Primate locomotor pattern repetitions, program clocks, and orientation to light

Biological time-keeping mechanisms: a need for broader perspectives?

Biological time-keeping mechanisms play fundamental roles in the regulation of behavior and physiology, disruption of which can lead to increased incidence of many disorders. Consequently, these mechanisms continue to be investigated intensively. For almost four decades they have been known to be able to store and program complex behaviors, and to be susceptible to the influences of light and day length. Yet present-day research concerns almost exclusively the means by which circadian and lesser time intervals are measured. Even within this narrow focus disagreements exist. Some early studies of small nocturnal rodents and primates running in exercise wheels illustrate the program-clock-like capabilities of endogenous time-keeping mechanisms and their degrees of susceptibility to external influences, such as artificial twilights and shortened light cycles. Broadening perspectives for research on biological clocks to take into account these often overlooked capabilities and susceptibilities could lead to a deeper understanding of them.

Effects of twilights on circadian entrainment patterns and reentrainment rates in squirrel monkeys

Entrainment patterns of the circadian rhythms of body temperature and locomotor activity were compared in 6 squirrel monkeys (Saimiri sciureus) exposed to daily illumination cycles with abrupt transitions between light and darkness (LD-rectangular) or with gradual dawn and dusk transitions simulating natural twilights at the equator (LD-twilight). Daytime light intensity was 500 lux, and the total amount of light emitted per day was the same in the two conditions. Mean daytime body temperature levels were stable in LD-rectangular but increased gradually in LD-twilight, reaching peak levels during the dusk twilight. Locomotor activity showed a similar pattern, but with an additional, secondary peak near the end of dawn. Activity duration was about 0.5 h longer in LD-twilight than in LD-rectangular, but the time of activity midpoint was similar in the two conditions. Reentrainment of the body temperature rhythm was faster following an 8-h advance of the LD cycle than following an 8-h delay, but did not differ significantly between the two LD conditions. These results provide no evidence that the inclusion of twilight transitions affected the strength of the LD zeitgeber, and suggest that the observed differences in the daily patterns reflected direct effects of light intensity on locomotor activity and body temperature rather than an effect of twilights on circadian entrainment mechanisms.

Circadian phase-response curves for simulated dawn and dusk twilights in hamsters

Phase shifts of the circadian rhythm of wheel-running activity were compared in Syrian hamsters maintained in constant darkness and exposed to 1-h naturalistic dawn or dusk twilight ramps (0.003-10 lx), or to 1-h rectangular light pulses (1 lx) providing equal photon exposure. The phase-response curves (PRCs) for dusk and rectangular pulses were virtually identical and resembled the PRC for dawn pulses, except that the mean phase advance caused by dawn pulses at circadian time 19 (CT 19) was approximately 1 h smaller. This difference could not be accounted for by differences in the amount of wheel-running observed during light pulse exposure, because the animals ran more during dusk pulses than during either of the other two pulse types. In a second experiment, 15-min rectangular light pulses (1 lx) immediately preceded by a 47-min dawn ramp caused smaller phase delays at CT 13 than rectangular pulses alone, despite a 40% increase in total photon exposure, but phase advances at CT 19 did not differ between the two light treatments. These results indicate that phase shifts of the circadian pacemaker in hamsters are determined primarily, though not entirely, by total photon exposure. They also indicate that dawn pulses may be less effective than dusk or rectangular pulses at certain circadian phases, possibly due to light adaptation during the early portion of the dawn twilight.

Circadian rhythms of drinking and body temperature of the owl monkey (Aotus trivirgatus)

The rhythms of drinking and body temperature of 4 male owl monkeys (Aotus trivirgatus) were examined under conditions of LD 12:12 (L = 100 lx, D = 0.1 lx), DD (0.1 lx) and LL (100 lx). For all 4 monkeys, the circadian pattern expressed in LD 12:12 continued in DD, with a free-running period averaging 23.6 hr. In LL the circadian component of both rhythms decayed and, in one monkey, a low frequency pattern arose. In at least two aspects, masking and persistence, the owl monkey circadian timing system appears to be unlike that of its diurnal relative, the squirrel monkey. Circadian rhythms of owl monkeys also differ in some respects from those of other nocturnal mammals.

Activity of nocturnal primates: influences of twilight zeitgebers and weather

The time of onset of activity of bush babies (Galago senegalensis), slow lorises (Nycticebus coucang), and an owl monkey (Aotus trivirgatus) living in outdoor enclosures usually fell in certain illuminance ranges of dusk and kept pace with seasonal progressions of sunset time. Influences of cloud cover were variable, but excessive heat and cold were inhibitory. Accurate endogenous timekeeping and reliance on the most stable zeitgeber apparently maintain activities in synchrony with the rhythmically changing environment.