ENERGY, THERMAL, AND WATER RELATIONS
OVERVIEW: We combine here several aspects of environmental factors. First, we need to consider the energy dynamics of animals as related to food needs and maintaining thermal balance. While water is an essential nutrient in terms of metabolic processes, we look at here with special emphasis on its role in preventing hyperthermia through evaporative cooling. Since our focus is on quadrupeds, we are concerned with both homeotherms and poikilotherms. An underlying issue in the environmental relations of both homeotherms and poikilotherms is how well they can adjust to threats from extreme ambient temperatures through use of external objects.
Without delving deeply into the physiology of heat balance or energetics, we need some understanding of strategies that animals employ to maintain a healthful thermal balance as conditions around them are constantly changing, often reaching extremes which they surely might not survive without making internal physiological adjustments, but also by using external sheltering structures -- if available. It is this latter response that most interests us here, because in dealing with habitat issues we define these functional structures as a form cover, namely thermal cover. (What other primary function does cover offer?).
"COVER" was not listed as an environmental factor per se, but rather is an resource used to solve certain environmental challenges, such as ambient conditions that lead to detrimental rates of heat exchange. This may seem confusing, but keep thinking about what "factor" is affecting the animal directly, and from there consider in this case that cover is a means by which the animal copes with that factor, i.e. the factor is environmental thermal conditions (we don't call this "air temperature" alone because other aspects are involved as well-- ground or water temperature, solar radiation, wind, humidity, wetting by rain, etc.)
MAINTAINING BODY TEMPERATURE: Homeotherms are animals that maintain a relatively constant body temperature, and this is most often either higher or lower than the ambient temperatures typically varying widely with weather, season and time of day.. Homeothermy serves to maintain an internal temperature that supports the most efficient metabolism for carrying on all phases of life-history, as well as permitting birds and mammals to exist in a far greater range of thermal environments than poikilotherms. In contrast, poikilotherms, e.g. reptiles and amphibians (and most all other animal forms), are much more limited in the scope of possible activities and thermal range, but at the same time their food needs per unit time are far less. While homeotherms can remain completely active even as ambient temperatures vary widely-- diurnally, within a season, and among seasons, there is a relatively high cost attached to this adaptation. Energy required is much higher; hence food is correspondingly greater than in poikilotherms whose body temperature is maintained primarily by inputs from the ambient temperature. When environmental conditions (thermal, that is) are unsatisfactory, poikilotherms just “sit it out,” waiting for the temperature to come into their range. In some regions it never does, so we find few if any herps in those places. Homeotherms also can “sit out” unfavorable conditions, but do so far less frequently: hummingbirds and some other birds go into “torpor” (a much reduced metabolic rate and a drop in body temperature) at night when they would be inactive anyway and could not “afford” to spend calories on maintaining their normal body temperature and high metabolic rate). Some rodents show deep hibernation, such as our “golden gopher” (actually a ground squirrel, the “thirteen lined g.s.) does so underground during winter. Bears go into a semi-hibernation dropping their heart rate to only 3-5/min, but not going into a deep sleep. Also nestling altricial birds apparently do not maintain as high a temperature as adults when they are sitting in the nest, growing fast but not at all active. And to a much less-dramatic degree, recall the periodicity of growth—hence metabolic rates-- in deer as found by Wood et al (Food Lecture II): the animals are up and remaining active during winter, but have essentially stopped growth and probably have dropped body temperature by a degree or two.
BODY SIZE AND METABOLIC RATE: Another key factor is the relationship between energy needs (through caloric intake) and body size. The graphic below shows clearly for homeotherms how, with increasing size, the caloric need per kg of weight decreases exponentially. The relationship would theoretically be
K (calories required or metabolic
rate) ~ WT2/3 2/3
assuming that the animal is likened to a sphere wherein energy production is a function of body volume and energy loss is function of the body surface area. However, empirical measurements indicate the relationship is more like
K (calories required or metabolic
rate) ~ WT3/4
in which the amount of surface per volume is greater due to appendages and elongate shape.
An example of this relationship relative to winter conditions for deer was modeled by Moen (1968). In the graph below, he has estimated differences in ability to maintain a thermal balance at or above thermal neutrality (below which the animal is loosing calories faster than it’s producing them on its current diet) according to their body weight. The hypothetical animals are standing out on a cloudless night, radiating to a – 40o C night sky at an air temperature of 0o C.
The graph shows the decreasing ability of each size class to maintain the balance as the wind velocity increases, with the smallest animal (e.g. fawns) being at the greatest disadvantage. Remember, if fawns cannot make it, there soon will be no population there. Of course the deer solve the problem by retreating to cover—preferably a canopy of conifers where wind (e.g. wind chill) is at or near 0, and loss of radiant heat is reduced also by the conifers.
DIET AND ENERGY BALANCE: In another graph in this set from Moen (and on reserve), we showed how for a deer of given size, improving quality of diet (e.g. digestible calories/unit intake) permits an increased ability to stand out there without going into a negative thermal balance). The three levels of diet quality there were :"full-fed deer" well above the best of available forage in a northern forest in winter (e.g. northern Minn.), "maintenance diet" typical of good quality of plants available in that northern forest, and "starvation diet" as in an area lacking decent amounts of the good or fair items comprising the maintenance diet.
THERMAL ADAPTATIONS: One more important point to help understand the energetics of maintaining thermal balance relates to species-specific adaptations above and beyond diet and body size. Some species are well adapted to handle cold and other to handle heat, so we need to be aware of what is “normal” for any given animal. Homeotherms show a minimum or constant energy expenditure over a range of ambient temperature known as the thermal neutral zone. This of course varies with their size and with the current state of their insulation – pelage, feathers, subcutaneous fat deposits, etc. Fat layers below the skin are critically important as insulation: many marine mammals could not survive continuous contact with icy waters without their heavy internal “coat” of fat. Mammals in tropical regions characteristically do not carry subcutaneous layers of fat: such might well make it difficult for them to reduce body heat when ambient temperatures exceed the maximum tolerated by the body. Beyond the physiological and anatomical adaptations, for any animal, when the environment exceeds the limits of its thermal neutral zone, it will act to avoid further exposure (i.e. energy cost) by moving to a more protected site.
Ecological physiologists in Alberta, Renecker and Hudson (1986) examined the relationship between ambient temperature and metabolic response in moose. The graph below shows respiratory rate relative to ambient temperature—but the pattern was exactly similar that of metabolic rate—i.e. the rate of burning calories.
The two types of circles represent two animals of the same size and sex being tested during winter. Note that the experimenters were unable to determine the lower limit of thermal neutrality, even as they dropped the temperature down to – 40 degrees C. This seems to explain how moose can winter in the Brooks Range of northern Alaska (as long as they have willows projecting above the snow to feed on). But the same moose began to experience “heat stress” at a couple of degrees below freezing. This surprise finding suggests the basis of a frequent observation that, in late winter as the sun is higher and reflecting with more energy off the snow, moose seek shelter in conifers. It had long been assumed this was a form of cold avoidance, but these experiments indicate that it is rather to avoid overheating. Obviously moose adapt physiologically to the increasing temperature of spring and summer, but it is known from zoo observations that this species is quite intolerant of summer heat levels that are readily tolerated by deer and bison. We described a case where, at Isle Royale on a sunny midsummer day in the 70s (F), a moose cow defending her calf against wolves, suddenly abandoned that defense and jumped in the water-- undoubtedly on the verge of severe effects of hyperthermy.
STORING ENERGY FOR TOUGHER TIMES: Another very important adaptation of many birds and mammals is their ability to store energy in a favorable season in order to get through the demands for heat production in seasons of greater cold and/or lesser food-availability. The annual fluctuations in body weight for animals at maturity in the graph of Wood et al’s (1962) experiments (see Food Lecture II) are accounted for mainly by accumulation and subsequent depletion of fat deposits.
AVOIDING HYPERTHERMIA AND A KEY ROLE OF WATER: Homeotherms maintain their bodies just below the temperature at which serious or fatal damages will occur (i.e. proteins are "cooked".. Homeotherms not only can produce needed heat to keep the body warmer than their surroundings, but also can dissipate heat to keep themselves below otherwise damaging ambient levels. Common behavioral and physiological strategies include minimizing physical activity at the warmest times as well as avoiding direct solar radiation. Shelter from heat therefore becomes an important environmental resource. Evaporative cooling is used by many birds and mammals by perspiring water onto the body surface where, with evaporation, it extracts body heat, or by exhaling water vapor that has taken up body heat after it had evaporated internally. In both cases here the cost is body water, the level of which cannot fall below a fixed minimum without serious damage and death to the animal. Hence, water as an environmental factor becomes highly critical with both increasing environmental temperatures and decreasing water availability. Recall the example of Uros, large kangaroos living in one of the hottest and driest regions of Australia. It was found through marking of all animals that visited a water source, that some never came there, as was assumed they had to. It was subsequently discovered that animals coming to the water were those that used shade of trees for protection from the midday sun, while those that never came had access to caves for cover. It was concluded that the better cover function of caves was sufficient protection from heat to permit those animals to survive without taking in free water. This example has some strong habitat-management implications (for either increasing or decreasing local carrying-capacity for these kangaroos).
Birds and mammals show a wide diversity of adaptations for surviving while taking in little or no water, just as they show wide differences in ability to tolerate high temperatures in the first place. Taylor (1969) studied strategies in the eland and the oryx, two medium-large bovids found in arid parts of Africa. The eland can go for many days without water, but must eventually drink, while the oryx in the hottest, driest periods can exist without free water. He showed that oryx confine their foraging, which was mainly on green leaves of Acacia trees, to evening hours, when, in contrast to midday, the leaves have a higher water content. Also, the oryx has essentially two, semi-separate cooling-systems-- one for the brain, the organ most sensitive to damage from overheating, and another for the body core that contains the greatest portion of blood but can tolerate a higher temperature:. The brain is cooled by blood circulating through the nasal septa, site of evaporative cooling during breathing, and through the horns which act as radiators. If the entire body was maintained down at the temperature critical for the brain, the cost in water might be such that the oryx would require free water-- and hence would be denied much habitat that is otherwise suitable.
In hot, dry areas, artificial provision of water is a key habitat tool in wildlife management (more often for game species, but that need not be the criterion). Starker Leopold (1977 Chap. 13) describes how discovery that, if water were supplied, California quail could use large regions in which they were otherwise not present.. This led to development of the widely used “gallinaceous guzzler” (See figs 75 and 76, pp 191-92, that book).This device holds for many months water flowing off an asphalt apron (today heavy plastic sheeting is probably used) during seasonal rains and is stored in a low-evaporation, bathtub-like tank covered to minimize evaporation. Birds and small mammals enter through a small opening that includes bars to exclude larger predators (e.g. coyotes) and walk down a slanted ramp that gives access to water regardless of its level. Fencing is used to keep livestock from breaking down the apron, and brush is piled on top to provide escape cover for quail from raptors. For quail and other gallinaceous birds the season of most critical need is before the precocial young are flying. After that, flocks fly relatively long distances to reach water.
Gordon Gullion, famous for his ruffed-grouse studies and management prescriptions in Minnesota, worked before that on game birds in the arid land of Nevada. In testing for optimal placement of guzzlers for Gambel's quail, he (1958) found that, when put out in clusters with spacing no more than 1-mile apart, each guzzler was visited by considerably more quail than were those placed alone with no others nearby. This not only indicated a greater use per device, but also reduced maintenance costs and proved more attractive to hunters who found a greater overall density of birds in the vicinity of these clusters than around lone units.
MANAGEMENT IMPLICATIONS: Again, the key here is the role of specific physical entities that can provide protection from excessive heat loss or heat gain associated with extreme (for the species in questions) ambient conditions. All this is easy to visualize in our own terms-when it's cold, not only dressing warmly but staying out of the wind, staying dry, and eating well; when it's too hot, taking cover in shade, in the water, inside cool structures, and avoiding vigorous activities. We take for granted home heating and cooling, all ranges of clothing, and generally our freedom to alter activities relative to weather, but the rest of the animal world has far less technology for assistance (a near exception does seem to be the beaver!). And when it's hot, we take off clothing, stand in the shade, seek a breeze, or go in the water; and we drink lots of fluids. The animals are in the same situation, and react correspondingly as well as habitat featuers permit.
REFERENCES
(If you want to look at items not on reserve, let us know)
Gullion, G. W. 1958. The "proximity effect" of water distribution upon desert small game populations. Proc. Conf. W. Assn. State Game & Fish Comms. 38: 187-189. (On reserve)
Leopold,
A.S. 1977. The California Quail. Univ. Calif. Press, 281
pp.
Moen, A.N. 1968. Energy balance of white‑tailed deer in the winter. Trans. N. Amer. Wildl. Conf. 33: 224-236. (On reserve)
Renecker,
L.A. & R.J. Hudson. 1986. Seasonal energy expenditures and thermoregulatory
responses of moose. Canad. J. Zool.
64: 322-327.
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