NOTE FROM PETER. Concerning the Moen article on deer thermodynamics, the introduction to calculating thermal balance is not information for which you’ll be held, i.e., reading that is simply optional. Focus, rather, on the “Results,” particularly as illustrated in the three graphs dealing with the winter scenarios of deer of differing size and diet-intake under varying wind velocities.
ALSO, you’ll note I use terms such as “environmental factor” that you’ve not encountered before in lecture or readings. With my first lecture, 9/21, I’ll briefly review a somewhat different conceptual approach used by me during the ancient history of this course. I’m not challenging Dan’s framework that reflects some of the best thinking in this business, but will simply mention an alternative angle for you to think about.
NOTE FROM DAN. In terms of the conceptual framework we use to characterize an animal’s “habitat” (see Hall et al. 1997; Morrison 2001), an “environmental factor” is equivalent to a resource, a constraint, or both. Be sure you understand which factors count as resources and/or constraints.
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 calories obtained from food on the one hand and maintaining their thermal balance on the other. Thermal balance involves a balance between calories produced metabolically from food and calories lost to the environment. Rate of loss can be moderated by habitat structures; and the ability to loose can be affected by quality of food and, in hyperthermic conditions, the availability of free water.
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 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.
How
do thermal dynamics of wildlife relate to their habitats?
MAINTAINING BODY TEMPERATURE: Homeotherms are animals that maintain a relatively constant body temperature that is ordinarily either higher or lower than the ambient temperature. The latter typically varies widely in a given locale with weather, season, or time of day. Homeothermy serves to maintain an internal temperature that supports the most efficient rate of 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 the “cold-blooded” animals or poikilotherms. These, including reptiles and amphibians (“herps”) and most all other animal forms, are far more limited by their thermal environment. While homeotherms can remain completely active over a wide range of ambient temperature, the cost this adaptation is relatively high, because the greater expenditure of energy involved requires a correspondingly greater food intake than for poikilotherms whose body temperature is maintained primarily by the external environment. When the ambient temperature is unsatisfactory, poikilotherms just “sit it out,” waiting for the surrounding temperature to come into their range. In some regions where this seldom happens, we find few if any herps there to begin with. Homeotherms also can “sit out” unfavorable conditions, but do so far less frequently: hummingbirds, whose rate of heat-loss per unit weight is quite high compared to other birds, go into “torpor” (a much reduced metabolic rate as well as body temperature) at night when they wouldn’t be active anyway and could not “afford” spending (or wasting) calories to maintain their normal or daytime 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 being at all active. And though much-less dramatic, deer and certain other temperate-region mammals, while remaining active during winter, essentially stop growth and probably drop body temperature by a degree or two (we’ll come back to that under “food”).
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 body 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, while energy loss is function of 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. The example is often given of how the shrew requires so much more food per unit body weight than does the elephant. Likewise we would expect that larger size makes it easier to maintain body temperature in a cold environment. In the same respect, smaller size should be a benefit where ambient temperatures regularly exceed body temperature.
Environmental Aids for Maintaining
Constant Body Temperature:
Homeotherm’s adaptations to exist under varying ambient temperatures are almost always supplemented by dependence on surrounding conditions and/or structures that either reduce loss of heat or reduce over-heating. Probably the most wide-spread set of physical objects for this is some form of “cover.” First, however, realize that cover can serve animals in two distinct ways—as protection from thermal stress i.e. “thermal cover”, or as protection from predators, including even members of one’s own species, “security cover.” Thus in using an environmental-factor approach for analyzing habitat relations, “cover” defines two distinct of resource types even if the same structure can potentially serve both functions. Also serving special reproductive functions, physical structures protecting nests or dens might be labeled “reproductive” cover.
Coping with cold: Considering cover’s thermal function, it’s not easy to quantify (e.g. in calories) the benefit provided by a certain physical structure in reducing heat loss for a given animal. This is well illustrated in your reading of the Moen paper. At the same time your personal experiences in Minnesota winters should have shown you the cold-reducing benefit of “getting out of the wind,” say by moving from a field into a forest, or from a ridge top to the leeward side of a hill. Habitat managers don’t often alter topography of the landscape, but can readily appreciate the role of human-made buildings and other structures that end up providing thermal cover. More universal is the result of altering vegetation—positively or negatively. Planting or protecting riparian woodlands has tremendous benefits to wildlife in winter; while clearing fields for crops and then, with plowing clean in the fall, all vegetation is removed leaving broad expanses thermally unsuitable for animals that otherwise would find adequate thermal cover if only dried grasses remained.
For the northern edge of white-tailed deer distribution, as in northern Minnesota, it clearly appears that dense conifer cover is preferred or even required by deer in winter. It’s readily observed that such vegetation always characterizes winter “yards” where deer have come in from wide regions to congregate during severe cold and deep snow. Another positive aspect of deer yards, however, needs mention here as well. Such vegetation leads to a major difference in substrate. Under conifers snow is less deep as well as not subject to crusting (shaded from solar radiation, snow surfaces are not melted to then refreeze as a crust). Together snow depth and crust make moving more difficult for deer, costing them more energy as well as giving advantage to pursuing predators as, in this state, the wolf.
Another key environmental factor for countering effects of cold is simply food. It’s said that our northern deer could survive winter without conifer cover if they had an ample supply of corn with its high concentration of readily digestible calories, compared to their low-calorie, low-digestibility natural winter forage of dormant deciduous twigs plus conifer needles. This wouldn’t solve the mobility problem for avoiding wolves except that better physical condition should afford deer an advantage in out-distancing the wolves.
Efforts to quantify actual and potential heat loss in cold weather are illustrated first by Verme’s development of the “chillometer,” as explained in lecture. Theoretically, this device offers an excellent means, almost better than with live animals, for evaluating the thermal quality of habitat sites during winter. Also, wildlife departments in this region have used a semi-quantitative measure known as a “winter-severity index” to compare how thermally challenged animals, such as deer, are among areas and years. This index combines temperature, wind, and snow measures to come up with a single number that supposedly reflects relative rate of accumulated heat loss.
Consider the implications of Moen’s hypothetical graphs of thermal balance in wintering deer of different sizes and nutritional states while subject to variations in micro-climates. In the open with no cloud cover and complete exposure to wind is a setting where heat loss by the deer would be maximal, so you can appreciate how important to the animals’ survival is use of whatever thermal can be found to reduce heat loss. Moen also has made the point from his field studies that when temperatures drop so low that, when moving and foraging deer may experience a negative heat balance regardless of available cover. At that point their best strategy is to lie down in soft snow (for insulation) with minimum surface exposure and no movement until it either warms up or the animal dies.
On the other hand, do not assume that cold-hardiness in every ungulate native to this state is similar to deer. Good research has shown that moose undergo little or no thermal stress even as temperatures drop way below freezing (i.e. down to – 40o C [ -40o F]), and such may also be true in caribou. However, unlike deer, moose cannot tolerate (or be very active) when summer temperature get up around 25o C (75o F). It is suspected that the tendency of moose to move into conifer cover in early spring is not a reaction to cold or snow depth, but rather is to avoid the increased intensity of solar radiation as the sun’s trajectory moves higher, causing them to actually over-heat while still in their winter adaptation mode.
The majority of birds that breed in north-temperate latitudes migrate to warmer regions for winter. However, it’s not easy to distinguish whether this is entirely driven by cold or to changes in availability of food. For example, robins will stay around quite far north during some winters when a good crop of berries or fruits remains on trees and shrubs. Also, several species are believed to have shifted from migrating to remaining all winter as a result of bird feeders plus horticultural plantings that produce and retain ample berries and fruits through winter. Ducks and geese may migrate more because their feeding sites become frozen than to avoid the cold. Where power plants and other human activities keep patches of water open, large numbers of Canada geese and mallards may not migrate, and this is accentuated when people put out food for the birds.
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
Coping with Excessive Heat: As with cold stress, thermal cover is critical to many wildlife species for avoiding hypothermy (overheating); such animals include herps as well as birds and mammals. If exposed to direct, mid-day sun for very long, dessert reptiles will die far sooner than would birds or mammals; hence habitat lacking structures that provide shade will be of low quality or unusable in mid-day for herps in the warmer seasons. Many amphibians gain their protection by simply remaining in water; but those that move far from that protection for much of the warm seasons (e.g. toads, tree frogs, and certain salamanders) must seek complete cover from mid-day solar radiation; hence you won’t find them in the middle of parking lots on hot days.
In hot regions, many small- and medium-sized mammals are mainly nocturnal, retreating into cover that offers complete shade and good security during daytime. Large mammals seek shade under trees, but those adapted to exist on the open plains must be able to withstand day-long, direct sun during the hottest of seasons. It has been clearly shown that members of the deer family (Cervidae) are far less adapted for such exposure than are the Bovidae (gazelles, cows, sheep, etc).
INGESTIBLE
WATER AS AN ESSENTIAL HABITAT RESOURCE
A critical factor for animals in hot-dry environments is the availability of
water, since evaporative cooling is a key mechanism for keeping the body below
what are often excessively high ambient temperatures. Anyone who has hiked in the open during hot
weather should be aware of how critical it is to consume plenty of water; but not enough of us appreciate how readily
we can die when out there too long without water. Just check the statistics on mortality of
persons hiking down or back-up the Grand Canyon in mid summer without carrying
ample water. This all relates to why we
link water with thermal effects rather than treating it as a food resource,
even though water can just as well be listed as a nutrient.
The water chapter in your text treats water as a single resource variable, but this tends to be misleading. Far better we consider it as having several important, but completely separate, functions. All of these are eventually relevant to our analysis, and the discussions of them in the text include excellent examples from various wildlife species in North America. For example, indirectly, water has a key influence on plant growth, with the plants themselves serving wildlife as food, or cover, or substrate, depending on each animal’s needs. Water is obviously an essential substrate for the wide diversity of aquatically adapted species, from otters to ducks and to essentially all amphibians. In some cases its presence is taken for granted, as in large deep lakes; however, in many situations water bodies come and go with the seasons or with climatic variations or human manipulations, and consequently many species are affected or adapted accordingly. In this section, however, we deal with water solely as a resource to be ingested for its essential role in animal metabolism.
Water, as consumed orally by birds, mammals, or reptiles as well as demally by most amphibians, and is lost from each animal in a wide diversity of ways, is a nutrient totally essential (non substitutable) for essentially all metabolic processes. Generally availability of free water tends to be more limiting as, among regions, climatic aridity increases. In a parallel sense, where available water is relatively sparse (or even totally lacking in some seasons or during times of drought), their adaptive specializations become a key determinant as to which species can exist in such habitats. Where free water is accessible only at widely separated point, then relative mobility of species is critical. In general, it is easier for birds to access distant sources than for reptiles and mammals (but not always as in the case of quail chicks before able to fly). Likewise most terrestrially adapted large mammals can manage long traverses for water that would be wholly impractical for medium and small-sized mammals. Consider then how local distribution within some species groups in arid areas may be strongly shaped by the spatial distribution of water sources.
Another key issue in meeting water needs is the ability of animals to obtain this resource from sources other than free-water. Live plant tissues show a wide range of free-water content, and this offers opportunities for some herbivore species; but of course chewing on leaves could not sustain you and me in the absence of free water. Plants also have additional water tied up in organic compounds, but that is not accessible until the plant matter is chemically altered during digestion.
Examples of management to provide
water:
Obviously to identify situations in which lack of available water may reduce or eliminate a species from a locale (i.e. is limiting), the habitat biologist need not explore all aspects of water availability along with the water-conservation physiology of animals that are or might be present. Suggestive information can come from a spatial inventory of species relative to their distance from free-water (as long as inconspicuous water sources are not being missed). And inventories before-and-after experimental placement of water can be even more informative. The point is, determining (or estimating) whether free water is limiting populations can result from a common-sense analysis of fairly straight forward information. A classical example of fortuitous evidence proving key evidence, along with development of a management device to provide the missing resource, is described by A.S. Leopold in his book on the California quail (see reading list). Development in that case of the “gallinaceous guzzler” was followed by widespread use of this device in the semi-arid west. Gullion’s monitoring of quail use comparing different spatial patterns of guzzlers in Nevada shows how fine-tuning of habitat alterations can lead to optimization of management investment. Other examples of devices for providing free-water sources are in your text (pp 234-236). There is no doubt that bird “baths” permit many song-bird species to be present in dry summers (usually meaning they are breeding there) in sites that otherwise would not support them.
.
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 euros (wallaroos), 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 features permit.
REFERENCES
(If you want to look at items here not on reserve, please 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.
Taylor, C. R. 1969. The eland and the oryx. Sci. Amer. 220(1) Jan. 88.
Wood, A.J., I. McT. Cowan,
and H.C. Nordan. 1962. Periodicity of growth in ungulates as shown by
deer of the genus Odocoileus. Can. J. of Zool. 40: 593-603.
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