Pacemaker of the Hares

By Carl Zimmer|Tuesday, June 01, 1993
Bunny booms and bunny busts have deeper roots than bunny lusts. When dark spots obscure the sun, the clock runs out on bunny fun.

The snowshoe hare of northern Canada and Alaska offers a lesson in bouncing back from hard times. Every ten years or so, its population goes through a complete boom-to-bust cycle, collapsing almost to zero, then soaring back up just as quickly. A square mile of Yukon woods that had supported 600 hares in 1980 might have less than a dozen in 1985, and then 600 again in 1990.

Ecologists think they understand how the booms and busts come about. Snowshoe hares feed on the shoots of tree saplings, and when shoots are plentiful, the hares breed extravagantly. After a while the animals that eat the hares begin to multiply as well. When the hares have wiped out their food supply, and their predators have become numerous, starvation and hunting combine to drive their numbers down. But then the predators themselves starve to death, the tree shoots start growing again, and the snowshoe population explodes anew.

So far, so clear, but a central mystery remains. The ecology of the snowshoe hare varies a lot--in Alaska it eats birch, in the Yukon willow or spruce; in some places it gets eaten by lynx, in others by coyotes--so you’d expect each population of hares to follow its own cycle. Amazingly, though, the snowshoe hares of the northern forests follow a single clock. From Nome to New Brunswick, no population of snowshoes is more than a year or two out of step with the others.

Now zoologist Anthony Sinclair and his colleagues at the University of British Columbia claim they’ve found the master pacemaker of the hare population. They say it’s sunspots--dark patches of intense magnetism on the surface of the sun, whose number waxes and wanes in a cycle that averages 10.6 years.

The theory has been proposed before and found wanting--in part, says Sinclair, for lack of good data on the snowshoe hare population. Sinclair’s group, however, found a way to trace the rise and fall of the hare population over more than two centuries in a single Yukon forest. When a hare nibbles the tips off a spruce sapling, the tree receives a scar that gets preserved in its rings; that scar can later be dated to a specific year. By counting these scars, and by assuming that more scars in a given year indicate a greater number of spruce-nibbling hares, the researchers were able to reconstruct the snowshoe population cycle back to 1750.

They found that it closely tracked the sunspot cycle--but not in a simple way. The sunspot cycle, as it happens, goes through a cycle of its own. Just as some high tides are higher than others, so too does the sunspot peak vary greatly in intensity. Sinclair and his colleagues discovered that during periods when the sunspot cycle is intense (such as during the past 45 years), the snowshoe hare population follows it almost exactly. But when the cycle is weak, something different happens: the snowshoe cycle still averages about ten years, but it drifts out of sync with the sunspot cycle.

This pattern, says Sinclair, is analogous to that of our own sleep cycle. Our bodies have an internal clock for sleeping and waking that’s not quite in step with our 24-hour day. But the external cycle of sunlight and darkness keeps us in sync by constantly readjusting our clock. When experimental subjects live in caves without sunlight, however, their cycles drift away from the norm--they may adopt a 25-hour day, for instance. Similarly, says Sinclair, snowshoe hare populations have internal clocks--set by their interactions with food and predators--which they follow when sunspot activity is low. But when sunspot activity is high, it forces the hare populations into synchrony.

How? Sinclair speculates that high sunspot activity might influence the weather in northern Canada and Alaska in such a way as to reduce the food supply. No matter where the hares were in their population cycle, the drop in their food supply would nudge them toward the nadir, and from then on they would follow the sunspot cycle until it weakened.

Although there is some evidence that the sunspot cycle may affect global weather patterns, the whole subject remains controversial. Connections between sunspots and terrestrial cycles--and many have been suggested over the past century--have had a tendency to crash and burn. But Sinclair has confidence in his tree-ring statistics. This is the first link between sunspots and animals, so it is unusual, he says. But I think there’s more evidence of the sunspots’ influence out there. We’ll just need a few more decades to find it.
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