Biological stability, or ‘homeostasis’, where an organism works to maintain an internal ‘steady state’ in response to the environment, is a concept familiar to all modern biologists. At all levels of organization, from cells to entire organisms, negative and positive forces act in a yin and yang fashion to dynamically respond to the environment and re-establish balance. In humans, for instance, our temperature ‘set point’ is ~98 °F. In response to an infection, our systems respond by raising the ‘set point’ via changes in neuronal activity in the hypothalamus (a key brain region in the regulation of temperature). This produces a fever, which facilitates immune system function and stifles the growth of the invading pathogen. When allowed to run rampant, however, fever can be detrimental to the health of the organism and eventually lead to death. This highlights the importance of precisely regulated homeostatic mechanisms in maintaining health and preventing disease.
If your education in the life sciences was similar to mine, a lot of emphasis was placed on homeostasis, with little if any placed on an equally important concept: biological rhythms. For a long time, researchers disregarded unexplained daily variations in what they were studying (like temperature, blood pressure, growth hormone levels…) as ‘noise’ rather than true biological phenomena. Indeed, until the latter half of the 20th century, research in ‘biological rhythms’ was lumped into an unfashionable category along with fringe subjects, including astrology and ‘mood rings’. I wrote this post aiming to address misunderstandings in the relationship between homeostasis and biological rhythms, and share some cool and interesting real world examples that highlight this relationship and the value of biological rhythms.
Temperature regulation shows how the set point (or “homeostat”) can adapt in response to challenges from the environment to aid in survival (a process called allostasis). To add another layer to this, wouldn’t it be even more advantageous for an organism to pre-emptively change its set point to coincide with predictable environmental challenges (i.e., those that occur with some predictable frequency)? In humans, for instance, body temperature and cortisol rise during the day to promote alertness, while growth hormone and melatonin secretion peak at night to aid in rest and recovery. At first glance, these variations seem to fly in the face of homeostasis, which dictates that each of these factors should remain at a fixed point to ensure optimum function. The reality is that this point must move up and down over time because the obstacles an organism must face change throughout the day and the year. Biological rhythms govern these changes and help explain why they occur.
Jürgen Aschoff, a fundamental figure in biological rhythm research, described the link between homeostasis and daily rhythms in the mid-1960s:
‘Homeostasis is a shielding against the environment, one might say, a turning away from it. For a long time, this phenomenon has been taken as the prime objective for an overall organization in physiology; and it evidently has great survival value. But there is another general possibility in coping with the varying situations in the environment; it is, instead of shielding, ‘to turn toward it’; instead of keeping the ‘milieu interne’ stable, to establish a mirror of the changing outside world in the internal organization. This has one clear prerequisite; the events in the environment must be predictable, which of course is the case when they change periodically.’
Nicholas Mrosovsky provides the example of temperature regulation in the camel to provide a real life example of Aschoff’s point. A camel faces a major problem every day of its life: how to keep cool. It is too big to bury into the sand, there is hardly any shade, and it would quickly die of dehydration if it were to utilize evaporative cooling to dissipate heat through sweating. There is a fundamental opposition between water balance and temperature regulation. Evolution has given the camel the necessary tools to deal with this situation.
During the day, the camel’s body temperature can rise as high as ~106°F – a wicked and almost certainly lethal fever if found in humans. At night, however, when water is scarce, the camel drops its temperature down dramatically to ~93°F, which would classify as dangerous hypothermia in people. The camel drastically reduces its temperature to protect itself from the next day’s heat. Because it drops it temperature so low at night, it now takes longer to heat up following day. In other words, the camels’ ‘set point’ is not fixed; it varies in response to predictable environmental challenges.
An organism’s physiology isn’t the only thing that needs precise timing; its behavior is set to a rhythm as well. Animals must not only adapt to a spatial niche (e.g., canopy, tide pools) but to a temporal niche (e.g., nocturnal, diurnal). It’s not only what an animal does that’s important, but when it does it. The ability to predict future events (either consciously, or in the case of biological rhythms, unconsciously) is of paramount importance in passing on your genetic information to future generations. One dramatic example of precise timing is the 17-year cicada, which emerges in a predictable fashion after lying dormant for nearly two decades. Another is the short-tailed shearwater, a bird that arrives at its breeding site in mid-autumn on small islands north of Tasmania. All the individuals in the population lay their eggs between November 24th and 27th each year, and they hatch at the same time. The cicada and shearwater make use of their exquisite timekeeping machinery to overwhelm potential predators with their progeny, allowing more newborns to survive than would if offspring emerged over a longer period.
Predator avoidance most certainly played a role in shaping the evolution of biological clocks. About 20 years ago, Pat DeCoursey and colleagues at the University of South Carolina conducted a study to investigate the adaptive function of rhythms in behavior. They lesioned the suprachiasmatic nuclei (SCN; a brain structure that acts as the primary ‘time keeper’ in vertebrates) of wild eastern chipmunks and then released them back into the wild and followed their survival for the next two years. To control for the effects of the surgery itself, they also “sham” lesioned several chipmunks, and left their SCN intact. After just 3 months, only a single intact chipmunk had become the target of predators, while 40% of the SCN lesioned animals became lunch. These deaths were attributed to the animals being active when they were not biologically inclined to be (i.e., their ‘clock’ was broken), making them easy prey.
In many rodents that live at non-tropical latitudes, the shortening amount of light each day signals the approach of winter months before the really cold weather hits. Siberian hamsters, for instance, have evolved to tell the time of year by measuring day length (photoperiod). With just two bits of information: (1) day length, and (2) whether days are getting longer or shorter; the hamster can tell what time of year it is, and if winter is coming or going. When days get shorter, males rapidly reduce their body size by ~20-30%, put on an extra layer of newly white fur, and all but eliminate their reproductive organs….they won’t be doing any mating when the weather hits -50°C. In response to short days, white-footed mice (commonly found in Ohio), actually reduce the size of their brain to putatively aid in saving energy. Every year, species like these need to radically reorganize their bodies to adapt to their changing environments, or die. This involves changing that ever stable ‘set point’ drastically throughout the year.
I hope some of the examples I’ve described above help provide context for thinking about biological rhythms. I also hope that a discussion of these rhythms in addition to homeostasis will facilitate the implementation of them into early lessons on the natural world. With the increased use of artificial lighting, shift work, and trans-meridian travel, our biological rhythms are being tested in contexts in which we have not evolved. Disruption of these rhythms is only now being appreciated as a contributing factor to many diseases including metabolic syndrome, depression, and cancer.
Nothing is more interesting than discovering the remarkable strategies animals have evolved to survive in their (sometimes) extreme environments. By adding the additional ‘wrinkle’ of rhythmicity in physiology and behavior, animals can exploit environments at one time of day or year that would be dangerous or even lethal at other times! I am excited for the future of the still new field of ‘chronobiology’ (aka the study of biological timekeeping), and can’t wait to see what nature has in store for us next!
Many of the examples I describe above are discussed in more detail in the excellent “Rhythms of Life: The Biological Clocks that Control the Daily Lives of Every Living Thing” by Russell G. Foster and Leon Kreitzman.
Jeremy Borniger is currently a doctoral student in the Neuroscience Graduate Studies Program at The Ohio State University. He received his BA in anthropology with a minor in medical science from Indiana University and has worked with chimpanzees, orangutans and gorillas. In his spare time, Jeremy enjoys playing the piano, scuba diving, cooking, and writing and reading as much about science as he can. You can follow Jeremy on Twitter: @JBorniger