Going by the clock

Originally published 19 February 1989

Some­time with­in the next week and a half the red-winged black­birds will return to East­ern Mass­a­chu­setts. In my town, the ear­li­est of the spring migrants will arrive on the 26th of Feb­ru­ary, give or take a few days. Not even an abnor­mal­ly warm win­ter will dis­rupt their sched­ule. Their bio­log­i­cal clocks are as punc­til­ious­ly set as those of the swal­lows of Capistrano.

I’ll rec­og­nize the day of the red-wings’ arrival the moment I open my eyes from sleep — when my own bio­log­i­cal clock wakes me at 6:30 a.m. pre­cise­ly (Sat­ur­days and Sun­days, unfor­tu­nate­ly, includ­ed). There will be a cer­tain — ah — tim­bre to the air, a cer­tain inef­fa­ble antic­i­pa­tion of spring, a cer­tain — a cer­tain 26ishness of February.

The birds will know too. They will know it’s the 26th. A chem­i­cal tick­ing in their brain will tell them so. They will be perched in the trees by Que­set Brook as I walk to work. I will hear their throaty chaaack even before I see them. They will hunch for­ward, lift their wings, and flash their gaudy epaulets. They will know the date as well as I do. Their cal­en­dar is in their brain.

Keeping in synch

Like oth­er birds, red-wings bear with­in their bod­ies bio­log­i­cal clocks that con­trol the tim­ing of migra­tion, nav­i­ga­tion, courtship, mat­ing, and molt­ing. These clocks are sen­si­tive to changes in the envi­ron­ment; it is prob­a­bly the increas­ing length of day­light, mon­i­tored by inter­nal clocks, that sets off the sig­nal for the birds to leave their win­ter range in the south­ern Unit­ed States and head north.

Changes in light and dark­ness adjust the clocks, keep­ing them in phase with the day or sea­son. But if birds are kept under con­di­tions of con­stant light and tem­per­a­ture their inter­nal clocks keep tick­ing, with a cir­ca­di­an (cir­ca-dian, rough­ly dai­ly) and cir­can­nu­al (rough­ly annu­al) rhythm.

Any­one who has suf­fered jet-lag knows that bio­log­i­cal clocks are not unique to birds. After a quick trip from Amer­i­ca to Europe, for exam­ple, our inter­nal clocks keep run­ning on the old time, and sev­er­al days will pass before they are altered to agree with the new envi­ron­ment. Humans, mos­qui­toes, morn­ing-glo­ries, and even bread molds pos­sess cir­ca­di­an clocks. Only the most prim­i­tive sin­gle-cell organ­isms are with­out inter­nal timekeepers.

The mimosa plant was one of the first organ­isms to reveal its innate time­keep­ing. In the 1700s French sci­en­tists main­tained the plants under con­di­tions of con­stant light and dis­cov­ered that the leaves con­tin­ued to open and close at approx­i­mate­ly dai­ly inter­vals — with a peri­od of about 22 hours (the rough­ly-dai­ly) instead of 24 hours. With­out adjust­ment by expo­sure to nat­ur­al pat­terns of day and night the mimosa’s clock runs some­what fast.

But what are these inter­nal clocks, where are they locat­ed, and what makes them tick? Some­thing wakes me at 6:30 a.m., day in and day out? What is it that caus­es the black­bird to arrive on Feb. 26th?

If the pineal gland in the brain of a spar­row is removed, the spar­row los­es its rhyth­mic­i­ty. If the pineal of a spar­row with 12-hour “jet lag” (raised in con­di­tions where the cycle of light and dark­ness is reversed) is trans­plant­ed into a spar­row whose pineal has been removed, the recip­i­ent takes on the rhythm of the donor. The pineal seems to car­ry the pace­mak­er. But per­haps the pineal is only nec­es­sary for the prop­er func­tion­ing of a clock which is locat­ed else­where, a clock which the recip­i­ent spar­row some­how quick­ly resets.

A report in a [1989] issue of Nature describes a more deci­sive search for the locus of the cir­ca­di­an clock in ham­sters. Ham­sters are per­fect for bio­rhythm research because their wheel-run­ning activ­i­ty, which nor­mal­ly takes place at night, is eas­i­ly mon­i­tored. Ham­sters con­tin­ue to take their dai­ly run on a wheel even if kept in con­stant light. It is thought that the ham­ster’s clock — and those of oth­er mam­mals — lies in that part of the brain called the suprachi­as­mat­ic nuclei (SCN). In the absence of exter­nal stim­uli, the free-run­ning rhythm of ham­ster cir­ca­di­an clocks is nev­er less than 23.5 hours.

A 22-hour hamster

Well, almost nev­er. Mar­tin Ralph of the Uni­ver­si­ty of Vir­ginia found a mutant ham­ster with a 22-hour cir­ca­di­an clock, from which he derived a line of short-rhythm off­spring. He showed that he could short­en the rhythm of wheel-run­ning activ­i­ties in nor­mal ham­sters by sur­gi­cal­ly excis­ing their SCN, and insert­ing SCN from fetal mutants — turn­ing 24-hour ham­sters into 22-hour ham­sters. A more con­vinc­ing demon­stra­tion of the loca­tion of the mam­malian cir­ca­di­an clock could hard­ly be imagined.

There is still much to be done in fig­ur­ing out what it is that ticks and how it ticks. Locat­ing the gene or genes that con­trol cir­ca­di­an rhythms will help researchers find the answers. Mean­while, the cer­tain­ty that cir­ca­di­an clocks are inter­nal and bio­chem­i­cal sug­gests whim­si­cal pre­dic­tions about the future of bio­log­i­cal rhythmicity.

How about anti-jet lag pills that could be used to tem­porar­i­ly sup­press the cir­ca­di­an rhythm after a long flight, allow­ing more effi­cient reset­ting of the human bio­log­i­cal clock? Or Sat­ur­day pills that slow the clock on week­ends, allow­ing an extra hour or two of sleep?

Or Feb­ru­ary-26th pills, bear­ing the ingre­di­ents of a fine­ly-tuned cir­can­nu­al rhythm (derived, per­haps, from the pineals of red-winged black­birds) that would wake me up an hour ear­ly on the day of the red-wings’ return, so that I could be there in the water mead­ow by the brook when the birds stake out their ter­ri­to­ry, hear the first chaaack of the new sea­son, and mar­vel again at the rhythms impart­ed to all crea­tures by a fine Swiss clock­mak­er called evolution?