Circadian clocks, which reset about every 24 hours, are common in organisms living on Earth’s surface. They control sleeping and eating patterns, the rise and fall of body temperature and blood pressure, hormone release and many other important body processes. But the clocks in some animals tick-tock to a different beat.
Along with its circadian clock, a marine worm called Platynereis dumerilii has a timer set to the moon, which controls when the animals spawn. No one has discovered yet which protein gears drive this lunar clock. But experiments have demonstrated that the lunar clockworks are different from those that keep the animal’s circadian clock ticking.
The speckled sea louse Eurydice pulchra follows tidal rhythms so it knows when to burrow deep into the sand to avoid being swept out to sea and when to emerge to forage. The tidal clock is paced by a protein called casein kinase 1, which may be a gear left from an ancient clock upon which all others have been built. (See “The origin of biological clocks.”) Antioxidants called peroxiredoxins also follow the tidal rhythm set by that protein, scientists reported in April in Current Biology.
Somalian cave fish, Phreatichthys andruzzii, haven’t seen the sun in more than a million years, but they still have the gears of standard circadian clocks. This clock no longer responds to light. It has wound down and now ticks out a 47-hour cycle, perhaps to synch with cyclical changes in the caves happening on that time scale. Or maybe the clock is slowly breaking because it no longer gives the fish an advantage.
Some cicadas follow even longer clocks, emerging from under ground every 13 to 17 years. No one knows how the insects mark time, says chronobiologist Barbara Helm of the University of Glasgow. Experiments suggest that animals don’t just count days ticked off by their circadian clocks, she says. That could mean that organisms have other timers that keep track of longer time spans, such as years.
In Arctic reindeer (Rangifer tarandus) the circadian clock has lost its rhythm or it beats only weakly. Going off the clock may help animals forage, sleep and carry out other activities in constant light and constant dark. The animals still follow seasonal cycles, such as mating and migration, dictated by a light-sensitive hormone called melatonin.
Voles (Microtus arvalis) feed in 2- to 3-hour cycles of activity, known as “ultradian” rhythms. Ultradian means less than a day. Following circadian rhythms seems to be less important for the voles, raising the possibility that a shorter timer controls the ultradian activity cycles. Researchers are still working out exactly what sets these rhythms.
A type of Mexican tetra (Astyanax mexicanus) that lives in dark caves has circadian clocks that are jammed in perpetual daytime. The unusual clock helps the fish save energy. (See “The origin of biological clocks.”) Other Mexican tetra, which live in surface waters, have clocks that keep normal 24-hour time.
Honeybees adjust their clocks according to their jobs in the hive. Nurses no longer follow the regular circadian cycles. Instead they care for larvae around the clock. Foragers work on regular circadian shifts. Put a forager in the nursery and she will adopt the nurse’s schedule, while nurses taken off duty will start following circadian rhythms.
Migrating birds and newborn killer whales and bottlenose dolphins and their moms don’t sleep for weeks and may put their circadian clocks on snooze. When birds reach their destination and when killer whale and dolphin babies grow a bit, they return to regular scheduled circadian programming, according to chronobiologist Helm. How they do this is still a mystery.