The red squirrel rested in his tunnel on a lumpy bed of walnuts, chestnuts, a scattering of hazelnuts, some hickory nuts – his stash for the winter. He had ventured far down the hillside, right to the border of the territory of an older more aggressive squirrel, to sniff out fallen acorns to add to his cache. Now the scents of nuts in his den filled his head. He would probably eat them first, when the coming cold set in and stopped his searches for whatever forage the forest might still conceal.
He sat in the dark in his excavation under a log, which was itself buried further under a pile of pine cones two feet deep. He had spent his summer afternoons high in the branches, cutting down those cones, then running down the tree trunks to retrieve and carry them to this hoard.
He had piled them against a dead tree stump, between a pair of fallen logs. This kept them in the shade, where they would not dry out, open up, and release their pine nuts. He would be counting on those nuts to sustain him through the rest of the cold season ahead. All summer he had worked to amass this cache, and now he would defend it with his life against all comers.
The morning air outside was chill – fall would be a short season here. The squirrel’s ears had already grown their winter tufts. In the branches far above, the remaining pine nuts were gone. The scales on the smaller cones he had left in place had dried and opened wide. All the pine nuts inside had floated off and dispersed across the forest.
There was not much left at ground level either. No more mushrooms or truffles waited to be found beneath the undergrowth. The squirrel had searched out all the fallen forage in his territory, even some nuts that had been resting in the grass for years and had lost most of their scent. He was now done searching – he would be spending the next half of this year right here hidden in his midden.
* * *
The snap of a twig brought the squirrel’s head up. He looked around to focus his ears toward the light at the end of his tunnel, and the sound of another, bigger branch breaking brought him to his feet. Immediately he tensed, whiskers twitching, and readied to rush out and confront whatever commotion was out there in his territory. But as he turned the ground shook, and the entrance to his tunnel caved in along half of its length. His fight-or-flight impulses were thwarted by the fact that he was now buried in his own den in pitch darkness.
The ground around him continued to quake under pressure from above. The solid roof over his head – the underside of a log – groaned and shuddered. Then through a landslide of earth, a streak of bright daylight flooded past one edge of the trunk. Finally, with a rush of falling dust and breaking sticks, the log lifted up and flew away.
The squirrel was out through the gap the instant the daylight appeared. The impact of a huge paw shook the ground just behind him. The spot where he had been resting seconds before was now a crater two inches deep, in the shape of the imprint of a bear track wider than he was long. The squirrel dodged through an avalanche of pine cones and dashed to open ground. He spun around and found a massive bear standing over his midden, working to dig up chickarees.
The squirrel let out a stream of invective. He ran back so close to the bear that he broadcast his staccato chirps and buzzes straight up at the mountain of fur. He bounced three meters away when the bruin stepped over a fallen trunk, then he rushed right back again, never stopping his harangue. He circled the ponderous beast with angry chatter, but this intruder took no notice.
When the squirrel had appeared, the bear had ceased her digging. She shifted her attention to an inspection of the cones she had trampled. The squirrel kept up his rant until all the forest knew what was going on, but to no avail. The bear was an immovable object, parked on top of his home, calmly chewing on his stores of nuts.
The squirrel exhausted himself and his scolding trailed away. He looked around, past the oblivious intruder, and realized he was out in the open. And that he had announced his presence to any hawk or stoat within earshot. He grew quiet, then backed farther off and sought cover.
* * *
With the afternoon, the sky shaded to overcast. Chill gusts of wind raced up through the canyon to scatter leaves across the hillside. The squirrel returned to his midden, but the site was unrecognizable. Eventually he realized that his dead stump had been reduced to shards half buried in plowed ground. Detached pine cone scales littered the area. He stood on his haunches and listened through the stiffening breeze.
Nothing remained of his pile of cones and his cache of nuts. The scent of bear hung everywhere. In minutes the big looter had devoured the larder the squirrel had spent the summer stocking. The bear was provisioning her own up-coming hibernation, at his expense.
He searched the slope, poking his nose down into the soil. The acorns, chestnuts, walnuts were all gone. After a while he found a few stray pine nuts, which he stashed in his cheek pouches. Then he set off to find a more secure site for another midden. He would have to discover a lot of food in a little time to last him through the dark days ahead. But there was not that much left to be found. He had already scoured his territory, uncovering seeds lodged in the most unlikely places. Elsewhere in the forest other squirrels had staked out the most fertile territories – they would defend their own nuts beyond the point of drawing blood.
Still, all he knew was how to gather and stash. So he busied himself with the search for provisions – inspecting the forest floor in hopes of finding more nuts where he had looked many times before. He paused again to test the wind. It was already carrying the scent of rain.
* * *
A month later the site of the bear dig was a muddy patch on the forest floor. A succession of storms had passed through the area and the ground was saturated. Below the surface one lost acorn was soaking in rain water.
Despite its tendency to fall straight down like a rock, that acorn had defied gravity and had risen up-hill to a site higher than the tree top it came from. The trip had taken it years. It succeeded passively, by being delivered to the new site in the cheek pouch of a chickaree. The squirrel had found the strength to carry the acorn up slope by eating other acorns. The mother oak tree had provided the energy that moved a few of her seeds out of her own shadow, and planted them in a sunnier spot.
This acorn was found resting in the rocks. Now it was coming to life in plowed soil. A matrix of dried proteins within the seed, packed in crystalline storage, was now starting to dissolve into the water the acorn had absorbed. In that process, a switch was being thrown. Those particular proteins controlled the activity of the cells in this seed. The cells were coming to life as they swelled with rain.
The germination switch synchronizes what happens inside the acorn with what happens outside. It stops the seed from breaking dormancy at the wrong place or time. The switch could detect the color quality of the light filtering down through the soil. The acorn would not germinate too deeply buried, it would also not germinate exposed on the surface where it would dry out in the sunshine; but then, it would not germinate in deep shade either.
Successful seeds send up their shoots only from spots that are unusually wet but also well-drained. The seedlings succeed because the germination switch can sense the reliability of the water supply. The seed could swell with rain water, but if that water did not linger, the seed would not germinate. Instead, it would dry out again, shrink back to dormant size, and wait for better conditions to come to it.
The germination switch predicts the springtime suitability of an acorn’s situation based on what happens in the winter. This ability to coordinate germination with environmental cues has been improved over millions of years of selective evolution. The big seed will rest in stasis for decades – until it dies – waiting for the ground to shift around it and provide better conditions.
Most of the weight of acorns is starch. That investment in starch serves two purposes. Most of it goes to feed squirrels – and jays, and acorn woodpeckers – and any other of the animals that move oak seeds around through the forest. But some of it survives to fulfill its other purpose – to power the germination of seedling oaks.
In this buried acorn the starch was already being broken down into sugar. The sugar was being built back into the walls of growing cells. A few weeks after the acorn had broken out of years of dormancy, its tip swelled and a tap root broke through into the soil. The root tip sensed the direction of gravity and commenced to grow straight down. By the time the winter began to thaw and a green growing oak shoot emerged into the air, the tap root was two feet long. It continued to grow down, staying level with the receding water table to support the growing shoot as it expanded into the light. By summer, a sapling oak was well established over the spot of the bear dig.
* * *
Fifteen years later the oak tree had grown wide and tall enough to cast its own trunk in deep shade. The tree had been growing faster every year. The ground at its base retained no trace of the circumstances of the founding acorn’s germination.
This season the tree would pause and shift its growth energy to the production of its own acorns. For years now, it had flowered and set seed. But each year it had aborted those fruits long before they matured, and had continued with its own expansive growth. This year would be different. Another switch had been thrown, and the tree’s energy would now be committed to carrying all of its seed to maturity. This switch was a counter. It would only commit the oak to total seed set if a minimum number of years had passed since the last total seed set. And, like the acorn’s germination switch, the seed set switch was attuned to environmental conditions. It sensed a series of cues – a late winter frost followed by an unusually wet spring – that were broad enough that they would also be noticed by the other oak trees across the area.
The seeding switch coordinates what happens inside the oak tree with what was happening outside of it. All the oaks in this forest carried the same switch. They were all counting the years since the last big seed set. They had all felt the same environmental cues. This meant that this year they would each be switching their energies from their own growth, to the production of all the seed they could, all at the same time.
The synchronization of seed production is another facet of oak life improved over millions of years of evolution. It allows the oak trees to manage predation on their acorns. There would be a surprisingly heavy fall of them this fall. More acorns would be available than all of the animals that customarily eat acorns would know what to do with. Acorns would be carried around all over the forest by all sorts of animals, and more of them than usual would be misplaced or forgotten. There would be a bumper crop of young chickarees in the spring – after high rates of over-winter success for squirrels in their middens. It would also be a good year for bears.
Goheen, J R & Swihart R K. (2003) Food hoarding behavior of gray squirrels and North American red squirrels in the central hard¬woods region. American midland naturalist 158:403-14