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.
The Earth’s oceans are mysterious and largely unexplored. Many of their inhabitants are familiar to us but their whereabouts and numbers are far less clear. This is starting to change. In two new studies, Boris Worm from Dalhousie University has revealed an unprecedentedly detailed portrait of the planet’s marine life, from tiny plankton to mighty whales. And with that knowledge comes concern, for neither study paints an optimistic picture about the fate of tomorrow’s seas, as changing climate slowly raises their temperature.
Graduate student Daniel Boyce focused on some of oceans’ smallest but most important denizens – the phytoplankton. These tiny creatures are the basis of marine food webs, the foundations upon which these watery ecosystems are built. They produce around half of the Earth’s organic matter and much of its oxygen. And they are disappearing. With a set of data that stretches back 100 years, Boyce found that phytoplankton numbers have fallen by around 1% per year over the last century as the oceans have become warmer, and if anything, their decline is getting faster. Our blue planet is becoming less green with every year.
Meanwhile, post-doc Derek Tittensor has taken a broader view, looking at the worldwide distributions of over 11,500 seagoing species in 13 groups, from mangroves and seagrasses, to sharks, squids, and corals. His super-census reveals three general trends – coastal species are concentrated around the western Pacific, while ocean-going ones are mostly found at temperate latitudes, in two wide bands on either side of the equator. And the only thing that affected the distribution of all of these groups was temperature.
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
If you think the stars of Pixar’s Finding Nemo had it rough, spare a thought for the plight of real clownfish. These popular fish may struggle to survive in oceans that are becoming enriched with carbon dioxide. High levels of CO2 dissolved in the water can muddle a clownfish’s sense of smell, preventing it from detecting both shelter and threats.
Philip Munday from James Cook University has shown that at levels of carbon dioxide within what’s predicted for the end of the century, a clownfish’s ability to sense predators is completely shot. Some larvae become literally attracted to the smell of danger and start showing risky behaviour. It’s not surprise that they die 5-9 times more frequently at the mouths of predators.
While the world wrangles over ways of reducing carbon emissions, some scientists are considering more radical approaches to mitigating the effects of climate change. Dumping iron dust into the world’s oceans is one such strategy. Theoretically, the iron should act as fertiliser, providing a key nutrient that will spur the growth of photosynthetic plankton. These creatures act as carbon dioxide pumps, removing the problematic gas from the air and storing the carbon within their own tissues. When the plankton die, they sink, trapping their carbon in the abyss for thousands of years.
It may seem like a fanciful idea, but as with much of our technology, nature beat us to it long ago. Trish Lavery from Flinders University has found that sperm whales fertilise the Southern Ocean in exactly this way, using their own faeces. Their dung is loaded with iron and it stimulates the growth of plankton just as well as iron dust does.
Sperm whales are prodigious divers, descending to great depths in search of prey like squid. When they’re deeply submerged, they shut down all their non-essential bodily functions. Excretion is one of these and the whales only ever defecate when they reach the surface. By happy coincidence, that’s where photosynthetic plankton (phytoplankton) make their home – in the shallow column of water where sunlight still penetrates. So by eating iron-rich prey at great depths and expelling the remains in the shallows, the whales act as giant farmers, unwittingly seeding the surface waters with fertiliser.
There are approximately 12,000 sperm whales left in the Southern Ocean. By modelling the amount of food they eat, the iron content of that food, and how much iron they expel in their faeces, Lavery calculated that these whales excrete around 50 tonnes of iron into the ocean every year. And based on the results of our own iron fertilisation experiments, the duo calculated that every year, this amount of iron traps over 400,000 tonnes of carbon in the depths, within the bodies of sinking plankton.
Previously, scientists assumed that whales (and their carbon dioxide-rich exhalations) would actually weaken the Southern Ocean’s ability to act as a CO2 pump. But according to Lavery, this isn’t true. She worked out that the whales pump out just 160,000 tonnes of carbon through their various orifices. All of these figures are probably conservative underestimates but even so, they suggest that sperm whales remove around 240,000 more tonnes of carbon from the atmosphere than they add back in. They are giant, blubbery carbon sinks.
However, their true potential will go largely unfulfilled thanks to our harpoons. Many sperm whales have been killed by industrial whalers, and the population in the Southern Ocean has declined by some 90%. On the bright side, the Southern Ocean’s population represent just 3% of the global total, so this species may have an even greater role as a warden for carbon than Lavery has suggested. Other seagoing mammals probably have a part to play too, provided that they feed at depth and excrete near the surface. Several other toothed whales do this, and some filter-feeding ones may do too.
Reference: Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2010.0863
Image by Cianc
Today’s mammals are facing the twin threats of a rapidly warming planet and increasingly intrusive human activity. As usual, the big species hog the limelight. The world waits on bated breath to hear about the fates of polar bears, whales and elephants, while smaller and more unobtrusive species are ignored. But smaller mammals are still vital parts of their ecosystems and it’s important to know how they will fare in a warmer world. Now, thanks to Jessica Blois from Stanford University and a hoard of new fossils, we have an idea. As they say, all this has happened before…
Today, the city of Angkor in Cambodia lies in ruins. But a thousand years ago, life there was very different. Then, Angkor was the heart of the Khmer empire and the largest preindustrial city of its day. It had a population of a million and an area that rivalled modern Los Angeles. And the key to this vast urban sprawl was water.
Radar images of the city by the Greater Angkor Project (GAP) revealed that Angkor was carefully designed to collect, store and distribute water. The “Hydraulic City” included miles of canals and dikes, irrigation channels for supplying crops, overflow channels to cope with a monsoon, massive storage areas (the largest of which was 16km2 in area), and even a river diverted into a reservoir. Water was the city’s most precious resource, allowing it to thrive in the most unlikely of locations – the middle of a tropical forest.
But water, or rather a lack of it, may have been part of Angkor’s downfall. Brendan Buckley from Columbia University has reconstructed the climate of Angkor over the last 750 years, encompassing the final centuries of the Khmer Empire. The records show that Angkor was hit by two ferocious droughts in the mid-14th and early-15th century, each lasting for a few decades. Without a reliable source of water, the Hydraulic City’s aquatic network dried up. It may have been the coup de grace for a civilisation that was already in severe decline.
Many theories have been put forward for the downfall of Angkor, from war with the Siamese to erosion of the state religion. All of these ideas have proved difficult to back up, despite a century of research. Partly, that’s because the area hasn’t yielded much in the way of historical texts after the 13th century. But texts aren’t the only way of studying Angkor’s history. Buckley’s reconstruction relies on a very different but more telling source of information – Fujian cypress trees.
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.
In 2005, corals in the large reef off the coast of Florida were saved by four hurricanes. Tropical storms seem to be unlikely heroes for any living thing. Indeed, coral reefs directly in the way of a hurricane, or even up to 90km from its centre, suffer serious physical damage. But Derek Manzello from the National Oceanic and Atmospheric Administation has found that corals just outside the storm’s path reap an unexpected benefit.
Hurricanes can significantly cool large stretches of ocean as they pass overhead, by drawing up cooler water from the sea floor. And this cooling effect, sometimes as much as 5°C, provides corals with valuable respite from the effects of climate change.
As the globe warms, the temperature of its oceans rises and that causes serious problems for corals. Their wellbeing depends on a group of algae called zooxanthellae that live among their limestone homes and provide them with energy from photosynthesis. At high temperatures, the corals eject the majority of these algae, leaving them colourless and starving.
These ‘bleached’ corals are living on borrowed time. If conditions don’t improve, they fail to recover their algae and eventually die. But if the water starts to cool again, they bounce back, and Manzello found that hurricanes can help them to do this.
In the movie Finding Nemo, the eponymous clownfish grows up in the security of his home reef and must find his way back after being fry-napped by an overenthusiastic diver. In reality, the larvae of clownfish spend their early lives adrift in the open ocean and only after weeks, or possibly months, do they return to the reefs where they were born.
Their journey is guided by several cues that help them navigate home. The sound of a reef may be one of these but it’s clear that the most important sense for a returning fish is smell. Young fish have very well developed smell organs and respond appropriately to the molecules given off by other fish, and by the sea anemones that they live in. They have a keen nose for things that smell fishy, and there is even some evidence that they can use smell to distinguish the water of their birth reef from that of any other.
But this uncanny homing ability isn’t foolproof. Philip Munday from James Cook University in Australia found that increasing the amount of carbon dioxide in the water muddles the senses of baby clownfish, sending them towards smells they would normally avoid. For an animal that relies so heavily on smell to find a suitable place to live, that could be catastrophic.
Sadly, the conditions in Munday’s experiment have a very good chance of coming to pass. As humans pump ever more carbon dioxide into the atmosphere, at least a third dissolves into the oceans, making them increasingly acidic. Over the past 200 years, the pH of the oceans has fallen 100 times faster than any time in the last 650,000 years. Marine life will suffer as a result – corals will find it harder to build their mighty reefs in water depleted of the ions they need. Shellfish that build limestone skeletons will also face trouble. Now, it seems that the popular clownfish joins the list of species at risk.