**1. Introduction**

194 Solar Radiation

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Solar radiation affects all aspects of the Earth's environment both directly and indirectly. Radiation from the sun produces a wide range of wavelengths that reach Earth and are absorbed or reflected from animate and inanimate surfaces. Visual, ultraviolet and infrared wavelengths, and solar intensity are the primary variables measured to evaluate their effects on the behavioral, thermoregulatory, and cellular responses of organisms. This review will focus on the effects of solar radiation on animal thermoregulation and the various methods used by scientists to assess the effects of infrared radiation on skin temperatures.

## **1.1 Ectotherms vs. endotherms**

All organisms regulate their internal body temperature (TB) to maintain a relatively constant temperature within a small range; in effect the rate of heat gained or produced must be balanced with heat lost to the environment. Animals are generally divided into ectotherms or endotherms based on how they maintain internal TB. Ectotherms, previously referred to as "cold-blooded", are animals such as invertebrates, amphibians, reptiles, and fish who regulate their TB externally primarily through behavioral mechanisms that alter heat exchange between their bodies and the environment. Endotherms such as birds and mammals, once described as "warm-blooded", are able to generate heat internally. Comparatively, leaky cell membranes, a greater amount of enzymes, and insulation (hair, feathers, and subcutaneous fat) contribute to endotherms having a higher basal metabolic rate than ectotherms, allowing them to maintain a relatively constant TB separate from their ambient surroundings—known as homeothermy or euthermy (Nagy, 2004). Notwithstanding, some larger invertebrates and fish employ muscle activity to perform some degree of homeothermy for short periods and some small mammals and birds relax their thermogenic abilities to enter states of torpor or may even hibernate at certain times of year—a thermoregulatory strategy known as daily or seasonal heterothermy.

### **1.2 Heat transfer and thermoregulation**

Each species has an optimal temperature where cellular processes are ideally maintained to optimize energy expenditure. Heat transfer across the body surface must be balanced lest an individual succumb to extreme heat gain or loss, a possibility especially in extreme environments such as polar or desert regions. Most organisms attempt to remain within a favorable range of temperatures. For homeotherms, this is known as the thermal neutral zone (TNZ). This optimal range of ambient temperatures typically lies between 30-42°C for 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 an integral part of an organism's energy balance.

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

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

Heat is transferred by four mechanisms: radiation, conduction, convection, and evaporation. Many species combine methods to absorb or transfer heat as efficiently as possible. Solar radiation plays a large part in determining not only ambient and body temperatures but animal behavior as well. A wide variety of species have developed methods to reduce the cost of thermoregulation by behaving certain ways: seeking shade, burrowing, panting, gular fluttering, wing flapping when exposed to temperatures above the UCT, entering short bouts of torpor or longer bouts of hibernation, increasing insulation, or migrating.
