Biological time clocks are ticking
As one telling of the story goes, at some point during Yogi Berr’s managerial stint with the Mets in the early 1970s, Hall of Fame pitcher Tom Seaver called over to him:
Tom: “Hey, Yogi, what time is it?”
Yogi: “You mean now?”
We humans are deeply interested — obsessed would not be too strong a word — with the passage of time, which over the years we’ve become really, really good at measuring. The NIST-F2 cesium atomic fountain clock housed in Boulder, Colorado, is expected to neither gain nor lose one second of ticking over the next 300 million years. (See YouTube: NIST Launches a New U.S. Time Standard.)
And yet, what exactly is it that Yogi’s Timex and the F2 were designed to monitor? What is this stuff we call time? Try this little exercise: Define the word “time” without using any of the many terms used to measure it such as second, hour, moment, period or duration.
So how’d that go for you?
While the substance of time remains one of the deep mysteries of life, biologists have nonetheless learned quite a lot about how various groups of organisms operate within its confines. Turns out, much of the diversity in which living beings — from bacteria to giant sequoias — make use of their allotted time on the planet boils down to issues of geometry.
The size and shape of an organism places physical constraints on its ability to obtain and make use of critical resources needed to power and nourish its body, and that in turn has all sorts of ramifications for how it spends its day.
That said, if we were to pose the ticklish question of how different organisms might actually perceive time, we might well start by checking the porch thermometer.
Insects’ experience of time, for instance, is entirely dependent on temperature. As ectothermic (cold-blooded) animals that can only generate a minimal amount of body heat, their metabolic processes are tightly tied to the surrounding temperature.
In fact, entomologists have discovered that each species requires a specific amount of heat to develop from egg to larva to adult. But because temperature can vary from day to day (and indeed hour to hour) the number of days for, say, a red-legged grasshopper to complete its life cycle typically fluctuates from one year to the next.
That’s why time for insects is measured not in days but in “degree-days.” One degree-day is defined as a 24-hour period in which the temperature is one degree above a species’ lower threshold temperature (below which development stops).
Example: Suppose our grasshopper egg can’t start developing into a larva until the thermometer reaches 56 degrees. Now let’s say the temperature hits 58 degrees and stays there for three days. So two degrees above the lower threshold times three days works out to six degree-days worth of heat.
Remember, it’s the total accumulated heat, measured in degree-days, that matters. And here’s the cool beans part … for a given insect species, the amount of heat (number of degree-days) to develop from one stage to the next is a constant. In colder years, it takes longer to accumulate the necessary degree-days worth of heat to attain adulthood than in warm years.
Scientists have determined degree-day constants for many insects with the practical application that by monitoring temperature, agronomists can predict the best time to apply pesticides to protect a crop against a given insect pest when it’s at its most vulnerable stage of life.
It’s not just insects whose lives are dominated by such time-temperature interactions. The metabolisms of other ectotherms such as reptiles, amphibians and fish are directly tied to the ambient (surrounding) temperature, as is the growth rate of plants. For all such life forms, biologists think in terms of “physiological time,” which essentially means growth and development take less “clock time” when it’s warm, longer when it’s cold.
In a sense, then, time goes faster for such organisms on warm days than on cold.
The study of biological time also has turned up a few mind-boggling, time-related patterns for endothermic (warm-blooded) animals. For instance, it’s long been known that the total number of heartbeats during the average lifetime for any mammal species, regardless of size or life span, is about one billion.
So the heart of a short-lived shrew is constantly racing while you’d have to wait a bit to pick up the long-lived elephant’s heartbeat with a stethoscope.
And here’s a last curious constant in time: Again, regardless of size and lifespan, it appears that all plants and animals deliver about the same amount of energy to each gram of tissue during the course of their lifetime — roughly 800 kilojoules per gram. We don’t know why.
Ken Baker is a scientist and a retired biology professor. If you have a natural history topic you’d like the author to consider for an upcoming column, email your idea to firstname.lastname@example.org.