To help set the stage for this series of posts examining different winter-coping strategies, Mother Nature gave us the first taste of winter last week. Air temperatures dropped to 11 degrees Fahrenheit, wind chills plummeted to below 0F, and we received about 4 inches of snow. I ventured out a couple of times in the hopes of collecting images that captured the collision between fall and winter and came away with shots of golden aspen coated in snow, steaming ponds, and frosted moose.
It was breathtakingly beautiful and brutally cold, but it was short-lived. Within two days temperatures rebounded into the 60s and most of the snow had vanished except for patches lurking in the shade of north-facing slopes. I think the foliage season is pretty much done, however, as the extreme cold temperatures really accelerated the leaf senescence process. Many aspen groves now stand naked, and others are dressed in crisp, brown leaves. Despite the loss of their leaves, aspen can continue to photosynthesize using the relatively high levels of chlorophyll in their bark. This is just one of many tricks that organisms at high elevations and latitudes use to deal with the relatively short growing season.
Caching food is a strategy that some animals use to take advantage of temporarily abundant food resources like seeds and nuts. Cachers generally store food in hidden locations across the landscape for consumption at times when other food isn’t available. A few weeks back I wrote about the impressive memory capabilities of some of these food-hiders, but how do these animals accomplish feats such as remembering where they cached 80,000 seeds, or avoid having their stashes raided by hungry thieves? Regarding the first task, animals that store food across the landscape typically have a highly developed hippocampus—this is the region of the brain responsible for spatial memory. In addition, the hippocampus can undergo seasonal changes, increasing in size during periods when the animal relies heavily on recalling where it placed its snacks, and decreasing for the months when it feeds on fresh food. This is handy because the brain eats lots of calories, and downsizing a portion of the brain that isn’t needed for months of the year can save a lot of energy.
In addition to seasonal plasticity, the hippocampus can exhibit neurogenesis of new hippocampal cells in response to certain types of stimulation. Research in birds has found that some forms of memory stimulation result in high levels of neurogenesis, and wild birds typically have much higher levels of neurogenesis than captive birds. Perhaps there’s hope for those of us who have a hard time recalling where they placed their keys, phone, or car. For some memory exercises I suggest hiding 100 M&Ms around your house, and then trying to find them again in a week. You’ll either have a better spatial memory, or your house will be overrun by rodents and/or children.
One thing to keep in mind when you’re hiding your candies is that many cachers use tricks to help reduce the memory load. One widely used tactic is stashing multiple food items in the same area. Clarke’s nutcrackers, for example, often stash four or five seeds together at the base of a given tree. Squirrels do this too. One of my squirrel-friends at the nearby Caribou Ranch Open Space has stashed dozens if not hundreds of pine cones in one 3-meter square area. This little fellow has taken caching to another level; I’ve sat and watched as he stuffed four large pinecones, one after another, into a single hole. Once a hole is full of pinecones, he carefully covers it up, tamps it down, and moves on to the next hole.
An important aspect of caching is that the food item being stashed needs to have a good shelf-life; perishable items that spoil rapidly are not good candidates for caching. Nuts and seeds, which are well-suited to long-term storage, make up the bulk of cached food items. In very cold or very dry areas, however, normally inappropriate food items like meat can be stored long-term without spoilage. In our neck of the woods, the primary cachers are the chickadees, nuthatches, jays, crows, ravens, magpies, red squirrels, and the champion cachers, the Clarke’s nutcracker. Many of these species visit backyard bird feeders, and you may have seen a nuthatch stashing sunflower seeds in bark crevices around your yard. This is caching in action.
Other than trying to remember where you placed your food, cachers have to contend with sneaky cache-thieves who would gladly dig up and eat the buried treasure. In fact, some individuals make a living as thieves, and invest a good portion of their time spying on others. What’s an honest, hard-working squirrel supposed to do when thieving spies are everywhere? Researchers have discovered that caching squirrels will engage in some deception of their own; if they think they are being watched, they will pretend to bury something in the ground. The would-be thief then wastes his time searching for a non-existent food item, while the honest cacher can go hide his food somewhere else. Birds also have to contend with thieves, and some species exhibit exceptional levels of awareness. Western scrub-jays, for example, keep track of which of their colleagues are watching them when they cache food. If an individual that is a known cache-thief was watching, the original bird will return to the cache, dig it up, and move it when they aren’t being watched by the thief. And for good measure, in addition to being able to identify likely thieves, and remember where they stashed hundreds or thousands of food items, scrub-jays keep track of the duration of time a food item has been stored, and if it has passed the “use by” date, they won’t return to dig it up. Not bad for a bird brain.
We all know some hoarders in our lives, but in this case, I’m referring to animals that stuff their larder with food so as to sustain them over the course of the winter. A number of rodents are fervid hoarders; if you’ve ever had a pet hamster or rat, you’re likely familiar with this behavior. I happened to have a trio of pet rats when I was younger, and in one of my more misguided moves, I decided that they could live a free-range life in my room. Things seemed to be working out for the first couple of weeks, and then I noticed one of the rats scurrying along the floor with some dog kibble in his mouth. I followed the rat (named Brownie), and he led me under the dresser to a massive pile of dog food. Turns out that in their spare time, my rats had found the food bin, gnawed a hole in it, and had been stockpiling kibble under my dresser for the impending rat Armageddon. But these rats hadn’t put all their eggs, or kibble, in one basket; they also had stashes in my desk drawers and under the desk. Thus ended the short-lived experiment with free-range rodents in my room.
It also happens that the second largest rodent in the world, the North American beaver, is a hoarder, with a twist. Beavers are well-known ecological engineers, modifying the landscape with the construction of their dams. It has been proposed that one reason beavers build dams is to create deep pockets of water in which they can store a winter’s supply of sticks and branches. Beavers subsist on a delightfully varied diet of twigs, branches, bark, and sticks. I’m kidding, of course; that’s just their winter diet. In the spring and summer they really spice things up by adding leaves and aquatic plants to the mix. Once the ponds freeze over, these industrious hoarders are able reach their larders under the ice via underwater entrances in their lodges. So in essence, beavers drastically alter the landscape in the process of making aquatic tree-branch refrigerators that only they can access during the winter. That’s a pretty neat trick for a rodent.
The term “hyperphagia” really just means elevated levels of food consumption and can be applied to animals that have increased appetites prior to migrating or hibernating. The supermigrants I covered last week experience pronounced periods of hyperphagia because they need to rapidly put on fat reserves before their marathon flights. But birds have to balance energy reserves with flight efficiency, and in case you’ve ever wondered, those fat turkeys that get served up for Thanksgiving meals are not flight capable. Being more earth-bound can free one up to pack on the pounds. Bears eat almost non-stop during the late summer and fall months in preparation for hibernating. Previously I covered some of the incredible statistics regarding caloric intake during the hyperphagia period, and caloric output during hibernation. But I didn’t cover this: Fat Bear Week. Alaska’s Katmai National Park and Preserve puts on a bracket-style competition in which the public votes for the fattest brown bear each October. These bears have been feasting on salmon for the previous month, and the results are impressive. Check out all the images in the link—the champion (Holly) was crowned last week, and it’s a crown well-deserved.
Yellow-bellied marmots also experience significant weight-gain prior to hibernation. These rotund ground-squirrels are close relatives of the woodchuck or groundhog (or better yet, the “whistle-pig”), and spend 7-8 months each year hibernating. Like the hibernating bears, marmots depend on their fat reserves to get them through this 200 day-long snooze. During this period, the marmot’s heart rate dips from 180+ beats/minute to 30 beats/min, their body temperature drops to 41 degrees Fahrenheit, and they take only one or two breaths/min. These physiological changes are crucial to helping them survive until the following spring. At that point they will slowly transition from zombie-marmot to living marmot, and resume their lives of socializing and eating.
Next week: TBD
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About the author:
Loren grew up in the wilds of Boston, Massachusetts, and honed his natural history skills in the urban backyard. He attended Cornell University for his undergraduate degree in Natural Resources, and received his PhD in Ecology from the University of California, Santa Barbara. He has traveled extensively, and in the past few years has developed an affliction for wildlife photography.