No abstract

The locomotor activity rhythm of different castes of the ant species Camponotus compressus was monitored individually under laboratory light/dark (LD) cycles, and under continuous darkness (DD). The colony of this ant species comprises two sexual castes, the queens and the males, and three worker castes, namely the major, media, and minor workers. The virgin males and virgin queens display rhythmic activity patterns, but the mated queens were arrhythmic while laying eggs, with the rhythmicity resuming soon after egg-laying. Under the LD regime, major workers showed nocturnal patterns, while about 75% of the media workers displayed nocturnal patterns and about 25% showed diurnal patterns. Under the DD regime, most major workers exhibited circadian rhythm of activity with a single steady state, whereas media workers displayed two types of activity patterns, with activity patterns changing after 6-9 days in DD (turn-arounds). The pre-turn-around tau of the ants that showed nocturnal activity patterns during LD entrainment was <24 h after release into DD, which then became >24 h, after 6-9 days. On the other hand, the pre-turn-around tau of those ants that exhibited diurnal patterns during LD entrainment was first >24 h after release into DD, and then became <24 h, after 6-9 days. The activity of the minor workers neither entrained to LD cycles nor showed any sign of free-run in DD. It appears that the circadian clocks of the ant species C. compressus are flexible, and may perhaps depend upon the tasks assigned to them in the colony.

Recent findings on neural and endocrine rhythms in infant mice and rats show that maternal coordination has an important role in setting the phase of the developing circadian clock both in the fetus and soon after birth. However, less information is available about the influence of the mother on activity/rest cycles of infants. Separation of the mother from infants in guinea pigs, monkeys and rats results in an increase in sleep disturbance (enhanced activity?). In this context it may be a common feature that during the postnatal period there is enhanced activity of pups during the hours when the mother is not nearby. Conversely, the social influences exerted by the mother while present with her young possibly leads to a relative rest stage. We have now tested this assumption in the night-active mouse Mus booduga. Our study addressed the postulate that the circadian activity/rest cycles of the pups are controlled by cyclic(?) presence and absence of the mother. The results reported here clearly indicate that the circadian locomotor activity of pups kept under continuous illumination or continuous darkness do entrain them to a regime of imposed 12:12-h cyclic presence and absence of the mother. The characteristics of this entrainment confer on the mother mouse the role of zeitgeber.

The effects of different light regimes on the fitness of organisms have typically been studied using mean or median adult life span as the sole index of physiological well-being. It is, however, known that life span is inversely related to reproductive output in many species. Moreover, the effects of a given environmental treatment on life span can be due to effects on either age-independent mortality or the "rate of aging," or a combination of both. Drawing evolutionary inferences from the effects of light regime on mean or median adult life span alone is difficult and, at best, speculative. We examined the effects of constant light (LL), alternating light-dark cycles (LD 12:12 h), and constant darkness (DD) on the life span of reproducing and virgin flies in four populations of Drosophila melanogaster and also estimated lifetime fecundity in the three light regimes. The light regime effects on life span were further dissected by examining the age-independent mortality and the Gompertz rate of aging under the three light regimes. While mean adult life span of reproducing males and females and virgin females was significantly shorter in LL compared to LD 12:12 h and DD, life-time egg production was highest in LL. Life span of virgin males was not significantly affected by light regime. The rate of aging in reproducing females was higher in LL as compared to DD, whereas age-independent mortality was higher in DD. As reproductive output, especially early in life, is a far more significant contributor to fitness than is life span, our results suggest that the earlier reported deleterious effects of LL on fitness are partly an artifact of examining life span alone, without considering other components of adult fitness that trade off with life span. Our results suggest that detailed investigation of the effects of light regime on the physiological and behavioral processes that accompany reproduction is necessary to fully understand the effects of different light regimes on adult fitness in Drosophila.

The oviposition rhythm of individual flies of Drosophila melanogaster from a population maintained in an aperiodic environment (with light, temperature, humidity, and other factors which could provide time cues, kept constant) for several hundred generations was assayed in constant light (LL), in light/dark (LD 12:12 hr) cycle, and in constant darkness (DD). More than 50% of the flies assayed exhibited rhythmicity in oviposition in all three light regimes. The results indicate that the phenomenon of egg laying is rhythmic in individual D. melanogaster females and is controlled by an endogenous time keeping mechanism. The persistence of the oviposition rhythm in a large proportion of individuals in the population after several hundred generations of rearing in a constant environment strengthens the view that possessing biological clocks may confer some intrinsic fitness advantage even to organisms living in aperiodic environments. J. Exp. Zool. 290:541-549, 2001.