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FIGURE 5.23 Daily rhythm of nociception. The graphs show the daily variation in pain sensitivity (nociception) of two golden hamsters maintained under a light-dark cycle (as indicated by the horizontal bars at the top). The data points correspond to measures of latency of the paw-licking response evoked by heating the floor (50°C or 122°F). Latencies are shorter (suggesting greater sensitivity) during the dark phase of the light-dark cycle in both specimens of this nocturnal species. (Source: Pickard, G. E. (1987). Circadian rhythm of nociception in the golden hamster. Brain Research 425: 395-400.)

FIGURE 5.23 Daily rhythm of nociception. The graphs show the daily variation in pain sensitivity (nociception) of two golden hamsters maintained under a light-dark cycle (as indicated by the horizontal bars at the top). The data points correspond to measures of latency of the paw-licking response evoked by heating the floor (50°C or 122°F). Latencies are shorter (suggesting greater sensitivity) during the dark phase of the light-dark cycle in both specimens of this nocturnal species. (Source: Pickard, G. E. (1987). Circadian rhythm of nociception in the golden hamster. Brain Research 425: 395-400.)

has an endogenous component. Indeed, rhythmicity in response latency was observed in hamsters free-running in constant light.349 The latencies were shorter during subjective day, however, rather than during subjective night, which is inconsistent with findings obtained with tests conducted under the light-dark cycle. Inconsistent results were also obtained in studies conducted under light-dark cycles in rats and mice: shorter latencies occurred at night,350351 during the day,352 or with two daily troughs — one during the day and the other during the night.353 A study in horses, which are diurnal animals, revealed shorter response latencies at the end of the day.354 A study in mice identified daily oscillation in pain reactions in tests conducted during the spring but not in tests conducted during the winter.355 Further studies are needed to clarify the inconsistencies.

Many studies have been conducted on the daily rhythmicity of temperature sensation as reflected in the thermoregulatory behavior of temperature selection. Consistent daily variation in the selection of ambient temperature along a temperature gradient has been documented in crustaceans,16'62356357 fishes,15 358-361 reptiles,80'362-374 rodents,30'123'154161'170'192'375-379 and other mam-mals.24,380,381 Figure 5.24 shows the data for a tree shrew

FIGURE 5.24 Daily rhythm of temperature selection. The graph shows the daily rhythm of temperature selection of a tree shrew (Tupaia belangeri) maintained under a light-dark cycle (as indicated by the horizontal bars at the top). The animal was housed in a long cage with a gradient of ambient temperature from 14 to 33°C. Tree shrews, which are diurnal, consistently select higher temperatures during the night than during the day. (Source: Refinetti, R. (1998). Body temperature and behavior of tree shrews and flying squirrels in a thermal gradient. Physiology and Behavior 63: 517-520.)

FIGURE 5.25 Basking lizard. Many animals select cooler or warmer environments at different times of the day in a consistent, rhythmic manner. (Source: © ArtToday, Tucson, AZ.)

maintained under a light-dark cycle (LD 14:10) in a temperature gradient similar to the one described in Chapter 2. The animal consistently selected higher temperatures at night than during the day. The reason a diurnal animal selects higher temperatures during the night, instead of during the day, is not immediately obvious. Chapter 10 discusses this issue.

Reptiles, such as lizards (Figure 5.25), rely much more than mammals on the selection of adequate thermal environments to optimize bodily functions. Consequently, their temperature-selection behavior has been studied extensively.80362-374 382 Figure 5.26 shows the temperature-selection rhythm of an iguana lizard (Iguana iguana) kept in constant light. The lizard was maintained in a cage at 22°C and was able to move on and off a "hot rock" (i.e., a small electric heater). Because lizards are cold-blooded animals, the circadian rhythm of ambient temperature

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