author: Julian Vincent

Keeping warm...

However, as the air is divided up into smaller and smaller parcels it is restricted from moving partly because of the physical barriers and partly because as the size gets smaller the flow properties of the air change and it appears more viscous. With fur this division is attained mainly by the shorter hairs beneath the longer guard hairs (which contribute the main waterproofing).Feathers are divided into two main parts - the vane (which is aerodynamically smooth and provides a low-drag outer surface) and the fluffy down which comes off the base of the feather In the feathers of most adult birds (other than flightless ones living in a warm climate) the vane is the larger part.

We have investigated penguin feathers where the vane is required simply to provide a smooth and waterproof outer covering. It constitutes only the distal third or so of the feather, the remainder being occupied by long strands of down. We calculated that the average size of air space within the down layer is only about 50 micrometers across and that the entrained air can account to the excellent insulating properties of penguin feathers - a gradient which can be as steep as 80°C across a layer 2 cm thick. The penguin has a problem, though. When it dives al l the air is only ~ driven out of the insulating layer by the water pressure. If the down layer were wetted it would lose its ability to subdivide the air layer into small parcels. So the feathers collapse and bend to lay close to the skin, and have massive hooks connecting them (rather 1lke Velcro) so that the out layer remains watertight. When the penguin returns to the surface the feathers are pulled back upright, partly by springing partly by muscles at their base, and the down layer fluffs out again. It manages this (rather than becoming lumpy) because the strands of down have barbs on them which interact with other strands, thus retaining the topological arrangement of the strands.

Presumably we could make a crush-proof insulator if it were made from sufficiently fine and rough fibre which could bend and interact in the same way.
The thickness of the layers of both fur and feathers is proportional to the size of the animal. So a small animal can have only a thin layer of insulation without restricting its movement too much. A smaller animal also has a greater area of skin per unit volume of its body and will therefore tend to lose heat faster, necessitating behavioural mechanisms for temperature control such as hibernation or construction of a nest or burrow.
Fat can be added to fur and feathers as a standard insulating material. Polar bears need it because their fur is such a bad insulator, especially when it gets wet. The same is true of seals. A thick layer of blubber is just about all they have to keep them warm. However, their flippers can' t afford to be too fat, nor can those of other animals such as dolphins These use a countercurrent heat exchanger to stop too much heat reaching the periphery of the body. The outgoing arterial blood loses its heat to the incoming venous blood, assisted by the arrangement of the blood vessels. A similar mechanism is found in birds legs, which explains why a duck can swim comfortably in ice-cold water without getting chilblains. The penguin is the same; when it emerges from the sea its feet and legs are yellow. In more northern (warmer) countries such as New Zealand they soon turn pink as the effective insulation of the feathers allows the bird to warm up and make it necessary to dump excess heat. The beat exchanger has an advantage over fur and feathers since it is controllable. It is even possible to lose heat through a blubber layer by adjusting the flow of blood through it.
Countercurrent heat exchangers are found in the so-called "cold-blooded" animals such as fish and insects.

For instance the tuna needs to keep its main swimming muscles at a reasonable working temperature, so controls this with a heat countercurrent called a rete mirabile or "wonderful net". Insects also make use of countercurrents to control the temperature of their flight muscles, which are capable of working harder than any other type of muscle, which therefore need to work at an elevated temperature and produce the most heat. On autumn mornings the last few "Silver-Y" moths can be seen shivering to warm up before taking off on their migrant flight southwards, and the later winter moth manages to retain nearly all the metabolic heat of his flight muscles by means of a countercurrent loop.
The heat from the wintery sun can be absorbed by the animal in direct proportion to the area it presents. Insects can spread their wings and bask which allows quite close control over the thoracic temperature and hence the degree of flight activity. Lizards and snakes can stand side-on to the sun's rays in the early morning. When they are warmer in the heat of the day they seek shade or stand end-on to the sun's rays, presenting as small an area as possible.

...and chilling out

Animals in hot deserts mostly avoid the heat of the day by being active only. at night. And apart from mad dogs and Englishmen, animals active during the day will seek shade. Otherwise, the best way to lose large amounts of heat is to evaporate water. This obviously requires water to be readily available, since the average sweating Englishman can lose l to 1.5 litres of water per hour controlling his temperature. The mad dog cannot sweat, so evaporates water from its tongue, moving air across it by panting. The truly adapted desert organism can allow its temperature to rise and its body fluids to become more concentrated. Ultimately it is possible for the organism to lose nearly all its water and become dry. The famous example is the larva of a midge found in Nigeria. When dry it could be broken into two pieces; these would have to be rehydrated so that they could wriggle around and have their death throes.
Some desert beetles can nucleate fog particles either on the tips of projecting piles of sand or on the end of their abdomen which they stick up into the air and collect the water. Many insects only ever breathe in and never breathe out, so conserving water. They lose their expiratory gases (carbon dioxide) by diffusion through the skin.
Flying insects have a problem in that their flight muscles are still producing large amounts of heat, but they have to radiate it. Insects cannot afford to lose much water So their blood system, like ours, becomes the equivalent of radiator liquid and the insect loses heat generated in the thorax by radiation from the abdomen. Fairly simple experiments in which the heart (basically just a long pulsatile tube going up the insect's back) is knotted and the scales are removed from the surface of the moth show that the insect can keep its temperature within the range 38° to 42°C over external temperatures which range from 12° to 36°C.

Structures - social thermoregulation

When animals group together they can control their temperature by bevioural mechanisms. A swarm of bees changes its behaviour as the temperature increases. At low temperatures the insects huddle and present a solid shell to the world. The core temperature is 35° although the outside of the swarm is colder At an external temperature of 30°C the swarm seems to have grown due to the incorporation of airways through the middle which allow cooler air to convect some of the heat away. Insects are best known for the structures they produce; mostly it's only the social insects (ants, bees, wasps and "white ants" or termites), although a number of caterpillars produce silken tents in which they shelter overnight. This enables them to keep a higher body temperature so that they are able to feed faster and earlier in the day.
Wasps make paper (carton) nests whose multiple layers provide very effective insulation allowing the nest to be built in shaded areas where they won't get direct sunlight and overheat. In a large wasps nest (the size of a large beachball) the internal temperature measured over a month varied from 9° to 34°C (average30.7°C) whereas the external temperature ranged from 9° to 34°C and averaged l8.4°C Wasps make a new colony and nest each year, whereas bees invest more heavily in collecting food, have a much larger colony, and overwinter using the design of the nest and the stored nectar to maintain a viable temperature. Clustered in the centre of the comb and shivering to produce heat they can easily survive long periods at sub-zero temperatures. During the summer the nest is cooled by forced evaporation Bees sit at the front entrance of the hive, which is always at a low position, and fan their wings so that air is driven through Water brought into the hive by the foraging bees (some of it in the gathered honey) evaporates and the nest retains its temperature of 30° to 35°C. Any undesired holes in the outside of the nest are blocked with a waxy material called propolis. This glues everything together and controls the air flow. The most famous of all insect nest builders are the termites. They have the largest nests with the greatest number of internal functional zones. Plants use changes in water content, effected by changes in humidity, to drive mechanisms; many seed and spore containers increase stored strain energy as they dry, finally breaking in a brittle fashion and shedding their seeds. Pine cones open and close with humidity changes so that the seeds are spread under the best conditions.

Some of these mechanisms have been modelled or plagiarised. There are examples of parallel evolution between nature and architecture such as ventilation towers similar to animal structures, and famously the termite's nest is a paradigm. Control in the natural systems is often at a local level, removing the need for integrated control. The sensors and effectors can be closely linked and the overall control is an emergent property of the organism or population.

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