Chapter Forty: Basic Principles of Animal Form and Function
A jackrabbit’s ear not only provide the animal with an acute sense of hearing but also help it shed excess heat via the blood flowing in each ears network vessels which transfers heat into surrounding air. When air is warmer than jackrabbit (exceeding 40º C), the jackrabbit’s pink ears turn pale, reflecting a narrowing of the blood vessels in response to the environment allowing their ears to absorb heat without affecting the rest of the body. Once the air cools, blood flow increases again, helping to release heat.
Anatomy: Study of biological structure Physiology: Study of biological function
Natural selection helps animals adapt to their environments and face challenges such as reproduction, infection, nutrition, climate, and predators.
The shape and size of the animal is a result of a pattern of development programmed by the genome, through millions of years of evolution. However physical laws that govern strength, diffusion, movement and heat exchange limit the range of animal forms.
Larger bodies for example need thicker skeletons to maintain adequate support, and a larger fraction of muscle mass otherwise mobility will be greatly reduced.Animals which need to swim have a streamlined body contour, a shape that is fusiform (tampered on both ends)- such as dolphins, seals, penguins, and sharks. This helps them overcome drag while swimming. As water in a thousand times denser than air and much more viscous, any bump on the animal’s surface would cause drag. This is an adaptation to the environment and is an example of convergent evolution.
Number of Cells
Animals exchange material (nutrients, waste products, gases) with the environment when substances dissolved in an aqueous solution move across the plasma membrane in each cell. The rate is proportional to membrane surface area while amount of material exchanged needed to sustain life is proportional to cell volume A single-celled organism has many opportunities to interact with the environment whereas multi-cellular organization works only if every cell has access to suitable aqueous environment inside or outside the cell. Examples include the hydra whose body consists of two layers of cells or a parasitic tapeworm whose thin flat shape exposes it to its environment. Animals with highly concentrated cells have specialized structures with extensive branches or folds to increase surface area (microvilli lining in intestines, sponge-like structure of lungs, narrow blood vessels packed into balls in kidney filtering) preventing dehydration and abrasion. Interstitial fluid fills spaces between cells, bathing them while other internal body fluids link exchange surfaces to body cells. Exchange with the circulatory fluid (blood) which is present in complex body plans and interstitial fluids allows cells throughout body to obtain nutrients and remove wastes. A complex body plan is more advantageous than a simple one particularly for terrestrial animals because of variable environments. The external system provides protection; the sensory system provides information related to surroundings, the internal digestive system can break down food gradually and release stored energy while the specialized filteration system can adjust composition of internal fluids that cells bathe in.
Hierarchical Organization of Body Plan
Cells Tissues Organs Organ Systems
Not all animals contain all these emergent features. Sponges lack organs and true tissues. Certain organs belong to more than one organ system (such as the pancreas which helps in digestive and endocrine systems).
Main types of tissue:
regenerates rapidly because new cells form by division near basil lamina.
Cells push outward, sloughing off older cells. Found in linings of mouth,
anus, and vagina (surfaces subjected to abrasion)
material by diffusion that lines blood vessels and lung air-sacs.
secretion or active absorption is required (intestines secreting digestive juices
and absorbing nutrients).
epithelium of kidney tubules and various glands (thyroid, salivary..)
widespread in vertebrate body and skin which binds epithelia to underlying tissue and holds organs in place.
(attach muscles to bones) and ligaments (connect bones to joints)
vertebrae. Bone-forming cells, osteoblasts, deposit matrix of collagen, calcium, magnesium and phosphate ions. Hard mammalian bones consists of repeating structures, osteons which has a concentric layer of mineralized matrix deposited around a central canal containing blood vessels and nerves.
many vertebrate embryos (later replaced by bones) and it remains in some locations acting as a cushion.
consist of filament containing the proteins actin and myosin, which enable muscles to contract. Vertebrae have three distinct type of muscle tissue:
axions (which are bundled together into nerves)
modulate neurons and nerve function.
Coordination and Control: An animal’s body systems must be kept in coordination (ex: seal diving has slower heart rate, collapsed lungs, and lowered body temperature while moving forward with it’s hind flippers). The endocrine and nervous system are responsible for coordination and control of the animal’s body. The pathways are the same for each system regardless of the distance the information needs to travel.
Endocrine System: Signaling molecules called hormones released into bloodstream by endocrine cells reach all locations in body. Different hormones have different effects and only cells with special receptors for a particular hormone can respond whether in single or multiple locations. Example: TSH can be received only by thyroid cells which in turn release thyroid hormones and cause cells in nearly every tissue to increase oxygen consumption and heat production. Hormones are slow-acting (it takes many seconds for TSH to be released) but long lasting as they stay in bloodstream for a while. It is well suited for coordinating gradual changes that affect entire body such as growth, reproduction, development, metabolic activity, and digestion
Nervous System: Neurons transmit signals called nerve impulses between specific locations in the body which have dedicated communication lines and consist of mainly axons. Neurons, muscle, endocrine and exocrine cells can all receive nerve impulses. The system, unlike the endocrine system can convey information based on the pathway it takes (example: human ears hear sounds). Communication involves more than one type of signal. Transmission is very fast and takes only a fraction of a second but it also only lasts a fraction of a second. This system is better for directing immediate and rapid responses to the environment particularly locomotion and behavior
Regulator: Animal which uses internal mechanisms to control internal change in the face of external fluctuation. For example; body temperature is largely independent of external temperature. Ex: River Otter
Conformer: Animal which allows its internal condition to change in accordance with environmental changes. For example body temperature changes with environment.
Ex: Large-mouth Bass
Note: Certain animals conform to stable and constant environments such as the spider crabs which let internal solute concentration (salinity) match the ocean environment.
Note II: Conforming and Regulating are extremes on a continuum. Most animals conform to certain aspects and regulate other ones.
Homeostasis: Maintenance of internal balance. Humans maintain fairly constant body temperature (37 ºC), pH level of blood and interstitial fluid (7.4 ± 0.1) and concentration of blood-glucose (70-110mg of glucose per 100mL of blood). Homeostasis relies on negative feedback (a control mechanism which reduces or “damps” the stimulus. Homeostasis is not instantaneous and therefore simply moderates change in internal environment but does not eliminate it.
Animals achieve homeostasis by maintaining a variable (ex: solute concentration) at a particular set point. Fluctuations above or below said set point serve as stimulus detected by a sensor (receptor) which sends signals to a control center who triggers a response to help return variable to set point.
Note: Sometimes a system has a normal range instead of a single set point which is acceptable.
Note II: Regulated changes are acceptable and even crucial to life. It can occur during phases (puberty) or be periodic (menstruation)
Positive feedback: Control mechanism which amplifies rather than reduces stimulus. It does not play a role in homeostasis in animals but rather drives processes to completion.
Example: when a baby is born the pressure of its head against receptors near mother’s uterus stimulates uterus to contract which result in more pressure heightening the contractions and so on.
Circadian Rhythm: A set of physiological changes that occur roughly every 24 hours. Example: Body temperature of humans undergoes a rise and fall of over 0.6 ºC every 24 hour period. Even if light level, room temperature, and human activity variations are minimized the biological clock still maintains this rhythm although more melatonin is secreted on longer winter nights. External stimulus can reset the clock but the effect is not immediate.
Acclimatization: Gradual process in which an animal adjusts to its environment (homeostasis changes overtime). This is not an adaptation but actually a temporary change. Example: Elk moves from sea level to mountains, breathes more rapidly and deeply, loses less CO2 through exhalation, raises blood (increased production of oxygen containing red blood cells), changes pH level above set point, changes in kidney function result in more alkane urine, returning blood to normal range. Birds acclimatize by growing thick coats of fur in winter and shedding them in warmer months. Ectotherms acclimatize by making adjustments at the cellular level (produce enzymes with different optimal temperatures, change proportions of saturated and unsaturated fat because unsaturated lipids help keep membranes fluid in lower temperatures, or produce anti-freeze).
Thermoregulation: Process by which animals maintain an internal temperature with a tolerable range. For every 10 ºC, decrease in body temperature, the rate of most enzyme-mediated activity decreases by two or threefold and also causes membranes to become too rigid. Increase in temperature speed up reactions but cause some proteins to become less active (hemoglobulin becomes less effective at binding oxygen with higher temperatures) and membranes to become too fluid. Internal metabolism and external environment are both sources of heat for thermoregulation.
Endothermic: Endotherms include: birds, mammals, a few nonavian reptiles, some fish, many insects and other animals which are mostly warmed by heat generated by internal metabolism. Endotherms are capable of being active in all conditions because they maintain a stable body temperature. Endothermic vertebrae have mechanisms for cooling themselves down enabling them to withstand heat loads ectotherms can’t take.
Ectothermic: Ectotherms include: amphibians, lizards, snakes, turtles, many fishes and most invertebrates which gain most of their heat from external sources (i.e. the sun) Ectotherms need to consume less food of endotherms of the same size and are much less active in colder temperatures. They are more tolerant to fluctuation in internal temperature. Their bodies do not allow them to produce enough heat for thermoregulation but they adapt behavioral traits such as seeking shade or sun basking.
Note: Endothery and ectothermy are not mutually exclusive terms of thermoregulation
Poikilotherm: Animal whose body temperature varies with the environment
Homeotherm: Animal whose body temperature is relatively constant.
There is no fixed relationship between stability of body temperature and source of heat. All ectotherms are not poikilotherms or vice versa. An ectotherm living in a stable environment can be a homeotherm. Similarly bats (endotherms) go into an inactive phase in which they maintain a much lower body temperature.
Ectotherms also do not have to be “cold-blooded”. The terms warm-blooded and cold-blooded are misleading. So-called cold-blooded animals can reach temperatures higher than “warm-blooded” ones by basking in the sun.
Heat Loss and Gain Methods
Heat is always transferred from object with higher temperature to lower temperature. The essence of thermoregulation is balancing rate of heat loss and heat gain. Animals do this by reducing heat exchange overall or favor heat exchange in a particular direction.
Several heat loss and heat gain methods involve the integumentary system (the outer covering of the body including hair, skin, nails, claws, and hooves)
Mammals and birds reduce the flow of heat between themselves and the environment through insulation through hair, feathers, and layers of fat formed by adipose tissue. Birds raise feathers to trap more air and make insulation more effective. They also secrete oily substances to reduce effect of water on insulation. Humans rely mainly of fat tissues for insulation but have goose bumps as an heirloom from their furry ancestors. Whales and marine animals depend the most on fat for thermoregulation because heat to water transfer is 50 to 100 times more rapid than to air. They have a thick layer of fat called blubber under their skin allowing them to maintain a body core of 36-38° C without requiring much more food than terrestrial mammals.
In response to changes in surrounding temperatures, many animals can alter the amount of (hence heat) flowing between their body core and skin.
Vasodilation: A widening of superficial blood vessels near the body surface by relaxation of muscles of the vessel walls which causes an increase in vessel diameter and blood flow in skin. In endotherms, it warms the skin and increases transfer of body heat to the environment by radiation, conduction and convection.
Vasoconstriction: reduces blood flow and heat transfer by decreasing the diameter of superficial vessels. It is the same process which allows jackrabbits to avoid overheating on hot days. It also helps marine iguana conserve heat
Countercurrent exchange: The transfer of heat or solutes between fluids that are flowing in opposite directions (usually in mammals and birds and in some shark, fishes and insects). Arteries and veins are adjacent to one another. Warm blood from core flowing to the extremities in the arteries is adjacent to cold blood from extremities back to the core in the veins. This maximizes heat transfer. It helps whales and tuna fish keep their activity levels high and insects like moths keep their thorax warm (where flight muscles are). To avoid over-heating some species have the ability to shut down countercurrent systems (passing heat from thorax muscles to abdomen and then releasing it to environment).
Cooling by Evaporative Heat Loss: Certain mammals live in climates warmer than their bodies and gain heat from metabolism and the environment, thus they rely on evaporation to cool them down. Panting is a behavioral adaptation in many birds and mammals. Sweating and bathing moistens skin and enhances evaporative cooling. Sweat glands are controlled by nervous system in many mammals.
Most animals exhibit behavioral responses to the environment which may be simple or extreme (migration or hibernation). Many terrestrial invertebrates can adjust internal temperature by the same behavioral mechanisms used by vertebrate ecotherms. They have certain postures which enable them to maximize or minimize heat absorption (desert locus). Honey bees cluster together and move between the cooler outer edges and warm center to circulate and distribute heat. They still need considerable energy to keep warm and store honey as fuel. In summer they transport water and fan their wings promoting evaporation and convection.
Thermogenesis: Ability (in endotherms) to match changing rates of heat loss by varying heat production. It can be increased by muscle activity (moving, shivering) Shivering helps chikadees keep body temperature at 40°C when its -40°C outside if they have sufficient food. Non-shivering thermogenesis: is the ability in some mammals to have certain hormones make mitochondria to increase metabolic activity and produce heat instead of ATP. Through both types of thermogenesis, mammals and birds can increase heat production by five to ten times then in the warmer months. The Burmese python produces heat by shivering when incubating its eggs (hinting perhaps some dinosaurs were endotherms-controversial) as well as insects getting ready to fly.
Brown fat: tissue in certain mammals in neck and between shoulders specialized for rapid heat production. It represents 5% if body weight for human infants and was discovered for the first time in human adults in 2009 (especially when outdoor temperatures were lower).
Humans and other mammals have sensors for thermoregulation concentrated at the hypothalamus region in the brain. When stimulus increases body temperature we sweat and increase blood flow to return to homeostasis but when stimulus decreases body temperature we decrease blood flow, directing it to our core and contract our muscles (shiver). Fevers (elevated body temperatures) occur from bacterial and viral infections in mammals and birds. A change in the set point of the biological thermostat by artificially raising hypothalamus temperature in an infected animal reduces fever in rest of body. Reptiles do not develop fever but the desert iguana when infected with certain organisms seeks warmer habitats and maintains an elevated body temperature. This feature is common to many animals including fishes, amphibians, and cockroaches.
Bioenergetics: Overall flow and transformation of energy in an animal. It is related to an animal’s size, activity and environment and also determines its nutritional needs.
Animals are heterotrophs and must obtain their chemical energy from food which contains organic molecules synthesized by other organisms in order to fuel metabolic activity. Food is digested by enzymatic hydrolysis and nutrients are absorbed by body cells to generate ATP which is produced by cellular respiration and fermentation, powering work and used in biosynthesis (synthesis of storage of material such as fat and production of gametes). The heat produced by the animal is generally given off to surroundings.
Metabolic rate: the sum of all energy used in biochemical reactions over a given time interval. (A kilocalorie=4184J). It can be found by a calorimeter measuring animals heat loss or by measuring oxygen consumption and carbon dioxide production. For long term figures, researches note down energy content of food, rate of food consumption and chemical energy lost in poop.
Basal Metabolic Rate (BMR): minimum metabolic rate for basic function (breathing, heartbeat, cell maintenance) of a non-growing endotherm at rest with an empty stomach and not experiencing stress. It is taken at a comfortable temperature range. Human adult males have 1,600-1,800 kcal per day while women have 1,300-1,500 kcal per day.
Standard Metabolic Rate (SMR): Metabolic rate of a fasting, non-stressed ecotherm at rest at a particular temperature. An alligator needs only 60kcal per day at room temperature (1/20th of human animal)
Metabolic rate is affected by: age, sex, temperature, and nutrition
The rate is proportional to size (m3/4) but scientists are still studying this. However the smaller an elephant the higher the metabolic requirements per gram (mice have 20 times higher requirements than elephant for each gram), which leads to a higher oxygen delivery rate, higher breathing rate, blood volume (relative to size), heart rate and food per unit body mass.
Doing any activity still raises energy requirements above BMR and SMR. The maximum metabolic rates (highest rates of ATP use) occur during peak activity such as lifting weights, running or swimming.
Terrestrial animals use 2 to 4 times BMR but humans only use 1.5 times the BMR indicating how sedentary we have become. Humans spend most of their energy for BMR activities, the activity then thermoregulation and reproduction and finally growth (fat). Penguins have more activity and less BMR no growth year after year of fat for adults.
Torpor: a physiological state of decreased activity and metabolism as an adaption that enables animals to save energy while avoiding difficult and dangerous situations. Bats feed during night and go to torpor during day (their temperature drops), while chickadees do the opposite. Endotherms that go into this state are relatively small, have high energy costs and metabolic rates.
Hibernation: long-term topor that is an adaptation to winter cold and food scarcity that causes animal’s body temperature to decline and thermostat to shut off. The artic ground squirrel reaches temperatures below 0 °C without freezing. Every two weeks or so animals wake up, become active and raise body temperature before resuming hibernation. It allows animals to save on energy (20 times lower) if food supplies are scarce.
Estivation: long-term topor for high temperatures allowing animals to survive with scarce water.
Research favors the hypothesis that the biological clock is turned off during hibernation
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