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Changes in natural plasma melatonin concentration. Melatonin plasma concentrations over time in normal LD conditions. Onset of night is indicated by ''lights off,'' end of night by ''lights on.'' Melatonin concentrations under normal nighttime conditions decreased as early as two hours prior to lights on. Groups with different letters are significantly different (Tukey's post-hoc test, p < 0.01).
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Melatonin is a key hormone in the regulation of circadian rhythms of vertebrates, including songbirds. Understanding diurnal melatonin fluctuations and being able to reverse or simulate natural melatonin levels are critical to investigating the influence of melatonin on various behaviors such as singing in birds. Here we give a detailed overview of...
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... analysis was done with JMP 10.0 (SAS Institute Inc.). To test for differences in the five treatment groups (Melatonin levels in different conditions) we used a REML (restricted maximum likelihood) analysis with treatment and sex as factors, and animal ID as a random factor. Melatonin levels were log 10 -transformed to meet requirements for parametrical statistical testing. Significance of specific differences was determined posthoc using a Tukey LSD test at alpha 0.01 to minimize the chance of false positives. Raw data were also analyzed using Friedman randomized blocks followed by post-hoc testing (Fig. 1). This yielded the same result as the parametric test after log-transformation. Fluctuations of blood plasma melatonin concentrations under normal LD conditions, and after treatment with both cream concentrations under LL conditions were compared using a REML with animal ID as a random factor. A full model including sex as factor and tarsus length and weight as covariates neither showed an effect of sex or interactions with it, nor did it show any correlation of tarsus length or weight with melatonin levels. These factors were therefore excluded from further analysis. Significance of specific differences was again determined posthoc using a Tukey LSD test at alpha 0.01. Additionally, to establish when plasma melatonin levels return to daytime levels after treatment, melatonin levels at all time points after treatment with the low concentration cream were compared to the control (0.5 h before treatment) using a Dunnett's test (Fig. ...
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... quantify how melatonin levels vary during a normal day/night cycle, we measured melatonin levels at 15 time points. Analysis of variance showed that levels differed significantly between sampling time points (F 14,81 = 24.21, P < 0.0001). Birds kept under natural conditions showed daytime melatonin levels in all measurements before lights off. Melatonin levels were already slightly increased in the first measurement 0.5 h after lights off, a significant increase as compared to daytime levels can be observed from 1h after lights-off (685.62 ± 221.498 pg/ml; P > 0.01, Tukey's post-hoc test; Fig. 2). Levels remained elevated for 7 h with small, non-significant fluctuations. Interestingly, a significant drop of melatonin levels compared to high nighttime levels already occurred as early as 2 h before lights-on in the morning (Fig. 2). Thirty minutes before lights-on, melatonin levels further decreased and reached average daytime levels (Fig. ...
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... quantify how melatonin levels vary during a normal day/night cycle, we measured melatonin levels at 15 time points. Analysis of variance showed that levels differed significantly between sampling time points (F 14,81 = 24.21, P < 0.0001). Birds kept under natural conditions showed daytime melatonin levels in all measurements before lights off. Melatonin levels were already slightly increased in the first measurement 0.5 h after lights off, a significant increase as compared to daytime levels can be observed from 1h after lights-off (685.62 ± 221.498 pg/ml; P > 0.01, Tukey's post-hoc test; Fig. 2). Levels remained elevated for 7 h with small, non-significant fluctuations. Interestingly, a significant drop of melatonin levels compared to high nighttime levels already occurred as early as 2 h before lights-on in the morning (Fig. 2). Thirty minutes before lights-on, melatonin levels further decreased and reached average daytime levels (Fig. ...
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... quantify how melatonin levels vary during a normal day/night cycle, we measured melatonin levels at 15 time points. Analysis of variance showed that levels differed significantly between sampling time points (F 14,81 = 24.21, P < 0.0001). Birds kept under natural conditions showed daytime melatonin levels in all measurements before lights off. Melatonin levels were already slightly increased in the first measurement 0.5 h after lights off, a significant increase as compared to daytime levels can be observed from 1h after lights-off (685.62 ± 221.498 pg/ml; P > 0.01, Tukey's post-hoc test; Fig. 2). Levels remained elevated for 7 h with small, non-significant fluctuations. Interestingly, a significant drop of melatonin levels compared to high nighttime levels already occurred as early as 2 h before lights-on in the morning (Fig. 2). Thirty minutes before lights-on, melatonin levels further decreased and reached average daytime levels (Fig. ...
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... levels showed a significant increase one hour after treatment compared to the control group (low concentration: 995.41 ± 103.216 pg/ml; high concentration: 6843.95 ± 4541.146 pg/ml; Dunnett's test; P < 0.01 for both concentrations) and remained elevated for 23 h after treatment with both cream concentrations (Fig. 3). The animals treated with the high concentration even showed levels that were significantly higher than normal nighttime levels (derived from Fig. 1) during both the first and the second 10 h after treatment (99% confidence interval not encompassing average nighttime level of 716.68 pg/ml; Dunnett's test P < 0.01). During these periods the normal nighttime level was within the 95% confidence interval of the low concentration animals. Twenty-three hours after treatment with the low concentration cream the first individuals show a drop to daytime Plasma melatonin concentrations under constant light conditions and after treatment with two different concentrations of melatonin cream (high: blue; low: red; control before treatment: light blue). Average daytime and nighttime levels of untreated birds (derived from Fig. 1) indicated by dashed lines. Levels in the high concentration group exceed normal nighttime levels (Tukey's post-hoc test, p < 0.01). Levels in the low concentration group were not significantly different to normal nighttime levels up to 24 h after treatment. After 24 h the levels of the low concentration group were variable (see Fig. ...
Citations
... As mentioned above, there is experimental evidence for a role of this hormone in affecting circadian and seasonal singing patterns (Cassone et al. 2008;Wang et al. 2012). However, documenting circadian rhythms of melatonin is extremely difficult given the rapid fluctuations in concentration that occur during the day and night, as well as the individual variation in cycling patterns (Seltmann et al. 2016). A common melatonin curve typically involves basal levels during the day with a sudden peak soon before sleep, a maintained plateau for some hours and a dramatic decrease some hours before dawn (Seltmann et al. 2016). ...
... However, documenting circadian rhythms of melatonin is extremely difficult given the rapid fluctuations in concentration that occur during the day and night, as well as the individual variation in cycling patterns (Seltmann et al. 2016). A common melatonin curve typically involves basal levels during the day with a sudden peak soon before sleep, a maintained plateau for some hours and a dramatic decrease some hours before dawn (Seltmann et al. 2016). Thus, melatonin can explain why birds are awake before dawn, but this does not tell us much about why they are singing at this time. ...
The bird dawn chorus has fascinated humans since ancient times, but still today numerous questions remain unclear. This chapter will explore this puzzling phenomenon, a communal display that likely involves the highest level of sound complexity found among animal signals. Covering from the first descriptive studies to recent multidisciplinary approaches, we review the physiological, behavioural and environmental factors affecting dawn chorus. In addition, we provide a critical assessment of the supporting evidence for the functional hypotheses proposed so far to disentangle its proximal and ultimate causes. We find that, despite the latest empirical and theoretical studies, there is still a good degree of confusion, and that four out of the nine hypotheses proposed so far in the literature have not been empirically tested. We show that most of these hypotheses are not incompatible with each other, and that their explanatory value changes depending on the species and the season. We argue that, at any rate, a single explanation may not be a reasonable expectation. The best-supported hypotheses for early singing provide three complementary lines of explanation: (1) singing at dawn has a relatively low energetic cost, most likely because it does not interfere with feeding; (2) is optimal to manipulate female mating or settle territory boundaries; and (3) may promote a handicap mechanism that prevents dishonest signalling. Thus, it follows that a combination of hypotheses based on both an optimality standpoint and costliness assumptions is needed to understand the phenomenon. We provide a series of specific suggestions for further research to refine our knowledge of this intriguing aspect of animal behaviour.
... The night hormone melatonin accurately encoded the duration of dark (and hence light) with higher night levels than day in zebra finches under 12 L:12D. A peak of melatonin rhythm at ZT 19 is consistent with a mid-night peak in captive zebra finches 3,39 . Furthermore, a significant reduction in night-time melatonin levels to near-day time levels along with an arrhythmic profile under LLbright was expected given the light sensitivity of the melatonin biosynthesis pathway 27 . ...
... Furthermore, a significant reduction in night-time melatonin levels to near-day time levels along with an arrhythmic profile under LLbright was expected given the light sensitivity of the melatonin biosynthesis pathway 27 . Thus, an absence of a daily rhythm in melatonin under LLbright was reflective of a lack of physiological differentiation between day and night and hence a state of circadian arrhythmicity 39,40 . Interestingly, the DLAN-induced suppression of melatonin was not as profound as under LLbright. ...
Increased exposure to light pollution perturbs physiological processes through misalignment of daily rhythms at the cellular and tissue levels. Effects of artificial light-at-night (ALAN) on diel properties of immunity are currently unknown. We therefore tested the effects of ALAN on diel patterns of cytokine gene expression, as well as key hormones involved with the regulation of immunity, in zebra finches (Taeniopygia guttata). Circulating melatonin and corticosterone, and mRNA expression levels of pro- (IL-1β, IL-6) and anti-inflammatory (IL-10) cytokines were measured at six time points across 24-h day in brain (nidopallium, hippocampus, and hypothalamus) and peripheral tissues (liver, spleen, and fat) of zebra finches exposed to 12 h light:12 h darkness (LD), dim light-at-night (DLAN) or constant bright light (LLbright). Melatonin and corticosterone concentrations were significantly rhythmic under LD, but not under LLbright and DLAN. Genes coding for cytokines showed tissue-specific diurnal rhythms under LD and were lost with exposure to LLbright, except IL-6 in hypothalamus and liver. In comparison to LLbright, effects of DLAN were less adverse with persistence of some diurnal rhythms, albeit with significant waveform alterations. These results underscore the circadian regulation of biosynthesis of immune effectors and imply the susceptibility of daily immune and endocrine patterns to ALAN.
... Such effects of continuous-release melatonin implants on the chronotype of entrained as well as periods of free-running circadian rhythms have indeed been demonstrated in songbirds [86 -89]. Furthermore, it has recently been shown that zebra finches (Taeniopygia guttata) decrease nocturnally elevated melatonin levels roughly 2 h before lights on, with actual times of melatonin decreases differing among individuals [90]. It is therefore possible that natural variation exists among males in the timing of their early-morning melatonin decline, which in turn may influence their activity onset. ...
Sexual selection favours the expression of traits in one sex that attract members of the opposite sex for mating. The nature of sexually selected traits such as vocalization, colour and ornamentation, their fitness benefits as well as their costs have received ample attention in field and laboratory studies. However, sexually selected traits may not always be expressed: coloration and ornaments often follow a seasonal pattern and behaviours may be displayed only at specific times of the day. Despite the widely recognized differences in the daily and seasonal timing of traits and their consequences for reproductive success, the actions of sexual selection on the temporal organization of traits has received only scant attention. Drawing on selected examples from bird and mammal studies, here we summarize the current evidence for the daily and seasonal timing of traits. We highlight that molecular advances in chronobiology have opened exciting new opportunities for identifying the genetic targets that sexual selection may act on to shape the timing of trait expression. Furthermore, known genetic links between daily and seasonal timing mechanisms lead to the hypothesis that selection on one timescale may simultaneously also affect the other. We emphasize that studies on the timing of sexual displays of both males and females from wild populations will be invaluable for understanding the nature of sexual selection and its potential to act on differences within and between the sexes in timing. Molecular approaches will be important for pinpointing genetic components of biological rhythms that are targeted by sexual selection, and to clarify whether these represent core or peripheral components of endogenous clocks. Finally, we call for a renewed integration of the fields of evolution, behavioural ecology and chronobiology to tackle the exciting question of how sexual selection contributes to the evolution of biological clocks.
