We can now turn to the second question raised by our new, capability-oriented approach. Must organs – units that deliver a capability – be permanently attached to the organism that they serve?
As long as organisms are viewed as physical phenomena whose special features differentiate them from inanimate objects, it is perfectly understandable why we consider unattached elements as not being bodily components. On the other hand, one can view organisms as entities whose physical structure is not an end in itself, but merely a means or precondition for certain capabilities that enable a steadily expanding process, namely life. In this case, there is no reason why organs that are separate from the body should not exist. If they deliver a required capability then they are part of the body, whether attached or not. It is common knowledge that human progress is largely built upon units that are separate from the cellular body. Does this necessarily mean that the organs of organisms are fundamentally different from man-made technical, economic, governmental and cultural structures?
In the animal kingdom, many species use secretions or environmental material to form "additional organs" that are not fused to the body. Before we evaluate humans and their products from the evolutionary perspective, it is perhaps appropriate to more closely examine such "predecessors". The examples I list are well known to science, but are viewed here for the first time in an evolutionary context in which natural selection acts on the entire capable entity rather than merely on the material structure of the organism itself. An impressive example of an additional organ is the web produced by many species of spider. The web represents a trap which helps these animals decisively improve the first two important capabilities of all living organisms, namely the gain of energy and vital substances.
Today, many spider species still hunt in the original manner without a web. Their prey, mostly insects, is overpowered by a rapid lunge or leap forward. In the course of evolution, species arose that were able to secrete threads from silk glands and construct webs. This development peaks in the round-web spiders, of which the cross spider is a member. They possess six different gland types located in paired spinnerets on the abdomen; the latter are supplied by no less than 800 individual glands. The different techniques used to construct the webs are innate.
The cross spider begins the process by stretching its abdomen upward at an angle and producing a thread bearing a fan-shaped terminal expansion. This allows the thread to be wafted away like a loose sail in the wind. Should the loose end stick to a solid object such as a branch, then this establishes a bridge which is the fundament for a future web. The renowned biologist and Nobel Prize winner Karl von Frisch wrote: " If the thread fails to attach itself to a solid object, then the spider reels it in and devours it in order not to waste the silk material. The spider then tries its luck again".
The technique used by the spider, whose eyesight is not well developed, has been studied down to the finest detail in many hours of patient observation. It initially involves a basic scaffolding attached to solid surfaces, followed by the production of the framework and spokes. The process is a rigid, innate behavior, but may be somewhat modified by local conditions. In the center of the web, the spider builds a platform (the so-called hub), from which it will later operate. While the initial phase involves non-adhesive threads, across which the spider can run with impunity, the final phase involves adhesive threads: the spiral threads running across the spokes represent the actual trap. The spider then lies in wait at the hub, with one leg touching a spoke at all times. If an insect flies into the web, the spider can determine its location based on the type of vibrations emitted by the hapless prey. The next step is to reach the prey as quickly as possible without, however, itself touching the adhesive threads and thus becoming a victim of its own trap. The prey is then immediately sprayed with a bundle of very fine threads ejected from additional glands, bitten with poison gland-containing fangs, and rapidly twirled around to wrap it in non-adhesive threads. The prey is then encased in silk and this food packet is cut from the net and transported to the hub, where it is suspended by a short filament. The spider extracts the substances and energy contained in the packet by injecting digestive juices with its mouthparts and later sucking up the dissolved nutrients. In this sense, it uses the insect's own armor as a digestive tract. During the night or in rainy weather, the spider seeks shelter at the margin of the net, where it remains informed about events in the web by vibrations of the silk threads.
Since the threads are not sticky for long, the web must be frequently renewed. The spider accomplishes this by consuming the threads and using the recycled material to build the next web. Whenever a male spider approaches a female’s web, he sends species-specific signals by plucking at the web; this allows the female to distinguish a potential mate from struggling prey.
I have gone into such detail here in order to point out just how many advantageous mutations were necessary for the innate programs behind such a complex device and such differentiated behavior. Before any change in the genome by mutation and sexual recombination of the genes becomes permanent, it must be accepted as progress by natural selection. The inescapable conclusion is that such a large number of innate program steps could never have developed if the web had not represented a decisive advantage for the spider. Even though the web is not firmly attached to the body, it is by no means far-fetched to view this complex device – whose production is, after all, coded in the genotype – as equal to any of the spider’s other organs.
An even stronger argument, one that cannot be easily brushed off, is the indisputable fact that a permanently attached web would be useless to the spider. This would thwart the prey-capturing process: the spider would neither be able to build the web nor use it to ensnare insects. Although it is too early at this stage to draw parallels to mankind and its artificially produced technical aids, it is worth mentioning that the above is equally true for all of our tools. If an ax, a pair of pliers, a pitchfork or a ladder were permanently attached to our bodies, they would surely do more harm than good.
A decisive advantage of functional extensions that do not inherently belong to the body is that they neither burden nor hinder the animal when not in use.
I would like to introduce a third argument here. The spider’s trap serves a particularly important task, namely energy gain. As mentioned above, no life process is possible without energy. For this reason, no organ composed of cells can fulfill its tasks without energy input. This applies equally to the spider’s organs. Thus, it is difficult to understand why the term "organ" should be rejected for the very unit that is the precondition for the function of all others. If one hinders round-web spiders from building their nets or repeatedly destroys these, then the spiders are doomed. Even if they manage to obtain food by other means for a while, they are at a clear disadvantage to free-foraging spiders and other competitors. These species cannot survive without their webs.
We may initially resist accepting such external units, which are not attached to the spider’s cellular body, as additionally formed organs; nonetheless, natural selection clearly argues against any such narrow interpretation based on subjective impressions.
Other tropical and subtropical spider species construct even more technically elaborate trapping devices that help shed light on our topic. The trap-door spiders (Ctenizidae, Actinopodidae and Barychelidae, Fig. 1A) coat holes that they either find or dig themselves with very thin, strong threads; they also use these threads to form a cushion-like lid which fits the conical entrance so tightly that it fully shuts out light and water. It is attached to the burrow with a hinge made of silk. Some species span threads in all directions from the opening: these tip the spider off about approaching prey (insects, millipedes and other small animals). Most of them, however, make do without such trip-threads and rely solely on their highly developed sensory organs to pick up telltale vibrations.
Fig. 1: Two examples of additional organs that animals form based on innate behavior control mechanisms. Both serve in prey capture.
A shows a trap-door spider, which uses silk threads to build a rigid tube inside holes. Only the males leave these structures to mate. The tube has a round opening with a lid that is also fashioned from silk. The spider lies in wait behind the lid, which it holds slightly ajar. If a small insect approaches, the spider flips the lid open in a flash, grasps the prey and drags it back into the tube, closes the lid, and devours the animal.
B shows the larva of a caddis fly (see arrow), which lives in
streams and constructs a net-like trap with silk threads. This is anchored
on aquatic plants and twigs; water currents hold it open and carry food
into the funnel. The larva takes up its position at the bottom end of the
funnel, where it is well protected against fish predators. From time to
time it wanders across the net and feeds on items trapped in the mesh;
after v. Frisch, 1974.
During the day, the spider holds the lid tightly shut with its palps and front legs. At dusk, the lid is opened slightly. If prey approaches, the spider flips the lid up in a flash and lunges forward, whereby the claws on its hind legs usually anchor it at the tube opening. It grasps, bites and drags the prey into the hole, shutting the lid as quickly as it was opened.
Interestingly, trap-door spiders spend their entire lives (some species live to be 10 years old) in these pitch-black burrows. The aranologist Wolfgang Crome reported that Conothele arboricola, which inhabits treeholes in the Bismarck Archipelago, shapes the size of its hole to fit the maximum size it can reach. As the spider grows, it never needs to change or enlarge its burrow. Only the males leave their hiding places to mate; they seek the burrow of a female, which proceeds to devour the male after copulation. The young remain with the mother for up to three years.
In these species, the separation between the trapping organ and body is less distinct than in web-building spiders. Their burrow represents additional armor, the perfectly functioning lid a highly efficient "body part" designed for camouflage and deceit. When it sheds its skin, the spider hermetically seals the burrow until the new cuticle has hardened.
The aranologist Harro Buchli studied the behavior of trap-door spiders in the Mediterranean region in 1969 with an automatic recording system. He set his equipment up next to a wall whose crevices contained a burrow of Nemesia caementaria: it recorded the spider’s every activity over a full year. During this time, the animal hunted on 252 nights, whereby it opened the lid a crack shortly after sunset, assumed an attack stance, and remained frozen in this position until dawn. The average time spent hunting per day was 8 hours and 37 minutes, with
the spider taking 5 breaks lasting a total of 2 hours and 45 minutes. On very cloudy days, the spider started earlier and extended its hunting activity by up to 4 hours in the morning as well. The longest uninterrupted lie-in-wait measured 12 hours and 57 minutes on an October night. From a total of 724 attack events, just above 10% were successful. Nemesia caementaria deposits the indigestible remains of its prey at the rear end of the burrow and covers them with silk. Other species form such remnants into balls, cover them with silk as well, and eject them from the tube opening.
In this animal, it is somewhat easier to view the additional organs formed by the cell body as being something that is not separate or different. Capability is increased through additional units; the fact that these were produced on instruction of the central nervous system rather than by cell differentiation hardly represents an insurmountable conceptual barrier, especially since natural selection clearly evaluates the capability of the entity as a whole.
Some trap-door spiders use small stones, twigs and leaves, i.e. environmental material, to camouflage their tubes if these are exposed. Suitable organic and inorganic objects from the immediate surroundings are spun around the tube and thus transformed into functional components of the capable entity. This also maintains the impression of unity that appears to be so important for our conventional categorization. In my opinion it is therefore incorrect to view these structures - which are not attached to the body - as being something principally different from organs that develop by way of cell differentiation.
Up to now, the use of the term "organ" in the biological sciences has been restricted to units formed of or by cells. According to the cell theory of Schleiden and Schwann (1839) the cell is defined as the "basic unit of living systems". In multicellular organisms it represents the "basic building block of animals and plants". Even if we should have to modify this dogma in the future, it may not be advisable to redefine or question an established term like "organ" without a compelling reason.
On the other hand, the two cases described above (to which I shall add more examples later) clearly show that cellular organs are by no means the sole or ultimate criterion for natural selection. We have failed to recognize that the formation of units of capability in animals is not restricted to one, but to two methods. Such units can arise by means of cell differentiation or by a more complex, indirect pathway. In the latter, the genome instructs the multicellular brain not only to develop programs for innate behavior, but also programs with which the animal forms additional functional units that are not firmly linked to the body. Simply put: in the first method the genome of the cells induces these to form organs directly, while in the second method it induces the highly specialized brain, consisting of billions of cells, to form additional organs from inorganic material.
In earlier publications (1969, 1970 and 1978) I used the term "artificial organs" to describe the latter because rather than arising by the "natural" process of cell differentiation, they are "artificially" formed products of the overall body. Experience showed that this term was suboptimal and prone to misunderstanding. After all, these additional structures are no less "natural" than organs arising via cell differentiation. Just like the latter, they are also based on instructions coded in the genome. In the first case, the building blocks are consumed as food and then used to build up organs. In the second case, the brain is given an additional task beyond controlling the body, namely to use secretions or environmental materials to form vital tools (Greek: "organa") that are separate from the body. We have already noted that this separateness enables capabilities that cellular organs could never deliver. An additional advantage, which we will deal with later, is that they need not be nourished by the bloodstream, nor need they be linked to nerve cells.
In my opinion, the term "additional organs" more aptly describes the essence of this key step in evolution. The fact that this second method is relatively rare and first appeared in more highly developed organisms is easy to explain: the mechanism involved is considerably more tedious. Specifically, it requires a two-fold transfer of information: first, from the genome to the behavioral programs stored in the brain, second from these programs to the executing organ. Furthermore, the first method produces only a single specialization, not a series of simultaneous, exchangeable specializations as in the case of humans. This will be discussed in more detail in later chapters. The bottom line, however, is that the product of this second method of organ formation in animals is as natural as the first. Such additional organs increase capability and improve fitness in the natural selection process. Thus, they promote the vitality of the life process just as efficient cell organs do.
These two examples allow one more conclusion to be drawn. The trap-door spiders, in particular, spend their entire lives in their additional organ (which has a protective, feeding, and reproductive function) and clearly demonstrate one point: a distinction must be made between the cellular (somatic) body and the capable entity itself. Our senses perceive and our brain categorizes the cellular body as the very organism itself. The key criterion for natural selection, however, is the capable entity. In all organisms that lack additional organs, this entity is identical with the cellular body. In those that enhance the capability of their cellular body with additional organs, this entity consists of the cellular body plus additional organs.
This unaccustomed perspective requires rethinking accepted
tenets. Additional examples of animal species that form such separate organs
may help promote this reorientation. Most of these examples are well-known.
Nonetheless, they have to date been viewed in the context of behavior patterns
and their consequences, without recognizing that they demonstrate a second,
important principle of organ formation in individuals. Each example drives
home the point that the essential unit is not the distinct cellular body
registered by our senses, but rather the capable entity, which is the sum
of all elements substantially contributing to overall capability.
The use of favorable environmental factors
We can begin by comparing two trapping devices formed by various insect larvae. The traps are quite different externally, yet comparing them sheds light on additional organs and their structure.
The first type of trap is formed by larvae of certain caddis flies (for example Hydropsyche) that inhabit slow-running streams. Much like spiders, they also secrete silk threads, although they use modified salivary glands opening at the mouth rather than at the hind end of the body. They construct funnel-like traps of fine-meshed threads that are anchored to aquatic plants and twigs. The water current keeps the trap opened and carries small organisms into the funnel, whose walls are periodically grazed by the worm-shaped larva. During the remainder of the time, the larva positions itself in the narrow funnel tip, where it is well protected against fishes and other predators (Fig. 1B).
The second trap that we can compare with that of caddis flies is well known to most children. This type is constructed along the forest edge or on embankments by the much more powerfully built larva of the ant lion (Myrmeleonidae). It digs funnel-shaped pits in flat, fine-grained sand surfaces. It lies in wait for ants at the bottom of the pit, with two-thirds of its body buried in the sand. Ants that venture too close to the funnel margin slide down into the conical pit. The larva foils their escape by bombarding them with sand grains that trigger miniature landslides which bring the prey within reach of its powerful jaws. The ant lion seizes, kills, and sucks out the victim, flinging the indigestible remains from the funnel.
Due to its oversized jaws, which make up one-third of the entire animal, the ant lion can only walk backwards. Accordingly, the bristles coating its body are directed forward. In building the funnel, the ant lion seeks a suitable site and proceeds to form a circular trench by walking backward. This involves wriggling into the sand and slinging sand in all directions with back-and-forth movements of its head and the anterior trunk segments. The gradually deepening circular trench defines the circumference of the funnel; the animal gradually removes the cone-shaped sand heap in the middle by turning ever-tighter circles and flinging the sand out. At the center of the circle (the deepest point of the funnel), the ant lion comes to rest and begins to lie in wait for prey. The same jerky movements that cast the sand out are now used to target ants attempting to flee from the funnel; this movement also serves to remove ant remains from the funnel. In the third year, the ant lion forms a spherical, sand-encrusted cocoon from which the delicate adult, which has a wingspan of 3 to 5 centimeters, emerges.
Viewing this sand funnel as an integral component of the ant lion – as its additional organ – no doubt presents considerable difficulty. While the caddis fly larvae along with the round-net and trap-door spiders construct their traps from material their bodies produce, the ant lion’s funnel is built of environmental material: we may initially be inclined to consider this as merely a useful environmental modification.
This represents an example of the fundamental capability we can term "use of favorable environmental factors". The common denominator of the sand funnel and the caddis fly trap is that environmental forces are harnessed to serve the species. The trap is kept open by water pressure, which also channels in microscopic organisms. The Earth’s gravity spells doom for ants in the sand funnel. Both animals use natural forces to reduce their own energy expenditure. More importantly, both cases functionally involve funnel-shaped structures into which outside forces direct food items. The caddis fly constructs its device with its own means, i.e. with thread production and innate behavior programs. The ant lion takes advantage of another favorable environmental condition: soil consisting of flat, very fine sand. It merely needs to form this sand, much like a potter would form a jug. Functionally, the key criterion is the required shape and a building material that prevents the prey from escaping. Sand has the ideal features. Thus, a second favorable environmental factor helps the ant lion build its trap, making it unnecessary for the organism to produce its own building materials.
In both species, the behavior programs behind these activities were achieved through mutation, recombination and natural selection. After all, the ant lion saves energy on two fronts. Viewed from this perspective, the sand funnel is very clearly an organ additionally formed by the animal to obtain food. It is an integral component of the capable entity we term ant lion, like the exposed stones that early man picked up and used as "ready-made" projectiles to subdue prey.
Favorable environmental conditions that can be used in their natural state include any kind of crevasse or cave that provides suitable shelter for animals and humans. Since they need not be reworked like the ant lion’s sandy substrate, we find it more difficult to view them as protective units that enhance the body’s capabilities. In this connection we should bear in mind that innate behavior is necessary in order to recognize such units as suitable protective organs. A case in point is the remora, which, as mentioned earlier, makes the shark into its protective organ. Humans, who rely on intelligence, may not immediately understand that for animals, even something as simple as recognizing a hiding place is not self-evident. Rather, it requires the corresponding programs, whether they be innate, instilled, or formed by previous experience.
As long as we consider organisms to be purely material
phenomena (the prevalent view today) it is in fact difficult to accept
a sand funnel or a natural cave as an integral part of an organism for
the duration of its use. The decisive factor for natural selection, however,
is competitive overall performance rather than physical shape or
behavior patterns. This performance can be achieved by very different methods
and body forms.
The great variability of additional organs
The best way to grasp the concept of additional organs and its validity is to examine a broad range of examples from this perspective. In all species, the ability to utilize favorable environmental factors is equally important in reproduction (a fundamental capability) as it is in repelling disturbing or hostile influences. In cases where the young are not fully developed at birth, protective structures that are not attached to the body become essential. Since cell differentiation alone rarely gives rise to such structures, additional organs formed by the cell body must assume this role.
Although many classes of animals contain viviparous species, these remain the exception rather than the rule. In the vast majority of species, the germ cell – provided with the appropriate nutrition and enclosed in a protective envelope – is discharged from the mother’s body as an egg and left to its fate. This is the case in most arthropods and most vertebrates, i.e. the fishes. In those cases where the progeny enjoy additional provisioning – from the mother, the parents, the pack, or from "states" (insects) – the number of brood care strategies are virtually unlimited.
From the functional perspective, the technically so ingenious honeycombs of bees and wasps are small, artificially formed protective units for embryos growing outside the mother’s body. The bees typically construct these from wax, which they secrete from special wax glands; other species use resin that oozes from trees. The combs of wasps, on the other hand, are built of wood fibers glued together with a cement produced by the body. The wall thickness of some of these combs measures a mere 0.0073 millimeters. The solitary pill wasp Eumenes forms delicate urn-shaped structures of clay. If the clay is too dry, the wasp tanks water in its stomach, spits it on the clay, and scrapes off enough to make a pill. This is then carried to the construction site and drawn out into a strip using the jaws and legs. A series of such strips is molded into a hollow sphere that is constricted distally to form the urn. Paralyzed larvae or caterpillars are squeezed through the opening as food for the larva. Before sealing the urn, the wasp forms a final pill from which it suspends an egg on a short thread. The freshly hatched larva can begin to feed immediately. The tropical South American oven bird also uses clay to form spherical containers with a side entrance. In the words of v. Frisch, they create "a chamber where none is provided by nature". Both the female and male work together to produce the structure. The task requires several weeks, as nearly two thousand small balls of clay must be transported to the nest as building material. Finally, the brood chamber, which is partitioned off by a wall, is cushioned with thin blades of grass.
The above examples all clearly show that it is principally irrelevant whether the additional organs are constructed entirely of material produced by the body (the bee’s honeycomb), only partially so (the wasp’s nest), or of foreign substances (the clay urns of the pill wasp). The decisive factor for natural selection or "the survival of the fittest" is an adequate brood protection function. This line of argumentation is cemented by the honeycomb toad (Pipa pipa), which forms similar protective units for its embryos on its back: these units consist of the toad’s own cells and are firmly attached to its body. In my opinion this is firm evidence that it is unjustified to interpret these protective units as components of the animal, and the others not.
In many cases, organs of other organisms serve as independent protective organs for progeny. Most bird nests, for example, are built of dead twigs and grasses. Living plant organs are also known to serve a similar function. The feces of other animals, environmental forces, and ultimately even the services of other animals can be utilized for this purpose. The tailorbird (Orthotomus sutorius) of southern China and India sews together large tree leaves using blades of grass which it pulls through holes it has punctured. The result is an open cone, which is subsequently filled with soft nesting material. The bird’s long, pointed bill functions as a needle for this complex task. The thread consists of silk filaments, bast, and cotton fibers that are twined into a thicker thread. A knot on both sides prevents the thread from slipping out. The beak and one leg work skillfully together to accomplish this feat.
A no less amazing counterpart is known in ants. In tropical South Asia, representatives of the genus Oecophylla form spherical or oval nests that are also made of living leaves joined together by a dense, silk-like tissue. The mechanism behind this nest building was initially unclear because only the larvae possess silk glands (in order to cocoon themselves after completing the growth phase). The riddle was solved when groups of workers were observed pulling together adjoining leaves. If the leaves were too far apart, these females formed chains, with one ant climbing over the other, its abdomen then being firmly held by the ant it crawled over. Other workers then bring a larva, which they hold in their jaws, and press them mouth-first against the leaf margins when these have been drawn close enough together. Jaw pressure induces the larvae to discharge their glandular secretion. The larvae therefore serve a two-fold additional function, once to produce thread and once as a weaver’s shuttle.
The weaver birds (Ploceidae) of Africa and Southeast Asia use their legs and beaks to construct particularly elaborate nests. The male grabs the margins of leaves and blades of grass, tearing off long strips as he flies away. These strips and other threads are then combined to form spherical nests that hang from trees like large fruits. The entrance is located on the lower side and often bears a tubular extension. Much like a basket weaver, the bird attaches its thread with a knot, forms it into loops, sticks the thread into the network, and pulls it out again at another position. The final product is a very durable home for the bird and its brood, one that also affords optimal protection. Once the nest is finished, the female is left to decide whether it meets her approval. If it stands the test, she will help complete the interior. If she rejects the nest, the male will destroy it after about a week: he undoes the knots and begins anew to construct an even better home for himself and his family.
The flying frog Rhacophorus reinwardti on Java builds an entirely different type of nest, although it uses living leaves as building material just like the weaver birds and tree ants. During the mating season, the males and females seek a large leaf along the shore of a river or lake, or choose a site between several smaller leaves. The eggs are deposited here and fertilized by the male. During this process, the female secretes a slimy fluid. After each egg is laid, the males and females stomp their feet in unison, whereby they dip their feet into the mucus and pat them together. After 30 to 60 minutes, 60 to 90 eggs lie in a foam mass measuring 5 to 7 centimeters in diameter. The female then proceeds to press the leaves against the foam heap, whose surface hardens and becomes glued to the leaves. Thereafter, the parents pay no further attention to their brood. During embryonic development, part of the foam becomes fluid, forming a small aquarium within the foam nest. The freshly hatched tadpoles can swim about here for several days until a stronger rainfall softens the outer layer of the foam nest and releases the young into the water.
During my "Xarifa" expedition to the Indian Ocean in 1958, our ship lay at anchor for nearly one month in an inlet of Grand Nikobar Island in the Bay of Bengal. We were the first to dive in these interesting waters and were able to observe a number of new phenomena. Directly under the boat, where our garbage began to collect on a flat, sandy bottom at 15 meters depth, I discovered a slightly gaping, upright cockle from which two eyes peered out. These eyes were much too highly developed for a bivalve. I brought the cockle to the surface and placed it in an a large aquarium, where it was soon surrounded by the many hermit crabs and other crabs living in the aquarium. It turned out that the cockle contained a female octopus (Octopus aegina), which had laid her eggs between the empty valves. She firmly attached herself to the inner surface of both valves with the suckers on her arms and was able to open and close the shell much like the living cockle had been able to do. Thus, the female octopus had transformed the unoccupied shell into her additional, brood-protecting organ.
In mouth-breeding fishes (for example certain catfish and cichlids), yet another strategy is used to protect the young. Rather than producing or using a structure that is separate from the body, an anatomical organ temporarily assumes a completely different role. In this case it is the mouth. Among the catfish, this method of brooding is done exclusively by the males. They safekeep the eggs deposited by the female in their mouths until the young hatch. The male Brazilian catfish Arius commersoni , for example, can accommodate between 30 and 40 eggs – each measuring 10 to 15 millimeters – in its mouth, forcing it to go without food for the entire brooding period. During this time they take no bait and their gut shows signs of degeneration. In mouth-breeding cichlids, the females provide the additional protective organ for the eggs; here, the freshly hatched fry dart into the mother’s mouth at the slightest threat. According to Eibl-Eibesfeldt they have developed an innate releasing mechanism to recognize this shelter, with the mother’s eyes playing a decisive role. "They attempt to gain entrance even into simple decoys of the mother’s head, orienting themselves according to the position of her eyes and heading toward a point between the two. Decoys whose eyespots are positioned on a horizontal plane are much more effective than when one eye is located on top, the other on the bottom".
In the Chilean bell frog (Rhinoderma darwini) the male takes the 10-14 yolk-rich eggs deposited by the female into his mouth and shifts them into the vocal sac, which opens into the floor of the buccal cavity. With this load of eggs, the bulging sac extends all the way to the back of the head. Inside this pouch, the eggs are arranged in two layers, one lying up against the dorsal wall, the other against the ventral wall. From there, they receive oxygen and apparently even food. The young remain here until after they have completed metamorphosis, leaving the sac as fully developed froglets. At this point, the brooding male has been reduced to skin and bones. von Frisch writes: "This is surely one of the most unique kindergartens in the animal kingdom. The father frog need not construct it – the kindergarten has already been provided as a gift of nature". In this example the protective function for the young is temporarily assumed by a natural organ formed of cells rather than by an additional organ. In marine turtles, on the other hand, the eggs deposited by the female are "entrusted" to the hot sand for brooding. Since these reptiles stem from terrestrial ancestors, their instinct drives them to return to land to lay their eggs. They arduously crawl up the beach, dig a pit with their hind flippers, and deposit their eggs in an egg chamber which is then covered up again with sand. Sand and sun are the factors that take over the protective function and control the brooding process. The energy savings afforded by this inorganic "foster mother" have a considerable effect on the turtle‘s overall energy budget – a vital factor in all animals.
The scrub fowl (Megapodiidae) of the Malay-Australian tropics exploit yet another energy source. They construct up to 5 m high "brood heaps" made of plant material: the heat of fermentation within this pile is sufficient to brood the eggs. The birds spend up to 11 months of the year maintaining the internal temperature of the structure at a constant 34° Celsius for the eggs inside. The temperature is controlled almost daily and fluctuations are generally kept within a range of 1° Celsius. The strategy is adapted to the season. In spring, they merely need to draw off excessive fermentation heat through air shafts and to close the openings on time. In summer, fermentation is slower, but the sun plays an increasingly important role. The birds counteract potential overheating by increasing the thickness of the sand layer covering the heap. As the sun’s heat gradually penetrates deeper into the structure, they implement an astonishing yet effective counter-strategy: in the cool of the morning they remove the upper dome, dig a deep crater right down to the top of the eggs, and spread out the sand. After the sand has cooled, they kick it back into the hole again and top it with a thick layer of the old plant material in order to regulate the temperature. It takes the bird 2-3 hours to complete this process each time. The Australian ornithologist Harold J. Frith inserted a remote-controlled heating device into such a nest. The scrub hens initially reacted correctly. In spring they tended to open the nest every 2-3 days. As soon as the temperature was artificially raised, they began to open it every day in order to keep the temperature under control. When the heat was turned on in summer, however, they failed to recognize that the heat was coming from below. Taking their cue from the season, they directed their activity toward preventing excessive heat build-up by the sun. The result was that the heap grew higher and higher; had a defective generator not ended the experiment, the heap may well have grown even taller.
The scrub fowl‘s capabilities are strongly reminiscent of how mankind technically manipulates the forces of nature. The more commonly known cuckoo bird, however, demonstrates that – beyond using leaves, sand, sun and heat of fermentation – other animals can also be enlisted to support the brooding process. The females begin by carefully observing the nest-building activity of other bird species. As soon as these birds have laid their eggs, the cuckoo female as inconspicuously as possible deposits one of her own eggs into the nest, often taking one of the original eggs with her. The affected birds typically show no reaction and assume the role of foster parents, brooding the foreign egg together with their own. Four hours after the young cuckoo hatches, an innate drive kicks in: the blind and helpless chick proceeds to evict other eggs and freshly hatched nestlings. It wedges itself under the other occupants and uses its head and feet to leverage them up over the rim of the nest. Its particularly large and conspicuous super-gape triggers a much stronger adult provisioning response than does that of the original young, should any of these manage to have remained in the nest. The foster parents are busy from dawn to dusk bringing food and even follow the young cuckoo for up to several weeks after it has left the nest in order to keep feeding it. This strategy is based on innate behavior programs in both the mother and the young: its success in inducing other birds to take on the task of brooding, i.e. in transforming these birds into additional organs of a capable entity (the cuckoo), is immediately apparent. The cuckoo bird family comprises over 140 species and is distributed over most of the world; more than 50 species are such brood parasites.
In my opinion, these case studies, to which many more could be added, clearly demonstrate that separate organs are by no means the prerogative of human inventiveness. A whole range of animal species has already developed such additional functional units which considerably amplify the capability of their bodies. They may be composed either of the body’s own secretions (the spider’s web) or of environmental materials (like the bird’s nest, the pill wasp’s clay urn, and the ant lion’s sand funnel). Our perception would lead us to believe that these objects are distinctly separate from the cell body: functionally, however, they clearly form a unit.
None of these additional organs can be formed without investing a corresponding amount of energy, just as in organs arising through cell differentiation. Similarly, the benefit that each such organ provides the organism must outweigh any costs involved in producing and maintaining it: additional organs must be controlled, upkept, repaired, and replaced as the need arises, just like organs composed of cells. Moreover, the above-mentioned examples show that virtually every additional organ would be unable to unfold its capability were it permanently fixed to the body that produced it. In most cases the body would not even be capable of forming them.
I have gone into considerable detail in presenting the
above examples because this highlights that very many advantageous mutations
and sexual recombinations of genes were required to produce such a variety
of body structures and behavior programs. Moreover, each intermediate stage
in such an evolutionary chain, which took place over millions of years,
was subject to natural selection, i.e. had to have a positive selective
value! In my opinion, this is further strong evidence that additional organs
were critical for the survival and development of the respective species
– it would be totally unjustified to view them as something entirely separate
from the cellular body, as something apart from them.