See a gallery of thermal images of warm- and cold-blooded animals
IF YOU stopped eating today, you wouldn't survive more than two months. A crocodile, on the other hand, might live for a year or more. Why the difference? You waste most of the food you eat generating heat.
The evolution of warm-bloodedness, or endothermy, is one of life's great mysteries. Sure, there are some advantages - staying active in the cold, keeping young cosy and warm, and avoiding having to go out into the open to soak up heat from the sun.
The thing is, you could get much the same advantages by turning up the heat only when and where in the body it is needed, as many animals do. So why do most birds and mammals keep the furnaces burning 24/7? Staying warm - which for birds means 40 °C on average - comes at a price. Some warm-blooded animals have to eat as much in one day as similarly sized reptiles do in a month, a dangerous and time-consuming strategy.
Biologists have long struggled to understand why we mammals and our feathery cousins are warm-blooded. The standard explanation is that it evolved in small carnivores to enable an active, predatory lifestyle. Last year, however, a radical new idea was put forward: warm blood evolved not in carnivores but in herbivores, as a way of balancing their nutrient requirements. Though it is early days, this idea could explain not only why we have such an apparently wasteful lifestyle, but also a long-standing question about the dinosaurs (see "Why did dinosaurs grow so big?").
In mammals and birds, heat is generated mainly by the visceral organs such as the liver and the brain. These organs are usually larger than their counterparts in cold-blooded animals (ectotherms), and their cells contain up to five times as many food-burning factories, the mitochondria. As a result, they generate heat continuously, keeping the entire body warm all the time.
This is extraordinarily wasteful, especially when you consider that many other animals have evolved more efficient strategies. For example, leatherback turtles conserve the heat they generate when swimming to keep their body temperature 10 °C or more above that of seawater, allowing them to forage in much colder waters than other turtles (Comparative Biochemisty and Physiology A, vol 147, p 323). Swordfish selectively warm their eyes and brain while hunting, while some sharks and tuna keep their long-distance swimming muscles well above the water temperature. Even a few insects, such as the hawkmoth, can generate heat when it's needed.
So why do most mammals and birds turn the thermostat to maximum all the time? The leading explanation was put forward three decades ago by zoologists Albert Bennett of the University of California, Irvine, and John Ruben of Oregon State University. They proposed that the evolution of endothermy was all about stamina. They noted that mammals and birds have a high aerobic capacity compared with other animals, which provides their muscles with lots of oxygen and keeps that supply going for long periods. As a result, they are able to sustain exertion for longer, whether chasing prey or fighting competitors. Nobody disagrees with that. Bennett and Ruben, however, went on to argue something more contentious: that high aerobic capacity inevitably leads to a high metabolic rate at all times. In other words, selection for stamina leads to endothermy.
Not so many people agree with that. There's no obvious reason why the two should be linked: aerobic capacity depends on the cardiovascular system and muscles whereas the resting metabolic rate depends mostly on the brain and visceral organs. On top of that there are a few reptiles, such as monitor lizards, which have high aerobic capacity but a low resting metabolic rate. Some mammals and birds achieve much the same by dropping their body temperature when inactive or when hibernating. The aerobic capacity hypothesis has never really been proved or disproved, despite quite a few attempts to do so (Physiological and Biochemical Zoology, vol 77, p 982).
There are other contradictions too. The theropod dinosaurs - the group that includes velociraptors - certainly had a high aerobic capacity. Most researchers agree that they lie on the evolutionary line to birds. But were they endotherms?
Ruben insists the answer is no, even though his own aerobic capacity hypothesis predicts a link. He agrees that the theropods were capable of great speed and exertion, but thinks they still had a slow resting metabolism. His conclusion is based on the fact that they lack so-called respiratory turbinates, elaborate whorls of bone or cartilage in the nasal passages of birds and mammals that reduce water loss through breathing, which is a big problem when you have a fast metabolism.
So if there isn't necessarily a link between stamina and warm blood, then why did it evolve at all? Enter Marcel Klaassen and Bart Nolet of the Netherlands Institute of Ecology. They study stoichiometry - how animals get enough of the various nutrients they need. Herbivores have a well-recognised problem here: how to get enough nitrogen, which they need for making proteins, DNA and RNA. Essentially, if you only eat leaves, you get too much carbon and not enough nitrogen. Some reptiles are herbivores but it is not a common lifestyle and those that are cheat, says Robert Espinoza of California State University, Northridge. "From time to time most herbivorous lizards eat bugs or small vertebrates," he says. "It undoubtedly helps them overcome nutrient deficits."
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