Back to the Syllabus
Genes, Development, and Behaviors
Stimuli and Innate Behaviors
Innate Releasers and Fixed Action Patterns
The Environment and Development of Behavior
Caste Determination and Task Allocation in Hymenoptera
Royal Jelly and Caste determination
Task allocation in the Honey Bee
Maternal effects in the development of Ant Castes
Movement and Mass Action in Foraging Army Ants
Song Development in Birds
Classic Song Learning
Conservation and Plasticity in Song Across Seasons
The Tudorless Song of Cowbirds
Late Onset Brain Development in Mice
We have already seen how single genes can have profound effects on what are considered highly integrated behaviors in our example of the fosB mutation in mice, the nuturing gene. It is of course simplistic to assume that a single gene is responsible for all of the complex actions of the mother mouse. It is undoubtedly governed by many genes, but a single gene may be instrumental in altering the cascade of events that lead to nurturing. Similarly, a single gene affecting levels of testosterone might have a profound effect on the phenotype and affect a suite of traits by virtue of the multitude of of effects that T has on a variety of organismal traits such as muscles, morphology, physiology, and behavior.
Simple mutations in key regulatory genes that affect behaviors might be how such traits evolve and how species with a single male mating type evolve to become a species with two or three mating types. Mary Jane West-Eberhard has indeed advocated such a mechanism underlying the evolution of developmental patterns. Rather than reconstructing an entire set of developmental pathways to create a new phenotype, a simple genetic mutation in a regulatory network might alter an existing developmental programme in large scale ways. For example, Andrew Bass speculates that the plainfin midshipman fish Type I male is the ancestral form. Recall the simple pathway. Gondatropin releasing hormone -> gonadotropin -> gonad growth -> testosterone. A simple mutation in GnRH may have produced the novel Type II males that mimic females. Development of the Type II phenotype was not just a short circuit of the Type I phenotype, but it was close to a short circuit, in that Type II males resemble females in many ways. The female morphology is considered the ground state upon which additional hormones (e.g., T) might exert their organizational effects. We have clear examples of a genes role in affecting behaviors, and we can see that they do not act in a simple way. Rather they act in an epigenetic process in which the genes trigger the building of complex structures (e.g., endocrine glands, the circulatory system) that then regulate the development of behaviors. There is no single gene that controls the development of the pituitary (GnRH)->gonadal regulatory axis.
Ethologists have always had a fascination for genetically programmed behaviors, and the differences between male and females in a single species illustrates such effects because of the genetic control of sex determination and differentiation. Many behaviors are thought to "hardwired" and ethologists have termed such behaviors as innate. Arguments made by some ethologists (e.g., Lorenz) contend that the basic neural circuitry for receiving stimuli impulses in the early stages of development control much of an animal's behavior. To be sure, a young animal does not have much experience.There is no need to learn a behavior, the animal could be born with an "instinct for it."
In contrast to this view, the students of the field of behavioral psychology believe that environment plays a major role in learning behaviors. They certainly do not discout that innate behaviors exist, but that the environment plays a major role. Differences between ethology and animal psychology led to a debate on the causes of behavior that has been captured in the phrase "nature versus nature". Are the behaviors seen in organisms a product of their genetic background, or their environment? As we will see, despite the controversial nature of this debate, it is not a question that can simply be answered in terms of either one or the other. Behavior can be the result of a complex interaction between genes and the environment. We have already seen how bird song is developed through a male hearing his own voice, thus a male must receive at the very least information from himself to produce a song. The song that he produces is quite poor unless he has been exposed to the species typical pattern during the critical period for learning song. Thus, the neural circuitry can be affected in interesting ways and experience can profoundly shape the "wiring diagrams" for behaviors. In this chapter, we will explore some of the classic ideas on nature and nuture, and the role of the environment.
Ethologist speak in terms of sign stimuli, innate releasing mechanisms (IRM), and fixed action patterns. A sign stimulus is an external signal that triggers a specific response from an organism. The response is triggered by the innate releasing mechanisms which is some neural circuit for mediating signal detection and motor actions. The fixed action pattern is the set of innate behaviors that are triggered by the innate releasing mechanism. From what we know of neural circuits, it is clear that such circuitry can be built up from genes, but this again requires epigenetic processes involved in neural development (see the next chapter).
A classic sign stimulus (see 3 Meg video) triggers the courtship display of male three-spined sticklebacks. The enlarged belly of a female triggers the zig-zag dance in male sticklebacks and this dance is used to entice the female to enter the nest that the male has built. In this video, the male is more likely to court a super gravid dummy (a more extreme or supernormal sign stimulus) than the normal gravid dummy. Jenny Jenkins of Indiana University set up this encounter of a male and two dummy females which differ in the degree of enlargement of the belly.
In the case of the distended fish belly simulating a gravid female (above), the male enters the courtship fixed action pattern (FAP) in which he does a zig-zag dance with his head down and approaches the female then circles back to point out his nest. The FAP response is highly stereotyped to the point of being repeated over and over and over (I only digitized a small part of the 15 minutes or so that the male attempted to copulated with the female).
FAP's can also be involved in aggressive encounters among male sticklebacks. If a small amount of red is added to the belly of the belly of a dummy stickleback, the males will engage in a stereotyped attack response -- the circle fight posture. This FAP is only the beginning of what may escalate into an all out battle between sticklebacks. Indeed, Tinbergen noticed that a male stickleback in one tank would rush over to the window and display aggressively at about the same time every day. One day Tinbergen realized the source of the stimulus that triggered the FAP. The local postal trucks were bright red, and the male was responding to the red image of the postal truck seen off in the distance.
Imprinting is a process that usually occurs within the first few hours of birth or hatching. During these early stages, offspring will imprint on a nearby object that is animated. In other cases, the imprinting might involve auditory cues found in the female's calls. They will literally imprint on moving stuffed objects. After such filial imprinting, the offspring will follow the mother, or respond to the mothers calls. Indeed, Konrad Lorenz had geese and duck chicks imprint on him (he was the first thing the chicks saw). These ducks followed Lorenz everywhere.
Sexual imprinting is a special kind of imprinting in which a young offspring imprints on members of its own sex, and when the offspring mature, they prefer their own species. However, mistakes occaissionally happen in the wild.
Recall that a tutor's song is often necessary for the proper recognition of individuals from his own population -- deme recognition. For example, the Grants have been working on Darwin's Finches on Galapagos Finches for over 20 years. There are several species of Finches on a single island. Two parents participate in the rearing of the young. The father's song is what the male and female offspring learn. This song is important for species recognition. Occaisionally, the father dies, and the birds only hear a neighbor's song. If the neighbor happens to be the same species there is no problem. If however, the neighboring birds are a different species, the Grants have found that the fatherless offspring seem to learn the wrong species song and attempt to court or mate with the wrong species when they mature.
Innate behaviors appear to be coded by relatively simple neural circuits, however, the motor circuits that they trigger are quite complex. The only external event that is required is the sign stimulus, which acts as the innate releasing mechanism and the fixed-action pattern is acted out by the animal. In this section, we will look at behaviors that are modified by the external environment to the extreme.
The first example, that we will consider is the case of worker bees and ants and their caste determination mechanisms, and the rules that they have for worker allocation. In these groups, determination of the different worker castes are modified by inputs from the environment. The environmental inputs are quite varied and include temperature, hormones provided by nurse workers, or even the egg size of the female. A second behavioral issue involving social insects involves the coordination of tasks in the colony. It is often argued that social hymenoptera colonies are some kind of super organism. There are no decision makers in a colony, rather colonies coordinate their activities by their interactions with other colony members and follow very simple rules that govern the outcome of interactions. Such individual behavior leads to an "apparent coordination" or self-organizing property to the colonies. To understand the workings of such insect societies, we will also consider how the:
The second example, that we will consider is the case of song production in birds in which song learning is modified by the social environment. We will first consider classic song birds in which tutor birds are present during the critical period. In the case of classic song learning, the young birds (male and female) usually requires a male tutor for learning the species typical song. We will then consider the unusual case of the cowbird that is parasitic on the parental efforts of other birds. The cowbird chicks are reared in nests with an inappropriate tutor so there must be some mechanisms for generating species typical song in the absence of a suitable tutor. In studying the cowbird, it is clear that the development of song in young male birds also requires interactions with the females. Even though females do not sing, they still give the male feedback in during the song learning process.
The social hymenoptera form some of the largest and most complex societies in the animal kingdom, and they accomplish many of such tasks without a defined leader. To be sure the queen of the colony provides some "guidance", but only to immediate workers who tend to her egg making machinery. As she drops eggs, the workers scurry about to bring the eggs to a nursery. The queen can produce fertilized or unfertilized eggs and this haplodiploid sex determination leads to either male or female offspring.
Besides sex determination, how are the foraging efforts and subsequent tasks allocated in such colonies of insects? In many societies there are also castes in which the sterile workers have strikingly different morphologies which have been suggested to be optimally designed for certain tasks. For example in army ants there are very numerous small attack/foraging ants, and larger sub-major and major classes. Surpisingly, the smaller ants do all the attacking of prey, but do so in extremely larger numbers. The sub-major and major classes of ants are specialized for hauling the large subdued prey items back to the bivouac along "super highways" at high speed.
Genetic control of caste determination would, in principal be very difficult (but see below). The breeding and non-breeding castes are largely environmentally determined. For example, in the case of the queen bee in a new colony, the following events lead to the formation of a new queen. The old queen lays special eggs prior to her departure, and the workers tending the several eggs that she has laid puts them in slightly larger chambers. The old queen recruits many workers of the colony to join her in establishing a new colony and leaves her old colony without a queen. The now queenless nursery workers are not receiving pheromonal cues from the old queen. They begin producing a royal jelly that they feed to the several queens that are growing up as larvae. The royal jelly contains the hormones that trigger the development of a fertile queen rather than a sterile worker. As soon as the first larval queen hatches, she moves to the other royal chambers where her sisters lie about to hatch, and she kills each and everyone with her functional stinger (recall that workers have a stinger that stays in the prey, the queen possesses a multi-use stinger). This queen then mates with a single male -- a drone that is the product of an unfertilized egg, and she then begins laying fertile eggs that develop into new sterile workers.
In the case of queen determination, the stimuli to re-organize the new queen's morphology and behavior comes from the environment. As the bee develops in the "nursery", special accomodations are arranged for the larger bodied "queen" larva, pupae, and eclosing adult. The chamber in which the egg is deposited is made much larger. As the larva develops, nurse workers provide a special royal jelly that seems to promote ovarian development, an event not normally found in the the sterile works.
A developing bee larvas sterile sisters provide royal jelly -- a clear environmental input on the development of fertile queen versus sterile worker behaviors.
The maintenance of the hive entails a number of tasks. In bees, there is generally not the sophisticated development of different castes to carry out the various tasks. There are just three morphological types:
For early studies of marked bees, it was clear that an individual worker performed a single task over the course of several days, seemingly specialized in one of the following roles:
Over the course of several weeks the worker might move on to the next task in the sequence.
How does specialization in such roles come about?
Environmental determination of tasks: "foraging for work". One theory holds that these defined developmental roles are largely due to a random process, that is spatial correlated with the center of the hive. Because all bees are born in the nursery, this is the first environment that they encounter. If there is a simple rule, that workers look around their local environment, and begin working on tasks that need to be done then young newly born workers will immediately take up tasks in the nursery. As a worker toils away at their task, by chance, all tasks may be filled in the nursery, and the worker moves outside of the nursery arena to "forage for more work". The next place she would encounter is the outer hive, where maintenance tasks might be open for her working instincts. Eventually, these individuals begin wandering and interact with the workers in the larder to get food for the growing embryos. She would continue with this task until displaced yet again to foraging for new tasks. After performing tasks in the larder, the worker begins interacting with foragers and assumes such tasks, especially if she is displaced by younger workers that have now moved into the larder. Finally, the foragers interact with the scouts and assume such tasks. The exposure to new environments and opportunities for work leads to the development of specialization with age. The task allocation is simple, forage for work. If no work is in the immediate vicinity, them move a little farther from the center of the hive and look for undone tasks.
Genetic determination of tasks. There are indeed other theories to account for the correlation between the age of a bee and the various tasks that occur further and further from the hive. Even though workers are very similar to one another and their relatedness is very high (r=0.75 vs the usual 0.50 for sexual offspring), they do receive different material from their female parent, the queen. These subtle genetic differences might pre-dispose them to different tasks in the hive (e.g., nursery vs scout or morge vs guard).
Age dependent determination of tasks. Yet another theory contends that worker development is programmed by changes that are dependent on age and physiology. For example, a young worker might have a different hormonal composition than older workers. Differences in the internal state of the workers then leads to differences in behavior. A key hormone might be involved in the progressive change in tasks with age. For example, injecting a "young" nurse worker with Juvenile Hormone will cause her to precociously shift her tasks and become a forager. Changes in levels of juvenile hormone with age may control task allocation.
Franks, N. R. and C. Tofts. 1994. Foraging for work: how tasks allocate workers. Anim. Behav. 48: 470-472.
Robinson, G. E. 1992. Regulation of division of labor in insect societies. A. Rev. Entomol. 37: 637-665.
Robinson, G. E., and R. E. Page. 1992. Genetic determination of guarding and undertaking in honey bee colonies. Nature 333 356-358.
Tofts, C. and N. R. Franks. 1992. Doing the right thing: Ants, Honeybees, and Naked Mole-rats. Trends in Ecol. and Evol. 7: 346-349.
The distinction between queen and worker in colonies of social insects leads to clear specialization for reproduction and colony maintenance. In the case of may species of social ants, the workers are further divided into specialized tasks such as small general purpose worker, large worker for hauling large objects, special warrior castes of various types, including some species in which individuals have a turret on their head and inject noxious chemical warfare toxins and repellents. The underlying selective reasons for such shifts in the number of workers with warrior castes, has to do with colony economics.
The most efficient colony is likely to leave more descendant colonies.
Where do such striking differences in behavior and morphology come from? Such differences clearly must arise very early in development. Holldobbler and Wilson have reviewed the literature and found evidence for at least six mechanisms that contribute to caste determination and I discuss 3 of these below:
Environmental temperature. Variation in the temperature of development has a dramatic effect on the relative number of workers compared to other specialized tasks. For example, early in the spring, it is necessary to produce more workers because attrition during the course of the winter has reduced the number of workers. By biasing the production of larger numbers of workers at the lower temperatures, the colony will get a more rapid growth spurt. As the temperature heats up during mid summer, and the colony is at close to its maximum carrying capacity, the number of eggs developing into workers begins to taper off.
Colony Size. There are clear limits to colony size that are dictated by the distribution of resources. Recall the problems faced by a central place forager and the Marginal Value Theorem. There are diminishing returns from too great a travel distance between the colony and the food source. A colony thus has an upper limit to overall size that is governed by:
Thus, an old colony should be less concerned with expanding its territory, and the number of workers begins to decline with colony age. Defense of the colony from raids from adjacent colonies becomes a more important issue. Hence a shift in focus from workers to soldiers in young versus old colonies.
Queen age. This shift can also be achieved by some mechanism of worker versus soldier allocation that is tied to the age of the female. As a queen grows old, and as her colony ages, she begins to lay fewer of the worker caste. A young queen should produce many more workers than an old queen. A young queen and hence a young colony must grow rapidly and thus relatively more workers are beneficial. However as a colony grows older and has expanded to its maximum size in both numbers and the "territory" from which it harvests food, the colony must also consider shifting more members to roles involving colony defense. Hence fewer workers and more soldiers are produced by an old versus a young queen.
Egg size. Certain of the larger castes come from definitely larger eggs, suggesting that maternal effects arising from the queen controls development of her offspring. There is no need for a genetic change in the offspring, egg size per se is enough to bias development, subsequent growth, and differentiation along a different pathway and lead to a different caste.
Many authors have advocated the view that the behavioral repertoire and sensory mechanisms of individual workers do not need to be very sophisticated in order to create the foraging patterns and task allocation patterns seen in such colonies (see foraging for work above). Indeed the stimulation for mass behavioral actions comes from interactions with other workers. For example, many species of army ants are blind or nearly blind, and they rely entirely on pheromonal cues for coordinating their activities. The importance of such pheromonal cues is evidenced by the fact that many ants have up to 6 glands for producing these cues. The cues themselves are laid down in extremely minute quantities, but their effects on other workers are both potent and self-reinforcing. Consider a simple rule for generating coordinated movement in army ant columns. Army ants leave on foraging raids early in the morning each day. There is no leader for these columns, rather the individual ants appear to mill about randomly.
Lets assume four simple rules:
Using these simple rules, let us look at the kinds of tracks that are possible for the ants. Shortly after exiting the colony, a defined column forms and begins to extend out. Many ants are always found at the leading edge milling about randomly. However, the column takes on a defined shape and runs in the same direction. As more and more ants are recruited down a path, they lay down a stronger trail for the followers. The movement along such columns picks up and they become larger freeways. Smaller routes that do not have as many workers shut down as the larger freeway takes hold. Thus, close to the colony there appears to be a single well defined route. As you move further away, a fan like fractal pattern is seen in which a single defined route has not been selected.
The ants milling about in at the many sub-fronts may encounter an arthropod prey item. At which point they swarm, attack, and kill the prey by virtue of their sheer numbers. After the prey has been subdued, the workers bring the item back to the bivouac. This return movement generates additional activity along the single more defined corridor and further establishes the pre-emminence of a single corridor.
All of this apparent "coordinated" activity does not involve any leader. There is no caste that provides direction. The coordination results from the mass action of individual colony workers that are following simple rules of interaction. These external inputs govern the behavior of the colony as a whole.
Recall that the basic song learning circuits of male song birds develops in response to early pulses of androgens. After all of the early wiring is activated by the effects of Testosterone that is aromatized to Estrogen, the song learning circuit is ready to begin recording the template songs of tutors. As we will see, the brains and basic neural circuits that birds develop for producing song are remarkably adaptable and strongly influenced by interactions with the external environment. We will first look at a classic pattern for song birds in which the species typical song is crystallized through a process of learning that entails a tutor that provides the chick with a template. We will then look at cases in which such a template can change during the lifetime of a bird owing to the remarkable properties of brain development in birds. The classic paradigm for thinking about vertebrate brain development is that after a certain point, most vertebrates are incapable of developing new neurons -- brains do not regenerate damaged areas very well after trauma and the nerves of the peripheral nervous system do not regenerate very well. However, birds, have a capacity for developing brand new neurons for learning new song.
The basic motor circuit for the production of song consists of the higher vocal center (HVC) which connects to the neurons that innervate the muscles of the throat. Such innervation is not by a direct route, but the circuit passes through two other parts of the brain.
A second circuit of separate brain regions is not directly related to song production as lesions of these circuits do not cut off song production. However, if the second circuit is deleted, the young male birds cannot learn song. The songs they learn are highly abnormal. Males copy a tutors song during the sensitive period for song learning (e.g., 20-65 days in the swamp sparrow), and this song learning circuit is necessary. During this period, the song circuits are growing in size and the number of cells in the circuit is increasing.
The male begins rehearsing and practising its song during the rehearsal period, and he has to hear himself to get it right. At times he sings sub-song which is consists of a near babble (very similar in concept to the babbling human infants undergo during the language learning process). He is presumably comparing his own voice to the song stored in memory during the sensitive period. Eventually, he develops a highly-stereotyped song that is crystallized from all his early efforts. Some species cannot change the song after this point, whereas "open-ended" learner species can change the song throughout life.
The impact of a tutors song can be long lasting and play a role in deme recognition through its effects on female choice for the song that the females heard as nestlings (usually the fathers song).
Another way to generate highly abnormal songs in individuals is to deprive them of contacts during the sensitive period for song learning. While such tutorless individuals do develop a song it is typically highly abnormal relative to the species typical song, but occaisionally some song elements of the basic species song are preserved.
Arnold, A. P. 1994. Critical events in the development of bird song: What can neurobiology contribute to the study of the evolution of behavior. In Behavioral Mechanisms in Evolutionary Ecology. edited by L. Real. University of Chicago Press, Chicago, pp 219-237.
The song production centers in most migratory song birds regress every season as the levels of testosterone drop. Prior to the beginning of a new breeding season, the song nuclei begin growing again, and new circuits are born, and brand new neurons are recruited in the song centers (Nottebohm). Recall the example of song recognition of neighbors that Rene Godard demonstrated in a migratory species of birds. These birds can clearly remember a template of their neighbors songs from the previous year.
Indeed, for such recognition to work, there must be some elements of a neighbors song that remain static even in the face of such new neuronal growth.
In other species of birds (and whales), individuals in the local population of birds change their song from year to year. In particular, less successful males model their song repertoire after the song repertoires of males that were successful the previous year (Greene). Such adaptive plasticity in song, clearly establishes that the song is not necessarily fixed after the song crystallizes in young birds. Such song plasticity could easily arise from the new circuits that are born with each new season.
This is a particularly clear example, of the environment external to the individual having a dramatic impact on the neural circuits of an individual. Such modification of neural circuitry in some organisms points to the routes by which novel behaviors and learning might be acquired during an animals lifetime.
Consider the parasitic lifestyle of Cowbirds. Cowbirds search out other species of birds that are tending eggs, and lurk around waiting for the parents to leave the nest unguarded. The cowbird then hops over, and lays a single egg in the nest, and leaves never to return. The cowbird, a brood parasite, flies off in search of the next host for her offspring. Meanwhile, her offspring are typically incubated and successfully hatched by the unsuspecting parents. Most of the 200 potential host species of cowbirds have not learned to reject cowbird eggs or evolved an innate rejection mechanism that is necessary to skewer the egg and eject the parasite from their nests. The cuckoo of europe has a similar parasitic lifestyle and a few species have learned to reject eggs of the cuckoo in this manner, so it is not totally impossible for some kind of egg parasite recognition to take place.
Given that the host birds rear the cowbird during the sensitive period for song, it must be the case that cowbirds have some kind of innate template for its own species specific song.
Meredith West and Andrew King isolated male cowbirds after they fledged from their hosts. They isolated the males from other male tutors, but they did provide each male with a female. Female cowbirds do not sing a note, thus, they thought that the female would not provide any feedback for the males song. Males that were isolated from other males, but not females, did not learn to sing the species typical song. Rather they sang an abnormal song. The loud whistle in the final phrase of the song, was the most divergent element. It appeared that cowbirds needed a male tutor to learn the species specific song, as do all the other song birds studied to date.
West and King then decided to assay the potency of the song of these males compared to undeprived males by playing the tutorless and tutored male songs to receptive females. They were astonished to find that the tutorless songs were more potent in eliciting lordosis from the females compared to most of the tutored males' songs.
How could this be if the un-tudored males only had the females for feedback? How could they learn a song without other experienced males to sing a template for them? Young male cowbirds do appear to have an innate template for the species specific song. However, the template does need to be refined by interactions with conspecifics. They do not need to hear other males to develop a sexually potent song, they only need interactions from the non-singing females.
Recall the videos on non-song communication between female and male cowbirds. We have already seen how females have a variety of gestures for communicating their lack of interest in copulation. West and King have found that females use a pointing gesture to give the males more subtle visual feedback regarding his song. The male presumably modifies his newly developing song in direct response to such visual gestures.
This kind of communication is a classic example of the "audience effect". The un-tutored cowbird males had a captive audience in the form of a female. The female provided the males with visual cues during his song learning that allowed him to modify his song into a really potent song. Their un-tutored songs were so potent that they out-performed the singing of most other birds in a flock. However, such un-tutored songs appeared on par with the songs of the most dominant males.
How do such un-tutored males behave in a more complex social environment. Cowbirds are quite gregarious creatures and they do aggregate during their first winter. In such aggregations, a young cowbird could easily learn and refine his own song by using both the males and females around him as tutors. West and King have clear evidence of young males associating with females in such flocks preferentially. However, they do appear to interact with males. There are clear dominance hierachies in such flocks. Dominant males are intolerant of males that sing a provocative song -- one that stimulates the female too much. For example, if West and King placed any of the un-tutored males (those that sang potent songs) into aviaries with established dominance hierarchies, the dominant birds immediately attacked them on the first few notes. A male needs feedback from females, but he also needs vital feedback from other male group members so that he learns his place in the dominance hierarchy of the flock. Once a young male can establish his dominance in the flock, he might be able to let loose a little and try out some of the more provocative song elements that are more stimulating to the females.
The classic paradigm underlying neuronal development in vertebrates is that neurons grow and proliferate during embryogenesis. In fact, many more neurons are produced than are found in the adult. Through a process of cell death, neurons that were not "connected" to other neurons would be removed from the system. Gerald Edelman came up with the idea of "Neural Darwinism" in which neurons compete to produce the "right connections" during early development. The actual behavior and motor control circuits reinforce the connections between nerves. Those that are participants in an "active circuit" would live, whereas those connections that are not particpants, would die. In this way the nervous system of a vertebrate would be wittled down to a small subset of the possible to connections, to those connections that work. It is clear that this a simple view of development and that it is purely epigenetic. Connections that work are those that survive, there is no real genetic code for the connections. It is now clear that many connections are indeed programmed by the interactions between gene products. Substances are laid down in the developing embryo that guide the development and growth of neurons such that the neuron correctly innervates the correct targets (e.g., sensory or motor neurons of the peripheral nervous system).
For many years, it was thought that all of these events were restricted to early development, and there couldn't be much new neuronal birth after an animal matures. The case of song neural development in birds in which song nuclei grow and retreat with the seasons has provided a startling counter-example to this view. Still others cling to the view that this was a special case and vertebrates did not have the capacity for further neuronal growth and proliferation after early development.
Marian Diamond began a series of experiments in the early 80's that was to provide additional evidence for a more plastic nervous system in adults. She began rearing mice in a variety of environments that she classified on the basis of enriched or depauperate in sensory information.
A enriched environment included a world filled with novel tactile cues, objects with strikingly different shapes (balls, squares, etc.), compared to environments that had none of these added stimuli. She found that a region of the brain, the hippocampus, showed pronounced increases in the number of new neurons compared to the brains of the mice at the start of the experiment. In contrast, no new neurons or connections were formed in those mice in depauperate sensory environments. These experiments have now been repeated and the evidence reinforces the view that the adult brain is not a static set of neural circuits, but it may have the capacity to adapt to challenges from the external environment. The studies also show an interesting link between the complexity of the environment and complexity of connections in the brain.
Back to the Syllabus