6.3.2 Endotherms compared with an ectotherm under natural conditions
But now suppose the body temperature of the ectotherm is recorded under sunny natural conditions, with the animal able to display its normal behaviour. Many lizards then have a body temperature considerably above that of the surrounding air, because they are able to bask in the sunshine. In sunny conditions, a modest-sized lizard – for example the common European lizard – can warm from 15 °C (close to the recorded air temperature) to 25 °C in about five minutes by basking side-on to the sun. By shuttling into the sun and back into the shade, this lizard can maintain a relatively even body temperature close to 30 °C, at least during the sunnier months of the year and during the day (Tb falls at night or on sunless days). So here thermoregulation is achievable by behavioural means.
By contrast, in mammals (and birds) thermoregulation can be achieved largely by physiological means – by internal adjustments within the body’s tissues and organs. Adjustments in metabolic rate are an especially important part of the process. Figure 20 helps to explain what’s involved. For the moment, concentrate on the line labelled ‘weasel’ – which, for convenience, can be thought of as a typical temperate mammal. Here, the vertical axis denotes metabolic rate. The horizontal axis is Ta, just as in Figure 19. When you've had a look at the graph, try answering the questions below.
What’s striking about the metabolic rate for the weasel between the ambient temperatures of 38 °C and 19 °C?
It’s constant, at what seems like the lowest possible value. Between these temperatures, there would be no change, therefore, in the amount of heat produced as a by-product of metabolism.
This minimum rate of metabolism is termed the basal metabolic rate (BMR). This would reflect the ‘at rest’ values recorded earlier for the mammals in Table 2.
Describe what happens when the ambient temperature experienced by the weasel falls below 19 °C.
The metabolic rate increases steadily, which means that the amount of metabolic heat produced also increases. (The fact that it’s a straight line means that the metabolic rate increases by the same amount for a standard drop of ambient temperature, of say 5 or 10 °C.)
Why might this boost in metabolic heat production be important?
When conditions become increasingly colder, the animal is likely to lose a greater amount of heat than it would in warmer temperatures. The extra heat production, therefore, might compensate for this increased loss, which helps explain why body temperature stays at a constant value in the way that we saw for the cat in Figure 19.
The type of elevation of metabolic rate seen in Figure 20 is termed a thermogenic response, and is one of the most important distinguishing features of mammals (and of birds too). The type and scale of the thermogenic response varies between mammals living in very different environments. When the metabolic rate in a tropical mammal like a sloth is measured against Ta, the BMR is evident over a much narrower range of Ta, and the rate of metabolism increases very sharply below about 31 °C.
Using the correct technical terms and symbols, briefly explain the way in which metabolism in the arctic mammal changes as ambient temperature drops. You can use the formatting buttons in the answer box below for subscript/superscript and italics.
As Ta falls, the BMR is maintained down to a Ta of about 10 °C, at which point metabolism increases comparatively slowly as Ta drops further.
The slopes of the three lines in Figure 20 differ. The steepest such slopes are typical of tropical mammals, especially those that are unlikely to encounter low temperatures. They lose heat relatively rapidly from their surface and their metabolism has to be cranked up very substantially to compensate. By contrast, mammals adjusted to very cold conditions (a polar bear cub or the arctic fox) are much more proficient at conserving the heat they produce, largely because of their superb insulation. The arctic fox can maintain its BMR down to a Ta as low as -30 °C, while the tropical forest-dwelling sloth is obliged to raise its metabolism at any Ta below 30 °C.
How then do mammals raise their metabolic rate in the ways shown in Figure 20? Some behavioural tactics will probably be familiar to you. Increasing activity – e.g. the stamping of feet – reflects an increase in voluntary muscular activity; since the muscles are working harder, they produce more metabolic heat.
Another part of the explanation is shivering, where opposing sets of muscles contract in a high frequency mode. Rather than movement resulting – the usual effect of muscle contraction – these ‘inefficient’ tiny contractions produce large amounts of heat. Shivering can be temporarily blocked by drugs (notably curare, which prevents muscle contraction) but when such drugs are applied, they don’t entirely remove the ability that animals have to increase metabolism in the cold. This suggests that there’s another component at work in such responses. It’s now known that many mammalian tissues (such as the liver of species used to living in the cold) have the capacity to raise their metabolism and burn off more fuel in the mitochondria. There is a range of circumstances when ‘turning on’ heat production in this way is especially advantageous, which can’t be explored in this course. But from a brief look at mammals well-suited to life in the cold, what’s striking is that they don’t just have visible ‘on the surface’ adaptations, such as thick fur. They can display ‘inner’ adaptations – in their metabolism, for example – of the type that only measurements such as those in Figure 20 can reveal.