Energetics and Metabolism

K.L. Paradis, L.A. Pyzocha, M.A. Gibson, N.R. Dion, K.L. Niessink, A.M. Konefal, D.E. Facey

(Saint Michael's College, Colchester, VT, USA )

In order for an animal to perform any functions it must have energy. Animals acquire energy through food, which can be acquired in several ways. After acquiring food, it must be broken down and absorbed. Animals use energy to support all life processes, including breathing, circulation, movement, nerve function, and temperature regulation. Animals use several physiological and behavioral mechanisms to maintain their body temperature and minimize the loss of energy.

Metabolism is the total of all physical and chemical changes that take place within an organism, and according to the Laws of Thermodynamics, all of these changes will ultimately release heat. Therefore, metabolic rate is a measure of the heat production of an animal. Very few organisms can survive in extreme thermal conditions (notable exceptions are bacteria). Less extreme but variable conditions are made tolerable through thermal regulation of body heat that is achieved through varying degrees of metabolic activity. For most animals it is essential that the body temperature be maintained within certain parameters, regardless of external thermal conditions.


What are the Different Methods of Feeding?

In order to sustain life all organisms must have some way of attaining food. This process can occur by means of filtering, piercing, seizing or grazing. The amount of energy put into attaining food can determine the efficiency of the method used for acquiring the food. If a lot of energy is put into acquiring food then the amount of energy coming from the food must also be great. Whereas, if very little energy is put into getting food then the amount of energy from the food does not have to be very great. The processes by which organisms acquire food are important because it is through these means that the organisms are able to survive.

One way organisms acquire food is by the process of filter feeding. Organisms such as sponges, brachiopods, lamellibranchs and tunicates are found in aquatic environments and use filter feeding to acquire food. This method requires very low energy expenditure if any at all. In some cases, water flowing across an opening in the organism can result in a decrease in pressure (the Bernoulli effect), causing water to flow out of this opening and drawing water into another opening, therefore bringing food to the mouth.  Mucus is also another important feature for filter feeders. This sticky mucus covers the ciliated epithelium of the organism and traps the microorganisms, phytoplankton or zooplankton. The trapped organism is then transported to the oral parts by the beating motion of the cilia. The extra water that is taken in is squeezed back out. The largest of these feeders are the baleen whales. This animal opens its mouth and sucks the water in. Its hair-like teeth, made of keratin, strain the food. When the mouth is closed the water is then squeezed back out through the hair-like teeth. The food that was obtained through this process is swallowed and digested for energy.

Another way organisms obtain food is through a process of fluid feeding. There are two main areas to fluid feeding, (1) piercing and sucking, (2) cutting and licking. The organisms associated with piercing and sucking are platyhelminths, nematodes, annelids and arthropods. These organisms have distinct mouth parts which bore into their prey. The organisms then suck out the prey's body fluids with their pharynx. Enzymes secreted from the pharynx help facilitate the digestion of the fluids. One of the most common piercing mouthparts, found in insects, is the proboscis which has two canals formed by the maxillae. The first canal carries in the victims blood and the other carries saliva and anticoagulants. The process of fluid feeding by means of cutting and licking are associated with some invertebrates (such as black flies) and a few vertebrates (such as vampire bats). The fluid feeder will feed by cutting the prey's body with sharp mouthparts or teeth and then lick the fluids as they flow from the body. Anticoagulants injected into the wound helps prevent blood from clotting.

A third method of obtaining food is by seizing of prey. The organisms associated with this form of feeding have jaws, teeth or beaks, and some also use toxins. Many invertebrates that don't have true teeth use a beak-like structure for feeding. The beak-like structure is driven into the prey, and some of these organisms also inject venom to paralyze their prey. Other non-mammalian vertebrates, such as sharks, tear their prey apart with successive rows of pointed teeth. The prey are then swallowed. Many poisonous snakes have modified teeth, called fangs, which can fold back when not in use. The fangs pierce the prey and inject venom. The snakes mouth, made of bones linked with elastic ligaments, spreads open and allows it to swallow its prey whole.

Mammals have various shapes of teeth, which serve different purposes. Some of the teeth types include incisors for gnawing, tusks for fighting and piercing, canines for tearing and piercing, and molars for grinding. Some gastropod mollusks have a radula, for tearing or grazing. Birds on the other hand, have horny beaks, which vary in shape and size according to the type of food they eat. The beak will capture the food which is then swallowed and passed into a crop for storage, and then on to the gizzard, which often contains small stones, for grinding.

Some animals also rely on toxins to immobilize prey for feeding. There are two different forms of toxins, which are made of proteins, associated with feeding. The first are nematocysts which are stinging cells which contain a toxin that immobilize prey and are found on tentacles of coelenterates. Another type of toxin is neurotoxin, which is associated with organisms such as scorpions and spiders. Neurotoxins interfere with nerve impulses at the motor endplate in muscles, which leaves the prey paralyzed. Alpha-bungarotoxin is one of the neurotoxins associated with this group of toxins. All of these toxins require great amounts of energy to make, and thus the amount of toxin used on a prey is given in measured doses. Special enzymes in the host protect it from its own toxin.


What Are the Different Sections Of the Alimentary System and How Do They Function?

The alimentary system plays a key role in the process of digestion, absorption, excretion and provision of energy/nourishment. The two forms of digestion are extracellular and intracellular. Extracellular deals with digestion which occurs outside of the cell in the true alimentary system. Whereas, intracellular digestion breaks down nutrients within the cells and is mainly associated with unicellular organisms.

Extracellular digestion can be categorized as either being a batch reactor (coelenterates), continuous-flow reactor (all phyla higher than flatworms),or a plug-flow reactor (most mammals). The batch reactor system brings food in batches. The food is then digested and excreted before another batch of food is taken in. The continuous-flow reactor continually carries in and mixes food and excretes food products. The last form of extracellular digestion is the plug-flow reactor. In this type, food is taken in and digested while it winds through long tubes in the digestive system.

The alimentary system is broken up into different structural divisions and within each division there is a specialization in the process of digestion. For each organism, the structure of each division varies with length, width, or winding. The first structural division of the digestive system is the headgut. It is the most anterior portion of the digestive system which includes the external opening and specializes in receiving ingested food. In this area digestion begins and salivary glands that secrete a lubrication, mucus, to help the organism swallow food. Tongues, a specialization found in the headgut of chordates, help in chemoreception, grasping food and manipulation during chewing. The foregut is the division which follows the headgut and specializes in conducting, storing, and digesting food. The esophagus brings the food to the area of digestion. This process of transportation occurs by peristaltic movement and in certain animals, like birds, the food is passed into a crop where it is stored before digestion. The food material is then passed into the stomach which is a storage site and the area where most chemical digestion begins in animals. In the stomach, enzymes and muscular movements mix and break down food particles. The stomach is specialized for the type of food that the organism eats, and it is either monogastric, which is associated with carnivores and omnivores, or digastric, which is associated with mammalian ruminants.

The midgut is the section following the foregut in the alimentary system. In vertebrates this is the primary site for chemical digestion of proteins, fats, and carbohydrates. Food leaves the stomach through the pyloric sphincter and moves into the small intestine. The vertebrate midgut is divided into the duodenum, jejunum, and the ileum. The liver and the pancreas secrete fluids, through a duct, into the duodenum that digest food particles. The liver secretes bile salts which emulsify fats and neutralize acidity in the duodenum. The pancreas produces a juice containing protease, lipase, and carbohydrates. The midgut varies from species to species in function and structure. Carnivores have a shorter small intestine than herbivores because the plants takes longer to digest than meat. The midgut is structured to allow for full absorption (see How does absorption occur while maintaining electrolyte balance?).

The hindgut functions mainly as a storage unit for undigested food particles. While food is in the hindgut, inorganic ions and excess water are re-absorbed into the blood. Organisms require water for many physiological functions, therefor they try to conserve as much water as they can. The feces of some insects are almost dry when they are expelled. This enables them to retain most of the water that they ingest. Also in the hindgut, bacterial digestion occurs in birds, herbivorous reptiles, and most herbivorous mammals. In all animals feces pass into the cloaca or the rectum and are expelled through the anus.


What are the three types of exocrine secretions and what are their functions?

There are various exocrine glands that secrete chemicals. These chemicals further digest and break down food particles so that they can be absorbed. Exocrine glands include the salivary glands, secretory cells of the stomach, intestine, liver, and pancreas. These glands are divided into three categories according to what they secrete: water and electrolytes, bile, and digestive enzymes.

Most exocrine secretions are largely water based enzymes and chemicals. An aqueous mucus solution is produced in the goblet cells of the stomach and intestine. The mucus lubricates to prevent mechanical and enzymatic injury.

Bile is secreted by the liver and is essential for the digestion of fats. Bile is a mixture of water and a weak basic solution of cholesterol, lecithin, inorganic salts, bile salts, and bile pigments. The bile is transported to the gallbladder via the hepatic duct where it is concentrated and stored. Bile buffers the acidity in the intestines introduced by gastric juices fro the stomach. Secondly, it facilitates enzymatic fat digestion. Finally, bile contains substances removed from the blood which is then digested or excreted.

Digestive enzymes aid in the hydrolysis of large food particles into smaller compounds enabling them to cross cell membranes of the intestinal barrier. Digestive enzymes are substrate specific, sensitive to temperature, pH, and ions. There are three types: protease, carbohydrases, and lipase. Proteases break peptide bonds of proteins and polypeptides. Whereas, carbohydrases break long chain carbohydrates and disaccharides into smaller carbohydrates. Lipase emulsifies fats, thereby decreasing the surface area. In addition, and with the aid of bile salts they produce fatty acids, monoglycerides, and diglycerides from fats.

Most glands of the alimentary canal are stimulated by another chemical secreted by another gland in the alimentary system. The secretion of saliva, in the mouth, is stimulated when food is present in the headgut. The secretion of gastrin is in turn stimulated in the lower stomach by the presence of saliva. Gastrin, then, stimulates the secretion of hydrochloric acid. Hydrochloric acid is a major secretion of the stomach. The secretion of HCl is stimulated by vagal motor discharges, gastrin, and paracrine actions synthesized in the mast cells of the gastric mucosa. HCl breaks down peptide bonds, activates gastric enzymes, and kills microorganisms in the stomach.

How does absorption occur while maintaining electrolyte balance?

Once food particles are broken down the products (amino acids) must be transported from the gut to cells and tissues of the organism. In multicellular organism amino acids must be transported across absorptive epithelium into the blood and then into tissues. The structure of the intestinal epithelium aids in the success of absorption. The small intestines of vertebrates are specialized to increase surface area. The first level are the folds of Kerckring that project into the lumen of the small intestine. They increase surface area and slow the progress of food, allowing time for digestion. Villi sit on circular depressions on the folds of Kerckring. These circular depressions are called the crypt of Lieberkuhn. Within each villus is a network of blood vessels and lymph vessels through which nutrients are absorbed. At the apical surface of the absorptive cells is the brush border.

The brush border contains digestive enzymes for the final digestion. The type of transfer depends on the type of molecule. Passive diffusion is used to transport fatty acids, monoglycerides, cholesterol, and other fat-soluble substances across the brush border. Water, some sugars, alcohols, and small water-soluble molecules passively diffuse through water filled pores. Carrier-mediated diffusion is responsible for transporting large molecules like monosaccharides and hydrophilic amino acids. Protein channels transport sugars by coupling sodium gradients and electrical gradients. A sodium/potassium pumps out Na+, creating a gradient that powers the transport of large or hydrophilic molecules. The molecules move down concentration gradients. Active transport can be one of four non-competing systems depending on the category of amino acids: 1) lysine, arginine, and histidine, 2) glutamate and asparatate, 3) glycine, proline, hyrooxyproline, and 4) the remaining amino acids. Each group uses a different method to transfer various types of amino acids across the brush border.

Lipids are an exception to the rule of absorption. Products of broken down lipids (fats, monoglycerides, fatty acids, and glycerol) diffuse through the brush border and are reconstructed on the other side. They are collected with phospholipids and cholesterol into chylomicrons. They’re coated with proteins and contained in vesicles formed by the golgi apparatus which are then expelled by exocytosis.

Digested particles enter the blood or the lymph system from the alimentary system. Lipids, for example, enter the lymph as chylomicrons. The lymph is returned to the blood via the thoracic lymph duct located near the heart. Sugars and amino acids generally enter directly into the blood.


Describe what metabolism is and explain the difference between anabolism and catabolism?

Metabolism is the sum total of all the chemical reactions occurring in an organism at any given time. Anabolism requires energy and is associated with building complex molecules whereas catabolism is involved in the breakdown of complex molecules into simpler ones.


How is metabolic rate defined, why is it useful to measure the metabolic rate of organism, and what is the difference among basal, standard, and field metabolic rate?

Metabolic rate is a measurement of the conversion of chemical energy into heat, or heat energy released per unit time. Measuring metabolic rate is not only useful to physiologists, but also to ecologists, animal behaviorists, evolutionary biologists, and many other fields of study because the metabolic rate can yield information on the energy requirements of animals. In order to learn more about the heat conserving and heat releasing mechanism of animals, metabolic rates can be measured at different temperatures. Metabolic rate can tell us how much energy an animal needs to expend to fly, swim, run, or walk. It can also tell us something about the processes of tissue growth and repair, chemical, osmotic, electrical, and mechanical internal work, and external work that is necessary for such things as locomotion and communication. The basal metabolic rate is the stable rate of energy metabolism measured in mammals and birds under conditions of minimum environmental and physiological stress, or essentially at rest with no temperature change. Standard metabolic rate refers to an animal’s resting and fasting metabolism at a given body temperature, whereas field metabolic rate is the average rate of energy utilization as the animal goes about its normal activities, which may range from complete inactivity while the animal is at rest to exerting maximum energy.


How is metabolic rate measured, is it measured directly, and what are some of the techniques that are used to determine metabolic rate?

To measure the metabolic rate of an organism directly, one must determine how much heat is flowing from the organism, for this is the true measure of metabolic rate. These measurements can be made using a calorimeter and the process is called direct calorimetry, but this method is often hard to employ, especially when measuring the metabolic rates of large animals. Thus, there must be other ways in which metabolic rate can be measured. These methods are called indirect calorimetry, which involves measuring the amount of food an animal ingested compared to the amount it excreted, respirometry, which involves measuring an animal’s respiratory exchange, and the isotopic technique, which measures the water flux in animals using deuterium or tritium labeled water.


Since animals cannot be hunting all the time, how do they obtain energy to sustain their basic biological functions while they are not feeding, and how do their metabolic rates change in the process of feeding?

Because animals do not ingest food steadily, there is not a moment to moment balance between the intake of food and the expenditure of energy. Food is obtained in bursts, and when an animal feeds, its immediate energy requirements are fulfilled with much being left over. The left over is not thrown out as waste but rather stored for later use, especially in the form of fats and carbohydrates. In 1885, Max Rubner concluded that an increase in metabolic rate was evident soon after the ingestion of food, independent of other activities. He gave this phenomenon the name specific dynamic action, or SDA. SDA has since been observed in all five vertebrate groups, along with invertebrates.


How does body size affect metabolic rates?

Body size is one of the most important factors that affects metabolism and the study of how body size affects physiological processes is called scaling. It has become clear that small animals must respire at higher rates per unit body mass than larger animals. Also, there is an inverse relationship between O2 consumption per gram of body mass and the total mass of the animal. When the log of the metabolic rate is plotted against the log of the mass for a variety of animals, the slope of the line always seems to correspond to .75, known as Kleiber’s law. Changes in body size affect metabolic rates immensely, namely because increases in body size are not always simple and proportional.


What is the surface hypothesis and what is the relationship between metabolic rate and an animal’s surface area?

The surface hypothesis, proposed by Max Rubner in 1883, states that the metabolic rate of birds and mammals that maintain a more or less constant body temperature should ultimately be proportional to their surface area. Since body size affects metabolic rate, and surface area is related to metabolic rate as well, there must be a correlation first between surface area and body mass. When animals of different size are compared to one another, their surface area varies to a .67 power of their body mass. This is due to the fact that different size animals within the same species follow the rule of isometry, or proportionality of shape regardless of the animal’s size. This concept does not hold for animals of different species and varying size. When this relationship is looked at, it can be found that the animal’s surface area varies to a .63 power of body mass. This is because when animals of different size and different species are compared to one another, they tend to follow the principle of allometry, which states that systematic changes in body proportions increases with a species size. When the basal metabolic rates of different size animals within the same species is plotted against body mass, the metabolic rate is proportional to the body mass raised to the .67 power, which is the same degree to which surface area and body mass are correlated among animals within the same species. But when basal metabolic rate in animals of different species are plotted against their body mass, the exponent relating the metabolic rate to the body mass is found to be .75. This is referred to as Kleiber’s law. The surface hypothesis holds true when metabolic rates are plotted against body size in animals belonging to the same species, but it does not hold true when metabolic rates are plotted versus body mass of animals within different species. When one is comparing different species, the metabolic rate cannot be determined simply on differences in body surface area.


How does an animals’ body regulate and influence temperature change?

The temperature of an animal depends on the amount of heat that is contained within the animals’ body. Body heat is generally defined as the amount of heat produced by the body through metabolism added to any thermal exchange between the animal and the environment. There are several ways that an animal can absorb or lose heat from or to the environment: conduction, convection, radiation, and evaporation. Conduction is the transfer of heat between objects and substances that are in contact with each other. The transfer of heat from the skin surface to molecules of gas in the air is an example of conduction. Convection is the transfer of heat contained in a mass of a gas or liquid, by the movement of that mass. A simple example of this is when you sit in front of a fan on a hot day and the movement of air takes the heat away. Radiation is the transfer of heat through electromagnetic radiation without objects coming into contact. An example of radiation is the warming that can occur to animals in sunlight. Evaporation is a change in state from liquid to gas, which requires energy. Heat is dissipated from the body during evaporation because large water molecules with high energy content accumulate at the skin surface and enter the gaseous phase. When the larger molecules vaporize, they take thermal energy with them and any water left behind becomes cooler.

Animals have the ability to gain and store heat from the environment which can result in an increase in body temperature. The amount of heat that will transfer into or out of an animal depends on surface area, temperature difference, and the specific heat conductance of an animals body surface. A large animal (elephant) has a small surface-to-mass ratio and will heat up more slowly in response to environmental heat, whereas an animal with a larger surface-to-mass ratio will heat up faster. If the temperature difference between the environment and an animal’s body is small, very little heat will flow into or out of its body. Insulators like fur, feathers, and blubber have a very low thermal conductivity and help to reduce the rate of heat flow. In addition to these thermal regulating characteristics, animals can also regulate the exchange of heat between themselves and the environment using behavioral control and/or autonomic control. Behavioral control is used by animals when they move to a part of the environment where heat exchange favors the attainment of optimal body temperature. An example of this is when snakes or other reptiles choose to bask in the sun during daylight hours, but may avoid basking during midday, when the suns rays are most intense. Autonomic control is being used when the temperature of the skin and extremities is controlled by the amount of blood flow to these areas of the body. Constriction of arterioles leading to the skin keeps warm blood from reaching the colder extremities and conserves heat at the body core. Autonomic control can also be used to absorb heat by increasing the amount of blood that flows to the surface tissues. Near the skin surface, blood will approach temperature equilibrium with the environment. The insulating properties of the blubber layer in whales is a good example of how autonomic control affects body temperature regulation. Whales at the surface of the ocean will increase the flow of blood to their blubber layer to absorb radiant heat from the sun. When diving into colder depths, whales constrict their blood vessels and decrease the blood flow to the blubber layer so that heat is conserved at the core and not lost at body surfaces. Acclimatization is also a method of regulating heat exchange, but it involves changes in insulation quality or heat dissipation effectiveness due to long term exposure to either high or low temperatures.


How do temperature changes affect individual molecules and enzyme function?

Temperature variation can cause changes at the molecular level. Homeoviscous membrane adaptations occur as a result of temperature changes. The cell membrane and many functions associated with the cell membrane are sensitive to temperature change. Low temperatures cause the membrane to enter a gel-like phase with very high membrane viscosity. High temperatures, on the other hand, cause the membrane to enter a hyperfluid phase with very low viscosity. Initially, acute temperature changes cause changes in membrane polarity and fluidity. On the other hand, after acclimation to both high and low temperatures, the resulting homeoviscous adaptations cause similar lipid polarization and membrane viscosity at both temperatures.

Temperature variation causes certain changes in enzymes. The rate of enzymatically controlled reactions changes due to temperature. This process is called enzyme acclimation. Enzyme acclimation can occur due to changes that occur in the molecular structure of one or more enzymes, changes in the quantity of the enzyme, or other factors that affect enzyme kinetics (for example nerve conduction).

What are Ectotherms and how do they regulate temperature?

Ectotherms are animals that produce metabolic heat at low rates and rely primarily on thermal conditions of their surroundings. They maintain body temperature that is often equal or dependent on the temperature of the surrounding environment, and tend to have lower metabolic rates at low temperatures and higher metabolic rates at higher temperatures. Ectotherms that normally live in cool temperatures have a low body temperature, maintain adequate metabolism at very low levels of enzyme activity, and have enzymes that can function at low temperatures. Ectotherms that normally live in warm temperatures function at very low levels of metabolism and rely heavily on behavioral regulation to control their body temperature. Certain reptiles also use autonomic control to regulate body temperature. For example the Galapagos marine iguana regulates its heart rate and blood flow during dives into cold water to reduce the amount of heat lost at its surface tissue (skin).

It is often more difficult for ectotherms to survive in extreme conditions of cold or hot environments. Extremely hot conditions are not frequented by ectotherms. Most ectotherms never experience such extremes due to the existence of the critical thermal maximum, a temperature above which long-term survival is not possible.


How do Ectotherms cope with extreme cold?

Freezing and cold temperatures are dangerous to all living organisms because if ice crystals begin to form within a cell, the crystals will continue to grow in size as freezing progresses until eventually causing the cell to rupture. No animal is known to survive complete freezing of its tissues, but some ectotherms have come close. Freeze tolerance mechanisms are used by some ectotherms that actually freeze and remain alive. Some species of beetles can withstand freezing temperatures because they have extracellular fluid that contains a substance that accelerates crystal formation. As a result of more rapid crystal formation outside of the cells, the solute concentration inside the cell increases, lowering the intercellular freezing point. Red blood cells, yeast, and sperm can withstand freezing temperatures using this mechanism, provided that intercellular ion concentrations do not get high enough to cause damage to the cell organelles.

Freeze avoidance strategies are used by other ectotherms to prevent freezing. Freeze avoidance is usually characterized by supercooling or the presence of antifreeze substances. Supercooling is observed in fish that live in water temperatures below the freezing point of their bodily fluids. They remain unfrozen because ice crystals are unable to form due to lack of nuclei which are needed to initiate crystallization. Antifreeze substances are characterized by their capacity to lower the freezing point of body fluids. Glycerol, which is present in various species of arthropods and insects, is an example of an antifreeze that has been shown to lower the freezing point to as low as -470C.


What are Heterotherms and how do they regulate temperature?

Heterotherms are animals capable of varying degrees of endothermic heat production, but they generally do not regulate body temperature within as narrow a range as endotherms. Heterotherms are broadly broken into two categories, temporal and regional heterotherms. Temporal heterotherms are a broad category of animals whose temperatures vary widely over time and may exhibit daily temperature fluctuations. Camels behave like temporal heterotherms because they allow their body temperatures to fluctuate throughout the day - absorbing heat during the day, and releasing it at night.

Regional heterotherms are characterized by the fact that they can achieve high core temperatures, while peripheral tissues and extremities approach ambient temperatures. The most common form of regional heterothermy is found among animals that are able to conserve heat generated by muscle activity. Certain species of flying insects require their flight muscles to be pre-warmed to a specific temperature before they are able to fly successfully. During flight, these insects can achieve relatively high body temperature, while at rest their body temperatures will reach equilibrium with ambient temperatures.

Certain fish, like tuna and some sharks are referred to as warm-bodied heterotherms because of their ability to maintain relatively constant, high core temperatures. The ability of these fish to attain such temperatures is dependent on the organization of the vascular system. Blood vessels in ectothermic fish are located centrally, while heterothermic fish have blood vessels located under the skin. The location of the blood vessels in heterothermic fish allows for countercurrent heat exchange to be utilized, minimizing the loss of heat. The location of swimming muscles in heterothermic fish also influences the retention of body heat. These muscles are located near the central part of the fish and since most of these fish do not stop swimming, the muscles do not cool down. Other types of fish have specialized thermogenic (see thermogenesis) structures that maintain a specific heat in a certain region of the body. An example of this is the swordfish which is able to maintain the temperature of its brain and eyes about 10 degrees higher than its surroundings.


What are Endotherms and how do they regulate temperature?

Endotherms are animals that generate their own body heat and are able to maintain internal temperatures that are often stable within a few degrees (between 370C - 380C in mammals and about 400C for birds). This is achieved through elevated basal metabolism in conjunction with heat-conserving and heat dissipating mechanisms. At stable, moderate temperatures regulation is maintained by enough metabolic activity to exactly balance the heat that is lost to the environment. The range of temperatures in which an endotherm can maintain the basal rate of heat production is called the thermal neutral zone. As the temperature drops below the endotherms’ lower critical temperature (LCT), the basal metabolic rate is no longer sufficient to balance heat loss to the environment. The endotherm must increase heat production by means of thermogenesis, which is the production of heat through either shivering or non-shivering methods. Heat production will increase linearly as temperatures continue to decrease so that colder temperatures yield greater rates of metabolic regulation to produce more heat. Eventually if the temperature drops too much, metabolic systems fail to compensate and the animal will die.

Endotherms are able to survive in cold environments through different mechanisms of adaptation. Fur or feathers are useful in reducing convection and the loss of body heat at the body surface of different animals that have fur or feathers. Thick layers of insulation can also reduce loss of heat. For example, blubber (fatty tissue found under the skin of whales) is a good insulator because it has a lower thermal conductivity than water, is metabolically inactive, and requires little profusion of blood. Endotherms are generally larger animals than ectotherms and this quality is useful in extremely cold environments because large animals generally have a lower specific heat conductance and relatively smaller surface areas which will reduce surface heat loss. Countercurrent heat exchange is also utilized by endotherms to drastically reduce heat loss from appendages.

As environmental temperatures rise above the endotherms’ internal temperature, metabolic rate decreases with increasing temperatures. When external temperatures exceed the thermal neutral zone the endotherm must be able to release, or dissipate excess body heat through passive or active heat-dissipating mechanisms, such as evaporative cooling by sweating or panting. All living animals are continuously producing heat at a basal level. If the body temperature can not be controlled and the heat dissipated, it is possible for the animal to overheat and die.


How does the body detect temperature changes?

Regulation of body temperature is established at temperature-sensitive neurons and nerve endings in the brain, the spinal cord, the skin, and sites in the body core. These sites provide input to thermostatic centers in the brain. Although a mammal may have several

thermoregulatory centers, the most important one, considered to be the body’s "thermostat" is located in the hypothalamus (see Endocrine System). Most mammals, fish, birds, and reptiles establish thermostatic regulation at the hypothalamus. When the hypothalamus detects temperature change, hormones are released that essentially carry information to other parts of the body. Different hormones convey a variety of messages that can include a signal to increase metabolic rate or initiate autonomic controls.

In experiments, cooling the hypothalamus produced an increase in metabolic rate. Warming the hypothalamus resulted in a decrease in metabolic rate and an increase in heat dissipating mechanisms.


What are the costs and benefits of Ectothermy?




What are the costs and benefits of Endothermy?




Why do animals hibernate?

Hibernation causes a reduction in body activities, including metabolic rate. Due to this reduction, hibernation offers an energetic advantage. During hibernation, thermoregulatory control continues with a lowered set point and reduced sensitivity, as in slow wave sleep. With the lowered body temperature characteristic of hibernation, body functions are greatly slowed. All true hibernators are mid-sized mammals weighing at least several hundred grams and large enough to store sufficient reserves for extended hibernation.

Most of the time, the rate of arousal from hibernation is higher than the rate of entry. Since the hibernators must warm themselves to achieve a functional body temperature, a large surge in metabolic rate is usually required. The ground squirrel, for example, takes 12-18 hours to enter hibernation but only three hours for arousal. The speed of the animals arousal depends on the rapid heating initiated by intensive oxidation of brown fat, accompanied by shivering. This process frequently requires a sudden increase in metabolic rate.


What are some of the physiological control mechanisms that operate in the regulation of biological rhythms?

Cronobiologists have identified the suprachiasmatic nuclei (SCN) as the "brain’s seat" of circadian cycling in rats. At the molecular level, fluctuating internal states can be examined in terms of oscillating protein levels. These oscillations are controlled by transcription factors that activate or deactivate particular genes. While research continues to narrow the focus on genes involved in biological rhythms, one in particular has been identified. The gene is called "period" (or "per") and experimentation has shown that mutations in this "per" gene alter or suppress the daily cycle of locomotor activities and eclosion (emerging from pupae) in Drosophila. Another gene called "clock" has been isolated in mice and also appears to directly relate to normal rhythmic functioning and daily activity cycles. Studies on humans have provided evidence that the release of a stress hormone called adrenocorticotropin plays a regulatory role in the sleep cycle, acting as a sort of "internal alarm clock". Studies on complex animals have also yielded evidence that the brain, pineal gland, and other tissues regulate biological rhythms. The persistence of true biological rhythms in unicellular organisms however, indicates that these are not necessary components to maintain such rhythms.

In fact, the endocrine system is closely linked to the intensity and duration of several biological functions and cycles such as sleep. It is important to make the distinction however, that in examining the biological rhythms related to daily cycles, termed circadian rhythms, certain criteria must be met.


What defines a circadian rhythm and distinguishes it from other physiological variables that just happen to coincide with daily cycles?

The rhythm must exhibit persistence in that daily cycles manifest an unchanged state of internal regulation even when the animal is removed from the natural environment and placed in a laboratory setting characterized by constant variables (i.e. light, temp, etc.). The rhythm must also be conditionally arrhythmic in that a specific combination of environmental cues such as light, temperature, or oxygen levels can be utilized to disrupt the normal cycle. Additionally, a true circadian rhythm can be entrained in a series of transient shifts of activity by zeitgebers such as light, temperature, or food availability. to the point of being in a phase completely opposite of the original rhythm. Circadian rhythms are those which typically manifest a 24 hour cycling period.

Circadian rhythms permit awareness of local environmental change, measure time passage, and allow for organismal maintenance of internal (temporal) order and anticipation of ambient change.


Besides circadian rhythms, what other biological rhythms exist?

Other endogenous biological rhythms are classified as infradian or ultradian. Infradian rhythms are typically related to aspects of cell function (i.e. cell division). Such cycles significantly affect animal energetics but are difficult to measure and quantify. Most infradian rhythms have yet to be correlated to any specific rhythmic environmental changes.

Ultradian rhythms persist for longer than one day and are quite common in animals. The three most easily identifiable are circatidal rhythms, circalunar rhythms, and circannual rhythms. Circatidal rhythms correspond to tidal cycles and are generally 12.4 hours in length. Circalunar rhythms correspond to lunar cycles and are usually 29.5 days in length. Circannual rhythms correspond to the earth year and its accompanying seasonal cycles and last for approximately 365 days. All are endogenous, physiologically speaking, and exhibit persistence as well as entrainment potential.


Hibernation involves very specific reductions in metabolism (body heat specifically). To what extent are body heat and other metabolic processes affected during normal daily cycles?

Biological rhythms, specifically circadian rhythms, tend to be body temperature independent. An increase in Tb (body temperature) usually results in little or no corresponding increase in the circadian rhythms exhibited by the animal. A decrease in Tb is generally a derivative of these rhythms rather than vice versa. Thermogenesis and other indirect physiological mechanisms regulating Tb typically require considerable amounts of energy. Since Tb directly affects an animal’s metabolism, careful maintenance of that aspect of homeostasis illustrates the inherent relationship between circadian (and other biological) rhythms and energy metabolism.

Small animals typically exhibit larger circadian variation in Tb relative to the metabolic costs expended in daily regulation of biological functions. In some cases, rhythmic increases and decreases can be attributed to varying degrees of locomotor activity. Usually however, there is a distinct intrinsic rhythm in Tb that is independent of activity. This is evidenced in experimental settings where 1) an animal’s activity levels are controlled or corrected for and 2) where temperature fluctuations (in humans) have persisted for extended periods of complete bed rest. The most important and specific affectation of daily cycles as reflected by an animal’s metabolic rate occurs during slow wave sleep. A drop in hypothalamic temperature sensitivity and consequently Tb along with a reduction of respiratory and cardiovascular reflexes.


What is extraocular photoreception and why is it an important aspect to understanding biological rhythms?

Extraocular photoreception is essentially characterized by phototactic movement towards or away from a light source (an example of behavioral control). Many animals distinguish between light and dark by means of light-sensitive receptors in their dermis. These receptors trigger muscle impulses (those involved in the behavioral control mechanisms). Extraocular photoreceptors are important because of their roles in informing animals of the presence of light, measuring its intensity, and selecting specific wavelengths for function. All of these variables influence photobehavioral responses in animals and biological rhythms, while often endogenously generated, are not independent of the affects of photoreceptive cues. Diurnal rhythms specifically, can be altered by the phase-shifting influence of light and dark reception.


What is the correlation between photobehavioral response and light patterns?

From microorganisms to human beings, light has a proven influence on biological rhythms. Photoperiodism is the rhythmic behavior patterns regulated by periods of light and dark. Although there is still research being done on photoperiodism, it has been established that retinal cones functioning at high light intensity and rods functioning at low light intensity are visual photoreceptors which likely have equivalent components which operate in the control of photoperiodic behavior.


What is an endogenous clock? How are circadian rhythms distinguished from photoperiodic behavior?

The term endogenous refers to the rhythms internally generated by an organism. Rhythms related to environmental periodicity should disappear under controlled, continuous darkness. Conversely, circadian rhythms, by definition, cannot be terminated but can be phase-shifted.


Special Topic Report: The Importance of Light in the Circadian Cycling of Vertebrate Systems (by Allison Konefal)

The manifestations of distinct internal regulatory mechanisms can be observed to varying degrees amongst vertebrates. Those biological rhythms which typically manifest a 24 hour cycling period are known as circadian rhythms. Set apart from other physiological variables characterized by cyclic patterns, circadian rhythms involve very specific relationships between zeitgebers (stimuli such as light, temperature, food availability, etc.) and the activation of sensory responses. One particularly significant correlation exists between the presence of light and the body’s physiological conditioning.

By virtue of its daily and seasonal variations, light is one of the most powerful aspects of environmental periodicity in terms of maintaining circadian systems. Organismal response to light is well documented and its effect in synchronizing circadian rhythms is a testable phenomenon. Periodic changes resulting from light-dark (LD) cycles have been shown to cause autonomic function adjustments in most vertebrates (Aschoff et al., 1982). Amongst the specific parameters that define true circadian rhythms is their capacity for entrainment. This refers to the process by which exposure to controlled zeitgebers in transient shifts can result in advancement or delay of the rhythm to the point of being completely opposite the original. In vertebrates, use of natural or artificial photoperiods for entrainment is mediated via photoreceptive systems located in the diencephalon of the brain (Aschoff et al., 1982).

There are several ways in which different photosensory input channels interact in the control of light-dependent autonomic events. First, the epithalamic pineal sense organ and deep encephalic photoreceptors manage the release of neuroendocrine signals. Some sensory elements of the pineal complex are capable of metabolizing indoleamines, which have been implicated in the control of daily changes in threshold photosensitivity levels. The light-sensitive diencephalon is also closely linked to neuroendocrine activity although no conclusive evidence has implicated this region as responsible for any one specific action. The management of photobehavioral response is largely accomplished by a series of interactions amongst various structures within the vertebrate brain (Aschoff, 1982).

It is necessary to understand that photoperiods are not always necessarily maintained by visually integrated light. Extraocular photoreception plays a very important role in regulating rhythmic behavior patterns in organisms by means exclusive of an eye structure. Studies involving photoperiodism have shown that light periods, termed photopic, alternating with periods of dark, termed scotopic, do impose a diurnal rhythm to some extent but this fails to account for the exhibition of rhythmic behavior in organisms removed from a visually detectable light source. In the latter cases, it is believed that diffused photosensitivity over an organism’s dermal regions are responsible for the upkeep of internal cycles (Wolken, 1986).

Extraocular photoreceptors are those which are bound to dermal and neural cells (including the brain) and initiate behavioral responses to light. Eyeless and blinded animals sense light cues through the detection mechanisms of these receptors. Regions of photosensitive cells are widespread and vary among animals. Not all exhibit equal sensitivities, making determination of their location and structural identity difficult to assess. The passage of information regarding light cues is directed to the integrative centers of the brain where structures such as the pineal gland, hypothalamus, pituitary, and rhinencephalon process and translate it into physiological responses.

Aside from understanding the mechanisms behind photoreception, it is necessary to quantify it somehow. Typically this is accomplished by measuring phototactic movement toward or away from the light source. Muscular impulses triggered by dermal photoreceptors are the key element in evaluating such responses. Another method of analysis comes in the form of monitoring melatonin levels. A unifying feature between sleep/wake cycles and other physiological activities is the pattern of temporal melatonin secretion. Melatonin appears to transduce action in endogenously regulated physiological systems, particularly those involved in measuring time passage (Aschoff, 1982).

There are various interpretations of how circadian oscillations underlying photoperiodic time-measurement function. A well-accepted theory holds that there exist periods of photoinducibility during which the photoreceptors located externally as well as in the brain of an organism which reach peak sensitivity levels for detecting and integrating changes in light patterns (Aschoff, 1982). The difficulty in assessing a potential photoinducible phase is that means of quantification such as monitored activity levels are specific to circadian systems and these are extremely varied in and of themselves. Attempting to draw general conclusions by means of activity rhythm measurement and extrapolating photoperiodic oscillations as causative is unreliable and inconclusive.

The exact structures and mechanisms regulating physiological response to light have yet to be completely understood. In terms of the correlation between these responses and circadian rhythms, much scientific research is being accomplished which further underscores areas of particular importance. Brain structures, the eye, the neuroendocrine system, and the whole of extraocular photoreceptive systems all provide links to an overall control pathway in the physiological response to light. The basic principle underlying the need to examine this pathway is the fact that light is an essential aspect of life processes. Cells, which are the body’s smallest "internal clocks," can lose the capacity to integrate light-generated information. As a result, organs and whole body systems risk altered rhythms. Such an alteration has the potential to cause severe stress for an organism (Orlock, 1993). This underscores the need for a comprehensive idea of how vertebrate physiology maintains "touch" with the rhythmic patterns of light.


Aschoff, J., S. Daan, and G. A. Groos. 1982. Vertebrate Circadian Systems: Structure and Physiology. New York: Springer-Verlag Berlin Heidelberg.

Orlock, C. 1993. Inner Time: The Science of Body Clocks and What Makes Us Tick.

New York: Birch Lane Press.

Wolken, J. 1986. Light and Life Processes. New York: Van Nostrand Reinhold Company.