Fluctuations In Length Of Day Research Articles

Adaptation of Temperate Seagrass to Arctic Light Relies on Seasonal Acclimatization of Carbon Capture and Metabolism.

Due to rising global surface temperatures, Arctic habitats are becoming thermally suitable for temperate species. Whether a temperate species can immigrate into an ice-free Arctic depends on its ability to tolerate extreme seasonal fluctuations in daylength. Thus, understanding adaptations to polar light conditions can improve the realism of models predicting poleward range expansions in response to climate change. Plant adaptations to polar light have rarely been studied and remain unknown in seagrasses. If these ecosystem engineers can migrate polewards, seagrasses will enrich biodiversity, and carbon capture potential in shallow coastal regions of the Arctic. Eelgrass (Zostera marina) is the most widely distributed seagrass in the northern hemisphere. As the only seagrass species growing as far north as 70°N, it is the most likely candidate to first immigrate into an ice-free Arctic. Here, we describe seasonal (and diurnal) changes in photosynthetic characteristics, and in genome-wide gene expression patterns under strong annual fluctuations of daylength. We compared PAM measurements and RNA-seq data between two populations at the longest and shortest day of the year: (1) a Mediterranean population exposed to moderate annual fluctuations of 10–14 h daylength and (2) an Arctic population exposed to high annual fluctuations of 0–24 h daylength. Most of the gene expression specificities of the Arctic population were found in functions of the organelles (chloroplast and mitochondrion). In winter, Arctic eelgrass conserves energy by repressing respiration and reducing photosynthetic energy fluxes. Although light-reactions, and genes involved in carbon capture and carbon storage were upregulated in summer, enzymes involved in CO2 fixation and chlorophyll-synthesis were upregulated in winter, suggesting that winter metabolism relies not only on stored energy resources but also on active use of dim light conditions. Eelgrass is unable to use excessive amounts of light during summer and demonstrates a significant reduction in photosynthetic performance under long daylengths, possibly to prevent photoinhibition constrains. Our study identified key mechanisms that allow eelgrass to survive under Arctic light conditions and paves the way for experimental research to predict whether and up to which latitude eelgrass can potentially migrate polewards in response to climate change.

Synchronization of photoperiod and temperature signals during plant thermomorphogenesis

ABSTRACTIt is well-known that even small changes in ambient temperatures by a few degrees profoundly affect plant growth and morphology. This architectural property is intimately associated with global warming. In particular, under warm temperature conditions, plants exhibit distinct morphological changes, such as elongation of hypocotyls and leaf petioles, formation of small, thin leaves, and leaf hyponasty that describes an upward bending of leaf petioles. These thermoresponsive morphological adjustments are termed thermomorphogenesis. Under warm temperature conditions, the PHYTOCHROME INTERACTING FACTOR 4 (PIF4) transcription factor is thermoactivated and stimulates the transcription of the YUCCA8 gene encoding an auxin biosynthetic enzyme, promoting hypocotyl elongation. Notably, these thermomorphogenic growth is influenced by daylength or photoperiod, displaying relatively high and low thermomorphogenic hypocotyl growth during the nighttime under short days and long days, respectively. We have recently reported that the photoperiod signaling regulator GIGANTEA (GI) thermostabilizes the REPRESSOR OF ga1-3 transcription factor, which is known to attenuate the PIF4-mediated thermomorphogenesis. We also found that the N-terminal domain of GI interacts with PIF4, possibly destabilizing the PIF4 proteins. We propose that the GI-mediated shaping of photoperiodic rhythms of hypocotyl thermomorphogenesis helps plant adapt to fluctuations in daylength and temperature environments occurring during seasonal transitions.

GIGANTEA Shapes the Photoperiodic Rhythms of Thermomorphogenic Growth in Arabidopsis

Plants maintain their internal temperature under environments with fluctuating temperatures by adjusting their morphology and architecture, an adaptive process termed thermomorphogenesis. Notably, the rhythmic patterns of plant thermomorphogenesis are governed by day-length information. However, it remains elusive how thermomorphogenic rhythms are regulated by photoperiod. Here, we show that warm temperatures enhance the accumulation of the chaperone GIGANTEA (GI), which thermostabilizes the DELLA protein, REPRESSOR OF ga1-3 (RGA), under long days, thereby attenuating PHYTOCHROME INTERACTING FACTOR 4 (PIF4)-mediated thermomorphogenesis. In contrast, under short days, when GI accumulation is reduced, RGA is readily degraded through the gibberellic acid-mediated ubiquitination-proteasome pathway, promoting thermomorphogenic growth. These data indicate that the GI–RGA–PIF4 signaling module enables plant thermomorphogenic responses to occur in a day-length-dependent manner. We propose that the GI-mediated integration of photoperiodic and temperature information shapes thermomorphogenic rhythms, which enable plants to adapt to diel fluctuations in day length and temperature during seasonal transitions.

The Exposure to Different Photoperiods Strongly Modulates the Glucose and Lipid Metabolisms of Normoweight Fischer 344 Rats.

Seasonal variations in day length trigger clear changes in the behavior, growth, food intake, and reproductive status of photoperiod-sensitive animals, such as Fischer 344 rats. However, there is little information about the effects of seasonal fluctuations in day length on glucose and lipid metabolisms and their underlying mechanisms in this model. To gain knowledge on these issues, three groups of male Fischer 344 rats were fed with a standard diet and exposed to different photoperiods for 14 weeks: normal photoperiod (L12, 12 h light/day), long photoperiod (L18, 18 h light/day), and short photoperiod (L6, 6 h light/day). A multivariate analysis carried out with 239 biometric, serum, hepatic and skeletal muscle parameters revealed a clear separation among the three groups. Compared with L12 rats, L6 animals displayed a marked alteration of glucose homeostasis and fatty acid uptake and oxidation, which were evidenced by the following observations: (1) increased circulating levels of glucose and non-esterified fatty acids; (2) a sharp down-regulation of the phosphorylated Akt2 levels, a downstream post-receptor target of insulin, in both the soleus and gastrocnemius muscles; (3) decreased expression in the soleus muscle of the glucose metabolism-related microRNA-194 and lower mRNA levels of the genes involved in glucose metabolism (Irs1, soleus, and Glut2, liver), β-oxidation (Had and Cpt1β, soleus) and fatty acid transport (Cd36, soleus, and liver). L18 animals also displayed higher blood glucose levels than L12 rats and profound changes in other glucose and lipid metabolism-related parameters in the blood, liver, and skeletal muscles. However, the mechanisms that account for the observed effects were less evident than those reported in L6 animals. In conclusion, exposure to different photoperiods strongly modulated glucose and lipid metabolisms in normoweight rats. These findings emphasize the relevance of circannual rhythms in metabolic homeostasis regulation and suggest that Fischer 344 rats are a promising animal model with which to study glucose- and lipid-related pathologies that are influenced by seasonal variations, such as obesity, cardiovascular disease and seasonal affective disorder.

Detection of different-time-scale signals in the length of day variation based on EEMD analysis technique

Scientists pay great attention to different-time-scale signals in the length of day (LOD) variations ΔLOD, which provide signatures of the Earth's interior structure, couplings among different layers, and potential excitations of ocean and atmosphere. In this study, based on the ensemble empirical mode decomposition (EEMD), we analyzed the latest time series of ΔLOD data spanning from January 1962 to March 2015. We observed the signals with periods and amplitudes of about 0.5 month and 0.19 ms, 1.0 month and 0.19 ms, 0.5 yr and 0.22 ms, 1.0 yr and 0.18 ms, 2.28 yr and 0.03 ms, 5.48 yr and 0.05 ms, respectively, in coincidence with the results of predecessors. In addition, some signals that were previously not definitely observed by predecessors were detected in this study, with periods and amplitudes of 9.13 d and 0.12 ms, 13.69 yr and 0.10 ms, respectively. The mechanisms of the LOD fluctuations of these two signals are still open.

Evidence for MAC waves at the top of Earth's core and implications for variations in length of day

Author(s): Buffett, B; Knezek, N; Holme, R | Abstract: Earth's liquid core hosts a diverse set of waves with periods ranging from days to thousands of years. One class of waves with periods of several decades is known to arise from an interplay between magnetic, Archimedes and Coriolis forces. These so-called MAC waves are thought to be relevant for interpreting historical fluctuations in the geomagnetic field. In this study, we show that MAC waves provide a good description of time-dependent zonal flow at the top of the core. The same collection of waves also offers a simple explanation for observed fluctuations in the dipole field. Both of these predictions require a stratified layer at the top of the core with a thickness of 130-140 km and a buoyancy frequency comparable to Earth's rotation rate. We extend these predictions to include changes in the length of day (LOD) and find that MAC waves can account for about half of the observed fluctuation at decadal periods. Larger fluctuations are possible when electromagnetic stresses couple MAC waves to flow in the interior of the core. In fact, an idealized model for the coupled motion overestimates the LOD fluctuations, probably reflecting limitations in this idealized model. Our results offer support for stable stratification at the top of the core and suggest a common origin for decadal fluctuations in the geomagnetic field and the LOD.

Solanum quitoense, a new greenhouse crop for Central Europe: Flowering and fruiting respond to photoperiod

The lulo (Solanum quitoense Lam.), an underutilized fruit crop originally from the Andes, is still widely unknown in Europe. Because of its delicious taste and high nutrition values it has high potential as greenhouse crop in Central Europe. But production of tropical crops in temperate regions might be limited by seasonal fluctuations in day length. Thus we studied lulo's reproductive development and yield in regard to seasonal photoperiod and supplemental lighting in winter. Therefore, we sow lulo seeds twice a year on 15 February and 20 June 2012 and grew them under natural light conditions so that bloom started in summer (natural long-day) and winter (natural short-day), respectively. Further lulos sown in June were exposed to an artificial long-day photoperiod as of 26 October 2012 (16h day, ∼150μmolm−2s−1 photosynthetic active radiation). We confirmed that in lulo flower initiation and fruit set occurs throughout the year and timing is not significantly different between plants grown under long-day (natural, artificial) and those grown under short-day conditions. However, flower development is affected by photoperiod as first time from bud to fruit set and therefore harvest begin was accelerated at long-days. Second, portion of hermaphroditic flowers increased in andromonoecious lulo in natural, not artificial long-days probably due to higher light intensity. Summer blooming plants bore more fruits (15 fruits per plant) than winter blooming ones (10 fruits). Supplemental lighting in winter only increased fruit number to a comparable amount as in summer but not yield. Surprisingly, blooming and fruiting period was conspicuously longer when flowering started under natural conditions in winter and therefore lasted until spring. So regarding this whole blooming period total yield was even higher when plants were grown during winter without supplemental lighting. In conclusion, reproduction of lulo plants is not restricted to a certain seasonal day length indicating that this crop is suitable for year-round fruit production under greenhouse conditions in Central Europe.

Control of flowering and storage organ formation in potato by FLOWERING LOCUS T

Seasonal fluctuations in day length regulate important aspects of plant development such as the flowering transition or, in potato (Solanum tuberosum), the formation of tubers. Day length is sensed by the leaves, which produce a mobile signal transported to the shoot apex or underground stems to induce a flowering transition or, respectively, a tuberization transition. Work in Arabidopsis, tomato and rice (Oryza sativa) identified the mobile FLOWERING LOCUS T (FT) protein as a main component of the long-range 'florigen', or flowering hormone, signal. Here we show that expression of the Hd3a gene, the FT orthologue in rice, induces strict short-day potato types to tuberize in long days. Tuber induction is graft transmissible and the Hd3a-GFP protein is detected in the stolons of grafted plants, transport of the fusion protein thus correlating with tuber formation. We provide evidence showing that the potato floral and tuberization transitions are controlled by two different FT-like paralogues (StSP3D and StSP6A) that respond to independent environmental cues, and show that an autorelay mechanism involving CONSTANS modulates expression of the tuberization-control StSP6A gene.

Air Temperature and Anthropogenic Forcing: Insights from the Solid Earth

Abstract Earth’s rotation rate [i.e., length of day (LOD)], the angular momentum of the core (CAM), and surface air temperature (SAT) all have decadal variability. Previous investigators have found that the LOD fluctuations are largely attributed to core–mantle interactions and that the SAT is strongly anticorrelated with the decadal LOD. It is shown here that 1) the correlation among these three quantities exists until 1930, at which time anthropogenic forcing becomes highly significant; 2) correcting for anthropogenic effects, the correlation is present for the full span with a broadband variability centered at 78 yr; and 3) this result underscores the reality of anthropogenic temperature change, its size, and its temporal growth. The cause of this common variability needs to be further investigated and studied. Since temperature cannot affect the CAM or LOD to a sufficient extent, the results favor either a direct effect of Earth’s core-generated magnetic field (e.g., through the modulation of charged-particle fluxes, which may impact cloud formation) or a more indirect effect of some other core process on the climate—or yet another process that affects both. In all three cases, their signals would be much smaller than the anthropogenic greenhouse gas effect on Earth’s radiation budget during the coming century.

Cycles in the scaling properties of length-of-day variations

Abstract Variations in the length-of-day (LOD) reflect the effects of several mechanisms in the Earth's rotation dynamics, including Earth–Sun and Earth–Moon line-up, geomagnetic effects and gravitational changes. Several studies showed that signatures of cycles occurring over a wide range of time scales are present in the LOD variations. The present work uses a fractal scaling study based on detrended fluctuation analysis (DFA) to study persistence of LOD variations and to provide insights in the different cycling mechanisms. The results showed that that the LOD variations are persistent over a wide range of time scales, meaning that an increment (resp., decrement) is more likely to be followed by an increment (resp., decrement). The temporal variation of the scaling exponent obtained from the DFA showed that several cycles already reported from the direct LOD variations analysis are inherited by the scaling properties. Inter-annual cycles, including 4.3 and 18.6 years cycles, are linked to the variations of the stochastic dynamics of LOD fluctuations. In this way, the 18.6 years cycle attains a period where variations are mostly affected by white noise effects, reducing the predictability of the LOD anomalies. The results are discussed in terms of the different lunar tidal and core–mantle mechanisms and related to recent results in the literature.

Long Term Variations in the Length of Day and Climatic Change

Summary The long-period (greater than about 10 yr) variations in the length-of-day (LOD) observed since 1820 show a marked similarity with variations observed in various climatic indices; periods of acceleration of the Earth corresponding to years of increasing intensity of the zonal circulation and to global-surface warming: periods of deceleration corresponding to years of decreasing zonal-circulation intensity and to a global decrease in surface temperatures. The long-period atmospheric excitation functions for near-surface geostrophic winds, for changes in the atmospheric mass distribution and for eustatic variations in sea level have been evaluated and correlate well with the observed changes in the LOD; although their total effect represents only about 10 per cent of the required excitation. The computed excitation lags the LOD variations by about 10-15 yr. Upper atmospheric winds or other meteorological related factors appear to be quite inadequate to provide the additional driving force that is required if the LOD changes are of meteorological origin. Instead, it appears that the LOD fluctuations and the climatic variations on a time scale of 20-30 yr may have a common origin as has been suggested by Anderson. That is, the results suggest that indirect solid Earth effects on climate may be important.

SEASONAL CONTROL OF TUBERIZATION IN POTATO: Conserved Elements with the Flowering Response

Fluctuations in day length determine the time to flower in many plants and in potato are critical to promote differentiation of tubers. Day length is perceived in the leaves and under inductive conditions these synthesize a systemic signal that is transported to the underground stolons to induce tuber development. Flowering tobacco shoots grafted into potato stocks promote tuberization in the stocks, indicating that the floral and tuber-inducing signals might be similar. We describe recent progress in the identification of the molecular mechanisms underlying day-length recognition in potato. Evidence has been obtained for a conserved function of the potato orthologs of the CONSTANS (CO) and FLOWERING LOCUS T (FT) proteins in tuberization control under short days (SDs). These observations indicate that common regulatory pathways are involved in both flowering and tuberization photoperiodic responses in plants.

Photoperiodic regulation of androgen receptor and steroid receptor coactivator-1 in Siberian hamster brain

Seasonal changes in the neuroendocrine actions of gonadal steroid hormones are triggered by fluctuations in daylength. The mechanisms responsible for photoperiodic influences upon the feedback and behavioral effects of testosterone in Siberian hamsters are poorly understood. We hypothesized that daylength regulates the expression of androgen receptor (AR) and/or steroid receptor coactivator-1 (SRC-1) in specific forebrain regions. Hamsters were castrated and implanted with either oil-filled capsules or low doses of testosterone; half of the animals remained in 16L/8D and the rest were kept in 10L/14D for the ensuing 70 days. The number of AR-immunoreactive (AR-ir) cells was regulated by testosterone in medial amygdala and caudal arcuate, and by photoperiod in the medial preoptic nucleus and the posterodorsal medial amygdala. A significant interaction between photoperiod and androgen treatment was found in medial preoptic nucleus and posterodorsal medial amygdala. The molecular weight and distribution of SRC-1 were similar to reports in other rodent species, and short days reduced the number of SRC-1-ir cells in posteromedial bed nucleus of the stria terminalis (BNST) and posterodorsal medial amygdala. A significant interaction between androgen treatment and daylength in regulation of SRC-1-ir was found in anterior medial amygdala. The present results indicate that daylength-induced fluctuations in SRC-1 and AR expression may contribute to seasonally changing effects of testosterone.

Photoreceptor regulation of CONSTANS protein in photoperiodic flowering.

Many plants flower in response to seasonal fluctuations in day length. The CONSTANS (CO) gene of Arabidopsis promotes flowering in long days. Flowering is induced when CO messenger RNA expression coincides with the exposure of plants to light. However, how this promotes CO activity is unknown. We show that light stabilizes nuclear CO protein in the evening, whereas in the morning or in darkness the protein is degraded by the proteasome. Photoreceptors regulate CO stability and act antagonistically to generate daily rhythms in CO abundance. This layer of regulation refines the circadian rhythm in CO messenger RNA and is central to the mechanism by which day length controls flowering.

Rapid ocean signals in polar motion and length of day

Non‐tidal variability in ocean currents and bottom pressure is substantial at short periods, but relating the associated signals in ocean angular momentum (OAM) to observed polar motion and length of day fluctuations has been difficult. Our simulations using a barotropic ocean model reveal a clear connection between the estimated OAM signals and both polar motion and length of day at periods of 3–10 days. The dynamic response of the oceans to barometric pressure plays an important role in the excitation of rapid rotation signals. The impact of OAM variability is most clear when using GPS‐based Earth orientation measurements with little temporal smoothing. Using the GPS series, and accounting for OAM in addition to atmospheric effects, yields substantial improvements in the excitation budget for rapid polar motion and length of day. The comparisons with Earth rotation data provide a useful check on the rapid ocean variability simulated by the barotropic model.

Angular momentum fluctuations within the Earth's liquid core and torsional oscillations of the core-mantle system

Summary Non-steady differential rotation is one of the main characteristics of the buoyancy-driven flow within the Earth's liquid metallic outer core that generates the main geomagnetic field by magnetohydrodynamic (MHD) dynamo action. Concomitant fluctuations in angular momentum transfer both within the core and between the core and the overlying ‘solid’ mantle are investigated by assuming that average departures from ‘isorotation’ on coaxial cylindrical surfaces are negligibly small. The interval of time covered by the analysis is the 15 decades from 1840 to 1990, for which requisite determinations are available of decadal fluctuations of the length of the day (LOD) and also of flow velocities just below the core–mantle boundary (CMB), as inferred from geomagnetic secular variation (GSV) data. Core angular momentum (CAM) fluctuations are most pronounced in the mid-latitudes, where they are generally out of phase with those occurring in equatorial regions. They are roughly in phase with decadal LOD fluctuations, especially after about 1870, with a dominant variability period of approximately 65 years, in keeping with previous analyses based on GSV and/or LOD data. The largest positive correlations (0.8 when data before 1867.5 are excluded) are found in the mid-latitudes, with a maximum at zero lag and with secondary peaks at 67 years and at −64 years, again implying a mode of approximately 65 years. Propagation of CAM anomalies from the equatorial to polar regions is evident in both the time-latitude dependence of CAM and its latitudinal correlation with length of day fluctuations. Future work on excitation mechanisms should establish the connection, if any, between the dominant timescale of approximately 65 years seen in the data and the gravest of the theoretically possible subseismic modes of torsional MHD oscillations of the core.

A study of the decadal fluctuations in length of day : WU Bin, PENG Bi-bio & ZHONG Min Institute of Geodesy and Geophysics, CAS, Wuhan 430077

Two very precise data sets, i.e. the variation of length of day (LOD) determined by space techniques and the geopotential coefficient ( Δ J 2) derived from satellite laser ranging (SLR), are used to study the decadal fluctuations in LOD. Instead of considering complex geophysical mechanisms, the authors use the observed Δ J 2 to calculate the contribution of mass redistribution of the Earth and find that this contribution amounts to 4% of amplitude of LOD at decadal time scale and hence is not large enough to excite the fluctuations. The results also demonstrate that the decadal fluctuations in LOD mainly come from the excitations of internal torques or relative angular momentum in the Earth.

The Atmospheric Dynamics of Intraseasonal Length-of-Day Fluctuations during the Austral Winter

Abstract The atmospheric dynamical processes associated with intraseasonal length-of-day (LOD) variability during the austral winter are examined with National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis and outgoing longwave radiation (OLR) data. The method adopted is to regress the relevant fields against the LOD tendency. All quantities in this study are bandpassed through a 30–70-day filter. The findings from an analysis of the OLR and 200-mb eddy streamfunction fields are consistent with the idea that large intraseasonal LOD fluctuations coincide with an active Madden–Julian oscillation (MJO). Further analysis suggests that the eddy response to the MJO heating drives both an anomalous meridional circulation that excites the anomalous global friction torque, and an eddy field that has the appropriate location relative to the topography for generating the anomalous global mountain torque. These results were obtained by calculating regressions of the anomalou...

Long‐period lunar fortnightly and monthly ocean tides

Long‐period lunar fortnightly (Mf) and monthly (Mm) tides in the global oceans are of particular interest to geophysicists because of their impact on the Earth's rotation, and they are of interest to oceanographers from the point of view of the response characteristics of the global oceans to low‐frequency forcing. Long‐period tides have long been quite controversial, but precision altimetry is providing an accurate means of measuring and modeling them in the global oceans. Here we describe Mf and Mm tides in the global oceans extracted from a 1° barotropic global hydrodynamic tidal model, which assimilates ocean tides estimated from cycles 10–130 of TOPEX altimetric data by an empirical tide model. The model results enable a better understanding of the energetics of these tides. Compared to the short‐period ocean tides, which dissipate a total of roughly 3490 GW of lunisolar gravitational energy, the dissipation rates for Mf and Mm, 0.369 and 0.023 GW, respectively, are quite insignificant from the point of view of the dissipation of tidal energy in the global oceans. Their energy content is also low, 0.381 and 0.049 PJ, respectively (compared to 580 PJ in short‐period tides). Also compared to the high quality factor Q values for short‐period tides (∼23 for semidiurnal and ∼13 for diurnal), the Q values for these tides are 5.9 and 6.2 for Mf and Mm, respectively. The corresponding unassimilated model Q value is 5.8 for both. These Q values correspond to a decay timescale of the order of a period and hence indicate that unlike its response to short‐period tidal forcing, the oceanic response to barotropic forcing at these low frequencies is like a heavily damped system. They are also consistent with the low values of Q (∼5.5) observed in the global oceans on timescales of several days. The relative potential energy to kinetic energy ratios for Mf and Mm tides (0.59 and 2.91 for assimilated and 0.83 and 3.20 for unassimilated, for Mf and Mm, respectively) are consistent with the dynamics of long‐wavelength Rossby waves. At amplitudes of ∼0.115 ms in universal time, UT1, for both Mf and Mm, the impact of these long‐period tides on the length of day fluctuations of the Earth is considerable.

Photoperiod-driven changes in reproductive function in male rhesus monkeys.

Environmental lighting, which regulates seasonal breeding in many animal species, has not been examined as a regulator of reproduction or puberty in man or nonhuman primates. In this study we examined the effects of controlled alternating long and short daily photoperiods, independent of other environmental variables, on testicular size and function in rhesus monkeys. Sixteen animals, some pubertal, others prepubertal, were individually caged indoors in light-controlled rooms. They were subjected to a 32-week "year" with alternating 16-week cycles of long (16 h of light and 8 h of darkness) or short (8 h of light and 16 h of darkness) days. Animals were examined every 2 weeks over four 32-week "years". Body weight, testicular diameter, and testicular volume were measured, and blood was collected for testosterone and PRL determinations. We found that although short days did not trigger testicular development in prepubertal animals, testicular growth was markedly enhanced by short days in postpubertal animals, accompanied by increased plasma testosterone levels and reduced PRL levels. In long days, testes regressed, testosterone levels fell, and PRL levels rose. The periodicity of testicular size, as determined by spectral analysis, showed a strong signal at a cycle length of 31 +/- 13 weeks, but no signal at intervals close to the natural 52-week year, indicating that the observed periodicity is induced by the changes in lighting, rather than by circannual changes in other variables. These studies establish that changes in photo-period alone can modulate reproductive function in a higher primate and suggest that the onset of puberty is not directly driven by seasonal fluctuations in day length.