8 conductors of nature

#1 Timing by length of day

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Deer traditionally mate in the fall. So why are their young born in May or June? A long pregnancy is not the reason. After mating, the fertilized egg remains in a state of suspended development until spring approaches and the days grow longer. At a certain ratio of day to night, embryonic development resumes. As a result, the fawns are born at a time of year with mild temperatures and abundant food.

The fertile phase in fall is also linked to day length. Deer prefer to get the stressful rutting period behind them before the strength-sapping winter when food is scarce. This precisely timed reproductive cycle has its downsides, however. In some parts of their range, deer are already ‘missing’ their optimal birth window because climate change is altering temperature and precipitation patterns. In the future, this may also disrupt the reproductive timing of many other animal species that rely on day length as a cue.

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#2 When trees count …

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The first warm day of spring leaves the apple tree indifferent – or so it seems, as its buds show no sign of change. In reality, however, the tree has already begun to ‘count’.

Only after it has registered a certain number of days above a minimum temperature will it begin to leaf out, and shortly thereafter, to blossom. This protects its tender new leaves and blossoms from late frosts, which are common in temperate climates in spring. Most trees and shrubs that live in these zones rely on cues from the weather.

Wherever there are seasons, temperature and precipitation serve as important prompts for nature. In forest trees such as beeches and oaks, a mild spring, combined with a species-specific internal clock, triggers a so-called mast year with particularly abundant seed production.

#3 Ancient internal clocks

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Life ticks: Even bacteria have internal clocks that follow a daily rhythm. These so-called circadian clocks are presumably as old as life itself and can be found almost everywhere. They regulate key cellular processes, metabolism, and gene expression in single-celled organisms, fungi, plants, animals, and humans alike. Ultimately, as in an orchestra, it is rhythm that creates a cohesive whole.

Although these circadian clocks run independently, they are continually calibrated by daylight, producing a meaningful day-night rhythm that alternates between activity and rest. In addition to circadian rhythms, the vast majority of living creatures also possess internal clocks that follow monthly or even longer cycles. These help coordinate processes such as reproduction. The precise timing usually results from an interplay between the internal clocks and various environmental factors.

#4 The rhythm of the moon

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Once a year, there is a mass wedding at the Great Barrier Reef. In the warm nights of the Australian summer, shortly after the November or December full moon, the corals spawn. During this event, all individuals of a species must release their eggs and sperm into the water almost simultaneously for successful fertilization. Clearly, it will not do to miss this ‘wedding night’! For precise timing, the corals use moonlight as a reliable natural clock. Light receptors help the individual corals determine the ‘right’ night based on the full moon.

The collective spawning of vast numbers of corals within a short timespan increases the chances of offspring. Although most eggs, sperm, and larvae are gobbled up by fish and other predators, a lucky few manage to settle in a suitable spot and found a new colony. Other marine organisms – including various species of bristle worms, sea urchins, mollusks, and fish – also use the phases of the moon to make a date for spawning, as it were.

#5 The pulse of the tides

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In the intertidal zone, the rhythm of high and low tides shapes everyday life – whether in the northern German mudflats or among tropical mangroves. When the water ebbs in such places, fiddler crabs go looking for food. The males also use the flat, muddy-sand surface as a stage for their courtship display. At the center of this dance, they wave their oversized claw in the air to impress females. Mostly at night, they drum on the ground to acoustically mark their territory. But no matter what the time of day or night, when the tide returns, they scurry back into their burrows to avoid being washed away.

As poor swimmers, mudskippers also take advantage of the low tide to move across the mangrove floor. These fish graze on algae and hunt small invertebrates – occasionally tearing off a crab’s leg. Other inhabitants of the intertidal zone lead very different lives. Mussels, for instance, simply wait for the next tide to roll in so they can filter nutrients from the water. For a few hours, their habitat once again belongs to the ocean.

#6 Smoke, flames, and new opportunities

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As destructive as fire can be, it also creates the conditions for new life. Take the Australian fire beetle, for example, which uses the burned trunks of eucalyptus trees as a nursery. Females locate suitable trunks – probably by the smell of charred bark – and use heat sensors to determine whether the dead wood has cooled enough for them to safely lay their eggs. The scorched wood provides the beetle larvae with ample food, and the damaged eucalyptus no longer has any defenses left.

Many plant species have also adapted to forest fires, such as the giant sequoias of North America. These trees keep their cones tightly closed until they sense rising hot air. Only then do they release their seeds. The young trees grow rapidly in the nutrient-rich ash of the cleared forest. Whether animals, plants, or fungi, many species that live in areas with regular natural fires have integrated fire into their life cycles – or at least learned to live with them. However, climate change is increasing the frequency of wildfires and pushing these strategies to their limits.

#7 Emerging by (prime) numbers

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In North America, in certain years, vast swarms of cicadas burst into life for a few weeks. When they do, their noisy buzzing and clicking is impossible to ignore. Their mission is to mate and produce the next generation of cicadas. However, the young live in the ground for quite some time before they emerge – for one population, thirteen years; for another, seventeen!

Why so rarely? And why these exact intervals? Various theories have been proposed, but the answer probably has to do with mathematics. Because 13 and 17 are prime numbers, the two populations emerge together only every 221 years – the least common multiple of the two numbers. This strategy reduces inter-species competition for food, while also preventing natural enemies like birds from getting used to this food bonanza and growing in numbers. After all, it is fiendishly difficult for the predators to adapt to the prime number rhythm.

#8 Dinner in the dark

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Probably the largest mass migration on Earth takes place underwater: the daily vertical migration of zooplankton in oceans and lakes. Although the creatures involved are tiny, together they form a huge biomass. At night, these animals – mostly crustaceans barely visible to the naked eye – rise from the depths to the surface to feed on microscopic algae. During the day, they retreat again from the light-flooded water layers. In this way, they avoid fish and other visual predators that hunt by sight. Feeding under the cover of darkness is simply safer.

To punctually begin their ascent and descent, water fleas, copepods, and other little zooplanktonic organisms have developed an internal clock that is chiefly calibrated by daylight. Unfortunately, artificial light sources can disturb this rhythm. Near large cities, light pollution is already measurably weakening the migration of zooplankton – with consequences for water quality.

More Research and Innovation:

• Against the clock: Sleep, hunger, alertness – the body’s natural processes are regulated throughout the day by our internal clock. But what happens when the body’s clock gets out of sync? Read now!
• 10 creatures that defy heat: From hydrothermal vents on the ocean floor to the deserts on all five continents: We present ten organisms and their tactics for dealing with heat. Read now!
• The rhythm of life: Axel Imhof studies how epigenetics regulate processes within the cell, preserve a molecular memory after division – while still allowing flexibility. Read now!

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