To mantain their structure and functions, living beings need a constant flow of energy from the environment (and to a sink, in the form of heat). Energy can exist as electromagnetic energy, thermal energy, chemical energy, mechanical energy etc., and each one of these forms can be exploited by organisms. Since life is most likely to be based on chemical reactions, the extracted energy will have to be stored as bond energy between the atoms of some simple molecule; every lifeform on Earth uses a sugar, glucose (C6H12O6) which releases this energy when combined with an oxidising agent.
Note on measurement units: Energy is measured in joules (J) or calories (cal): a calory is equal to 4.18 J, dietary calories (Cal) a thousand times as much. Power is the rate of energy production/consumption per time: one watt (W) is equal to 1 J/s (one joule per second), or 0.24 cal/s. The energy consumption over a day (assuming days of 24 hours) can be measured in kilocalories per day: 1 W = 20.7 kcal/d, 1 kcal/d = 0.0483 W.
Depending from the way an organism gets its energy, it can be classified in three different ways: the primary source of energy (phototrophs for light, chemotrophs for chemical compounds, etc.); the source of the electron donor that gets oxidized (organotrophs if it comes from organic compounds, lithothrophs otherwise); the source of the carbon or other basic element (heterotrophs if they obtain them from other organisms, autotrophs if they synthetize the molecules themselves). Thus, plants are photolithoautotrophs, animals and fungi are chemoorganoheterotrophs, sulfate-reducing bacteria are chemolithoautotrophs, and so on.
Earth plants produce glucose through photosynthesis: 6CO2 + 6H2O → C6H12O6 + 6O2. Since the energy stored in glucose comes from light, carbon from an inorganic source (carbon dioxide) and electrons used to reduce it from another inorganic compound (water), plants are photolithoautotrophs. Most ecosystems on Earth ultimately receive their energy from photosynthesis; since this process is so simple, every world with Earth-like biochemistry is likely to develop it.
During synthesis, a reaction occurs in which an electron donor (reducing agent) cedes electrons to the carbon source, thereby reducing it, and being oxidized in return. The inverse reaction is respiration, in which glucose (or the equivalent compound) is combined with an electron acceptor (oxidizing agent) takes electrons back from it, oxidizing the compound and being in turn reduced. The most efficient oxidizing agent is most likely molecular oxygen; see here for possible alternatives. There's a simple form of respiration without electron acceptor, called fermentation, for example, glucose to ethyl alcohol: C6H12O6 → 2C2H5OH +6CO2, which releases 74.6 kJ for each mole (that is, 6.022×1023 molecules) of glucose, but it's far less efficient: oxigenation of glucose (C6H12O6 + 6O2 → 6CO2 + 6H2O) releases 2820 kJ per mole of glucose.
It being understood that carbon will still be central to every chemical process with such a biochemistry, glucose can be synthetized with a variety of other reactions. For example, purple sulfur bacteria add hydrogen sulfide to the reagents and replace oxygen with sulfuric acid (their cytoplasm is coloured yellow by the residual sulfur crystals): 6CO2 + 6H2O + 3H2S → C6H12O6 + 3H2SO4. Since sulfuric acid is hydrogen sulfate, this lithotrophic reaction can be identified as sulfide-to-sulfate reduction, with the oxidizing agent being oxygen.
Other examples are hydrogen sulfide synthesis: 6CO2 + 12H2S → C6H12O6+ 6H2O + 12S and methanogenesis: 4H2 + CO2 → CH4 + 2H2O, which does not result in glucose being synthetized. Eponan metabolism is based on the reaction 6H2O + 2N2 → 4NH3 + 3O2, which produces energy but requires carbon to be fixed in a different reaction.[1]
Examples of metabolic reductions
Name
Electron donor (synthesis)
Electron acceptor (respiration)
Initial
Oxidised
Initial
Reduced
Plant photosynthesis
H2O
O2
O2
H2O
Aerobic hydrogen bacteria
H2
H2O
O2
H2O
Methanogens
H2
H2O
CO2
CH4
Carboxydotrophic bacteria
CO
CO2
H2O
H2
Purple sulfur bacteria
S2−
S0
O2
H2O
Sulfur-oxidizing bacteria
S0
SO2− 4
O2
H2O
Thiobacillus denitrificans
S0
SO2− 4
NO− 3
Sulfate-reducing H2 bacteria
H2
H2O
SO2− 4
H2S
Sulfate-reducing PO 3 bacteria
PO3− 3
PO3− 4
SO2− 4
Anammox bacteria
NH3
N2
NO− 2
Nitrosifying bacteria
NH3
NO− 2
O2
H2O
Nitrifying bacteria
NO− 2
NO− 3
O2
H2O
Iron bacteria
Fe2+
Fe3+
O2
H2O
Bond-dissociation energy is the amount of energy needed to build or dissolve a bond between atoms, assuming no participation of external temperature, in each mole of compound. This table contains the bond-dissociation energy of some common bonds, mostly inorganic; see here a table that presents more examples, including the more complex organic bonds. We can se from the table that carbon double- and triple-bounds and nitrogen triple-bounds are very rich in energy.
Bond
kcal/mol
Bond
kcal/mol
Bond
kcal/mol
O-O
43
H-Br
88
O-H
110
H-NO
50
H2-S
91
C-F
117
C-S
62
Si-H4
92
O=O
119
H-I
71
N-H
93
H2-O
119
C-N
72
C-H
98
H-CN
126
C-Cl
79
C-H2
101
H-F
136
Ge-H4
81-85
H-Cl
103
C=C
146
C-C
83-85
H-H
104
C=O
175
C-O
84
C-H4
105
C≡O
200
S-H
84
N-H3
107
N≡N
226
Note: more lines between the atoms show a stronger chemical bond. O = oxygen; H = hydrogen; N = nitrogen; C = carbon; S = sulfur; I = iodine; Cl = clorine; Ge = germanium; Br = bromine; Si = silicon; F = fluorine.
Energy Consumption[]
As discovered in the 1930s by the biologist Max Kleiber, in most animals the metabolic rate (energy required per unit of time) is proportional to the ¾ power of the mass: that means that a cat, a hundred times heavier than a sparrow, needs an amount of energy 100¾ = 32 times greater (and thus it means that metabolic rate per unit of mass is lower in bigger animals). The precise exponent can vary, though: in plants it's closer to 1. This is a consequence of the surface-volume ratio, since a bigger organism can hold a larger fraction of his weight in reserves.
The basal metabolic rate (consumption of energy at rest) can be estimated with the formula r = KM¾, where M is the body mass in kg and K is a constant that depends from the organism type (in the table above are given two values for each type, one for calculating the rate in kilocalories per day and one for calculating the rate in watts). That allows us to estimate the metabolic rate of a 10-kg reptile in 10·10¾ = 56 kcal/d (or 2.7 W), and that of a 5-kg passerine bird in 129·5¾ = 431 kcal/d (or 20.8 W).
Organisms
K (kcal/d)
K (W)
Sponge, fungus, plant
~2
~0.1
Reptile, amphibian, invertebrate
10
0.48
Marsupial mammal
49
2.4
Placentate mammal
72
3.5
Birds other than passerines
78
3.8
Passerine birds
129
6.2
Another value linked to the metabolic rate is the heart rate: it can be inferred from the body mass with the formula r = 10(2.9 - 0.2·logM), where M is the mass in grams.[2] This formula gives 19 beats/minute for a 120-ton whale, 81 b/m for a 90-kg human and 200 b/m for a 1-kg rabbit (the real values are 20, 60 and 205). A 1-gram animal would have a heart rate over 800 beats per minute, and in fact some hummingbirds can have 1200 b/m, but under this size the circulatory anatomy becomes so different that the formula loses its efficacy. Besides, an animal can be expected to live for 1-2 billions of heartbeats.
Metabolism, heart rate and lifespan of various animals
Note: lifespan is given both in years and in billions of heart beats (Gb). The number of Gb per lifespan remains constant (between 0.7 and 1.5, except in humans) for animals of very different sizes.
Given as example a mammal weighing one ton, we can thus conclude that it will consume 12 800 kcal/d (or 619 W), that it will have a heart rate of 50 beats/minute and that it will live for 40-80 (Earth) years.
Thermoregulation[]
The usage of energy to fuel biological processes necessarily releases waste heat, which often builds up fast: during the glucose oxidation employed by Earth animals, each litre of oxygen consumed (see below), which equals to 0.045 moles, produces 21 kJ (5000 cal). Let's assume that all of this energy is destined to muscular work; since animal muscle has only an efficiency of roughly 25%, that means that 75% of that energy, or 15.8 kJ, are going to be dispersed as heat: that's enough energy to heat up a kg of animal tissue[3] of 4.5°C.
Organisms deal in different ways with the problem of excess heat, often simply letting it disperse in the environment. They can be classified in two different ways, one of them according to the tolerance to internal temperature variation.
Poikilotherm organisms have a
Power output of a poikilotherm lizard and a homeotherm mouse. The lizard can survive with a wide variety of temperatures, but it never outputs as much energy.
variable internal temperature; they can have several different enzyme systems that work at different temperatures. Since they don't spend much energy to produce or remove heat, they need as little as a tenth of the food homeotherms eat. Given these characteristics, it's likely that they'd be precluded from energy-consuming activities such as large-scale powered flight, high intelligence or prey pursuing: poikilothermic predators (spiders, mantises, frogs, etc.) prefer ambushes. They often invest most of their energy in reproduction, produncing quickly a large offspring; they have short lives and are usually preyed upon by homeotherms.
Heterotherm organisms are intermediate between poikilotherms and homeotherms. Temporal heterotherms change their temperature during the day: the smallest homeotherms (bats and hummingbirds) save energy reducing their metabolism and lowering their temperature when they rest. Regional heterotherms have a different temperature in different parts of the body: tunas and many penguins and seals have retia mirabilia that heat up blood incoming from the appendages and cool down blood coming from the heart, thus reducing heat loss.
Homeotherm organisms, such as birds and mammals, keep constant their internal temperature, often with a precision of a tenth of a degree, to allow the optimal functioning of enzymes. This adaptation requires a large amount of energy, especially in a medium with a low specific heat capacity (see the table here), such as argon, sulfur oxides, nitrogen and carbon dioxides etc., which absorb heat quickly. Gigantothermy is a particular form of homeothermy that isn't produced by metabolic activity but by size: larger organisms don't disperse heat as easily, they tend to have a reliably high internal temperature (organisms in cold environment tend thus to be bigger than those in warm environments). Gigantotherms include sharks, great sea turtles, ichthyosaurs and mosasaurs, and perhaps the largest sauropod and ornithopod dinosaurs.
The other parameter is the ability of organisms to change their own temperature:
Ectotherm organisms include all the poikilotherms: they're those who regulate their temperature through external means. There are, though, many ectothermic homeotherm: water is such a good insulant that allows to animals without an internal thermoregulation system to keep their temperature constant. Many mammals also thermoregulate themselves with both internal and external means. These include:
Convection. Warm air rises, cold air falls: an animal can move to meet air at the right temperature.
Conduction. Touching warm or cold materials; many lizard bask on sun-heated rocks.
Radiation. Large surfaces such as wings, crests, dewlaps or membranes of exposed skin can absorb or leak heat. Dimetrodon's "sail" and stegosaurid crests had probably this function.
Evaporation. Evaporating water is an excellent heat carrier. Humans sweat (up to 10-12 litres per day in some conditions); dogs, rabbit and many birds pant, losing water through the oral mucosa; bats, rodents and several marsupials lick themselves; pigs, hippos and elephants wallow in water or mud.
Endotherm organisms keep their temperature stable by internal mechanisms: a simple method is shivering (quick vibration of muscles), which is though very inefficient, as most of the energy is turned in useless cyclical movement; brown adipose tissue, a type of fat found in most mammals, performs a series of reactions that produce more heat than usual cellular respiration. To avoid dispersing the produced heat through the surface, endotherms cannot be very small: the smallest weigh at least a few grams. Endotherms, at least in cold environments, also need some form of insulation, given by a layer of matter that doesn't carry heat well, such as fat (animals that need fat as a food storage but need to avoid overheating concentrate it in a small area, as camels do), or air: fur and feather retain air close to the body, limiting the cooling. Furahan woolly haired shuffler has skin with the fur on the inside, making the insulation more effective.
The most obvious morphological difference between ectotherm and endotherm organisms will be their skin cover: ectotherms generally have naked skin, cuticle, thin scales, etc., while endotherm usually have an insulating fibrous coat, such as hair, wool or feathers.
Energy Storage[]
Autotrophy[]
Autotrophs produce complex organic molecules (carbohydrates, fats, proteins, nucleic acids) from simple chemicals absorbed from the environment. As said above, autotrophs perform at least a double reaction: first they produce energy-rich molecules such as glucose (both to store energy and to produce more complex chemicals) and then they break them down to extract the energy within.
Examples of autotrophic energy prouction (see below)
Water falling from a 10 m waterfall (Earth gravity)
329 kcal/s/m2
Notes: most of the table derived from one in Cosmic Biology[4]; the solar irradiation considers the amount of photons that hit Earth's Equator as an average, night included; the energy derived from wind and water flow is proportional to the 3rd power of its velocity, and directly proportional to its density.
Phototrophy[]
Also see: Productivity
Light should be abundant on the surface of any planet, even more so if it's located inside the water habitable zone. An extremely thick and opaque atmosphere (perhaps rich in fluorine, chlorine, nitrogen dioxide and/or methane) could effectively block most of the light at the ground level, but we'd still expect to see a layer of phototrophic aeroplankton at the upper boundary of the atmosphere. Oceans completely covered in ice, such as those on Europa, abyssal ecosystems and deep caves would also be among the few environments devoid of light.
Phototrophs use compounds called photosynthetic pigments that capture energy from light of certain wavelengths, reflecting the light of the other wavelengths, and appearing of that colour. For example, chlorophyll absorbs mostly blue and red light; therefore, it reflects green light, and thus it tinges leaves green. Other plausible photosynthetic pigments, many of them used by Earth organisms (see here) can appear black, brown, red, orange, yellow, blue or purple; the colour of an alien "plant" should be a wavelength that it has no reason to exploit. Darker colours absorb more light: plants on a planet orbiting a red dwarf could be dark purple or black, while on a planet orbiting an extremely bright star they could be silver or white to reflect excessive radiation and heat.
The energy content of light increases proportionally to its frequence (and thus inversely to wavelength): it's lowest in radio waves, it increases through microwaves, infrared, visible light, ultraviolet and X-rays, and it's highest in gamma rays; still, light above ultraviolet is unlikely to be exploited, because they're both dangerous to molecular structures (though examples of adaptation to strong radiations are known) and blocked by most likely atmospheres. The light most likely to be employed includes infrared, visible and ultraviolet, roughly between 100 and 10 000 nm of wavelength.
Tree-, bush- and grass-like forms are highly likely for any extraterrestrial phototroph: wide and thin parts such as leaves are ideal to absorbe light without adding unnecessary mass, a large number of small leaves is more resistent to mechanical stress than few large ones, and "plants" will likely compete in height to overshadow their neighbours, as far as structural constraints allow them to do so. As analyzed on the Furaha blog[5], multiple layers of leaves are useful if the upper ones don't cast too much shadow: in Earth-like conditions, the vertical length of the umbra (the dark zone where other leaves shouldn't be placed) is about 108 times the width of the upper leaf, though this value is inversely proportional to the apparent diameter of the star in the sky. Smaller, branched, indented leaves would cast a smaller umbra and allow a multi-layered canopy.
Chemotrophy[]
Chemotrophs obtain energy from the oxidation of reducing chemicals in their environment. Animals are chemoorganoheterotrophs, as they oxide organic molecules (-organo-) extracted from other organisms (-hetero-). The greatest diversity in chemotrophic diet is found in chemoautotrophic bacteria, which oxide hydrogen sulfide, elemental sulfur, ferrous iron (FeO), hydrogen and ammonia (since these are inorganic molecules, these bacteria are considered chemolithoautotrophs); they're most common around extreme environments such as hydrothermal vents, which emit iron, manganese, sulfur and other useful elements. For example, certain microbial organisms, such as Mariprofundus ferrooxydans andAcidithiobacillus ferrooxidans, survive by oxidizing iron.
When one of the reducing compounds is oxidized, it releases one or more electrons (increasing their oxidation number, for example from Mn2+ to Mn4+, since each electron has negative charge). Some examples of synthesis of organic molecules, which can be driven by chemical energy as well as by light, are given above.
Thermotrophy[]
A very simple form of autotrophy based on heat. A thermotroph organism would sit on the boundary between a very cold and a very hot area, getting its energy from the flux of heat towards the colder zone. The best location for thermotrophy is a hydrothermal vent, though it could exist - with less effectiveness - in lesser temperature gradient (for example at the boundary of the atmosphere), provided there is always a colder heat sink, such as the space.
The book Life in the Universe[6] provides some examples of thermotrophs that could inhabit an alien hydrothermal vent (which presumes a planet with significant tectonic activity, radioactive decay and/or tidal flexing, see here), migrating up and down, flexing an appendage or transferring a vacuole inside their body. Given that water has a heat capacity of 4.4 J/kgK at the most common temperature and pressure conditions, cooling a kg of water by 1°C allows to extract 4.4 joules. Let's suppose that 1 kg of water has to be lifted from the vent for 20 cm to be cooled by 1°C: at Earth's gravity, lifting 1 kg for 0.2 m requires 9.8*1*0.2 = 1.96 J, and since the cooling gives off 4.4*1*1 = 4.4 J, the net gain is 4.4-1.96 = 2.44 J = 0.58 calories.
While easy to exploit, heat flow is a very inefficient source of energy. It's likely to evolve on environments devoid of light, such as oceanic abyss or seas entirely covered in ice, such as those on Europa.
Osmotrophy[]
As speculated in Cosmic Biology[7], life-sustaining work might also be obtained from osmotic gradients - the difference in salt concentration (tonicity) between layers on liquid. Europa-like oceans can have a seafloor that ejects large amounts of minerals and a surface diluted by the melting ice on top: in the salt-rich (hypertonic) bottom, the interior of a cell or sac would expel water (or other solvent) or draw ions inside, while in the salt-poor (hypotonic) top the cell would draw water inside or expel ions. This motion could power chemical reactions or directly alter the structure of energy-storing molecules. Osmotrophs might want to have a large surface area to exchange more water/ions, though a greater volume would defend them against extreme variations of their internal chemical environment.
Electrotrophy[]
Life could theoretically survive directly on electricity. Existing organisms, such as Oriental hornets, can metabolize sunlight by converting it into electrical energy[8], then using the electrical energy. A photovoltaic cell can convert light in electricity through a layer of semiconductors (such as silicon, germanium, some aromatic hydrocarbons, zinc tin and titanium oxide, etc.) which lose electrons when hit by photons, therefore producing a current. Organisms with photovoltaic "leaves" would be electrotrophs, as true phototrophs use directly sunlight to power chemical reactions, without converting it to different forms of energy.
Ionotrophy is a process similar to osmotrophy, in which ions flow through the membrane towards the region where they're less concentrated. The strength of the flow is proportional to the ratio between the two concentrations: if the concentration inside is a 100 times higher (or lower) than the concentration outside, the ion flow produces 2.77 kcal per mole of ions. Being electrically charged, the ions work as in a chemical battery, creating an electric current. This source of energy could be exploited by organisms that live in hypersaline waters.
Yet another possible source of electricity could be found in magnetotrophy, through Lorentz force: when an area containing electrically charged particles (i.e. ions) is subjected to an external magnetic field, electrical charges are moved in a specific direction, creating a current. Most planetary magnetic fields are too weak to produce a useful current, though magnetotrophs could arise on the inner moon of giant planets, such as Io.
Radiotrophy[]
Also see: Radiophiles
Fungi such as Cladosporium and Cryptococcus (unrelated) are known to grow as black mold inside the collapsed reactor of the Chernobyl Nuclear Power Plant[9], where they absorb gamma rays thanks to a thick coating of melanin, a brown-black pigment also found in human skin. Gamma rays excite the atoms of melanin freeing electrons, which are used to produce energy through a still unknown process. Since high-energy radiations are quickly blocked even by atmospheric gas (and are highly dangerous to complex molecules), radiotrophy should not be expected to be very common, except perhaps in close proximity of radioactive ores and similar sources of natural radioactivity.
Kinetotrophs would feed on mechanical energy, and the best way to do this is through piezoelectric materials, which generate an electric charge when mechanical stress is applied to them: if one of these materials is bent or deformed, a current is produced. Piezoelectric materials include both common crystals such as quartz, Rochelle salt and sucrose (table sugar) and organic tissues such as bone collagen, silk, enamel, DNA, etc. While piezoelectricity is not likely to produce large amount of energy, it probably could at least sustain populations of simple plant-like organisms.
A kinetotroph "prairie" could be a crystal-like paving stepped upon by moving animals (perhaps they could even create smooth roads that would finally make wheels a useful adaptation), or elastic fibres moved by wind, river currents, rain and tides; other proposals on the forum include grasslands where each stalks transmits motion to its clones, cup-like plants that fall over when they're full of rain water, and parasites that exploit the animals' joints and muscle movement.
A particular (and perhaps less likely) form of kinetotrophy is phonotrophy or audiovory, that is, feeding on sounds. Audiovores (for example, this one) could cluster around a noise source; since sounds are vibrations in a material medium, their energy can be caught by hairs or membrane with piezoelectric elements; however, the amount of energy gained from them would be extremely small.
Heterotrophy[]
General[]
Classification of consumers based on their food: green and brown (left) are plants, purple and red (right) animals; green and red (outer ring) are alive, brown and purple (inner ring) are dead; in the upper half, consumers eat the whole organism, in the lower half only parts of it
Heterotrophs are organisms that aren't able to fix carbon (or whatever else the central element or their biochemistry is), and therefore have to consume the tissues of other organisms, which can be autotrophs or other heterotrophs (see here). On Earth, they're roughly divided in herbivores (which eat mostly plants) and carnivores (which eat mostly animals).
In ecology, autotrophs and heterotrophs are referred to as producers (of organic molecules) and consumers. Consumers/heterotrophs can be classified in different dimensions: herbivores and phytophages eat plants, carnivores and zoophages eat animals; totivores (or "gatherers") eat whole organisms or consistent parts such as fruits or leaves, holophages (or "miners") eat part of tissues and organs; croppers and parasites eat live organisms (or part of them), scavengers and saprophages eat dead ones; and so on.
More specifically, victivores eat whole, living plants; lectivores eat whole, dead plants; bestivores eat whole, living animals; carcasivores eat whole, dead animals; zontanophages eat parts of living plants; thanatophages eat parts of dead plants; sarcophages eat parts of living animals; necrophages eat parts of dead animals. The image on the right summarizes this complex classification.
Other useful criteria include the mode of ingestion:
Pharyngeal jaws of a moray eel
Filter feeders eat tiny particles suspended in a fluid (water, another thalassogen, or perhaps in the atmosphere, if it's dense enough). They include sponges, crustaceans such as barnacles and krill (which is in turn small enough to be eaten by larger filter feeders), fishes such as herrings and whale sharks; among land vertebrates, only baleen whales and flamingos. Whales have comb-like structures (baleens) that sieve water, while flamingos have similar hair-covered lamellae. Manta rays have fins that push plankton towards their mouth, while nightjars have stiff hairs around the beak with the same function as they fly after insects.
Bottom or deposit feeders eat detritus, extracting food particles from the soil (see "detritivores" below).
Fluid feeders ingest the liquid tissues of a organism, typically blood, nectar and lymph.
Ram feeding can exist only in water or another dense fluid: it involves the predator launching itself towards the prey swallowing it whole with the surrounding fluid. Snapping turtles and rorquals have an elastic throat that expands holding a large amount of water, while sharks and herrings expel it quickly through the gills; garfish and sea snakes have a narrow muzzle that reduces the amount of water ingested. A rapid movement in a dense fluid produces turbolence which can push the prey away, but it can be corrected by a backwards movement: thus, squids open their arms when attacking a prey.
Suction feeding is a variety of ram feeding: the predator opens wide its mouth, which gets filled with fluid, dragging the prey inside. Some moray eels have developed a second pair of jaws in the throat (pharyngeal jaws) to bite the swallowed prey.
Bulk feeders simply eat a prey whole or consistent solid parts of it.
In addition to ingestion, there of digestion, relatively to the cells:
Extracellulardigestion happens outside cells and tissues thanks to the secretion of digestive enzymes (proteases, lipases, carbohydrases, nucleases) through the cell membrane, usually inside a body cavity. Most animals use this method; some organisms, such as spiders, fungi, lichens, etc. resort to "external digestion", introducing their enzymes inside their food and then reabsorbing the digested chemicals, thus reducing the amount of waste inside the body. Organisms that do this are called liquivores: most of Darwin IV fauna, such as the arrowtongue, are liquivores.
Myzocytosis consists in piercing the membrane or cell wall of a prey, sucking out organelles and cytoplasm. It's common among ciliate protozoans, and it's possible only on cellular scale.
Phagocytosis involves creating a cavity in the cells to engulf food matter and prey cells, which are digested inside a vesicle. The immune defence system of multicellular organisms uses phagocytosis to rid the body of microbes; among animals, it's found only in Trichoplax adhaerens.
The table below gives the amount of energy stored in various foods: since most plant matter contains much less energy per unit of mass than animal matter, herbivores - especially graminivores and folivores, see below - will need to spend much more time eating than carnivores (and even some specialized herbivores). As for pure chemicals, fibers contain 2000 kcal, carbohydrates and proteins both 4000 kcal, ethanol (alcohol) 7000 kcal and fat 9000 kcal, but actual foods contain undigestible matter that reduces the total energy content.
Energy content of different foods
Food
Energy
Content
Leaves, grass, sprouts
130 kcal/kg
Carbohydrates, mosty cellulose
Roots and tubers
300 kcal/kg
Carbohydrates, mostly starch
Fruit
500 kcal/kg
Many carbohydrates, such as fructose
Milk
800 kcal/kg
Proteins (caseine), lipids, lactose
Blood
900 kcal/kg
Carbohydrates, some lipids and proteins
Beans
1000 kcal/kg
Many carbohydrates and proteins
Fish and shellfish
1000 kcal/kg
Many proteins and lipids
Bird meat
2000 kcal/kg
Many proteins and lipids
Mammal meat
3000 kcal/kg
Many proteins and many lipids
Grains
3000 kcal/kg
Much starch and other carbohydrates
Honey, nectar
3000 kcal/kg
Almost pure sugars
Insects
5000 kcal/kg
Many proteins and carbohydrates
Nuts and hard fruit
5000 kcal/kg
Carbohydrates and many lipids
Butter, fat, vegetable oils
8000 kcal/kg
Almost pure lipids
Herbivores[]
Examples of specialized herbivorous totivore (gatherer) diets:
Among the main kinds of herbivores there are folivores, leaf-eaters. These are very abundant, and provide food for a wide variety of species ranging from grasshoppers to elephants, but they're also very poor in energy, and most of their volume is made of cellulose, a complex molecule which is very hard to digest. Folivores have a long and twisted intestine, full of symbiotic bacteria (gut flora) that help breaking down cellulose into simple sugars[10]. Even tree-living folivores tend to be slow and relatively large, such as sloths and koalas; for this reason, flying folivores are very rare, including only the hoatzin and some bats.
Graminivores are other classic herbivores: they eat grass, that is, the stems and leaves of the small Poales and other low-growing plants. Grass stems are rich in silica, and thus they quickly wear out teeth, which in most graminivores grown continuously; for the rest, they have similar adaptations as folivores.
Frugivores eat fruit. Since many plants purposefully let animas eat their fruits so that they'll disperse the seeds, desirable, nourishing and easily digestible fruits were developed: they're usually very rich in water and simple sugars, though often poor in proteins and lipids. Some frugivorous birds have a very short intestine to expel seeds unharmed. Animals that eat fruit to disperse seeds include many birds (toucans, cassowaries, parrots), maned wolves, orangutans, diurnal bats.
Granivores eat seeds. These, since they contain the material necessary to the plant embryo to survive in the first days, are packed with calories (some seeds, such as nuts, also contain a great amount of lipids); for the same reason, they're usually heavily defended by woody shells, thorns, poisons such as tannins and alkaloids (see here), or simply by making them so rare they're not worth the effort. Granivory is a form of durophagy (eating hard, resilient food), and thus main adaptations include short jaws or beaks (which can apply a greater force) and large molar teeth.
Nectarivores are usually very small, since their food - nectar - is only available in small quantities. On the other hand, being mainly composed of simple sugars (mostly glucose, sucrose and fructose), it's extremely rich in energy and doesn't require a complex digestive system to be digested. Most nectarivores are insects (butterflies, bees, etc.) or birds (sunbirds, hummingbirds, etc.), but they also include some North American bats, the honey possum and the Phelsuma geckos.
Since pollen is also a nutritious food, though as quantitatively limited as nectar is, some small animals are palynivores. Bees and a few wasps are the only true palynivores, given that pollen is the only solid food they eat, but some mites and thrips also eat pollen grains; many flies, butterflies and beetles are palynivores in the larval stage.
Examples of specialized herbivorous olophagous (miner) diets:
Many insects, especially larvae, are xylophages, that is, wood eaters. Wood is very difficult to digest, being mostly composed by energy-poor fibres of cellulose and lignin, and thus requires both a strong buccal apparatus and a complex digestive system to break down the molecules.
Exudativores eat sap, a fluid carried between plant cells composed mostly of water, sugars, hormones and mineral elements. Despite being energy-rich and easy to digest, exudativores are found only among true bugs, because of two specific issues: sap is poor in amino acids (exudativorous bugs produce them through symbiosis) and the sugar content can exert osmotic pressure, sucking water out of the cells (they quickly turn the sugar in long-chain carbohydrates)[11].
Carnivores[]
Examples of specialized carnivorous totivorous (gatherer) diets:
Piscivores eat fish. They can have a generic carnivore set of teeth or a sharp beak, but some develop specialized needle-like teeth, pointed backwards, that allow to hold slimy preys such as fish and squids. They typically also have a long and narrow muzzle, as gars and gharials do.
Insectivores or entomophages eat insects, which are an extremely abundant and nourishing source of food, with tissues rich in nitrogen and proteins; insect exoskeleton is made of chitin, a carbohydrate. Specialized insectivores have very small teeth (long and pointed if the insects are large in comparison to the insectivore, as is the case for shrews). They're usually small, though the sloth bear can weigh up to 190 kg.
Myrmecophages are a particular group of insectivores specialized in social insects: ants, termites, or less often bees (in this case, they usually eat all the comb, which contains lipids from the wax and sugars from the honey). Specialized myrmecophages, such as aardvarks, pangolins, numbats, echidnas and true anteaters share several adaptations, such as strong claws on the forelegs and very long and sticky tongues, often in a tube-shaped snout.
Molluscivores eat mollusks, or more specifically those protected by a shell. They follow two strategies: pufferfish and placodonts (extinct reptiles) have chisel-like front teeth to detach shellfish from the rocks and, being durophages, strong and wide back teeth to crush their shell; molluscivore loaches and oystercatchers have a long and narrow snout or beak to grab the soft flesh inside the shell.
Other common carnivorous specializations include avivory (eating birds), vermivory (eating worms), ophiophagy (eating snakes, e.g. secretary birds, mongooses, some herons, king cobras), spongivory (eating sponges, e.g. hawksbill turtles and emperor angelfish), etc.
Examples of specialized carnivorous olophagous (miner) diets:
Hematophages eat blood. They include leeches, lampreys, female mosquitoes, several bugs and vampire bats. Their most obvious adaptation is found in buccal parts: lampreys and leeches have suckers that tear through skin, vampires have tiny needle-like teeth and a ribbed tongue to lap blood; mosquitoes have a more complex apparatus that pierce the skin with a needle and suck out blood with a hollow proboscis. This kind of perforation is called phlebotomy. Blood is nutritious and easy to digest, since the food it contains has already been digested, but it's not usually abundant, and thus most hematophages are very small.
There are many species of lepidophagous fish, that is, they eat the scales of other fish. These contain a surprising amount of food: layers of enamel and keratin, parts of skin, calcium phosphate and protein-rich mucus. They're also very small, seldom larger than 20 cm.
Mucophagous parasites, such as sea lice, eat the fresh or dry mucus that covers the skin of fish and marine invertebrates. Mucophages are also small, and don't need a complex stomach or buccal parts.
Keratophages eat horny material such as scales (many snakes eat their own skin after moulting) or parts of horn, hair, feathers, claws, etc. This material is mainly composed of keratin, a rather energy-poor protein. They also include dermatophyte fungi and cloth moths.
Other Diets[]
Diets that do not fall under strictly "herbivore" or "carnivore" categories:
Omnivores eat both plants and animals. Humans are a good example of omnivores: their teeth can be used to cut soft fruit, pierce through flesh, crush seeds, etc. Omnivores often survive more easily than other species, especially during times of crisis. While very few species are limited to only one source of food, the more adaptable omnivores include primates, pigs, badgers, bears, skunks, raccoons, rats, opossums, chickens, crows, many lizards, catfish, etc.
Fungivores or micophages eat fungi, whose tissues are mainly composed of chitin, like the insect exoskeleton; incapable of growing shells or thorns, fungi mostly defend themselves with poison. Among fungivores there are mites, flies, ants, slugs and roundworms; the northern flying squirrel, the black snub-nosed monkey and potoroos are among the few mammals that feed mostly on fungi and lichens.
Many species of amoebae, ciliates and other protozoans are bacterivores: they eat bacteria, of which they consume the cytoplasm, the nucleic acids and the lipids from the cellular membrane.
Diets involving decaying or inorganic matter:
Necrophages or scavengers consume parts of animals or plants dead by a substantial amount of time, usually days, or the leftover food from a predator. Typical examples include blowflies, vultures, raccoons, several varieties of wasps and beetles; many animals that otherwise hunt live preys (e.g. lions, hyenas, dogs, ravens) can occasionally eat carrions. Usually, scavengers don't need adaptation different than for eating the same (living) food, but those that eat a corpse from the inside, such as many vultures (or Dixon's gholes) have a head and neck devoid of fur or feathers so they don't get soaked in blood, while others have durophagous adaptations to break open bones and eat the marrow.
Osteophagy is a variety of necrophagy specialized in bones. More specifically, it includes herbivores that sometimes consume bones to extract calcium and phosphorus, as giraffes do. Hyenas are the only mammals able to fully digest bone tissue.
Coprophages eat feces; they're especially common among insects and roundworms. This behaviour also exists in many mammals, for different reasons: folivores and graminivores that can't ruminate, such as rabbits, hamsters and capybaras, eat their own feces to digest cellulose a second time (caecotrophy); hamsters, guinea pigs and chinchillas also do it to recover lost vitamins B and K; folivores that need a substantial gut flora, such as elephants, pandas, hippos and koalas eat their mother's feces to gain intestina bacteria; gorillas and other primates eat them for their content of seeds and mineral salts.
The mushroom Mycena interrupta growing on detritus.
Detritivores or saprophages consume detritus, a particulate composed of decaying organic matters, fragments of corpses and feces, rotting plant matter, etc. Assembled on the ground or on the seafloor, detritus is rich in complex carbohydrates and inorganic molecules such as nitrates. Detritivores have the ecologic role of decomposers, the organisms that break down any residual organic tissue into simple molecules that can be riabsorbed by producers. All non-parasitic fungi are saprophages; among animals, they include millipedes, woodlice, slugs, sea stars, sea cucumbers, crabs, etc.
Geophagy is the practice of eating inorganic matter. By definition, no heterotroph can survive only on inorganic food, but many mammals, reptiles, woodlice, butterflies and birds (especially parrots) ingest chalk and clay to obtain important minerals (mainly calcium and sodium), and to absorb and neutralise toxins such as the alkaloids found in unripe fruit[12]. It's also common among human cultures. A particular case is given by the annelid Olavius algarvensis: it lacks a digestive system, but ingests sulfate-rich soil to feed symbiotic chemoautotroph bacteria that reduce sulfate to sulfur, thereby gaining energy from them[13].
Opportunistic or sporadic diets:
Cannibalism involves eating individuals of one's own species; recorded in several species from each major taxonomical group, it's especially common in aquatic environments (where the 90% of species is estimated to be cannibalistic), and surprisingly present among herbivores and detritivores. Besides the usual alimentary cannibalism, there are several specific forms:
In sexual cannibalism, an individual belonging to the gender that takes are of offspring (usually the female) eats the partner after, during, or even short before the mating, Most likely, this behaviour exists to supply more resources to take care of the young, and removes from competition an individual that isn't usful to its own genes anymore[14]. It's found in praying mantises, some scorpions and many spiders; the crustacean Gammarus pulex is a rare example of male eating the female.
In size-structured cannibalism, larger and stronger individuals in a population consume the smaller ones. Again, this trasnfers resources to the individuals that are more likely to survive anyway. Its effect varies between species: it's responsible of the 8% of mortality among ground squirrels, and the 95% among dragonfly larvae.
Filial cannibalism is connected to reproductive r-strategy (producing a vast offspring with little to no parental care), and it involves consuming part of one's own offspring, as an extreme variety of size-structured cannibalism. It's very common among fish (a species of goby can devour up to 40% of its eggs), but it's found even among K-strategy-following mammals: cats, lions, dogs, bears, pigs, baboon and chimpanzees.
In some salamanders and teleost fish and in the lamnoid sharksintrauterine cannibalism can occur, in which an embryo consumes or devours its siblings as eggs (oophagy) or as embryos themselves (adelphophagy or ovophagy).
Autosarcophagy is an extreme form of cannibalism, consisting in consuming one's own body parts. For obvious reasons, it can be employed only in exceptional circumstances: some cephalopods eat their own tentacles when food is scarce, to regrow them once properly fed; some species of crickets have been observed eating their wings, while sea squirts digest their brain when growing from free-swimming larvae to fixed-living adults. Reabsorption of cells is present in every living organism.
Kleptoparasitism consists in stealing from another animal the food it has found, catched or produced. It's found in several spiders and insects (wasps, flies, heteropter bugs, ants, etc.), and in birds such as gulls, frigatebirds and skuas, which often take with force fish from other seabirds. Among mammals, hyenas and lions often chase other predators (often each other) away from the killed preys. Usually, kleptoparasites steal from closely related species (Emery's Rule).
Trophallaxis is the mouth-to-mouth (stomodeal) or anus-to-mouth (proctodeal) transfer of food. It's almost always intraspecific and used as a method of food sharing: it's most developed in social insects (bees, wasps, ants) and also in vampire bats; wolves and many birds use trophallaxis to feed their young. In bees and ants is also a form of communication.
Digestive System[]
Respiration and Circulation[]
Respiratory System[]
The simplest way to convey necessary gases inside the body is diffusion - letting the molecules spontaneously pass through the external boundary of the body. Diffusion requires a high surface/volume ratio: all unicellular organisms breathe through the membrane, and the smallest invertebrates, such as mites, through their skin. Amphibians also absorb oxygen through the skin, but this forces them to keep it constantly wet, to remain small-sized and to limit expensive activities such as movement, intelligence and internal thermoregulation. Diffusion through skin is actually more effective in air than in water, since water can carry only a smaller amount of dissolved gases[15]; oxygen, or any other gas, is easier to absorb with a higher partial pressure, which is directly proportional both to the atmospheric pressure and to its concentration in the air.
Plants have a very low metabolic rate, but they need a large amount of carbon dioxide to synthetize glucose. Air is brought in through stoma, microscopic "mouths" found on the surface of leaves and stems, which can be opened (through osmotic swelling of the constituent cells) and closed to limit the loss of water.
The first specialized respiratory organs developed by animals are gills: strands, plates or branches of tissue rich of blood vessels. The thin and moist gill covering allows a very effective and localized diffusion; locally increasing the surface/volume ratio allows the animal to develop a large-sized body. The gills of crustceans and polychete worms look like feathers, while fish have overlapping laminae between which there's a constant flow of water. Several groups of arachnids have book lungs, laminae very similar to fish gills, found inside a chamber filled with hemolymph (a blood-like fluid).
Other arachnids, myriapods and insects employ tracheae, a system of tubes that bring air from opening on the outside (spiracles) carrying oxygen and water directly inside the tissues, branching between the cells instead of dissolving it in hemolymph. This system is effective only at small sizes; in some insects, such as bees and grasshoppers, aerial sacs actively pump air inside the body, controlling its flux. Aquatic insects have bristles around the spiracles that retain air bubbles (another effect that wouldn't work on a larger scale), the surface of which allows an exchange of gases with moving water.
Diagram of bird respiration
Land vertebrates, finally, have developed lungs from a diverticulum of the digesting tract: they're sacs with a finely folded inner surface, rich in blood vessels, which allow again a localized diffusion. Lungs are directly connected to the heart (see below), to pump oxygenated blood all around the body. An inner respiratory system such as this, with only a small opening on the outside, allows a fully impermeable skin and thus good thermic insulation. Mammals employ "tidal respiration": the two lungs expand and contract simultaneously, drawing air in and then pushing it out through the same passage. Hermit crabs and snails also have lung-like structures.
Birds have a more refined system: different aerial sacs work as bellows, drawing in air that flows from the posterior air sacs to the anterior air sacs flowing through the lungs. These contain parallel tubes (parabronchi), that are then closed by a valve; the aerial sacs contract, and air is expelled without passing through the parabronchi again. This means that air flows in them always in the same direction, without pause: because of this system, bird respiration is much more efficient than mammals respiration, and sustains a higher metabolic rate (see above).
Circulatory System[]
In smallest organisms, especially if they have a flat shape, diffusion can also take care of the distribution of water, mineral salts, food, etc., but as size increases a system to carry chemicals around the body, preferably in liquid form, becomes necessary. Cnidarians such as jellyfish have a branched gastrovascular cavity which occupies most of the body volume, used both to digest food and to disttibute it to the tissues.
Arthropods and some mollusks have an open circulatory system: there is a large body cavity, the haemocoel, filled with a watery fluid (haemolymph) containing nutrients, oxygen, carbon dioxide, organic compounds and ions (Na+, Cl-, K+, Mg2+, Ca2+), in which all internal organs are bathed. In insects, haemolymph is moved around by a number of hearts connected by a dorsal vessel (aorta); in crustaceans, a single heart is enclosed in a pericardial sinus, which also helds gills.
Generic two-chambered fish heart
Ringed worms, cephalopods and vertebrates have a closed circulatory system: all the blood is held and carried by closed vessels that branch into cell-sized capillaries to reach each cell, and cyclically pumped by one or more hearts. In fish, each heartbeat pushes the blood through one cycle: through gills to receive new oxygen, and then through all the body; the fish heart has four chambers (sinus venosus, atrium, ventricle and conus arteriosus, see image on the right), though it's considered a two-chambered heart because only the atrium and the ventricle pump blood. Land vertebrates have a double circulation, in which blood is pushed through the lungs (by the right side of the heart), then is drawn to the heart again, and then pushed in the rest of the body (by the larger left side).
Lungfish and amphibians have a muscle wall that divides the atrium in two parts; in lungfish it also partially separes the halves of the ventricle. This allows the separation of the oxygenated and unoxygenated blood, and thus improves the efficiency of oxygen distribution (in amphibian the separation is less clear, probably because the skin respiration produces more oxygen anyway). Crocodiles, birds and mammals have two completely separated atria and two completely separated ventricles, though in crocodiles the foramen of Panizza connects the two circulation to regulate blood pressure during immersion in water.
Since blood cannot carry much more dissolved oxygen (or any other gas) than water, animals have molecules, often enclosed in specialized cell, that capture oxygen atoms thanks to highly reactive metals, most commonly iron and copper. See here for more details and more exotic possibilities.
Blood pressure, exerted by the heart(s) to push blood in the most distant extremities, can be computed[2] as a funcion of the vertical difference between the heart(s) and the highest point of the body, typically the head, with this formula: p = 1.203 + 0.377·log(h), where h is the height difference in mm and p is expressed in mmHg. Of course, it will be directly proportional to the planet's gravity. See above a similar formula to compute the heart rate.
Note: while height differences are the single most important factor to determine blood pressure, an animal with its head at the same height as the heart will still have a non-negligible blood pressure.
↑At 1 atm and 14°C, a litre of water contains 10 mg of oxygen. The amount of oxygen dissolved in water, in mg/L, can be computed as D = (515·P·c)/(35+t), where P is the atmospheric pressure in atm, c the oxygen concentration relative to modern-day Earth (21%) and t the temperature in °C. This formula works best between 0°C and 30°C; with a temperature between 30°C and 50°C, it becomes D = (628·P·c)/(49+t). Remember that with glucose oxidation each gram of oxygen (700 mL at 1 atm pressure) yields 3.5 kcal, or 14.8 kJ.
