**1.2.1 Radiation**

196 Solar Radiation

homeothermic or euthermic organisms (Speakman, 2004). Within this range, metabolic rate is minimal. Outside the TNZ, metabolic energy is required to maintain TB within the optimal range. Metabolic activity involving temperature-dependent enzyme activity will not function properly if the animal becomes hypo- or hyperthermic. Thus, thermoregulation is

Visual and ultraviolet wavelengths of solar radiation may be reflected or absorbed by animal surfaces, producing distinctive coloration and/or synthesis of vitamin D in terrestrial vertebrates, but infrared radiation is absorbed directly. Infrared radiation can increase TB near to or excelling the upper critical temperature (UCT). The UCT is the greater temperature limit where behavioral modifications are not enough to inhibit heat absorption and therefore energy must be expended in the attempt to dump excess heat. The temperature range from the UCT to the upper lethal temperature (ULT), the TB where an organism can no longer thermoregulate and dies of overheating, is known as the zone of evaporatory cooling where evaporation of metabolic water is the most efficient, and sometimes only, method available for transferring excess heat. On the other hand, the lower critical temperature (LCT) is the turning point at which more heat is lost to the environment than is normally metabolically produced. The lower lethal temperature (LLT) is the extreme cold temperature where an animal can no longer produce enough heat and dies of hypothermia. The range of temperatures between the LCT and LLT is known as the zone of metabolic regulation, where heat production through metabolic processes such as shivering or non-shivering thermogenesis is necessary to increase TB. Figure 1 depicts a general TNZ

Fig. 1. Thermal neutral zone graph showing lower and upper critical temperature, zones of metabolic regulation and evaporation. Source: Randall/Eckert Animal Physiology 5ed.

an integral part of an organism's energy balance.

graph, LCT, UCT, and zones of energy use (Randall, 2002).

The sun is Earth's principal source of radiative energy. With a surface temperature of approximately 6000K, the sun's electromagnetic radiation transmits some of this heat in the form of infrared radiation to our atmosphere and to varied surfaces on Earth (Speakman, 2004). Infrared radiation, undetectable to the human eye, is the peak wavelength emitted by objects with a surface temperature of -20° to 40°C. Special cameras sensitive to infrared wavelengths allow humans to record thermal images and videos of infrared radiation emitted by both inanimate objects and organisms, making it possible to effectively measure the surface temperature of whatever is within the camera's field of view (McCafferty, 2007). The amount of radiated heat detected depends upon factors such as the emissivity and absorptivity of an inanimate object or organism. Emissivity is defined relative to what is known as a black body, a perfect emitter with an emissivity of 1.0. Most animals have an emissivity value within 0.90-0.98, often dependent on surface properties such as fur or skin color. Surfaces either reflect or absorb light to varying degrees contingent on pigment levels and texture that in turn affect emissivity. Dark colors absorb energy within the infrared spectrum, increasing the absorptivity of inanimate objects or organisms compared to light colors which reflect visual wavelengths of solar energy. Water content, often high in living organisms, additionally contributes to an animal's elevated emissivity, since water is itself an excellent emitter.

### **1.2.2 Conduction**

Heat transfer between two solid objects in contact with one another is known as conduction. Heat energy travels down a thermal gradient and thus is conducted from a warmer object to a cooler one. This attribute also allows for conduction within a single body, if the core of an object or organism is warmer than the surface layer for example, heat will be conducted along the gradient. The rate at which this transmission occurs depends on several factors such as the material properties of both items, distance heat must travel, and the actual surface area that is physically in contact. Like radiation, thermal conductivity relies on certain properties of the materials in question. Insulation is the opposite of conductivity, meaning objects or organisms having little to no insulation may have a high thermal conductivity. This increases the rate and likelihood of heat transfer through conduction. Ectotherms such as insects, amphibians, reptiles, and fish have little insulation, making them more likely to gain or lose heat to their surroundings. In a given environment, a sun warmed rock in an otherwise cool environment can be critical to an ectothermic animal trying to remain active. In fact, insulation would be detrimental to ectotherms relying on external heat sources, because any such barrier would slow the rate of heat transfer into the body (Speakman, 2004). Endotherms such as birds and mammals have varying degrees of insulation made from feathers, fur, and/or fat deposits allowing them to retain their internal heat and thus rely less upon external heat sources.

The ratio of surface area to the volume of an organism is also an important component of heat transfer through conduction. Animals not only gain or lose heat via conduction with surfaces and objects in the environment they come in contact with, but also from the core of the body outwards toward the skin surface. Larger animals create thermal inertia, requiring less energy to balance heat loss and so have a low thermal conductance. Smaller animals lose heat rapidly and need more energy per gram of tissue to maintain heat balance with their higher thermal conductivity. Behavioral adjustments that change the amount of exposed surface area as well as physiological responses can increase or decrease an animals' thermal conductivity in either short periods of time or for seasonal modifications.

#### **1.2.3 Convection**

Rather than two solid objects adjacent with one another, convection requires one fluid coming in contact with another fluid (water or air) of a different temperature. Rather than heat transferring at the molecular level or through electromagnetic waves, heat is dispersed via the bulk motion of fluids (Cengel, 2003). Once again the temperature gradient between the two surfaces influences the rate of heat transfer. Conduction and convection along the human body is illustrated in Figure 2. Water conducts heat 23-25 times greater than air, making water an efficient medium for heat loss exhibited by both animals and humans moving towards sources of water during periods of intense solar radiation and high ambient temperatures (TA). Wind is also an efficient mode of convection. When cool air flows over the warmer skin surface of an animal, this may allow that organism to remain active longer on warm, sunny days. Convection even occurs within the body. Blood transfers heat throughout the body, bringing warmth from the core to the extremities, or bringing cooled blood from the surface back to overheated organs or muscle tissue.

#### **1.2.4 Evaporation**

Evaporation uses the energy required to convert liquid water to gas, allowing organisms to transfer heat even if the TA is greater than TB. This is often the only mechanism available to organisms during prolonged exposure to solar radiation that efficiently dumps excess heat in an effort to maintain a safe TB. Both environmental as well as metabolic water (Figure 2) can be used to transfer heat from an organism's body to the surrounding air (Gupta, 2011). Though highly effective, if the air is saturated with moisture already, such as during periods of high relative humidity or in constantly humid environments, evaporation as a thermoregulatory mechanism is rendered almost useless because net evaporation ceases when the air can no longer absorb additional moisture (Speakman, 2004).

Fig. 2. Heat loss via evaporation, convection, conduction, and radiation using the human body as an example. Source: Gupta, 2011.
