Warm Welcome

Natural History, March, 2001 by Peter J. Marchand

A beaver's lodge is its castle, particularly when ice covers the pond.

Northern winters can be long for a beaver--longer than for most nonhibernating mammals. Though equipped to gnaw through the hardest wood, beavers show little inclination to chisel through ice and thus are seldom seen from the time their ponds freeze over until spring melt, often long after snow has left the land. Yet beavers rarely die during the winter from cold stress or a shortage of food. The key to their success in the North seems to lie, in large measure, with their lodge--the most massive communal nest constructed by any animal.

Not until I actually crawled inside a lodge did I understand the full implications of this structure for a beaver's life under ice. My opportunity came early one winter in northern Vermont as a result of an unusual circumstance. Just before freeze-up, a local highway maintenance crew had rifted a colony's dam to prevent the flooding of a road. When the pond's water level dropped below the entrances to their lodge, the beavers, rather than attempting to repair the dam, vacated the premises. A month later, my students and I ventured onto the frozen pond to investigate the lodge and found the exposed entrances. We couldn't resist the temptation. Shedding some of our bulky clothing, two of us wiggled and squirmed (with an occasional push from behind by others) into opposite tunnels until we met in the middle.

That firsthand inspection gave me an insight into the winter lives of beavers that none of my previous studies had ever provided. Outwardly, a beaver lodge appears to be nothing more than a mud-plastered pile of woody debris--an unkempt heap of odd-sized sticks that occasionally reaches twenty feet in diameter and rises four or more feet out of the water. In this case, the interior turned out to be a marvel of neatness and cleanliness. The earthen floor of the lodge was worn smooth by the countless comings and goings of wet feet on silky clay. The walls and ceilings were trimmed evenly; not a single nub protruded to discomfort a huddling animal (which explained why the beavers in lodges I'd previously attempted to study had been so quick to chew off the ends of some temperature probes I'd inserted). A small chamber branched to the side of one entrance tunnel, apparently having served as a feeding platform just above the water level. It held a single scrap of food: a frozen aquatic quillwort, still whole and green. The main nest chamber, roomy enough to permit us to raise our heads and pass a camera back and forth between opposite tunnels, was devoid of any detritus--not a trace of food, fecal material, or odor.

The lodge had been empty for some time, but my initial sensation upon entering it was one of subtle warmth. Heat, conducted upward from the unfrozen water beneath the floor, maintained the temperature near the freezing point--considerably warmer than the snowy world outside. It was easy to imagine the relative comfort of an occupied chamber. A family of beavers crowded into a small space, huddling and grooming, can generate significant heat (it is not unusual for a pair of adults and two litters of kits to winter together). On a zero-degree day, I once recorded a lodge temperature of 60 [degrees] E well within a beaver's thermoneutral zone (the temperature range within which an animal can remain comfortable without raising its metabolic rate) and warm enough to melt a hole in the snow at the top of the lodge.

But there's another, colder side to this picture. Beavers face an energy dilemma every time they slip into the icy water for an excursion to their food cache. (In the fall, beavers stockpile tree branches underwater.) The compression of their fur in the water and the resultant displacement of air from the otherwise superbly insulated pelt, coupled with water's high capacity to conduct heat, greatly accelerate heat loss from the beaver's body and can render the animal hypothermic within about thirty minutes. The need to procure food can become a repeated trauma for the kits, which may enter the water daily in feeding forays lasting from less than five minutes to more than forty. (Adults may make fewer trips, subsidizing their energy needs with fat stores in the tail.) A foraging beaver benefits by being able to return to a warm lodge, where its body temperature can be quickly restored to the requisite 98 [degrees] E

But herein lies a potential problem. Four hundred cubic feet of earthen lodge, if allowed to cool, can quickly turn into a massive heat sink instead of a life-saving refuge. One way to prevent this might be for family members to stagger their foraging trips, ensuring that the lodge is occupied at all times--and this is where an unusual aspect of beaver behavior comes into play.

To maximize effectiveness in functions such as feeding and mating, virtually all animals maintain biological rhythms that are precisely cued to day length and seasonal cycles. Scientists aren't sure why or how, but beavers' biological clocks (and thus their activity patterns) drift out of phase with the day/night cycle in winter. Although long isolation in the lodge without external light cues would promote such drift (and I can attest to the darkness of a lodge interior), even infrequent excursions underwater should recalibrate internal clocks, since light easily penetrates ice and snow cover on a pond. Yet all across southern Canada and the northern United States, beavers display winter activity patterns based on a twenty-six- to twenty-nine-hour cycle, resulting in a considerable shift, over time, in their daily schedules. Having a free-running internal clock probably carries little risk under the ice, where predators are not a threat, but what is the advantage? One possibility is that in winter, staggered foraging tunes may maintain an equable indoor temperature, guaranteeing a warm welcome whenever a beaver returns to its lodge.