本文作者：小红猪小分队

Source：http://www.newscientist.com/article/mg21729032.500-withering-heights-why-animals-are-shrinking.html?full=true

It might sound incredible, but many animals are shrinking – and they will become ever tinier in the centuries to come

See more in our gallery: "Six animals changing size before your eyes"

THE polar bear is the largest land carnivore on the planet. Male bears typically weigh around 400 kilograms, and a few grow up to 600 kg. At least, that's how big they used to be. In Alaska, and perhaps elsewhere, polar bears are no longer growing as large as they did just two decades ago.

At the other end of the spectrum, the saffron-bellied frog of Borneo is so small - just 20 millimetres long - that it is difficult to spot despite its marbled blue back and brightly spotted underside. And it is becoming even harder to spot, because it too is shrinking.

Right across the animal kingdom - from fish to amphibians, reptiles, birds and mammals - reports are coming in of changes in body size, usually of animals getting smaller. This matters because body size affects everything, from the ability to catch food to the chances of escaping from predators to finding a mate. So why is the size of many animals changing, and will the trend continue?

Many biologists think the answer is related to an observation made in 1847 by the zoologist Christian Bergmann. He pointed out that if you look at closely related species of mammals and birds, you find the smaller species tend to live in warmer climes. This pattern, Bergmann reasoned, was to do with surface area and volume. Smaller animals have a higher surface area-to-volume ratio, so they lose heat faster and struggle to maintain their body temperature when it is very cold. Larger animals have the opposite problem - they struggle to stay cool when it's hot.

Cool running

Bergmann's "rule", as this tendency has been misnamed, doesn't mean large warm-blooded animals do not live in warm climates. Even very big animals can evolve to cope with heat, as elephants prove. Rather, for a given set of adaptations to heat or cold, a smaller body is better in warmer conditions. The implication is obvious - as the world gets warmer, animals will get smaller.

This seems to be exactly what is happening to the semi-wild sheep on the Scottish island of Soay, in the St Kilda archipelago. More smaller individuals are surviving as winters become milder, resulting in a decline in the sheep's average size (Science, vol 325, p 464).

Many birds are shrinking too. For instance, Janet Gardner, an ecologist at Monash University in Melbourne, and colleagues studied museum specimens of eight small Australian birds collected before and after 1950. They found a decline in body size in four species. "The body size of a bird found in Brisbane in 1900 now occurs round Sydney," says Gardner. The birds haven't moved, she says; instead, they have downsized in response to their warmer world.

The observed changes in size are small - around half a per cent in the case of the Soay sheep, for instance. But this is not surprising given that the world has warmed less than a degree so far, and that wildlife has had little time to respond. What will happen over a longer period?

The only way to answer this question is to turn to the past. The closest parallel to today's warming is the Palaeocene-Eocene Thermal Maximum (PETM) about 56 million years ago when the world warmed by 5 or 10 °C before slowly cooling again, although the warming probably took place over tens of thousands of years rather than a few hundred.

Many mammals did become smaller during the PETM and larger afterwards, but with only a handful of fossils to go on, it wasn't clear if this was a response to the changing climate. Last year, however, a team produced a far more detailed fossil record of several species of horse - the size of cats at this time - that lived in what is now the Bighorn basin in Wyoming. This showed that their body size did correlate with temperature, shrinking by 30 per cent as the climate warmed, then growing 70 per cent as it cooled (Science, vol 335, p 959).

These were huge changes (see graph). And while correlation doesn't prove causation, the temperature change is the most likely explanation. It is already clear that climate change will have a massive effect on wildlife by altering where plants and animals can survive, and when they do things. Now it seems animals that continue living where they are could undergo dramatic changes in body size. What's more, it is not just warm-blooded vertebrates that will be affected.

A study, also in Wyoming, of the fossilised traces left by soil-dwelling invertebrates found that they shrank too, with the diameter of burrows reducing on average by 30 to 50 per cent as the climate warmed. "I was kind of shocked when we found the extent of the decrease," says Jon Smith, a geologist at the University of Kansas. "It's entirely possible that soil biota are going to respond to modern climate change in a similar way," he says.

No one is claiming that all animals will shrink, though. While there is some validity to Bergmann's "rule", there are plenty of exceptions. The way in which animals are affected by climate is much more complex than Bergmann envisaged. "There are examples of organisms shrinking, some not shrinking and a few even getting bigger," says Jennifer Sheridan, an ecologist at the University of Alabama in Tuscaloosa.

Short-term responses to warming could also be quite different to long-term responses. Take the polar bears in Alaska, for instance. The most likely explanation for their declining body size is a lack of food (Ecological Applications, vol 20, p 768). The bears hunt seals and other prey on and around Arctic sea ice, and hunting is becoming harder because of the dramatic loss of sea ice.

For some animals, though, food availability has increased, which may explain why a few species are growing larger than normal. These effects are probably temporary: population sizes are likely to increase or decline until their numbers are back in balance with the food supply. So if polar bears avoid extinction, their body size could recover. Animals that are shrinking as a direct result of rising temperatures, however, are likely to stay small as long as the climate remains warmer.

There is a lot of debate among biologists about whether the changes seen so far are merely due to the development of animals being influenced by the changing environment, or are a result of natural selection, that is, genetic. There's no easy way to tell, and the answer may be different for different animals. Clearly, though, evolutionary changes will be longer-lasting.

Yet rising temperatures do not always favour animals with a smaller body. Mass die-offs of small birds and bats in extreme heat have long been reported, and they are becoming more frequent as extreme heat becomes more common. The reason, it seems, is that smaller animals with a relatively high surface area do not just lose heat faster; they also lose water faster. When the thermometer soars and there is no easily accessible water, tiny birds can become dehydrated in as little as 2 hours, even when resting in the shade (Biology Letters, vol 6, p 253).

So although warmer average temperatures favour smaller body size, heat extremes favour larger bodies when water is scarce. The pressure of selection can thus swing from one direction to the other depending on the weather. Whether populations grow larger or smaller in stature, or don't change much at all, depends on which factor predominates.

For cold-blooded animals, including invertebrates, a warming climate poses some distinctive challenges. For starters, development is faster when it is warmer, so animals reach adulthood sooner. But growth does not usually keep up, meaning most end up smaller.

The speed of cold-blooded animals' metabolism also depends largely on the ambient temperature. This might seem like an advantage, allowing them to be more active, but there can be big drawbacks, too. A faster metabolism burns up more energy even when animals are not active - which means they need to eat more.

Sheridan and colleagues have made some rough calculations of the consequences for amphibians and reptiles. If the climate warms by 6 °C this century, as some have predicted, a tiny species like the ornate narrow-mouthed frog of India, which typically weighs just 0.3 grams, would need to eat up to 75 per cent more, or an additional 426 termites a year, she predicts. A 90-kilogram Komodo dragon would have to find an extra 224 chickens a year to maintain its body size.

"If organisms can consume more resources to match their increased metabolism, they can maintain their growth rates and they'll be the same size," says Sheridan. But finding more to eat can be hard when it's hotter. Astudy of lizards found that they are having to spend more time in the shade to avoid overheating, leaving less time for hunting or foraging.

And there might not be any more food to find. If the same number of animals are competing for the same amount of food, then something has to give. "They will have to sacrifice something, and growth is likely to be one of the first things to go," Sheridan says.

This could be why some amphibians and reptiles are already shrinking. A 20-year study of toads in Dorset, UK, found that their body size is declining, for instance. In her work on frogs in Borneo, Sheridan has found that the number of species that are getting smaller outstrips the number that are growing.

These findings may seem surprising given that giant reptiles and amphibians thrived during many epochs when the planet was hotter than it is now. The best known, including dinosaurs, pterosaurs and marine reptiles, were probably warm-blooded, but plenty of cold-blooded behemoths thrived in the heat, too, such as Titanoboa - a stonking 13-metre-long snake weighing over a tonne.

"When you are a great big, giant reptile, you need to have warmer environments just to maintain your body temperature," says Jason Head, a palaeontologist at the University of Nebraska-Lincoln, and a member of the team that described Titanoboa. "I don't know of a time interval in the fossil record where body size has decreased in either amphibians or reptiles in response to warming."

"Over long timescales, you will get the evolution of gigantism," says Sheridan. But the world has never warmed as fast as it is doing so now, and the short-term result is that reptiles and amphibians are likely to get smaller, she says.

Fish, too, will shrink. They face a double whammy: rising water temperatures will speed up their metabolism and thus their need for oxygen, but there will be less and less of it because oxygen is less soluble in warmer water. The consequences are inevitable. "The fish will run out of oxygen at a smaller body size," says William Cheung of the University of British Columbia in Canada, whose team has studied the impact of warming on marine life (Nature Climate Change, doi.org/kbn).

Adapt or die

For animals struggling to adapt to warmer conditions, the obvious answer is to move somewhere cooler. In the oceans, many species are already doing exactly this. But migrating can be difficult or impossible. Paths may be blocked by natural barriers such as mountains or seas, or by fences, roads, cities and farmland.

Some animals must adapt to a warmer world, or die. And adapting by changing your body size can bring its own problems. One is that smaller individuals have fewer offspring, says Sheridan. "If you take that to the extreme, you could potentially have a population that gets so small that it becomes vulnerable, endangered or even extinct."

Another is that smaller animals may find it more difficult to capture some prey, and may also become more vulnerable to predators. "If everything were shrinking at the same rate then you'd basically just have a miniaturised version of the world, and everything would just be a little bit cuter," says Sheridan. But with different species responding to climate change in different ways, there will be winners and losers, which by itself will cause considerable ecological ripples.

"Body size is, in my view at least, the most important characteristic of an animal, linked to all kinds of other traits," says Martin Wikelski, an ecologist at the Max Planck Institute for Ornithology in Germany. Changing it has myriad knock-on effects.

And changing body sizes is just one of many challenges facing wildlife. We're not only causing the world to warm faster than it ever has since animals appeared, we're also making it incredibly difficult for them to survive this change. By overexploiting animals, destroying their habitat and spreading diseases, we have already driven many species extinct - especially large ones that breed slowly - and eliminated much of the genetic variation in surviving populations. This greatly reduces the chances of these species evolving fast enough to keep up with the challenges of a rapidly changing world.

We will almost certainly feel the consequences, too. Although the effects of reduction in body size in the oceans are unpredictable, says Cheung, one result could be a collapse in fish stocks. If other ecosystems we depend on find themselves in similar turmoil, then the looming food crisis will be even more serious. Perhaps, like polar bears, people will end up shrinking too.

This article appeared in print under the headline "Shrink or sink"

Henry Nicholls is a freelance writer based in London. Follow him on @WayOfThePanda