Ecology: The Biology of Interaction. V-19. Adaptations of Organisms

Appendices: Curriculum. Questions. References. Personalities. Glossary. R commands.

V-19. Adaptations of Organisms
For living organisms whose entire organisation is deeply pervaded by correlations and interdependencies, the compromise between the conflicting demands of optimising different adaptive functions must be particularly acute. A stable epigenotype (a reliable system of ensuring individual development for a given set of conditions) must therefore be organised according to the principle of a deeply worked-out compromise between the conflicting requirements of maximising all adaptive functions.
A. P. Rasnitsyn
Many living organisms attain perfection with respect to one parameter or another. Let us consider only the most widely known examples. You are aware of how perfect the eagle’s vision is, the dog’s sense of smell, the tortoise’s longevity, the swift’s speed, and the camel’s endurance. Why do these remarkable qualities occur separately rather than being combined in a single organism? Presumably because they are poorly compatible with one another. As regrettable as it may be, the improvement of adaptations along one parameter entails a reduction in the level of fitness along others. By gaining in one respect, organisms are inevitably compelled to lose in another.
You are no doubt aware that organisms which provide parental care produce far fewer offspring than those which leave the next generation unattended. The smaller the roe of fishes, the greater the number of eggs (all else being equal) that each female will shed. To understand why this occurs, one may assess the amount of energy that each species expends on reproduction. In doing so, one will find that the costs of producing offspring are approximately comparable across different species. This quantity of energy may be spent on producing a small number of energy-rich eggs or a multitude of small ones; beyond egg production, energy may be spent on caring for the eggs and fry (nest construction, protection, feeding). In each case the reproductive characteristics of a given species represent a compromise between the necessity to leave offspring and the necessity to conserve energy. For a given expenditure on reproduction, the strategy of each species is a compromise among the various forms of such expenditure.
Are the conditions in which organisms live always optimal for them? Of course not. Unfavourable conditions may characterise a particular habitat permanently, or they may arise periodically (over the course of a day, a year, or a solar cycle) or irregularly (for example, weather anomalies). How do organisms respond to an adverse environment? They develop one or another form of adaptation to it.
Adaptations are adjustments to particular environmental conditions, manifested in accordance with the morphological, physiological, and behavioural characteristics of an organism and with its mode of life under specific environmental conditions. Adaptation may be regarded as the correspondence between the characteristics of an organism and the features of its mode of life in a given environment. Adaptations are observed as a consequence that can be investigated through the purposiveness of living organisms and other biosystems (according to Aristotle, the problem of purposiveness is the central problem of biology). The same term is sometimes used for the process of developing adaptations, which may introduce a degree of terminological confusion.
Preadaptations are characteristics that arose as adaptations to one factor but prove useful for overcoming the effects of another. The lungs of the first tetrapods arose as an organ for breathing air from organically rich water, while the limbs with digits developed as a means of locomotion in shallow water. Together they proved to be a preadaptation for the colonisation of land.
If environmental conditions are extremely adverse, resistance to them proves impossible. For example, neither amphibians nor reptiles are found in either the Arctic or the Antarctic—overcoming the cold proves impossible for these poikilothermic animals. Under less severe environmental conditions, three groups of adaptations of organisms to unfavourable conditions can be identified: overcoming, avoidance, and endurance. Let us consider each in turn.
Overcoming. We noted that neither amphibians nor reptiles occur in the fauna of polar regions. Mammals and birds, however, are numerous there. The principal pathway of their adaptation to severe cold consists in the active maintenance of a constant body temperature. Even if the greater part of the food energy is expended on maintaining a constant body temperature, if the remainder suffices for the satisfaction of basic vital needs, survival among perpetual ice is possible.
Beyond cold, one of the principal problems limiting the diversity of Arctic fauna is the near total absence of plant food on land. Higher plants require prolonged periods of positive temperatures for development; where these are absent, all terrestrial photosynthesis is associated with algae and cyanobacteria dwelling on the surfaces of rocks and ice. Polar waters, by contrast, are rich in life, especially animal life. Owing to its extraordinarily high heat capacity, water prevents both aquatic animals and algae from freezing. The good solubility of gases in cold water provides favourable conditions for their functioning. Thus many terrestrial mammals and birds of polar regions subsist on the productivity of the sea. One of the examples of remarkable adaptations to life in an extraordinarily inhospitable environment is the polar bear.
Among the order Carnivora, representatives of the family Ursidae are characterised by the highest proportion of plant food in their diet. The sole exception is the polar bear, an exclusively carnivorous animal. Certain bear species (such as the brown bear) are capable of supplementing their diet with fish caught in rivers (for example, salmon during their spawning runs), but they feed predominantly on land. The polar bear’s diet is associated primarily with the sea, and this predator has perfectly mastered swimming and diving in icy water. For the polar bear, “dry land” is to a considerable degree the ice floating on the surface of the Arctic Ocean (regrettably, the reduction in ice extent due to global warming is proving catastrophic for polar bears). The large body size, the thick layer of subcutaneous fat, and the high metabolic rate allow the bear to maintain a constantly elevated body temperature during the most severe frosts. Even the white fur of the bear is a marvel of adaptation: its colour camouflages the bear on ice and snow, while its permeability to solar radiation allows light to pass through to the animal’s dark, heat-absorbing skin.
As one may appreciate, the case examined above is far from the only example of organisms overcoming adverse conditions. And what is human behaviour and the human way of life, if not the overcoming of countless environmental constraints?
Avoidance of adverse conditions. The forms of avoidance of adverse conditions are diverse. One may consider that the marmot retreating from rain into its burrow, or the desert lizard moving around a saxaul shrub in accordance with the movement of its shade, are avoiding conditions unfavourable to them. On the other hand, these forms of behaviour are better regarded as the selection of favourable, preferred conditions. What is essentially occurring is that within the permanent home range of a given animal, local conditions change, and the animal selects those that are most suitable for it. But what is to be done if conditions throughout the entire home range have deteriorated? One of the most widespread “solutions” is migration.
Migrations are defined as regular movements of animals between different habitats, separated from one another by distances that are considerable for the animals in question. The qualification regarding the significance of distances for the animals concerned is not accidental. Migratory birds may travel to the opposite side of the globe, while migratory soil invertebrates may move from the leaf litter layer to a depth of several tens of centimetres. Both may be regarded as migrations proportional to the locomotory capacity of the animals under consideration.
As is evident from the examples given, migrations may be horizontal (geographical) or vertical. Depending on whether the cause (regularly operating or irregular) that gives rise to these movements is regular or not, migrations may be divided into periodic and non-periodic.
The most widely known category of migration is the seasonal movement of birds. For the majority of our migratory birds, the cause that compels them to set out on their journey is winter food scarcity (while the signal factor that triggers the corresponding behaviour is the shortening of day length). Among the longest migrations are those of the Arctic tern—a bird that nests in the Arctic and winters in the Antarctic (where the “warm” season occurs during the Northern Hemisphere winter). The length of this migration (in one direction alone) exceeds 30,000 km! In cases where changes in conditions are not very abrupt, birds may undertake not true migrations but nomadic movements, during which they remain within a single region and move about in response to weather or the availability of appropriate food.
Seasonal migrations are also characteristic of many ungulates of Africa and Central Asia. Their cause is the change in moisture levels and the availability of accessible food.
Vertical migrations are typical of the inhabitants of mountains, soil, and especially bodies of water. Thus daily movements to the surface at night and to depths of tens and hundreds of metres during the day are highly characteristic of zooplankton. The food of zooplankton is phytoplankton. The distribution of phytoplankton is associated with the surface layers of water—at greater depths there is insufficient light for photosynthesis. In order to feed on phytoplankton, zooplankton must be in proximity to it. During the day, however, the surface waters expose one to easy predation by fishes that rely on vision for feeding. Therefore many planktonic animals, including crustaceans, conceal themselves in the darkness at depth during the day and ascend to the surface at night in pursuit of their own food. Incidentally, these movements make a rather significant contribution to the mixing of ocean water layers, influencing the cycling of many biogenic elements.
It should be noted that migrations are not always a form of avoidance of adverse conditions. For example, anadromous fishes are characterised by spawning migrations from marine to freshwater environments (anadromous migrations, as in salmon) or from freshwater to marine environments (catadromous migrations, as in eels). Of course, these migrations may also be viewed as avoidance of adverse conditions (seawater is unfavourable for the eggs and larvae of salmon, while freshwater is unfavourable for the early developmental stages of eels), but on the whole these migrations more accurately reflect the evolutionary history of the species and its strategy of adaptation to its habitat. Such migrations are termed ontogenetic. The ontogenetic migrations of sea turtles, which in their journeys may circumnavigate the globe, are not inferior in extent to the migrations of Arctic terns.
Endurance of adverse conditions. We have already discussed how difficult it is for poikilothermic tetrapods to live under conditions of severe cold climate. Nevertheless, some amphibians and reptiles have adapted to life beyond the Arctic Circle. The Antarctic is surrounded by ocean, whereas both Eurasia and North America extend into the Arctic. For example, the ranges of the common viper and the viviparous lizard extend beyond the Arctic Circle. However, the species that penetrates furthest north among all amphibians and reptiles (to 72° N latitude!) is the Siberian salamander. This small (approximately 15 cm including the tail) caudate amphibian crosses the Arctic Circle in the Northern Urals, in the Magadan region, and in Yakutia. Siberian salamanders inhabit even the environs of the “old pole”, the Yakutian city of Oymyakon! What characteristics help them survive in such severe conditions?
First and foremost, it is the capacity to tolerate cold. These animals withstand being frozen into permafrost (the recorded record for time spent in a frozen state is 90 years) and remain active at 0°C. The capacity to endure cold is associated with a low metabolic rate, cold-tolerant enzymes, and the presence in tissues of cryoprotective substances that inhibit the formation of ice crystals, which disrupt and desiccate cells.
If environmental conditions are unsuitable for active vital activity, many organisms can enter one or another physiological state in which their capacity to withstand the effects of adverse factors is enhanced. Such states include anabiosis (or cryptobiosis)—a state in which vital processes are so decelerated that they proceed without any external manifestations. Anabiosis is an adaptation to cold, desiccation, and other adverse factors; when conditions improve, the organism returns to physiological normality.
For example, when mosses and lichens are moistened, tardigrades—eight-legged invertebrate animals, representatives of the small phylum Tardigrada belonging to the same evolutionary lineage as the arthropods—become active. The length of these animals is most often measured in fractions of a millimetre. Moving unhurriedly through moss, tardigrades feed on plant cells, bacteria, or invertebrate animals. Tardigrades are characterised by exceptional resistance to adverse environmental factors in the resting stage. When the moisture content of the moss decreases, the tardigrade contracts, opens its mouth, withdraws its limbs, and enters the “tun” stage—it enters anabiosis. In this state the tardigrade is capable of withstanding prolonged desiccation, low temperatures, the action of toxic substances, vacuum, high doses of radiation, and many other adverse factors. In the “tun” stage, tardigrades can probably even survive a journey through open space! Animals may spend the greater part of their lives in this stage. In rarely moistened locations the lifespan of a single individual (less than a millimetre in length!) may probably exceed 100 years! Upon entering a moist environment, the tardigrade exits the resting stage and becomes rather sensitive to many adverse environmental factors.
Some animals, for example heterothermic mammals and birds, are capable of entering hibernation—a deceleration of vital processes accompanied by a significant reduction in body temperature and a decrease in heart rate. Nevertheless, during hibernation, unlike in anabiosis, manifestations of vital activity remain detectable. Hibernation may be seasonal (in marmots, hedgehogs, and others), daily (in bats and hummingbirds), or irregular, in response to an unexpected deterioration of conditions (in swifts and raccoon dogs).
Paradoxically, bears are often cited as an example of animals that enter anabiosis or hibernation—states that are in fact uncharacteristic of them. Bears (like badgers) endure the cold in a state of winter sleep (simply a deeper sleep than normal), not anabiosis. These animals are sufficiently large (recall Bergmann’s rule) to maintain a constant body temperature for prolonged periods without receiving energy from food. A hunter who stumbles into a bear’s den will quickly rouse it. Female bears give birth to cubs and suckle them with milk while in this state! Unlike the bear, a marmot in true hibernation, for example, has an extremely reduced body temperature and a heart rate decelerated many times over, and can awaken only after sufficiently prolonged and energy-costly physiological processes (heat production by brown adipose tissue cells).
Hibernation (and also torpor) is the term commonly applied to the state in which amphibians and reptiles of temperate latitudes overwinter. For example, the European pond turtle dives to the bottom of a body of water in autumn (in October–November), from which it will not resurface until spring (in April). The turtle’s physiological processes are sharply inhibited, and for its vital activity it is sufficient to exchange gases through the surfaces of the oral cavity, pharynx, and cloaca. A turtle in this state can display a certain degree of activity (for example, moving from one spot to another on the bottom of a body of water).
The Central Asian tortoise in the deserts of Turkmenistan emerges from its winter hibernation (hibernation) in February and remains active until late May. When the desert grass desiccates, the tortoises enter summer dormancy (aestivation). In late September to early October, autumn rains may briefly stimulate a small amount of herbaceous vegetation. During this period the tortoises briefly emerge from dormancy. If there is insufficient rain, aestivation transitions into hibernation (a regime of reduced activity at high body temperature gives way to one at low temperature). Under particularly severe conditions the total duration of the active period of the Central Asian tortoise does not exceed 3 months!
Hibernation is of course not part of the normal human life cycle, but our fairly distant ancestors were probably capable of it. Even today, certain mammals that do not normally hibernate can be artificially induced into this state. For example, by compelling laboratory mice to inhale a gas mixture containing hydrogen sulphide, one can induce hibernation in them (and subsequently bring them out of it by ventilating their lungs with an oxygen-enriched mixture and warming their bodies).
From a number of accidents it is known that a person can be restored to life if the body cools under conditions in which the thermoregulatory centre does not attempt to maintain normal body temperature. This can occur during abrupt chilling (a child falls through ice), in a carbon dioxide atmosphere (a refrigerated lorry driver accidentally confined himself in a compartment with “dry ice”—solid carbon dioxide), or under certain other, insufficiently studied conditions. People who have frozen (but have not cooled to negative temperatures) in such a state may, given appropriate rescue measures, be revived even several hours later.
The potential capacity of humans for physiological inactivity has provided the basis for the idea of placing cosmonauts in such states during long-duration spaceflight. Without entirely dismissing this idea, it should be noted that it currently remains in the realm of pure science fiction.
For the purpose of extracting money from wealthy clients, some Western countries offer the service of post-mortem freezing (cryonics). Immediately after death, the body is deeply cooled and immersed in liquid nitrogen (−196°C). It is then placed in a Dewar vessel (essentially a thermos), where it is stored in nitrogen, with fresh nitrogen added to replace that which evaporates. This activity is typically funded from the interest on capital that the frozen client has left to the company responsible for storing the body. The bodies of clients, rigid with cold, are kept in special repositories in a vertical, head-down position, so that in the event of a drop in nitrogen level, the feet rather than the head will be affected. One is expected to take on faith that when, in the future, people learn to cure the diseases from which the frozen clients of such companies died, they will thaw their bodies, cure the consequences of the fatal diseases, and grant them the opportunity for a new life in a happy future. Leaving aside the extraordinary difficulty of predicting the capabilities and actions of our descendants, the body-freezing technology employed by such companies probably results in irreversible destruction of the cells of those whose fear of death has manifested in such a peculiar form.
Finally, resting stages of the life cycle may assist organisms in enduring adverse periods. These include cysts of protozoa, spores of bacteria, fungi, and some plants, seeds of plants, resting eggs of animals, and many others. Beyond enduring adverse conditions, resting stages may also perform other functions, such as dispersal. For example, the eggs of tadpole shrimps (crustaceans of the subclass Branchiopoda, which also includes the more widely known water fleas, Daphnia) serve both to endure the dry season and to disperse by wind-borne dust.
Many plants exhibit the characteristic that when favourable conditions arrive, their seeds do not all germinate simultaneously but instead form a resting soil seed bank. Even if a catastrophe destroys a forest, if the forest soil is preserved even somewhere, it will harbour seeds from most populations of flowering plants that formed that forest (as well as spores of the bacterioflora—the community of bacteria—and mycobiota—the community of fungi—characteristic of that forest).
In concluding this review of detailed examples, we note that the capacity of organisms to endure adverse conditions is of extraordinary importance for maintaining the observable state of the biosphere.
Additional materials:
Column: On skin colour: an attempt at discussing one human adaptation with justification of far-reaching conclusions about the specifics of the action of ecological factors
Column: More on skin colour: why we are white and why we tan