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Many organisms have a body temperature that differs little, if at all, from their environment. A parasitic worm in the gut of a mammal, a fungal mycelium in the soil and a sponge in the sea acquire the temperature of the medium in which they live. Terrestrial organisms, exposed to the sun and the air, are different because they may acquire heat directly by absorbing solar radiation or be cooled by the latent heat of evaporation of water (typical pathways of heat exchange are shown in Figure 2.11). Various fixed properties may ensure that body temperatures are higher (or lower) than the ambient temperatures. For example, the reflective, shiny or silvery leaves of many desert plants reflect radiation that might otherwise heat the leaves. Organisms that can move have further control over their body temperature because they can seek out warmer or cooler environments, as when a lizard chooses to warm itself by basking on a hot sunlit rock or escapes from the heat by finding shade.

Figure 2.11 The avenues of heat exchange between an ectotherm and its environment.

Source: After Fei et al. (2012).

Amongst insects there are examples of body temperatures raised by controlled muscular work, as when bumblebees raise their body temperature by shivering their flight muscles. Social insects such as bees and termites may combine to control the temperature of their colonies and regulate them with remarkable thermostatic precision. Even some plants (e.g. Philodendron) use metabolic heat to maintain a relatively constant temperature in their flowers; and, of course, birds and mammals use metabolic heat almost all of the time to maintain an almost perfectly constant body temperature.

An important distinction, therefore, is between endotherms that regulate their temperature by the production of heat within their own bodies, and ectotherms that rely on external sources of heat. But this distinction is not entirely clear‐cut. As we have noted, apart from birds and mammals, there are also other taxa that use heat generated in their own bodies to regulate body temperature, but only for limited periods; and there are some birds and mammals that relax or suspend their endothermic abilities at the most extreme temperatures. In particular, many endothermic animals escape from some of the costs of endothermy by hibernating during the coldest seasons: at these times they behave almost like ectotherms.

endotherms: temperature regulation – but at a cost

Birds and mammals usually maintain a constant body temperature between 35 and 42°C, and they therefore tend to lose heat in most environments; but this loss is moderated by insulation in the form of fur, feathers and fat, and by controlling blood flow near the skin surface. When it is necessary to increase the rate of heat loss, this too can be achieved by the control of surface blood flow and by a number of other mechanisms shared with ectotherms like panting and the simple choice of an appropriate habitat. Together, all these mechanisms and properties give endotherms a powerful (but not perfect) capability for regulating their body temperature, and the benefit they obtain from this is a constancy of near‐optimal performance. But the price they pay is a large expenditure of energy (Figure 2.12), and thus a correspondingly large requirement for food to provide that energy. Over a certain temperature range (the thermoneutral zone) an endotherm consumes energy at a basal rate. But at environmental temperatures further and further above or below that zone, the endotherm consumes more and more energy in maintaining a constant body temperature. Even in the thermoneutral zone, though, an endotherm typically consumes energy many times more rapidly than an ectotherm of comparable size.

Figure 2.12 Examples of the thermoneutral zone. (a) Thermostatic heat production by an endotherm is constant in the thermoneutral zone, between b, the lower critical temperature, and c, the upper critical temperature. Heat production rises, but body temperature remains constant, as environmental temperature declines below b, until heat production reaches a maximum possible rate at a low environmental temperature. Below a, heat production and body temperature both fall. Above c, metabolic rate, heat production and body temperature all rise. Hence, body temperature is constant at environmental temperatures between a and c. (b) Mean resting metabolic rate (measured in units of power) versus ambient temperature in nine Japanese quail, Coturnix japonica (each bird has a different symbol). The thermoneutral zone extends between 23.2 and 36.0°C and the birds’ minimum body temperature within this zone was 40.7°C.

Source: (a) After Hainsworth (1981). (b) After Ben‐Hamo et al. (2010).

ectotherms and endotherms coexist: both strategies ‘work’

The responses of endotherms and ectotherms to changing temperatures, then, are not so different as they may at first appear to be. Both are at risk of being killed by even short exposures to very low temperatures and by more prolonged exposure to moderately low temperatures. Both have an optimal environmental temperature and upper and lower lethal limits. There are also costs to both when they live at temperatures that are not optimal. For the ectotherm these may be slower growth and reproduction, slow movement, failure to escape predators and a sluggish rate of search for food. But for the endotherm, the maintenance of body temperature costs energy that might have been used to catch more prey, produce and nurture more offspring or escape more predators. There are also costs of insulation (e.g. blubber in whales, fur in mammals) and even costs of changing the insulation between seasons. Temperatures only a few degrees higher than the metabolic optimum are liable to be lethal to endotherms as well as ectotherms (Section 2.3.6).

It is tempting to think of ectotherms as ‘primitive’ and endotherms as having gained ‘advanced’ control over their environment, but it is difficult to justify this view. Most environments on earth are inhabited by mixed communities of endothermic and ectothermic animals. This includes some of the hottest – e.g. desert rodents and lizards – and some of the coldest – penguins and whales together with fish and krill at the edge of the Antarctic ice sheet. Rather, the contrast is between the high cost–high benefit strategy of endotherms and the low cost–low benefit strategy of ectotherms. But their coexistence tells us that both strategies, in their own ways, can ‘work’.