a protein used to reabsorb sodium in the kidneys).
And penguins don't actually have taste buds, as such, either.
Another adaptation to aquatic living is the loss of the gene for the enzyme chitinase, which is used by bug-eating land animals to digest the hard chitin in insect exoskeletons.
Penguin evolution and climate change
The researchers also built a comprehensive family tree of penguins and compared it to geological and climate records.
They were able to see the impact of cycles of warm and cold on the evolution of different penguin species, how the population of different penguins changed over time, and their rate of evolution.
They found there was a burst of new species evolving after the initiation of a strong ocean current that began encircling Antarctica around 30 to 40 million years ago, on which penguins could have hitched a ride to disperse.
Then there was another burst after a global cooling that happened about 14 million years ago (during the middle Miocene), which is when the researchers estimate the ancestor of modern penguins evolved. This is more recent than previous estimates.
An important finding was that in warmer temperatures, penguins' rate of evolution slowed. They now have one of the slowest evolutionary rates among birds, something that is of concern in the context of climate change.
Adaptations in response to climate change
Jane Younger of the University of Tasmania, who was not involved in the study, was impressed by the documentation of genes that help penguins dive deep and live in cold conditions.
"I think what's really cool about this study is it shows us, in the genomic work, the actual changes in the genes that make this sort of lifestyle possible."
And, like others, she welcomed the analysis of penguin evolution over time.
"This is definitely the most complete record of the evolutionary history of penguins that we've ever seen, and it very nicely shows how diversification events have lined up with climate events," she said.
Sankar Subramanian, a geneticist from the University of the Sunshine Coast, said the study showed penguins were able to adapt in response to climate change in the past but this would not necessarily apply in the future.
"These climate-driven adaptations took a very long time," said Dr Subramanian, who has studied penguin evolution.
"The current speed of rising temperature far exceeds the adaptive capabilities of penguins. I think this is the important message from this paper."
Dr Subramanian said while some species have previously moved further south during warming periods, they could only go so far.
"These penguins will have nowhere to go when the temperature increases further in the Antarctic Peninsula due to global warming."
It's a message echoed by others.
"The slow evolutionary rates among penguins, and the speed at which Earth's climate is currently changing, does not bode well for penguins," Australian Antarctic Division seabird ecologist Barbara Wienecke said.
"Given that most penguin species are already threatened with extinction, the chances that they will survive climate change are limited."
Dr Younger pointed to the Emperor penguin, which is particularly well adapted to the cold, and is very vulnerable to a warming climate.
"Once it gets above zero degrees Celsius, they start eating snow to cool themselves down," she said, adding they are also vulnerable to loss of sea ice.
Meanwhile, the populations of little (fairy) penguins are also declining on both east and west coasts of Australia, in part due to warming waters, said Peter Dann, research manager at Phillip Island Nature Parks, another co-author on the new study.
Habitat loss, pollution, commercial fishing, and predation by introduced animals are other factors that can make matters worse for penguins.
Dr Dann said while efforts to control foxes, domestic dogs, and some human infrastructure has boosted penguin numbers on Phillip Island, not enough is generally being done to help penguins — and this is despite our "great affection" for them.
