Nature – Circadian Rhythms of the Animal Kingdom

Human circadian rhythms tend to be closely tied to the rise and fall of the sun each day, occurring in fairly regular 24-hour cycles. But by no means is this how all organisms’ internal clocks operate. Science News recently compiled seven of the strangest circadian rhythms. These work to keep different organisms in tune with their (sometimes-extreme) environments.

Lunar Clock

Some species operate not by the rise and fall of the sun but by the rise and fall of the moon. This includes a marine worm called Platynereis dumerilii. The moon controls when the worms spawn in a type of lunar clock that appears to be separate from the animal’s circadian clock.

Tidal Clock

Certain marine species, such as the speckled sea louse, use tidal rhythms to help them decide when to burrow into the sand (so they’re not swept out to sea) and when it’s safe to come out to forage.

Prolonged Clocks

Not all species operate on a 24-hour cycle. Some, like the Somalian cave fish, have a longer cycle; theirs is about 47 hours. This might be because of slower changes that occur in the dark caves where the fish live, or it could be that their clocks are slowing down and “breaking” simply because they’re not providing a survival advantage.

It’s been shown, for instance, that the eyeless Mexican cavefish save about 27 percent more energy due to the lack of circadian rhythm.

No Clocks

Aside from species living in dark caves or the deep sea, Arctic reindeer may also have lost their circadian clock. This may help the animals forage, sleep, and survive during periods of constant light or darkness.

Even with no circadian rhythm, however, the animals are still in tune with seasonal cycles of mating and migration due to melatonin, a light-sensitive hormone.

Short Clocks

Some animals actually have short (less than 24 hour) clocks, which are known as ultradian rhythms. This includes voles, which feed and follow cycles of activity that last just two or three hours.

Social Clocks

Honeybees are able to adjust their clocks depending on their job in the hive. While forager bees have regular circadian shifts, nurse bees stop following a circadian rhythm so they can care for larvae around the clock.

Snoozed Clocks

Some species, such as migrating birds and newborn killer whales and their moms, don’t sleep for weeks on end. It’s thought that they hit the “snooze” button on their internal clocks during this time, then return to their regular circadian rhythm as their circumstances change, such as when a migratory bird reaches its destination or a newborn killer whale grows up a bit.

Emerging research suggests many animals have altered or absence of circadian rhythms with no apparent ill effects.

7 Circadian Rhythms of the Animal Kingdom, Healthy Pets, Dr. Karen Becker, November 3, 2015.

“All the creatures of the woods and hills are a part of His [God’s] great household. He opens His hand and satisfies ‘the desire of every living thing’ (Psalm 145:16).” Child Guidance, 58.