**4. Reproduction**

208 Solar Radiation

water is often observed in livestock, compensating for the water lost through evaporation. Some species of goats and cattle are able to reduce their TBs in the early morning as a preparatory strategy for the increased amount of solar radiation they will be exposed to

Many animals are able to use their own body parts, fluids, or environment to reduce heat absorption or expedite heat loss. Since water transfers heat 25X faster than air, metabolic as well as environmental water is used in evaporative cooling. It is beneficial to have an external water source nearby as the rate of body water turnover increases with solar radiation in most species, though at different levels depending on body mass and other characteristics (Figure 6) (King et al., 1975). Environmental water is an excellent source if available and many animals, including humans, use rivers, lakes, ponds, and the ocean to transfer excess heat. South Australian, Northern fur seals and Stellar sea lion females are able to tolerate solar radiation by keeping themselves wet, via mass movements to the shoreline while New Zealand sea lions immerse their hind flippers in tide pools or stay within the splash zone (Beentjes, 2006). Humans and other organisms that have sweat glands secrete the plasma portion of their blood onto the surface of their skin where it evaporates, the heat energy being used to transform liquid to gas. Organisms lacking sweat glands must devise other means of cooling their TB effectively. Dogs, cats, even sea lions, pant when they become overheated, using the evaporation of their saliva and moist tissue to dump heat. Kangaroos lick their own wrists where the skin is thinner and blood vessels are closer to the surface, aiding heat balance in the high solar and arid regions of Australia (Dawson et al., 2000). South Australian fur seal males unable to gain access to water due to territorial defense use their own urine for evaporatory cooling. After urinating on the rocks, males wet their ventral side and rear flippers, then lie on their side and raise a hindflipper into the air to enable convective heat loss. Female Stellar sea lions are often seen to huddle

during the warmer parts of the day (Al-Tamimi, 2007; Brosh et al., 1998).

around small tidepools with increasing temperatures (Gentry, 1973).

Fig. 6. Relation between daily total body water turnover and solar radiation in domestic

Eland and oryx under African ranching conditions. Source: King et al., 1975.

**3.3.2.1 Evaporation** 

Breeding demands a large amount of energy intake by both sexes, even more so when males must physically compete with one another to gain access to one or several females. Pinnipeds all have blubber layers of varying thickness to insulate them against cold water. Elephant seals are no exception, with males weighing over 2 tons. Male northern elephant seals must compete for reproductive access to females and physical combat among competing males can last upwards to 45 minutes (Norris et al., 2010). Blubber acts as insulation both in and out of water but is much heavier on land. Combative males produce a great amount of extra heat that must be expelled during and immediately after physical interactions with other males. Weaker males may retreat to the ocean for multiple cooling avenues that aid in thermoregulation but alpha males, if they wish to maintain their access to females and increase mating success, cannot leave their harem and thus are subjected to environmental variables such as TA, solar radiation, wind, and humidity. The only heat loss mechanisms readily available to alpha males on land are conduction and radiation to a cooler substrate and convection from prevailing winds (Norris et al., 2010).

Solar radiation has a significant effect on the circulatory physiology of male northern elephant seals. Infrared thermal images show that certain areas of skin function as thermal windows, vasodilating the blood vessels and shunting warm blood directly to the skin surface, facilitating heat transfer to the air by convection or substrate by conduction. By increasing skin temperature in specific areas, males are able to increase the temperature gradient more so than if blood was perfused along the entire body surface. On warm days, conduction and radiation work against the males, thus on days with high solar radiation and low wind the males are inactive, allowing skin temperature to rise 42°C (Figure 7) (Norris et al., 2010). In this manner they can lower their metabolic rate and conserve energy. Even pups of the California sea lion have been observed to sleep and stay still during periods of intense solar radiation for the same reason. On days when wind speed is high and/or clouds block direct solar radiation, male elephant seals are more likely to be active or engage in combat behavior, enabling them to rely on convection or even evaporation via precipitation to increase the rate of heat transfer (Norris et al., 2010).

Fig. 7. False color thermal image of a male northern elephant seal; max skin temperature of 42°C. Source: Norris unpublished data

Males of some pinniped species claim a territory rather than a harem and must rely on the habitat within their territory to entice or gain access to reproductive females. For example, South Australian fur seals exhibit several behavioral mechanisms to dissipate excess heat before finally abandoning their territory for access to water as TB approaches a certain thermal threshold often correlated with substrate (rock) temperature. Reproductive success of male New Zealand fur seals may depend on how much water is present within their territory. Males with areas including tide pools or along the shoreline have more access to females than males with no water in their territory. Male Northern fur seals that abandon their territories suffer a 50% reduction in mating success, whereas males of Southern Australian fur seal with no access to water average only 1.7 copulations per male compared to 3.6 copulations per male near a water source (Gentry, 1973). Southern sea lion males with territories lacking water access experience half the copulation frequencies of males with available water. During their forays to the water line, female Southern Australian fur seals expose themselves more frequently to males versus females that remain stationary and thus may contribute to an increased pregnancy rate (Campagna & Le Boeuf, 1998).
