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Wednesday, Oct. 4, 2000

WILD WATCH

NATURE'S GUIDING LIGHT

Many life cycles under the moon


A fluttering of powdery wings, silent in the night, and the moon moth came, drawn to the proverbial candle flame. Its guidance system, fine-tuned over millions of years of evolution to a satellite system predating our GPS systems by billions of years, was overwhelmed and confused by a modern source of light mere hundreds of meters away. Captured in its beam it fluttered, misguided by the light.

The moon moth and its relatives rely on the moon as a beacon. Bright electric lights, however, can send the wrong messages.

While capable of tracking their flight in terms of their angle to celestial bodies, moths have no means of knowing that the light they flutter toward is not the moon. The time since our discovery of electricity is a mere wing-beat in evolutionary terms.

Moths are used to tracing their flight at a given angle to the light; when that source is 380,000 km away there is no chance of confusion, nor any likelihood of moving substantially closer to it, and a steady course results. When, instead, the source is an electric light atop a pole, situated at a forest camp site, then the moth, in trying to keep a constant angle to it, merely approaches it in a spiral until it is battering at the glass of the impossible -- a moon up close!

Why doesn't it just go away? one person remarked. That, of course, is exactly what it is trying to do, but programmed into its tiny brain is a remarkable ability to navigate around the night sky using natural sources of light, which, from a moth's perspective, are infinitely distant. In trying to fly away, it can only use as a direction beacon the very source of light that is confusing it; so once again it spirals toward the light.

Some creatures, though, have learned to take advantage of our artificial moons. Earlier this summer, a light near a rotenburo I favor drew in not merely moths, but also a brown hawk owl, a first for my bath list.

The moon's influence is felt by many more creatures than merely those relying on its light as a navigational aid. Through its driving of the tidal cycle, it affects enormous numbers of marine species. They all depend on the daily rising and falling of the tides to measure their lives and co-ordinate their behavior. Most shore crabs and other intertidal creatures have body clocks that are constantly re-adjusted by the lunar tides.

Watch any mudflat and the creatures responding to the tidal influence of the moon are soon apparent. In the Ariake-kai, the great inland sea of Kyushu, mudskippers are so famous that there is even a museum/aquarium devoted to them. Their life cycle is tidally driven too; there are various small crabs that fit into the same category. And what the prey do, so too must the predators. People who hunt the mudskippers must follow them when the tide is out. The mudflat-crab-hunting specialist, the rare Saunders' gull of China, must also follow the tides, for only when the crabs are on the surface of the mud is it able to pounce on them from the air.

The moon's effects are, anecdotally at least, far reaching. These effects influence the activity patterns and behavior of such diverse creatures as the West Indian scarab beetle, the spawning habits of a fish called the Californian grunion, and the tropical marine palolo worms, all of which rise to the sea surface to breed on the same night.

Experiments have shown that creatures such as oysters and fiddler crabs are sensitive to tidal gravitational forces. Their sensitivity stems from what are known as statocysts, which consist of a tiny mass anchored by sensory hairs within the cavity of a cyst. As changing gravitational forces cause the statocyst's tiny mass to move, the animal can, by way of the sensory hairs, detect the motion.

Personally I'm glad I can't. Imagine if we were equipped with statocysts; we would have the benefit of being able to feel seasick miles from the sea, as our sensory hairs would tell us what state the tide was at.

Night-flying insects are far fewer, reduced by as much as 80 percent, around the nights of the full moon. Reasoning has it that there is a clear survival benefit to be gained by being less visible to nocturnal predators. On the one hand, it makes sense that a usually nocturnal species would be less active on brightly moonlit nights, because for them the amount of light more closely resembles daylight, which they normally shun. But wouldn't that apply to predators too?

On the other hand, those nocturnal birds known colloquially as nightjars and nighthawks apparently hunt more on moonlit nights because they can see insects better. The moon affects other night flyers in a different way. Tropical fruit-eating bats prefer to fly on moonless nights, feeding more frequently then. This strategy is perhaps to help them evade their main predators, the owls.

An unusual species of shorebird found in Japan, the painted snipe, is renowned for its sexual role reversal. It is the female that is brightly and attractively colored, and in addition she has a bright white stripe on her wings. She typically calls at dusk and dawn; however, she will call throughout the night around the time of the full moon, accompanied by distinctive wing-flicking behavior. During her display her white wing bars reflect the moonlight for her drab mate to see.

Many birds are night migrants, and the moonlight around the full moon increases their restlessness. Studying night-flying migrants without using radar is a well-nigh-impossible task, but one man I know has spent weeks sitting staring at the full moon -- counting migratory birds passing in front of its disk. Some would claim there is always a lunatic fringe to any science; others, of course, insist that all birdwatchers are lunatics.

In an ancient ritual, horseshoe crabs emerge from the sea by the thousand. Like spawning frogs, males jostle around the females, instinctively eager to compete with each other. When the tide is at its highest, driven by the aphrodisiac moon, they mate, releasing their gametes onto the sand. Two weeks later there is a mass hatching and millions of tiny larvae are washed out to sea.

Whereas the horseshoe crab emerges from the sea at the bidding of the moon, a land crab in Japan emerges from the forest. Sesarma haematocheir lives on wooded hillsides in warmer regions of Honshu, such as above the Ogamo River, near Kyoto. These crabs mate on land during summer, and the females carry the developing eggs under their abdomens.

When the embryos are sufficiently developed, the gravid females await the moon's bidding. Then, at dusk on days around the full and new moon, each female lumbers toward the river, wades into the water, grips the stones and wriggles to release her young into the water which are carried away to the sea.

Mass breeding events, triggered by moon-generated tides, are a common feature in nature. Tiny creatures living solitary lives must somehow signal to each other their readiness to breed; they must all receive some clue that warns them to be prepared for an orgy of reproduction. The subtle cyclic nature of the moon's movement, light and gravitational pull provide those triggers; there are many that respond, each to a different part of the cycle perhaps, but each in its own way to the lunar influence.

E-mail questions and comments to MarkBrazil@compuserve.com or by snail-mail c/o The Japan Times.


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