Ectotherms and endotherms

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

Figure 2.6 Schematic diagram of the avenues of heat exchange between an ectotherm and a variety of physical aspects of its environment. (After Tracy, 1976; from Hainsworth, 1981.)

Radiation from atomsphere

Figure 2.6 Schematic diagram of the avenues of heat exchange between an ectotherm and a variety of physical aspects of its environment. (After Tracy, 1976; from Hainsworth, 1981.)

Ectotherms And Endotherms Diagram

Radiation from atomsphere

Conduction exchange

Metabolism»

Conduction exchange pathways of heat exchange are shown in Figure 2.6). 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.

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.

Birds and mammals usually maintain a constant body temperature between

35 and 40°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 2.7), 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.

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 endotherms: temperature regulation - but at a cost

Figure 2.7 (a) Thermostatic heat production by an endotherm is constant in the thermoneutral zone, i.e. 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. (After Hainsworth, 1981.) (b) The effect of environmental temperature on the metabolic rate (rate of oxygen consumption) of the eastern chipmunk (Tamias striatus). bt, body temperature. Note that at temperatures between 0 and 30°C oxygen consumption decreases approximately linearly as the temperature increases. Above 30°C a further increase in temperature has little effect until near the animal's body temperature when oxygen consumption increases again. (After Neumann, 1967; Nedgergaard & Cannon, 1990.)

Population Size Endotherms Ectotherms

Figure 2.7 (a) Thermostatic heat production by an endotherm is constant in the thermoneutral zone, i.e. 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. (After Hainsworth, 1981.) (b) The effect of environmental temperature on the metabolic rate (rate of oxygen consumption) of the eastern chipmunk (Tamias striatus). bt, body temperature. Note that at temperatures between 0 and 30°C oxygen consumption decreases approximately linearly as the temperature increases. Above 30°C a further increase in temperature has little effect until near the animal's body temperature when oxygen consumption increases again. (After Neumann, 1967; Nedgergaard & Cannon, 1990.)

seasons. Temperatures only a few degrees higher than the metabolic optimum are liable to be lethal to endotherms as well as ectotherms (see 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, crudely, 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'.

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Responses

  • vanna
    Why would endotherms increase oxygen consumption?
    6 years ago
  • rose
    Are plants ectotherms or endotherms?
    6 years ago
  • almaz
    Are worms endotherms or ectotherms?
    6 years ago
  • Klaudia
    Is a bumble bee an endotherm or ectotherm?
    6 years ago
  • HAMILCAR
    Is a bivalve an endotherm or ectotherm?
    6 years ago
  • immacolata
    Is coral endotherm or ectotherm?
    6 years ago
  • CHANTELLE MOORE
    Are whales ectotherms or endotherms?
    6 years ago
  • mirella
    Does temperature affect species richness in ectotherms?
    6 years ago
  • Kaisa
    Are sponges ectotherm or an endotherm?
    6 years ago
  • michael
    Is an eastern chipmunk ectothermic?
    6 years ago
  • russell
    Are mammals ectotherms or endotherms?
    6 years ago
  • konsta
    Are ectotherms or endotherms more susteptible to pesticides?
    6 years ago
  • rahel
    Are krill ectotherms or endotherms?
    5 years ago
  • sebastian
    Are sponges ectothermic or endothermic?
    4 years ago
  • Medhanie
    Is a bumble bee an endotherm or an exotherm?
    4 years ago
  • Minnie
    Are eastern chipmunks endotherms, ectotherms, or poikilothermic?
    4 years ago
  • samuel
    Is a whale endotherms ectotherms?
    4 years ago
  • marco
    Are penguins endotherms or ectotherms?
    4 years ago
  • peter
    Is krill an ectotherm?
    4 years ago
  • Diamond
    Are bumblebees ectotherms?
    3 years ago
  • Simone
    Are bivalve endotherms or ectotherms?
    3 years ago
  • maximilian
    Are earthworm ectothermic?
    2 years ago
  • katariina
    Are fleas ectothermic or endothermic?
    2 years ago
  • marcus sandyman
    Are segmented worms endotherms or ectotherms?
    2 years ago
  • Janne
    Can extotherms have a thermoneutral zone?
    3 months ago

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