杏吧原创

AS EUREKA moments go, it wasn鈥檛 the most dignified. David Lambert lost his footing and face-planted into a patch of expired penguins. He had been taking blood samples from living birds at a nesting site, but as he scrambled to his feet, it dawned on him that he was standing on a mass grave. 鈥淚n those penguin colonies you are literally walking on matted bodies,鈥� he says. 鈥淲hen you scratch around, you just find bones after bones after bones.鈥�

Lambert鈥檚 insight was to realise that he had stumbled on a deep-frozen archive. The remains belonged to Ad茅lie penguins, which return to the same spots to nest year after year, often for centuries. And this was Antarctica, the coldest, driest place on the planet, offering the ideal conditions for preserving DNA. By digging into this repository, he could unearth the story of Ad茅lies and their evolution.

That鈥檚 not all. This frozen treasure trove has the potential to give new insights into the past, present and future of the Antarctic, too. This promise is what鈥檚 drawing scientists like Lambert to the bottom of the world, braving seat-of-your-pants helicopter rides and vicious polar storms to sift through layers of mummified penguin bodies and reeking semi-fossilised bird faeces. And what they are finding has exceeded expectations. The preserved Ad茅lie remains are providing clues about past climate conditions, changes in ice shelves and sea ice, the impact of historical human activities such as whaling, and even the mechanism of evolution itself. Not bad for a short, stout bird with a reputation for belligerent curiosity.

Three quirks of Ad茅lie life make all of this possible. First is the location of their colonies. Unlike Emperor penguins 鈥� the only other truly Antarctic penguin species 鈥� Ad茅lies can鈥檛 nest on ice. They need bare ground with easy access to the sea and a plentiful food supply. So if you find a relic colony and can work out how old it is, you can be pretty confident there was no ice around at that time.

Next, there is the content of the nesting sites. Forget the idyllic pictures on Christmas cards 鈥� real-life penguin colonies aren鈥檛 pretty. 鈥淭hey are incredible places of birth and death,鈥� says Lambert, an evolutionary biologist at Griffith University in Brisbane, Australia.

Ad茅lie penguins spend the winter offshore, foraging. Each October, as the Antarctic summer takes hold, monogamous pairs reunite and rebuild their nests 鈥� mounds of small stones that keep their eggs off the freezing ground. Colonies can be huge, containing hundreds of thousands of pairs. Over the coming weeks, the debris builds up: guano, eggshells, rotting food scraps, and the sorry little bodies of chicks lost to the elements or injury, tangled with those of dead adults.

鈥淔orget the idyllic pictures on Christmas cards 鈥� penguin colonies aren鈥檛 pretty鈥�

Once the chicks are fledged, the survivors depart, returning next summer to repeat the cycle. In this way, successive generations of these waddling curators have unwittingly amassed an archive of Ad茅lie existence extending back 50,000 years or more 鈥� to when humans were still chipping away at stone tools.

Cryogenic preservation

Crucially 鈥� and this is quirk number three 鈥� unlike other creatures that have left frozen remains, such as woolly mammoths, Ad茅lies are still around, so ancient samples can be compared with modern ones. 鈥淚t鈥檚 a remarkable record,鈥� says Steven Emslie at the University of North Carolina Wilmington.

Evidence that penguin remains might reveal past environmental conditions first came in the mid-1990s. That was when a team led by Carlo Baroni at the University of Pisa in Italy compared the age of relic colonies along the Ross Sea coast determined by radiocarbon dating with temperature records stored in Antarctic ice cores. between 3000 and 4000 years ago, indicating that a reduction in ice had given the birds easier access to the coast. This penguin heyday coincides with warmer temperatures recorded in the ice cores.

Subsequent studies have helped build a picture of Ad茅lie antics since the Ross Sea was in a warm period some 44,000 years ago. Intriguingly, from the Ross Sea coast between about 27,000 and 13,000 years ago. This corresponds with the last cold period and confirms the hypothesis that the world鈥檚 largest ice shelf, the Ross ice shelf, expanded at that time, closing off the coast, before retreating once the climate warmed, allowing the penguins to return. Where they went in the intervening millennia remains a mystery.

Penguins haven鈥檛 just left a record of themselves, though. Mingled with their remains are those of their prey. 鈥淭hey are essentially sampling the marine environment for us,鈥� says Emslie. Which sounds very kind, until you realise that the samples are provided as smelly deposits of ancient penguin faeces, or guano. It is worth holding your nose and delving in, however, as prey remains reveal past foraging conditions. This helps explain why colonies appear and disappear from the fossil record. 鈥淭hey are blinking in and out throughout an area, based on the local conditions, sea ice conditions, access to open water, krill availability and fish availability,鈥� says Emslie.

鈥淪melly deposits of ancient penguin faeces reveal past foraging conditions鈥�

Careful sifting of guano yields bits of squid beaks, fish bones and 鈥渙toliths鈥� 鈥� tiny stone-like structures from the inner ears of fish 鈥� which provide clues about the size of Ad茅lie prey, their relative abundance and how this shifted over time. Emslie and others have found that the ratios of certain isotopes 鈥� heavier or lighter versions of elements such as oxygen 鈥� in otoliths can reveal the surface temperature of the sea at the time the fish lived.

Guano also contains DNA, which can open a window on ancient ecology. Lambert鈥檚 team, for example, is sequencing DNA from a series of guano layers to find out how the birds鈥� diet has changed, over the past 30,000 years. 鈥淭he only way you can do that is from the penguin guano, because the penguins feed out there and they collect it all,鈥� he says.

Historical human activity has left its mark, too. Emslie鈥檚 studies of isotopes in ancient penguin eggshells show that between 200 and 300 years ago, Ad茅lies were eating less fish and more krill. Around that time, explorer James Cook reported sub-Antarctic islands heaving with fur seals and elephant seals, prompting a stampede of hunters, followed by commercial whalers. The mass killing of these animals removed the main consumers of krill from the ecosystem. Emslie鈥檚 findings lend weight to the idea that krill populations boomed, shifting the balance of the food web.

Back to the future

Putting all these pieces together is helping us predict how penguins 鈥� and the Antarctic 鈥� will respond to future human activity and climate change. For example, Emslie has been charting the changing fate of Antarctica鈥檚 biggest Ad茅lie colony, on the Cape Adare coast in the northern Ross Sea. It was founded some 2000 years ago, and around 800 years later swelled to about twice its present size, probably thanks to marine conditions producing rich pickings. The million-strong colony spilled up onto a rocky terrace above the shore 鈥� which was abandoned as the colony dwindled. Rising sea levels within the next 50 years are likely to inundate the beaches at Cape Adare, and the birds may once again shift to the terrace. They may also move south to old stomping grounds as retreating ice makes them accessible once more. 鈥淭here鈥檚 going to be ,鈥� says Emslie.

Changing behaviour is one way of coping with a changing environment. Evolving is the other. From the start, Lambert could see that penguin remains offered a unique opportunity to peek under the bonnet of evolution. 鈥淲e realised that this was a really good way to measure how fast DNA changes over time,鈥� he says.

To do this, he and Baroni extracted DNA from a series of birds dating from the present to more than 6000 years ago, then worked out the rate at which random mutation has occurred. Such 鈥渕olecular clocks鈥� are used to estimate the timings of key evolutionary events such as when new species emerged. The Ad茅lie鈥檚 鈥渃lock鈥� turned out to be than previous calculations had suggested.

The team also studied a small number of DNA stretches to see how different versions of genes 鈥� alleles 鈥� behaved over time. They found that some had spread, while others had died out, and linked this to colonies interbreeding, perhaps when giant icebergs restricted their movements and forced them to mix. This is the first glimpse of over such a time frame. Now Lambert and his colleagues have their : sequencing whole genomes of lots of long-dead penguins and comparing them with those of their living descendants, to see how natural selection has shaped genes over millennia, and how this relates to the changing environment.

A lot has been discovered about Ad茅lies since Lambert had his slippery epiphany. Much of this is down to hard graft and ingenuity, but the scientists know they also owe a huge debt to a legion of feathered research assistants.

This article appeared in print under the headline 鈥淧enguin archaeology鈥�