Polar bears have become a poster child for the impacts of climate change on wildlife. Their future may be bleak but their past is altogether more glorious. Polar bears are an evolutionary success story. They’re a recent addition to life’s repertoire, splitting off from their closest relatives – the brown bears – as recently as 150,000 years ago. Within just 20,000 years, they accumulated many adaptations that have made them the masters of their icy realm. But some of these same adaptations could now be their undoing.
The polar bear is the only bear that eats nothing but meat. It is wonderfully adapted to live off the flesh of seals. Graham Slater from the University of California Los Angeles has found that the shape of the polar bear’s skull has evolved at around twice the rate of the bear family as a whole. It’s flatter and more slender than those of other bears, all the better for thrusting into the dens and breathing holes of seals. Their eyes sit high up on their skulls, as is often the case for animals that spend a lot of time swimming.
But this rapid change has come at a cost for this specialist seal-killing skull turns out to be surprisingly weak. Slater used a technique called finite element analysis to put the skulls of a polar bear and a brown bear through a ‘digital crash-test’. He used a medical scanner to create virtual models of real skulls and then subjected them to different forces in his computer.
Not Exactly Pocket Science is a set of shorter write-ups of new stories with links to more detailed takes by the world’s best journalists and bloggers. It is meant to complement the usual fare of detailed pieces that are typical for this blog.
Frogs evolved to jump before they perfected landings
Most frogs are can leap large distances in a single bound, jumping forward with a thrust of their powerful hind legs and landing gracefully on their front ones. But it wasn’t always like this. A study of one of the most primitive groups of frogs suggests that the first frogs landed in an awkward belly-flop. These animals evolved to jump before they perfected their landings.
Virtually all frogs jump and land in the same way. But Richard Essner Jr from Southern Illinous University discovered a unique leaping style in the Rocky Mountain tailed frog. This species belongs to a group called the leiopelmatids, more commonly (and accurately) known as the “primitive frogs”. Using high-speed video footage, Essner showed that the tailed frog’s landings are an awkward mix of belly-flops, face-plants and lengthy skids. Only when it grinds to a halt does it gather its outstretched limbs together. By contrast, two more advanced species – the fire-bellied toad and the northern leopard frog – rotate their limbs forward in mid-air to land gracefully. The tailed frog managed to jump a similar distance, but its recovery time was longer.
These results support the idea that frogs eventually evolved their prodigious jumping abilities to escape from danger by rapidly diving into water. Landings hardly matter when you’re submerged and the ability to pull them off elegantly only evolved later. Essner thinks that doing so was fairly simple – if the tailed frog starts pulling its legs in just slightly earlier, it would land with far more poise. This simple innovation was probably a critical one in frog evolution. The primitive frogs never got there, but they have other ways of coping with their clumsy crash-landings. They’ve stayed very small to limit the injuries they sustain, and they have large shield-shaped piece of cartilage on their undersides to protect their soft vital organs.
Reference: Naturwissenschaften http://dx.doi.org/10.1007/s00114-010-0697-4; Video by Essner; soundtrack by me.
Changing climate fattens marmots
The media is rife with tales of animals from polar bears to corals suffering as a result of climate change. But some species stand to gain from the rising global temperatures. In Colorado’s Rocky Mountains, warmer climes allow the yellow-bellied marmot to awaken from its winter hibernation earlier. With more time available to eat, they become bigger and so do their populations. In just three decades, their numbers have tripled.
Arpat Ozgul from Imperial College London studied a 33-year census of Colorado’s marmots, where individuals have been tracked over their entire lifetimes. These rodents spend the winter hibernating in their burrows. But since 1976, they have been waking up earlier and earlier in the year, presumably because of a rise in warm days. That gives them more time to eat and grow before their next hibernation, and the adults have become around 10% heavier. Ozgul found that being fatter offers many advantages for a marmot – females are more likely to breed, youngsters grow more quickly, and adults are more likely to survive their next bout of hibernation.
It’s no surprise that their population has shot up dramatically, although surprisingly, this wasn’t a gradual process. Their numbers seemed to be fairly stable but they passed a tipping point in 2000 and have skyrocketed ever since. By modelling the changes in their bodies over time, Ozgul concluded that the marmots haven’t changed much genetically – their extra pounds are the result of their response to environmental changes. For example, the bluebells that they like to eat declined after 2000, which might have prompted them to seek other fattier foods.
But Ozgul worries that this boom period has a bust on the horizon – it’s a short-term response to warmer climate. These are animals that are adapted to chilly mountainous temperatures and they don’t fare well in heat. If temperatures continue to rise and summers get longer and drier, their health might suffer and their populations might crash.
Reference: Nature http://dx.doi.org/10.1038/nature09210; image by Ben Hulsey
Around 15,000 years ago, North American was home to a wide menagerie of giant mammals – mammoths and mastodons, giant ground sloths, camels, short-faced bears, American lions, dire wolves, and more. But by 10,000 years ago, these “megafauna” had been wiped out. Thirty-four entire genera went extinct, including every species that weighed over a tonne, leaving the bison as the continent’s largest animal.
In trying to explain these extinctions, the scientific prosecution has examined suspects including early human hunters, climate change and even a meteor strike. But cracking the case has proved difficult, because most of these events happened at roughly the same time. To sort out this muddled chronology, Jacquelyn Gill has approached the problem from a fresh angle. Her team have tried to understand the final days of these giant beasts by studying a tiny organism, small enough to be dwarfed by their dung – a fungus called Sporormiella.
Sporormiella grows in the droppings of large plant-eating mammals and birds, and it leaves tell-tale spores in its wake. More spores mean more dung, so Sporormiella acts as a rough indicator of the number of herbivores in a given area. The fall of these beasts is reflected in falling numbers of spores.
Gill counted these spores in the sediment of Indiana’s Appleman Lake, and compared them to counts of fossilised pollen and charcoal from the same soil. That allowed her to match the numbers of plant-eaters at any given time with the local plant species and the frequency of forest fires.
Using this fungal index, Gill has produced a detailed timeline of the changes in the Pleistocene. Her revised history argues against a role of climate change or alien rocks, but fails to clear early humans of the blame. More importantly, it suggests that many events that happened around the same time, such as an upheaval in the local plant communities and a rise in large infernos, were the result of the beasts’ decline, rather than the cause of them.
The spores revealed that the fall of the megafauna began in earnest around 14,800 years ago. By the 13,700 year mark, their numbers had fallen to less than 2% of their former glory. They never recovered, but it clearly took a few more millennia for the stragglers to succumb – the last bones of the great beasts date to around 11,500 years ago.
Changes in the local vegetation happened after the beasts started disappearing, around 13,700 years ago. Before this point, the environment was open grassland with the odd tree. Fires were a rarity. But without the suppressive mouths of the big plant-eaters, trees grew unchecked, producing a combo of vegetation you just don’t see today. Large numbers of temperate deciduous trees like elm and ash happily coexisted with cold-loving conifers like larch and spruce.
And with them came fires, large infernos that broke out around 14,000 years ago and returned every century or so for the next few millennia. The pollen and charcoal of Appleman Lake tell the story of these changes, and also show that they came after the beasts’ disappearance.
Right away, this timeline rules out the possibility that a collision with a large space object killed the megafauna. The proponents of that theory place the collision at around 13,000 years ago, after the giants had started to decline. And it’s clear that extinctions were long, drawn-out affairs, rather than the relatively rapid annihilations you’d expect from an extraterrestrial impact.
Likewise, changing climate becomes an unlikelier suspect. The megafaunal extinction predated a rapid, millennium-long chill called the Younger Dryas that took place around 11,500 and 12,800 years ago. When the megafauna started dying, the Earth was going through a warming phase. That might well have affected them, but it didn’t do so through the most obvious method – changing the plants they ate. After all, Gill’s work tells us that the beasts’ disappearance changed the plants, not the other way round.
What about humans, those pesky slayers of animals? Some scientists believed that North America’s Clovis people specialised in hunting big mammals, causing a “blitzkrieg” of spear-throwing that drove many species to extinction. But these hunters only arrive in North America between 13,300 and 12,900 years ago, around a thousand years after the population crashes had begun.
If people were responsible, they must have been pre-Clovis settlers. There’s growing evidence that such humans were around, but they weren’t common or specialised. They may have contributed to the beasts’ downfall, while Clovis hunting technology delivered a coup de grace to already faltering populati0ons.
By analysing the sediment at Appleman lake – spores, pollen, charcoal and all – Gill has replayed the history of the site, spanning the last 17,000 years. Her data rule out a few theories, but as she says, they “[do] not conclusively resolve the debate” about climate causes versus human ones. It’s possible that similar studies at different sites and other continents will help to provide more clues.
Meanwhile, her study certainly tells us more about what happened in Earth’s recent history, when a large swathe of hefty plant-eaters died off – a change from savannah to woodland, and more fires. This isn’t just a matter of historical interest. The same events might be playing out today, as the largest modern land mammals suppress fires by eating flammable plants, and are facing a very real threat of extinction. History could well repeat itself.
Reference: Science 10.1126/science.1179504
More on megafauna:
The island of Hirta, on the western coast of Scotland, is home to a special breed of sheep. Soay sheep, named after a neighbouring island, are the most primitive breed of domestic sheep and have lived on the isles of St Kilda for at least a millennium. They’re generally smaller than the average domesticated sheep, and that difference is getting larger and larger. Over the last 20 years, the Soay sheep have started to shrink.
They are becoming gradually lighter at all ages such that today’s lambs and adults weigh around 3kg less than those from 1986. Their hind legs have also shortened to a similar degree, suggesting that they have indeed shrunk, rather than fallen increasingly ill.
The reasons behind this downward trend have now been revealed by a group of British scientists led by Arpat Ozgul from Imperial College. Using decades’ worth of data, the team showed that natural selection normally favours larger sheep, as the odds of survival increase with body size. But this evolutionary pressure has been overwhelmed by the effects of climate change. Warmer winters have led to easier conditions, and less need to pile on the pounds in the first years of life. The lambs can afford to grow more slowly and they become smaller adults, who are only physically capable of raising small young themselves.
Soay sheep live in a closed population that doesn’t have to deal with human interference, predators, migrants (either in or out), or significant competitors. That makes them an ideal population to study if you’re an evolutionary biologist interested in how animal populations change over time. One such group, including Ozgul and his colleague Tim Coulson, have been studying the Soay sheep since 1985 and have brilliantly called themselves SLAPPED (short for Studies in Longitudinal Analysis of Population Persistence and Evolutionary Demography).
The group wanted to work out the extent to which the sheep’s shrinking size is due to the influence of natural selection and to what extent it is just an ecological response to changing environments. To that end, they developed a mathematical job designed to analyse their 24 years of data and tease apart these contrasting effects.
The emperor penguin – caring parent, extreme survivor and unwitting movie-star – could be marching to extinction by the turn of the next century. In its Antarctic home, the penguins frequently have to deal with prolonged bouts of starvation, frosty temperatures of -40 degrees Celsius, and biting polar winds that blow at 90 miles per hour. And yet this icy environment that so brutally tests the penguins’ endurance is also critical to their survival. This is a species that depends on sea ice for breeding and feeding.
So what will happen to the emperor penguin as Antarctica’s sea ice shrinks, as it assuredly will in the face of a warming globe? Stephanie Jenouvrier from the Woods Hole Oceanographic Institution have tried to answer that question by combining the forty years of census data on a specific emperor colony with the latest models from the Intergovernmental Panel on Climate Change (IPCC).
The results are not encouraging. They suggest that the number of emperors in a large colony at Terre Adelie (where March of the Penguins was filmed) will fall from about 6,000 breeding pairs at the moment to a mere 400 by 2100. There’s even a one in three chance that the population will drop by 95%- a level described as “quasi-extinction”, when the population is so small that it’s unlikely to sustain itself.
Image copyright of Samuel Blanc
It’s not a good time for corals. Last year, a third of coral species went straight into the endangered lists after being assessed for the first time, and it looks like 2009 isn’t going to bring any reprieves to the doom and gloom. In particular, a new study provides hard evidence that the mightiest of coral super-colonies – the Great Barrier Reef – is in trouble.
Like reefs across the world, the Great Barrier Reef faces many threats, including pollution, physical destruction, predatory starfish and perhaps most importantly, the many effects of climate change. Glenn De’ath and colleagues from the Australian Institute of Marine Science have found that the corals among this greatest of reefs are starting to yield under these multiple assaults, adding new material to their limestone skeletons at ever-declining rates. The Reef’s growth is slowing to a worrying degree, the likes of which are unprecedented in at least the last 400 years.
De’ath’s group focused on one group of corals called Porites. They are a widespread and important group, and like most of their kin, they build reefs by laying down external skeletons of aragonite, a version of calcium carbonate or limestone. Like trees, they have annual growth rings that reveal how quickly they expand. And because coral growth depends on a variety of environmental conditions, the skeletons of the Porites provided a potted history of environmental changes, recorded in unchanging limestone.
Did our ancestors exterminate the woolly mammoth? Well, sort of. According to a new study, humans only delivered a killing blow to a species that had already been driven to the brink of extinction by changing climates. Corralled into a tiny range by habitat loss, the diminished mammoth population became particularly vulnerable to the spears of hunters. We just kicked them while they were down.
The woolly mammoth first walked the earth about 300,000 years ago during the Pleistocene period. They were well adapted to survive in the dry and cold habitat known as the ‘steppe-tundra‘. Despite the sparse plant life there, the woolly mammoths were very successful, spreading out in a belt across the Northern hemisphere.
Their fortunes began to change as the Pleistocene gave way to the Holocene. The climate around them started to become warmer and wetter and the shrinking steppe-tundras greatly reduced the mammoth’s habitats. The species made its last stand on the small Wrangel Island in Siberia before finally succumbing to extinction.
But climate change isn’t the whole story. About 40,000 years ago, those relentless predators – human beings – started encroaching into the woolly mammoth’s range in northern Eurasia. Which of these two threats, climate change or human hunters, sealed the mammoth’s fate?