Physio

How animals have adapted to high temps

The tissues of endotherms have substantially higher mitochondrial densities and higher activities of mitochondrial enzymes than do similarly sized ectotherms (Fig. 8.31), reflecting this ability to produce metabolic heat.

Diurnal species are warmer by day, and nocturnal species warmer by night,

For birds and mammals at rest, the thoracic and abdominal organs (gut, liver, kidney, heart, lungs, etc.) produce up to three-fourths of the metabolic heat, whereas when active, with the metabolic rate perhaps elevated 10fold, it is principally the muscles that produce extra heat.

Heat production in animals is continuous and inevitable; the important distinction is that in endotherms it occurs at a higher rate (4–8 times that of ectotherms at the same Tb) because of a greater concentration of mitochondria and perhaps also a different kind of mitochondria in which oxidative phosphorylation is always partially uncoupled so that more heat is released for a given level of ATP generation.

For birds and mammals at rest, the thoracic and abdominal organs (gut, liver, kidney, heart, lungs, etc.) produce up to three-fourths of the metabolic heat, whereas when active, with the metabolic rate perhaps elevated 10fold (see Chapter 6), it is principally the muscles that produce extra heat.

Regulating heat gain and keeping warm

the thermoneutral zone (TNZ) is a useful concept, being the range of ambient temperatures over which the animal’s metabolic rate and thus heat production is not varied.

Below some specific lower temperature, a mammal will show a rise in metabolic rate with decreasing temperature in a linear fashion. Above some specific higher temperature, metabolic rate may again rise due to the direct effects of temperature on metabolic processes

An Arctic mammal does not need to increase its metabolic rate to keep warm unless the external temperature is well below 0°C (e.g. −40°C for an Arctic fox), and even then the metabolic rate/ambient temperature plot is very shallow. In contrast, a tropical mammal may show an increased metabolism if the ambient temperature only drops to 25°C, and the rate of increase will be much sharper with declining temperature.

a large endotherm can tolerate a much greater degree of cooling without increasing its heat production; it will inevitably have an extended TNZ at the lower end.

These differences in TNZ are largely due to reductions in the thermal conductance of the cold-adapted endotherms they have thick insulating pelts (see sections 8.5 and 8.7.4) that greatly increase their thermal tolerance. Larger mammals and birds have thicker fur or feather layers, and/or subcutaneous fat layers, and therefore are better protected against heat loss;

Heat production can be increased according to need in three ways. Muscular activity by physical exercise greatly raises the metabolic rate as we have seen, and could be used to warm the body. Involuntary muscular activity could also be initiated, this being what we perceive as shivering.

As a third possibility, thermogenesis could be achieved without muscular contractions, by a variety of chemical means, so-called non shivering thermogenesis.

Voluntary muscular activity to achieve warm-up is not uncommon in animals, and is of course used by humans. Some insects may use short periods of flight for no other purpose than to warm their bodies up,

However, activity is not a simple solution to being cold for most animals, as exercise may in itself accelerate some avenues of heat loss: it tends to involve high surface area positions, reduces the thickness of boundary layers, and most seriously may reduce the value of insulation layers by wetting them (in sweating mammals) or making them dishevelled (as with bird plumage).

Shivering is used by nearly all adult endotherms, and by many ectotherms as well. It is a high-frequency reflex operating via efferent nerves to the muscle spindles, to produce oscillatory contraction of mutually antagonistic skeletal muscles, giving little net movement. Thus the ATP hydrolysed to provide energy for contraction is producing minimal physical work and being diverted into heat instead. The intensity of shivering is linearly related to oxygen consumption.

animals can still exhibit an increase in metabolic rate for heat production when cooled. This results from non shivering thermogenesis (NST), It certainly occurs during arousal from torpor in specific mammalian tissues such as brown fat (brown adipose tissue, or BAT),It refers mainly to the increased metabolism of brown fat, It probably also happens in the liver, kidneys. Some semiaquatic mammals also use it to warm up after diving or prolonged swimming.

Nonshivering thermogenesis equally clearly occurs in at least some ectotherms, including leatherback turtles and a few insects, where for example warm-up can occur in immature forms (social wasps and others) lacking any proper skeletal muscle to shiver with.

The best understood mechanism of NST is that in the brown adipose tissues of mammals (see Fig. 6.10). Under resting conditions the BAT mitochondria function in the normal way, protons flowing back into the mitochondrial matrix through the ATP synthase enzyme, generating ATP and some heat (see Chapter 6). But when stimulated by norepinephrine from the sympathetic nerve endings the BAT mitochondria switch to a different proton route (Fig. 8.35d), due to the activity of uncoupling proteins (UCPs) on the mitochondrial inner membranes. This route is normally inhibited in the resting state by purine nucleotides. Once this inhibition is released, the protons flow back into the matrix via proton translocase channels (“thermogenin” sites, now identified as one isoform of UCP), and since there is no associated ATP synthase all the energy is liberated as heat.

Heat is produced in all body tissues, but particularly in the core organs of a resting animal, and in muscles during activity. This must be distributed around the body, principally in the blood or other body fluids, and dissipated suitably at the surface according to need.

The simplest form of control is to manage the rate and volume of the flow of blood to the surface relative to the core. In a cool and resting animal where most of the heat is generated in the core it can be effectively retained there with minimal peripheral vasodilation, keeping most of the circulatory flow in arterioles and venules and away from superficial capillary beds (Fig. 8.36). There may be direct arteriole to venule pathways (known as arteriovenous anastomoses) in vertebrates, as we saw in Chapter 7, allowing this bypassing of capillaries and keeping heat in the core. In an active or hot animal, peripheral arterioles dilate, sometimes with specific valve actions, and the capillary beds are incorporated into the circulation.

Freeze-tolerant animals

Freeze-intolerant animas