快猫短视频

TOUCH someone else鈥檚 skin and you will feel human warmth, no matter what the ambient temperature is. A cold body is a sick body. A hot body is a sick body. A healthy human body, like the storybook porridge, needs to be just right.

We take our body temperature for granted, but in fact our predictable 37 掳C is distinctly odd. Most species do not produce their own heat. Lizards are cold-blooded, or ectotherms, frogs too, as are insects, plants, fungi, protists and bacteria. As the sun sets, their bodies chill; as it rises they warm.

Ectothermy is the 鈥渘ormal鈥� way to be; warm-bloodedness, or endothermy, is found only in mammals and birds. We know it evolved independently in each group at the very beginning of their respective histories, but the mystery is why. It has long been argued that the ability to generate your own body heat allows you to remain active even when it is cold. But can this benefit outweigh the costs?

These costs are considerable: maintaining a constant high temperature requires an extraordinary expenditure of energy. Truth be told, warm-blooded species are not warm, they are hot. The average bird has a mean body temperature of 40 掳C. For mammals, it is a couple of degrees lower. At 37 掳C, your body is warmer than the mean annual temperature of any habitat on Earth, including the Sahara desert and the Amazon rainforest. To fuel this biochemical fire you must eat nearly 50 times more frequently than a boa constrictor of equivalent size, and consume up to 30 times more calories overall. Why so wasteful? There have been several attempts to answer this question (快猫短视频, 7 February 2009, p 42). Now Arturo Casadevall at the Albert Einstein College of Medicine in New York City has come up with a new and elegant explanation. It can be summarised in a single word: fungi.

鈥淭o fuel your biochemical fire you must eat nearly 50 times more frequently than a boa constrictor鈥�

We tend not to think about fungi except when we eat them or find them growing on our shower curtains or between our toes. But for close to a billion years fungi have been central to the story of terrestrial life. They grow in essentially every terrestrial habitat and on almost every species. Plants have fungi on their leaves, on their roots and nearly everywhere else. Amphibians and reptiles harbour thousands of fungal pathogens, several of which now threaten to extinguish entire frog species (快猫短视频, 11 December 2010, p 14). Beetles, ants and termites all farm fungi. More often, however, fungi get the upper hand in their dealings with insects 鈥� sometimes dramatically so. Several lineages of fungus are capable of taking over the brains of insects and making them do terrible things (快猫短视频, 27 August, p 37). In short, fungi are everywhere. Well, almost.

One place they are distinctly lacking is in humans. Casadevall has spent the past three decades studying fungi that invade the human body. They are his nemesis, a kind of daily quarry, yet he couldn鈥檛 help noticing how very few in number they are. The most common ones include a few species of Trichophyton, which cause athlete鈥檚 foot and ringworm, and Pneumocystis, which can lead to pneumonia in people with compromised immune systems. Most of the time, however, Pneumocystis lives peaceably in the lungs of nearly all of us and Trichophytons are more often guests than pathogens. A few other fungi, including Candida, Cryptococcus, Aspergillus and Histoplasma, tend to live only in people with compromised immune systems. While they can cause harm, they did not evolve to be pathogens. Instead they are simply ready to take advantage of weakness if given the opportunity.

Fungi, it would appear, are puny in the face of our defences. Every day, countless airborne spores land on you, but as long as you are healthy, you are essentially immune to them. In most cases, the bacteria on your skin and the cells of your immune system defeat the new arrivals, employing one biochemical parry at a time. However, the sheer effectiveness of our defence against them led Casadevall to suspect there must be some other force at work. This is where the story of fungus meets the story of warm-bloodedness.

It turns out humans are not the only species virtually untroubled by fungi. The same is true of all mammals and birds. Of the 1.5 million species of fungus thought to exist, fewer than 500 are known to grow on mammals 鈥� despite there being more than 4000 species 鈥� and most of these do not cause disease. For comparison, the human belly button alone is host to thousands of species of bacteria. No comprehensive tally of the fungi that live on birds seems to exist, but a quick search for 鈥渇ungal pathogens of birds鈥� in the online citation index yields precisely zero hits. Surely, thought Casadevall, it is no coincidence that mammals and birds are both relatively immune to fungi and are the only two groups of warm-blooded animals. He suspected that our tropical body temperature is what keeps fungi at bay, and that this is why we evolved to be so warm.

If his idea is correct, you would expect to find that the body temperatures of warm-blooded animals exceed the maximum temperature tolerated by most fungi. To test whether this is the case, Casadevall teamed up with Vincent Robert from the Fungal Biodiversity Centre in Utrecht, the Netherlands. Using the centre鈥檚 collection of fungi, they examined the temperatures that 4082 fungal species can survive, and compared these to the body temperatures of birds and mammals.

This is usually the point in the story where a wild theory comes tumbling down. Instead, the pair found exactly what Casadevall鈥檚 hypothesis had predicted. Most of the fungi thrived at temperatures between -4 掳C and 30 掳C. Beyond that, however, they started to feel the heat, with fewer than a third of species able to survive beyond 37 掳C and just 5 per cent able to grow at 41 掳C. In other words, the majority of birds and mammals maintain their body temperature above 鈥� though only just above 鈥� the maximum temperatures tolerated by the majority of fungi ().

In a second study, Casadevall modelled the optimum temperature for mammals as a function of their body size and the costs of maintaining a high temperature. He then combined this initial model with one showing the benefits of additional temperature in terms of a reduced exposure to fungal pathogens. He found that the temperature that best balances the costs of being warm with the benefits of defence against fungi is 36.7 掳C 鈥� almost exactly where humans and other mammals pitch it ()

What鈥檚 more, those few mammals whose body temperatures drop lower than this 鈥� either occasionally or full-time 鈥� do seem to be more susceptible to fungi. That is certainly true of the platypus, which maintains a relatively chilly 32 掳C. Across North America, meanwhile, hibernating bats are being killed by the fungus that causes white nose syndrome, when their bodies are cold with torpor (快猫短视频, 27 March 2010, p 42). And although rabbits have few fungal pathogens, they are prone to them in their testes, which at 35 掳C are 4 to 5 掳C colder than the rest of their bodies. Whether this is also the case for other mammals with warm bodies and cool scrotums is, as yet, untested.

Casadevall says his idea doesn鈥檛 necessarily undermine the theory that warm-bloodedness evolved to allow animals to be more active. Endothermy, he says, would probably have given birds and mammals an advantage on cold days and evenings and in cold months, when ectotherms could only give up and go to sleep. But why be so hot 24/7? He thinks that once organisms began to be able to raise their body temperature, it was fungi that pushed the thermostat higher than it might otherwise have been 鈥� perhaps close to the limit of what is possible for vertebrate metabolism. The temperature we think of as normal, then, is the temperature warm enough to kill most fungi without killing us. This balancing act could be what makes high fevers 鈥� which themselves may have evolved to fight other pathogens 鈥� so dangerous. They push our bodies right up to the edge.

鈥淭he temperature we think of as normal is the one high enough to kill most fungi without killing us鈥�

Whether or not fungi did drive the evolution of warm-bloodedness, what appeals about the idea is its recognition that other species made us what we are. Our bodies, with all their particulars of temperature, chemistry, organs, size and everything else, evolved not just in ancient environments, but also from ancient species (see 鈥淭he beasts that made us鈥�).

History may come to regard Casadevall鈥檚 idea as a curious possibility raised by a mind otherwise dedicated to practical answers. If he is correct, however, it may have more implications than are superficially apparent. By 2100, global warming could make large areas of the world as warm as our bodies. As the environment heats up, Casadevall speculates that more fungi will evolve to tolerate warm conditions. So, along with all the other trouble it will cause, global warming could mean that we start to lose our immunity to fungi.

The first signs, Casadevall says, may be 鈥渁 rise in fungal diseases in mammals as some species that are currently considered non-pathogenic become pathogens鈥�. Instead of the canary in the coal mine, we may need to watch for the fungus in the mammal.