I remember the first time I questioned myself about movement; I was with my parents on the family farm, and a flock of birds flew in a particular formation over us. They were migrating, but I didn’t know it at that moment. I turned and asked them how those extremely intelligent animals could fly without colliding with each other; well, how they fly even.
Their answer was the most natural for any person to that question: it’s their instinct.
And that answer was enough for the next several years. If birds fly it’s because of their instinct, if cats climb and dogs run after them it’s because of their instinct; but, do people move for the same reason? That’s something I wouldn’t think about until a long time later when I started to question what made us walk on two feet, run chasing a ball with colleagues in the neighborhood, leave our home at a certain age, and even migrate.
The perfect biomechanical machine
Some time ago I started a university degree that I abandoned after a few years to follow my dreams in another career. However, from those few early years, I remember a couple of things.
First: human beings are the perfect biomechanical machine that so much has been imagined and written about in Sci-Fi stories.
Systems tend to entropy, to disorder; that’s one of the main axioms of thermodynamics. But the counterintuitive chronology that allows us to be born with the gift of movement seems to reveal itself to this. From the appearance of somites, primal structures that are the basis of our skeletal system (driven by a fibroblast growth factor), to our last cardiorespiratory movements before leaving this world.
Second: this series of processes that are fulfilled in time and form, so similar to the evolution of our species in its random nature, also tend to fail and remind us of the great reality of the natural systems; they don’t always work in the same way. That’s the reason of so many diseases and syndromes that directly influence movement, but also in the behavior and neurological processes of people.
According to a group of scientists, authors of the book Mental and behavioral dysfunction in movement disorders, who make a historical review of the relationship between movement disorders and diseases of the nervous system, the interaction between these conditions is profoundly closer than what we think. As evidenced in cases such as Parkinson’s disease and Huntington’s disease, in which disorders that lead to dementia, depression, and even psychosis can be observed.
So, do we move because we think and our nervous system works properly? Or perhaps we should also consider that motion is an intrinsic component of the physics of the natural systems of our world? I mean, even if we are lifeless bodies, we will move by simple inertia, right?
Well, there is a very cool approach to the relationship between movement as we know it in our daily lives and the physics of movement; to be more precise, the dynamics.
In 2002, Oxford published a book called Physics and the art of dance: understanding movement. This work is based on the following idea: dance exists thanks to the movements of the human body, which are understood within the same framework of the universal physical principles that apply to all moving objects. In such a way that it is possible to analyze dance from the conceptualization of the physical principles that interact in it. For example, in this way we understand that to achieve the necessary balance in a pirouette, it’s required that the body’s center of gravity be directly over the support area on the floor.
Up to this point, we understand that movement requires a little more than instinct. It requires complex interactions between biochemical elements and principles and magnitudes of Physics. Maybe a little luck too.
And it’s precisely these elements, interacting so that we can move, that initially drove the emergence of our own species. An event that was driven by a particular phenomenon of large-scale movements: migration.
We migrate
Another thing that I remember from my university lessons (from the second and final attempt) is that organisms migrate seeking to satisfy two primary needs: food and shelter. And if so, what made us humans so different that we went out to explore the world?
The Uruguayan songwriter Jorge Drexler, in a song called Movimiento, says the following:
As soon as we got on two feet, we began to migrate through the savanna, following the herd of bison, beyond the horizon, to new distant lands.
Something that is, in fact, slightly accurate, but at the same time difficult to explain.
It turns out that evolution is not that linear process in which we go from monkeys to men in a classic sequence, as is shown in the famous scene from The Simpsons in which Homer goes from being a uninuclear organism to … well, the yellow bald man that we all love. On the contrary, the evolutionary process is much more boring and takes thousands and even millions of years. So, trace the exact moment when we started to migrate (and by ‘we’ I mean our phylogenetic lineage) is difficult and at least confusing. So we will narrow down the history of migrations of our species at three key moments.
In the 19th century, the anatomist Eugène Dubois found on the riverbank of the Solo River in Indonesia the skeletal remains of a creature that he presumed to be the missing link between the ape and man, and named his find Anthropopithecus erectus, although it would be historically known as the Java man, species that when being dated with an antiquity of at least bigger than one million years, represents the first hominid to migrate outside the African continent. However, it does not represent a direct ancestor to our species. In this sense, the modern human being that arises from the African continent (the other evolutionary branches were degenerating with time) registers two great migrations towards the Eurasian region; one of approximately 130 thousand years ago, and the second and final one that occurred approximately 60 thousand years ago.
The third, more than a moment, is composed of a series of migratory events that I hope to address extensively in the future. Some of these would be the migratory hypotheses of the American continent, and the great migrations in the Eurasian steppe. Each one of these events built and diversified the culture of the modern world.
If we observe these moments through the logic of Ecology, we could imagine that all these great human mobilizations occurred due to seasonal climatic causes, which led to food shortages and the necessity to survive. But here comes a bit of the short answer that I received at first: perhaps it was instinct, an inexplicable instinct to go out to explore and discover the world, something that made our species what it’s today.
And if the above is true, why does migration in our contemporary times cause so many problems? There are many opinions on this that I prefer not to touch on in this storie, but I highly recommend three books on the subject: Migration in a globalized world edited by Amsterdam University Press, Making people illegal edited by Cambridge, and The immigration crisis edited by Altamira Press.
Understanding the peculiarities of the migratory phenomenon in our species, we could return to the initial question a bit: why do birds migrate?
The migration ecology of birds
To finally solve the enigma we have to contextualize a little more.
As narrated in the natural history of humanity, many other species have also learned to migrate in search of new and more suitable regions to live in, based partly on their adaptive capacity and their individual tolerance range. But there are certain distinguishable patterns in these migratory processes, some of the most notable are: terrestrial organisms are surrounded by a low-density atmosphere, this causes them to use a huge part of their energy to move and stay upright; on the contrary, marine organisms are surrounded by water, which has a density closer to theirs, giving them support and causing that the cost of energy when moving is much lower compared to that of terrestrial organisms. That same relationship is transferred to migration processes and metabolic expenditure during them.
Also, it’s necessary to consider that the environmental factors that affect the migration of species by sea and land may be different, and geological events may affect the two realms in different ways.
The relationship between migratory events and adaptation phenomena is slightly unclear; sometimes both are antagonistic, and at other times they are complementary. Migration can serve as a short-term solution to environmental changes; that is, a particular population of organisms may have to move to another location when their original home is in extreme conditions, such as heavy snowfall. On the other hand, populations that remain in one place despite environmental changes and manage to adapt after a long time and hundreds or thousands of generations, achieve a long-term solution; They no longer need to move and expend huge amounts of energy. However, in the vast majority of cases, lineages have to use both processes in order to survive.
Ok, so migrating is important and necessary, to migrate you have to move, to move you need a sequence of biochemical processes that exist within the logic of physical systems, but … the question remains, how and why do birds migrate?
The great differentiator of most existing birds is flight, that aerodynamic marvel that allows them to travel much greater distances than the rest of the organisms. And that, despite representing a huge amount of energy expenditure per unit of time, is highly efficient due to the area they cover. Even some birds can use much less energy with a particular type of flight, by soaring–gliding. And this natural phenomenon would not be possible without body peculiarities such as: larger pectoral muscles (which even comprise a third of the total body mass of many birds), lower bone density, and highly efficient metabolic, cardiovascular and respiratory systems. In short, their unique body configuration allows them to fly, and consequently, migrate in a very particular way.
Ian Newton, in the book The migration ecology of birds, categorizes the way in which birds move into 6 types:
1. Routine movements that occur in all birds at their place of residence, between feeding, nesting, and roosting sites.
2. Dispersal movements, whether in migratory or sedentary species, occur when birds move in different directions from their birth sites, adapting even to other populations.
3. Migration, which depends on many extrinsic (generally environmental) and even intrinsic factors (I will talk about this below), and involves more specific directions and long-distance travel, even thousands of kilometers.
4. Dispersive migration, which combines the last two types of movements.
5. Invasive migrations, which are similar to seasonal migrations, but the proportion of individuals that leave the breeding range and the distances they travel is much more variable. Some even form new resident populations in these migration regions.
6. Nomadism, which is characterized by almost random movement of birds in search of places where food is sufficient and breeding is possible.
So if we focus on migratory movements (our initial interest), we are left with only two specific questions to answer. How do birds orient themselves during this phenomenon? And what causes them to fly in such particular ways sometimes?
Answering the first question is a bit more difficult than it sounds due to the lack of consensus in the science community. There is an ecological term that can partially answer this; spatial memory. Just as when we walk in the city in which we were born we recognize places, buildings, and colors; birds also recognize patterns on their migratory routes, visual guides that allow them to identify the correct path and the remaining distance to reach their temporary homes. However, metabolic and neurochemical processes are also involved, since many species of birds alter their behavior in the days prior to migrating, or save fat reserves for their journey, during which they barely eat. We know that this is not a rational act (or at least we suppose that), so it’s logical to think that these types of events occur at the same time and synchronously with the change of environmental patterns that alter the ecology of the species.
And regarding the second question, the answer is nothing less than wonderful. This phenomenon is explained by the chorus line hypothesis, which compares the orderly formation of flocks of birds with a human chorus line (dance-related formation), in which individuals observe the approaching maneuvering wave and they calculate their own execution to match their arrival. In birds, these processes unconsciously obey physical magnitudes that also indirectly allow them to save energy when flying. That is, the upward airwave generated by the first individual with his flapping serves the individual immediately behind as an aerodynamic advantage, and so on with the following individuals until completing the flock. In the same way, thanks to these particular formations, birds can protect themselves from predators and find food more efficiently, compared to birds that are separated from the flock.
Thus, in a little less than complex way, an innocent mystery from my childhood is solved: how do birds fly?
And yet, if I could travel back in time and find myself, the answer I would give myself would be: it’s our instinct.
