388 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
the exact moment of high tide. These fish normally reproduce by 
depositing eggs and sperm in pits they dig in the wet sand just out of 
reach of most high tides. Here the young fish develop and are ready 
to take off to sea at the time of the next monthly high high tide. 
We do not have to select illustrations from animals alone, since even 
seaweeds are known which have lunar-monthly reproductive cycles. 
These are critical for the species; they synchronize the production 
of eggs and the sperm which must fertilize them. 
Examples of solar-day and annual rhythms of living things in 
nature are legion. Every one of us can think of organisms that are 
habitually nocturnal, diurnal, or crepuscular in their daily time of 
activity, and axiomatic are our spring and fall waxing and waning 
of the activities of innumerable plants and animals. And along the 
shores of our oceans where the tides monotonously rise and fall with 
lunar-tidal frequency, 50 minutes later each day, are corresponding 
rhythms of activity of the organisms that live there: some, like oysters, 
which filter their food from the sea water are active only when 
covered by water at flood tide; others, like the foraging fiddler crabs, 
scour the beaches for food exposed at ebb tide. 
Animals, furthermore, do not have to dwell on the seashore to dis- 
play a lunar-tidal rhythm. There is the often-quoted observation of 
the reef heron, which lives on the mainland of Australia up to 30 miles 
away from its feeding ground on the reef where it preys upon animals 
exposed at low tide. Hach day this bird departs from its roost on its 
trip to the reefs just at the proper time to take advantage of the low 
tide—50 minutes later each day. This phenomenal timing ability 
led one writer to remark, “Here is a sixth sense if ever there was one.” 
Another kind of example of a daily rhythm is seen in birds such as 
the starlings, which, in their north-south migrations in Europe, have 
been demonstrated to utilize the sun as a compass through holding 
their bodies at a fixed angle relative to it. But the sun during a day- 
long flight gradually moves across the sky from east to west. Jramer 
and Matthews both reported that the birds continuously corrected for 
the changing position of the sun in order to maintain a fixed, straight, 
compass course. Kramer spectacularly demonstrated this by placing 
birds in a domed enclosure possessing an artificial sun fixed in one 
position. Under these conditions, the direction of orientation of the 
birds relative to the artificial sun gradually changed during the day, 
exactly as one would have predicted on the basis of the rotation of the 
earth. Some beach fleas which migrate up and down the beaches have 
similarly been proven by Pardi and Papi to use both sun and moon 
as their compass, correcting with equal facility and accuracy to the 
normal movements of each. 
