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The majority of organisms are exposed to environmental fluctuations, including seasonal change in climate. In this chapter, we focus on the effects of winter.
Summary of Sections 1.1 and 1.2
Organisms have evolved a range of strategies to cope with winter. Thus this
common environmental variable has led to a diversity of responses.
Summary of Sections 1.1 and 1.2
The strategies for coping with winter can be considered with respect to
different levels and types of explanation.
Summary of Sections 1.1 and 1.2
Molecular and cellular level responses to winter include the prevention of
freezing through the production of antifreeze molecules such as peptides.
Summary of Sections 1.1 and 1.2
Physiological and behavioural responses include detecting the onset of winter
through changes in the L:D ratio which prompts alteration of sexual
behaviour. The reproductive cycles of many organisms are linked to the L:D
ratio. Many of these effects can be investigated by experiment.
Summary of Sections 1.1 and 1.2
The life histories of organisms can be viewed as the products of trade-offs in biological processes.
Summary of Sections 1.1 and 1.2
Coniferous trees are an example of plants that remain active during winter, with adaptations such as reduced water loss.
Summary of Section 1.3
Bird plumage and mammal hair are highly effective insulators, reducing heat loss in winter. Heat loss is also controlled by heat exchange mechanisms and reduction in blood flow near the body surface.
Summary of Section 1.3
Endothermy allows some birds and mammals to remain active during cold winters, but places physiological and energetic demands on the organism,
e.g. the need to maintain a high metabolic rate and/or reduce heat loss.
Summary of Section 1.3
The energy needed to sustain a high metabolic rate may be stored through either physiological and biochemical processes (adipose tissue) or changes in behaviour (hoarding or caching of food).
Summary of Section 1.3
Some animals change their social behaviour during winter, becoming more gregarious.
Summary of Section 1.3
Deciduous trees avoid the problems of winter by shedding their leaves.
Summary of Section 1.4
Plants can store nutrients over winter in a variety of structures.
Summary of Section 1.4
Amphibians have evolved behavioural responses (e.g. burying themselves)
and physiological responses (e.g. different types of antifreeze in the body fluids) to winter.
Summary of Section 1.4
Hibernation occurs only in certain small mammal species and one species of bird and is accompanied by marked physiological and behavioural changes.
Summary of Section 1.4
Prior to hibernation, animals build up their fat reserves and frequently possess larger amounts of brown adipose tissue than non-hibernators.
Summary of Section 1.4
There may be a trade-off between hibernation and dispersal in some animals.
Summary of Section 1.4
Some annual plants and insects can spend the winter at juvenile stages, such as seed, egg, larva or pupa. Butterflies in Britain display a variety of juvenile overwintering strategies.
Summary of Section 1.5
Migration often results in high mortality, but completion of the journey results in higher breeding success, due to increased availability of food and fewer competitors.
Summary of Section 1.5
Birds increase their body mass, sometimes by up to 50%, prior to migration. The body mass increase is related to the power margin of the birds.
Summary of Section 1.5
Early arrival of migratory birds at the breeding site confers important advantages on those individuals.
Summary of Section 1.5
Intestinal epithelium lines the gut except around the mouth and anus, and in vertebrates and advanced invertebrates is surrounded by muscles that propel and/or grind the food. The muscles are controlled by the autonomic nervous system.
Summary of Section 2.1
Most vertebrates have jaws, and arthropods mouth parts, often armed with hard and/or sharp teeth, which fragment food before it is taken in. Gizzards and stomachs also grind large particles.
Summary of Section 2.1
The stomach and intestine differ in muscularity, and secretory or absorptive capacity. In more complex animals, the secretory functions of the gut itself are supplemented by associated glands. The effective surface area of absorptive regions is greatly increased by numerous villi.
Summary of Section 2.1
Animals can prey upon each other in a huge range of ways, and predators have a variety of habits and body form.
Summary of Sections 2.2 and 2.3
Primitive vertebrates, such as crocodiles, stalk prey and then seize it with stout teeth, tearing and shaking it with their powerful jaws and body. Large chunks are swallowed whole.
Summary of Sections 2.2 and 2.3
Spiders build webs which trap flying or walking arthropods by secreting several different kinds of silk and associated materials.Web building requires a significant outlay of body reserves, which cannot be sustained unless enough prey are caught and eaten.
Summary of Sections 2.2 and 2.3
Poisonous snakes inject venom into prey. It contains enzymes that break down tissues and red blood cells, and, in more advanced families, neurotoxins. The fangs are modified teeth. The jaws are mechanically weak but can be disarticulated to swallow immobilized prey whole. Digestion of large prey may take weeks.
Summary of Sections 2.2 and 2.3
The main consumers of terrestrial plants are insects, gastropod molluscs, nematodes, a few birds and many kinds of mammals, whose guts are modified to harbour microbial symbionts.
Summary of Sections 2.2 and 2.3
Digestion of organic material involves mechanical shredding and churning, and enzymatic breakdown of large molecules.
Summary of Section 2.4
The stomach and intestine, and associated glands, secrete digestive enzymes. Some are highly specific and attack only certain kinds of bonds in certain positions within substrate molecules.
Summary of Section 2.4
Bile salts secreted from the liver emulsify lipids, thereby facilitating their digestion by lipases. The capacity for digesting waxes is low in most birds, mammals and insects, but can be much higher in animals whose diet includes large quantities of wax.
Summary of Section 2.4
Efficient digestion depends upon adjusting the concentrations of food and enzymes. Water secreted and mixed with food in the mouth or stomach is reabsorbed in the hindgut. Very dilute food may be useless to animals that cannot excrete the excess water quickly and efficiently.
Summary of Section 2.4
Most vertebrate herbivores depend upon symbiotic microbes to digest tough or toxic plants. A forestomach or modifications of the hindgut that accommodate microbes have evolved several times among mammals, but only ruminants chew the cud.
Summary of Section 2.4
Microbial fermentation in the rumen alters energy metabolism in all the other tissues and improves protein nutrition by recycling of urea and/or synthesizing essential amino acids.
Summary of Section 2.4
Intestinal reflux and/or coprophagy allow absorption of the products of microbial digestion in hindgut fermenters, thus improving nutrient extraction from plant food.
Summary of Section 2.4
Absorption involves specialized cells which in vertebrates are found in the small intestine. The absorptive surface area is greatly increased by being frilled out to form villi.
Summary of Section 2.5
In most animals, a large fraction of the amino acids and sugars released by digestion and many vitamins and minerals are taken up by specific transporters, some of which utilize ATP. They are located in the membrane of the epithelial cells lining the small intestine, but similar transporters may occur on the outer body surface of animals without guts. Fatty acids and monoacylglycerols enter cells by diffusion, often facilitated by carrier proteins.
Summary of Section 2.5
In mice, the capacity for active transport takes at least 1 day to adapt to large changes in diet composition.
Summary of Section 2.5
Many herbivores compensate for the nutritional insufficiencies of plants by eating mineral-rich soil or small quantities of animal food.
Summary of Section 2.5
Unicellular protoctists that cannot avoid taking up too much iron sequester it in an insoluble form.
Summary of Section 2.5
The intestine is not impermeable, and many other substances, including potential toxins, can cross its epithelium and enter the bloodstream.
Summary of Section 2.5
Glucose, amino acids and short-chain fatty acids dissolve in blood but triacylglycerols form chylomicrons.
Summary of Section 2.5
Most aphids harbour one species of endosymbiotic bacteria which synthesize essential amino acids and enable the insects to breed rapidly on a nutritionally meagre diet of sap. Other insects have gut symbionts that aid digestion as well as metabolism.
Summary of Section 2.5
The lining of the mammalian intestine is replaced every few days as new epithelial cells mature and pass along the villi, where they are sloughed off. The absorptive capacities of newly formed cells are adapted to changes in diet.
Summary of Section 2.6
Enzyme secretion and active transport are energetically expensive. Animals that eat large nutritious meals very infrequently may save energy by partially dismantling the gut while fasting. Reassembling it after feeding requires high rates of energy usage sustained for many hours.
Summary of Section 2.6
Rebuilding the gut and digestion release large quantities of heat, which speeds up digestion and metabolism but may overheat other tissues when in excess.
Summary of Section 2.6
Many animals can reabsorb and break down their tissues during starvation but most vertebrates and more complex invertebrates have specialized storage tissues.
Summary of Section 2.6
Glycogen is a short-term energy store often used by the tissue that sequesters it.
Summary of Section 2.6
The arthropod fat-body and vertebrate adipose tissue are specialized for storing lipid for use by other tissues. These tissues can become massive, enabling animals to fast for long periods and/or travel long distances. Mammalian adipose tissue is split into many depots, some of which have specialized site-specific properties.
Summary of Section 2.6
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