ABSTRACT Investigating the ability to entrain to auditory stimuli has been a powerful approach to uncovering the comparative rhythm abilities of different animals. While synchrony to regular simple rhythms is well documented, synchrony to complex stimuli, with multiple components at unequal time intervals, is rarer. Several katydid species with simple calls have been shown to achieve synchrony as part of their natural calling interactions in multi-individual choruses. Yet no study so far has demonstrated synchrony in any insect with a complex call. Using natural calling behaviour and playback experiments, we investigated acoustic synchrony and the mechanisms underlying it in the katydid Mecopoda sp. ‘Two-part caller’. This katydid has a complex call consisting of a long trill followed by two or more chirps. We found that individual males synchronized trills and, to a lesser extent, chirps. Further investigation of trill synchrony showed that the timing of trills is modified by external trills but not chirps. Chirp synchrony is modified by external chirps, but also by trills. We suggest a qualitative two-oscillator model underlying synchrony in this species and discuss the implications for the evolution of acoustic synchrony.

ABSTRACT Even though the rhythms in adult emergence and locomotor activity are two different phenomena that occur at distinct life stages of the fly life cycle, previous studies have hinted at similarities in certain aspects of the organisation of the circadian clock driving these two rhythms. For instance, the period gene plays an important regulatory role in both rhythms. In an earlier study, we have shown that selection on timing of adult emergence behaviour in populations of Drosophila melanogaster leads to the co-evolution of temperature sensitivity of circadian clocks driving eclosion. In this study, we investigated whether temperature sensitivity of the locomotor activity rhythm evolved in our populations separately from the adult emergence rhythm, with the goal of understanding the extent of similarity (or lack thereof) in circadian organisation underlying the two rhythms. We found that in response to simulated jetlag with temperature cycles, late chronotypes (populations selected for predominant emergence during dusk) indeed re-entrained faster than early chronotypes (populations selected for predominant emergence during dawn) to 6 h phase delays, thereby indicating enhanced sensitivity of the activity/rest clock to temperature cues in these stocks (entrainment is the synchronisation of internal rhythms to cyclic environmental time cues). Additionally, we found that late chronotypes show higher plasticity of phases across regimes, day-to-day stability in phases and amplitude of entrainment, all indicative of enhanced temperature-sensitive activity/rest rhythms. Our results highlight remarkably similar organisation principles between circadian clocks regulating emergence and activity/rest rhythms.

ABSTRACT Considerable advances in chronobiology have been made through controlled laboratory studies, but distinct temporal rhythms can emerge under natural environmental conditions. Lab-reared Nematostella vectensis sea anemones exhibit circadian behavioral and physiological rhythms . Given that these anemones inhabit shallow estuarine environments subject to tidal inputs, it was unclear whether circadian rhythmicity would persist following entrainment in natural conditions, or whether circatidal periodicity would predominate. Nematostella were conditioned within a marsh environment, where they experienced strong daily temperature cycles as well as brief tidal flooding around the full and new moons. Upon retrieval, anemones exhibited strong circadian (∼24 h) activity rhythms under a light–dark cycle or continuous darkness, but reduced circadian rhythmicity under continuous light. However, some individuals in each light condition showed circadian rhythmicity, and a few individuals showed circatidal rhythmicity. Consistent with the behavioral studies, a large number of transcripts (1640) exhibited diurnal rhythmicity compared with very few (64) with semidiurnal rhythmicity. Diurnal transcripts included core circadian regulators, and 101 of 434 (23%) genes that were previously found to be upregulated by exposure to ultraviolet radiation. Together, these behavioral and transcriptional studies show that circadian rhythmicity predominates and suggest that solar radiation drives physiological cycles in this sediment-dwelling subtidal animal.

SUMMARY An unusual characteristic of nitrogen excretion in the ammoniotelic mangrove killifish Rivulus marmoratus is that urea is excreted( J urea ) in a distinct diurnal pattern, whereas ammonia is excreted ( J amm ) at a steady rate. In this study we tested the hypothesis that the diurnal pattern in R. marmoratus is an endogenously generated pattern that is characterized as a circadian rhythm. This hypothesis was tested by measuring J urea and J amm following manipulation of feeding or lighting regimes. The diurnal J urea pattern in food-deprived R. marmoratus had a 24 h periodicity under normal conditions of 12 h:12 h light:dark (12:12 L:D) with 72% more urea excreted during 12:00 h and 18:00 h. In contrast, there was no significant pattern in J amm . Fed fish (12:12 L:D) demonstrated a diurnal pattern in both J urea and J amm with up to an eightfold increase in excretion rates compared with rates obtained from food-deprived fish. Patterns of J urea were free running with a 24 h period under conditions of continuous darkness (0:24 L:D). Exposure to an inverse photoperiod (12:12 D:L) resulted in entrainment of the J urea pattern to the new photoperiod, with the highest rates of excretion occurring during midday of the new photoperiod. In contrast to R. marmoratus , nitrogen excretion rates in the zebrafish Danio rerio remained constant over time. The results of this study show that J urea in R. marmoratus demonstrates the characteristics of a circadian rhythm: a 24 h periodicity, a free-running rhythm in continuous conditions, and entrainment to new photoperiods.

Electrical stimulation of an epibranchial vagal ganglion, which innervates the gill region, had a marked influence on the respiratory rhythm of the carp Cyprinus carpio . Vagal input could initiate ventilation in fish displaying intermittent respiration. In fish breathing steadily, vagal stimuli could reset the respiratory rhythm by modifying the existing breathing cycle. An increase of stimulus intensity evoked a cough-like movement, thus delaying the onset of following cycles. Rhythmic stimulation, at intervals not more than 10° longer or shorter than the breathing cycle period, could entrain the respiratory rhythm in a one-toone ratio. Larger differences between the stimulation interval and the period of the breathing cycle resulted in either a cyclic modification of the respiratory cycle period or entrainment patterns with coupling ratios of 2:1 or 2:3. Coughing decreased in frequency or even stopped during rhythmic vagal stimulation.

Artificial oscillation imposed onto the gill arches could modify the respiratory rhythm in the carp Cyprlnus carpio . The degree of modification depended upon the frequency and amplitude of the applied movement. Oscillation at frequencies close to the spontaneous respiratory rhythm and at amplitudes in the range of 2–3 mm resulted in a cyclic pattern of lengthening and shortening of the breathing cycles. When the gill arches were maximally abducted artificially at the end of inspiration or early in expiration, the breathing cycle was shortened. A slight lengthening occurred when maximal gill arch abduction fell during the second half of expiration or during the first half of inspiration. An increase in oscillation amplitude led to entrainment of ventilation at a one-to-one ratio with the imposed movement. After oscillation stopped an aftereffect was seen. It is argued that gill arch proprioceptors contribute to the observed responses.

To examine the effects of locomotor frequency and respiratory drive on the incidence of locomotor-respiratory coupling (i.e. entrainment), we examined the relationship between locomotor and ventilatory patterns in trained Canada geese that were: (1) running at three speeds (0–40, 0–52 and 0–72 ms −1 ) on a treadmill, (2) free-running at three velocity intervals (0–40–0–72, 1–0–1–5 and 1–5–2–Oms −1 1) overground, and (3) breathing hypoxic (12 % O 2 / l % CO 2 ) and hypercapnic (5 % CO 2 /30% O 2 ) gas mixtures while running at one speed (0–52ms −1 ) on the treadmill. The portion of the time that the locomotor and respiratory systems were coupled was significantly greater during overground locomotion (41.1±3.2%) than at comparable speeds on the treadmill (29.0±2.6%). In addition, a significant increase in coupling with velocity was only observed during overground locomotion (41%1 ±3.2% at 0.40–0-72ms −1 vs 57.7 ±6.3% at 1.5–2.0ms −1 ). This increase in entrainment appeared to be the direct result of an increase in locomotor frequency per se and not due to the increase in metabolic rate associated with greater locomotion velocity. The effects of hypercapnia on the degree of entrainment were unclear owing to inter-animal variability; however, hypoxia caused a substantial decrease in entrainment from air-breathing values (34.8 ± 3.9 %) to levels not different from those possible due to chance alone (20%). Thus, it appears that locomotor pattern does entrain ventilatory rhythm and that the degree of entrainment is partially dependent on limb movement frequency. However, the increased respiratory drive associated with hypoxia appears to override the influence of locomotor rhythm on respiratory pattern, with a subsequent decrease in entrainment.

To elucidate the importance of telencephalic structures and the effects of metabolic rate in the production of locomotor-respiratory coupling, we examined the relationship between locomotor and ventilatory patterns in: (1) intact trained geese, and (2) brainstem-stimulated (medullary reticular formation) decerebrate geese, that were walking on a treadmill. The decerebrate geese, however, were not completely self supporting. Thus, while the two groups walked with similar stride frequencies ( f s), they did so at two different work rates. While at rest, tidal volume ( V T), breathing frequency ( f v) and minute ventilation ( V E) were very similar in the two groups. V E increased 120% during walking in the intact geese, primarily as a result of increases V E, while both V T and f v increased to produce a smaller 40 % increase in V E in the decerebrate birds. Although the magnitude of the increase in V E was three times greater in the intact geese, the relationships between V E and oxygen uptake ( VO 2 ) and V E and CO 2 output ( V CO CO2 ) were similar in the two groups. Significant coupling between locomotor and respiratory patterns was found in both intact (28.3%) and decerebrate birds (28.9%), suggesting that the telencephalon is not essential for the coupling of locomotor and respiratory rhythms during walking in geese. In addition, the incidence of locomotor-respiratory synchrony was virtually identical in the two groups in spite of a threefold difference in metabolic work rate.