For warm-blooded animals, the ability to maintain relatively constant body temperatures, called “homeothermy”, is critical to life. Regardless of outside temperatures, body temperatures must be maintained within a relatively narrow range. When outside temperatures drop below internal temperatures, body heat is lost. To counteract this heat loss, animals may increase heat production (increasing fuel), decrease heat loss (improving insulation), or lower internal temperature needs (adjusting the thermostat). Biologists refer to these responses as maintaining energy balance.
The biology of energy balance has always fascinated me: it is an interesting lens from which to view animal behavior, particularly during the long, Montana winters. Many of an animals’ physical attributes and behaviors are directly related to balancing this energy budget. For example, body size; because heat loss is a function of surface area, small mammals and birds lose a greater proportion of their body heat than large ones under the same conditions. Thus, all things being equal, larger animals are better suited to colder environments than smaller ones. But our winter world is filled with animals of all sizes, owing to the diverse and remarkable strategies for solving energy balance problems. This winter, I have been contemplating this large versus small contrast, using two of my favorite locals - Grizzly Bears and Golden-Crowned Kinglets. Let’s take a “deeper dive” into how one of our largest and one of our smallest residents survive Montana’s winter energy crunch.
Grizzly Bears - Inactive and Unfazed: Hibernation is often used to describe any dormancy during the winter, and for practical purposes bears do hibernate. However, “true” hibernation is met by three specific conditions: reduced metabolism, slower heart rate, and lowered body temperature. Bears meet two of the three requirements for hibernation: their metabolism drops to roughly 25% of normal levels. Although remarkable, this is a relatively small savings compared with the 95-98% metabolic reductions of some true hibernators, like the little brown bat. While bears’ heart rates and breathing rates slow down to almost 20% of normal levels, they only minimally reduce their body temperature; less than 10%, from roughly 99°F to 92°F. Conversely, true hibernators reduce body temperature to just a few degrees above their surroundings, which minimizes heat loss. For example, the Golden-mantled Ground Squirrel reduces its body temperature during hibernation by as much as 50°F! Instead of trying to make themselves the same temperature as their surroundings, bears simply put on a thicker winter coat, which nearly doubles during winter. Their thick layer of fat helps with insulation too.
What I find most fascinating is that bears enter hibernation in a state of obesity and then remain in a state of inactivity without eating, drinking, or excreting waste for 4-5 months. They live off their fat reserves, which when metabolized, release water that is recycled internally. They do this without suffering any long-term detrimental effects that humans would experience under such conditions, such as bone loss, muscle atrophy, bed sores, diabetes, heart disease, and kidney failure. In fact, grizzly bears have nearly 100% survival while hibernating. When spring arrives, they emerge from their dens almost as physically fit as when they entered.
Golden-crowned Kinglets - Living on The Edge: This diminutive bird is one of the smallest songbirds in the world, weighing about 5-6 grams, or roughly the weight of two pennies. Given their size (~1/45,000 the weight of the average male grizzly bear), the Kinglet faces some severe challenges in balancing winter energy budgets. Yet, they remain here all winter through a daily dance on the edge of death. Given their size, they are remarkably adapted for winter, with feathers making up about 8% of their body weight. Furthermore, about 80% of those feathers are dedicated solely to insulation and play little to no role in flight.
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Despite this insulation, to survive the long nights and temperatures as low as -40°F, the Kinglet must remain almost perpetually active from sunrise to sunset, foraging for dormant invertebrates in the forest canopy and needing to eat up to 3 times their body weight each day.
Unlike bears, Kinglets can’t afford to increase their insulation or put on heavy layers of fat; they need to stay light for flight. Instead, they fluff out their feathers to nearly an inch in thickness to trap heat - basically, putting on a down coat. Without thick fat reserves there is a perilously small reserve for energy production, and even a few hours without food during the day can mean death. They must find enough food to not only survive the day but provide enough fat storage to fuel heat production through the night. As temperatures drop, Kinglets huddle together for warmth. Fluffed up, they shiver through the night, depleting their fat reserves until the sun rises and the daily struggle to not freeze begins again. One bad day of foraging or single prolonged storm can spell disaster. In fact, almost 90% of individuals will not survive the winter and some local populations may not survive at all.
Kinglets counteract this high mortality rate with a high reproduction rate. Broods of 8-9 young, nearly double that of most songbirds, Kinglets engage in what biologist call double-clutching. While males feed the young of the first clutch, females build a second nest and begin incubating a second set of 8-9 eggs. With average nestling success around 80%, Kinglets produce lots of young, especially compared to Grizzly Bears; it can take a female Grizzly Bear 10-years to successfully raise her first cub.
As you are enjoying wildlife this spring and summer, take the time to learn about how those species make it through the winter. You’ll almost certainly learn something cool! With winters as harsh and cold as ours, I guarantee every critter has something remarkable to reveal about maintaining energy balance.
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