Seasonal Variation In Photoperiod Research Articles

Well-being in healthy Icelandic women varies with extreme seasonality in ambient light.

Seasonal variation in photoperiod may affect psychosocial and physical well-being in healthy persons. We tested this hypothesis in healthy pre-menopausal women, without a history of mood disorders, living year-round in Reykjavik, Iceland (64.1°N). Participants reported daily self-assessments of well-being throughout a complete ovulatory menstrual cycle in summer and/or winter (70% participated in both seasons). Scores for mood, cognitive acuity, social support, physical health and a composite of these four indicators were each significantly higher in summer than in winter (linear mixed effects models: p < .001 for each model); tiredness did not differ by season. The effect of season was not significantly changed by inclusion of body mass index and/or age as covariates. Some prior studies have been hampered by sparse time sampling, inattention to covariates and/or relying on recalled data. This is to our knowledge the first investigation to test the study hypothesis with daily real-time data spanning complete ovulatory menstrual cycles in each of two seasons. This dense sampling has revealed modest seasonal variation in well-being in healthy women. Daylength (sunlight exposure) is likely a major, but not necessarily sole, factor in these seasonal differences in well-being; temperature is likely less important given Iceland's relatively moderate (for its high latitude) seasonal temperature swings.

Effect of Natural Seasonal Changes in Photoperiod and Temperature on Immune Function in Striped Hamsters.

What environmental factors contribute to seasonal variation in immune function in striped hamsters (Cricetulus barabensis) remains unclear. How immune responses would respond to natural seasonal changes in photoperiod and temperature was investigated in the present study. Twenty-nine male and 30 female hamsters were randomly assigned to the winter, spring, summer, and autumn groups. Spleen mass was the highest in male hamsters during autumn, while it did not differ among seasons in females. Regardless of sex, bacteria killing activity (BKA) was the lowest in the spring, whereas phytohaemagglutinin (PHA) responses at 12 h, 24 h, and 48 h were all highest in the winter among the four seasons. Females had the lowest titers of immunoglobin (Ig)G5, 10, and 15 in winter, while they did not show seasonal variation in males. Compared with male hamsters, females had higher levels of IgG10 and IgG15 in the spring and autumn, but had lower BKA in autumn. Blood glucose was the lowest in the spring in both sexes. Males had higher leptin level in winter than in summer and autumn, while leptin level was higher in winter and spring than in autumn in females. Corticosterone level was higher in winter and summer than in spring and autumn in males, while it was higher in winter than in the other three seasons in females. Males had higher levels of leptin in winter, and corticosterone in summer, than females. In summary, distinct parts of the immune system respond differently to natural seasonal variations in photoperiod and temperature.

Photoperiod controls wing polyphenism in a water strider independently of insulin receptor signalling.

Insect wing polyphenism has evolved as an adaptation to changing environments and a growing body of research suggests that the nutrient-sensing insulin receptor signalling pathway is a hot spot for the evolution of polyphenisms, as it provides a direct link between growth and available nutrients in the environment. However, little is known about the potential role of insulin receptor signalling in polyphenisms which are controlled by seasonal variation in photoperiod. Here, we demonstrate that wing length polyphenism in the water strider Gerris buenoi is determined by photoperiod and nymphal density, but is not directly affected by nutrient availability. Exposure to a long-day photoperiod is highly inducive of the short-winged morph whereas high nymphal densities moderately promote the development of long wings. Using RNA interference we demonstrate that, unlike in several other species where wing polyphenism is controlled by nutrition, there is no detectable role of insulin receptor signalling in wing morph induction. Our results indicate that the multitude of possible cues that trigger wing polyphenism can be mediated through different genetic pathways and that there are multiple genetic origins to wing polyphenism in insects.

Interaction between photoperiod and variation in circadian rhythms in tomato

BackgroundMany biological processes follow circadian rhythmicity and are controlled by the circadian clock. Predictable environmental changes such as seasonal variation in photoperiod can modulate circadian rhythms, allowing organisms to adjust the timing of their biological processes to the time of the year. In some crops such as rice, barley or soybean, mutations in circadian clock genes have altered photoperiod sensitivity, enhancing their cultivability in specific seasons and latitudes. However, how changes in circadian rhythms interact with the perception of photoperiod in crops remain poorly studied. In tomato, the appearance during domestication of mutations in EMPFINDLICHER IM DUNKELROTEN LICHT 1 (EID1, Solyc09g075080) and NIGHT LIGHT-INDUCIBLE AND CLOCK-REGULATED GENE 2 (LNK2, Solyc01g068560) delayed both the phase and period of its circadian rhythms. The fact that variation in period and phase are separated in tomato provides an optimal tool to study how these factors affect the perception of photoperiod.ResultsHere we develop tomato near isogenic lines carrying combinations of wild alleles of EID1 and LNK2 and show that they recreate the changes in phase and period that occurred during its domestication. We perform transcriptomic profiling of these near isogenic lines under two different photoperiods, and observe that EID1, but not LNK2, has a large effect on how the tomato transcriptome responds to photoperiod. This large effect of EID1 is likely a consequence of the global phase shift elicited by this gene in tomato's circadian rhythms.ConclusionsOur study shows that changes in phase that occurred during tomato domestication determine photoperiod perception in this species, while changes in period have little effect.

Melatonin and osmoregulation in fish: A focus on Atlantic salmon Salmo salar smoltification.

Part of the life cycle of several fish species includes important salinity changes, as is the case for the sea bass (Dicentrarchus labrax) or the Atlantic salmon (Salmo salar). Salmosalar juveniles migrate downstream from their spawning sites to reach seawater, where they grow and become sexually mature. The process of preparation enabling juveniles to migrate downstream and physiologically adapt to seawater is called smoltification. Daily and seasonal variations of photoperiod and temperature play a role in defining the timing of smoltification, which may take weeks to months, depending on the river length and latitude. Smoltification is characterised by a series of biochemical, physiological and behavioural changes within the neuroendocrine axis. This review discusses the current knowledge and gaps related to the neuroendocrine mechanisms that mediate the effects of light and temperature on smoltification. Studies performed in S. salar and other salmonids, as well as in other species undergoing important salinity changes, are reviewed, and a particular emphasis is given to the pineal hormone melatonin and its possible role in osmoregulation. The daily and annual variations of plasma melatonin levels reflect corresponding changes in external photoperiod and temperature, which suggests that the hormonal time-keeper melatonin might contribute to controlling smoltification. Here, we review studies on (i) the impact of pinealectomy and/or melatonin administration on smoltification; (ii) melatonin interactions with hormones involved in osmoregulation (e.g., prolactin, growth hormone and cortisol); (iii) the presence of melatonin receptors in tissues involved in osmoregulation; and (iv) the impacts of salinity changes on melatonin receptors and circulating melatonin levels. Altogether, these studies show evidence indicating that melatonin interacts with the neuroendocrine pathways controlling smoltification, although more information is needed to clearly decipher its mechanisms of action.

Seasonal variations in photoperiod affect hepatic metabolism of medaka (Oryzias latipes).

Organisms living in temperate regions are sensitive to seasonal variations in the environment; they are known to accumulate energy as fat in their livers during the winter when days are shorter, temperatures are lower, and food is scarce. However, the effect of variations in photoperiod alone on hepatic lipid metabolism has not been well studied. Therefore, in this study, we analyzed lipid metabolism in the liver of medaka, Oryzias latipes, while varying the length of days at constant temperature. Larger amounts of fatty acids accumulated in the liver after 14 days under short‐day conditions than under long‐day conditions. Metabolome analysis showed no accumulation of long‐chain unsaturated fatty acids, but showed a significant accumulation of long‐chain saturated fatty acids. Short‐day conditions induced a reduction in the levels of succinate, fumarate, and malate in the tricarboxylic acid cycle, decreased expression of PPARα, and decreased accumulation of acylcarnitine, which suggested inhibition of lipolysis. In addition, transparent medaka fed on a high‐fat diet under short‐day conditions exhibited greater amounts of fat accumulation and developed fatty liver. The findings of our study will be useful for creating a medaka hepatic steatosis model for future studies of hepatic steatosis‐related diseases.

Suppression of voluntary ethanol intake in mice under constant light and constant darkness.

Seasonal variations in photoperiod are associated with alterations in human mood and behavior. Similarly, manipulation of the environmental lighting regimen can exert pronounced effects on affective behavior in experimental animals. These observations may be due, in part, to light-induced alterations in circadian rhythms, but it seems likely that other, non-circadian factors also contribute. Several studies have shown that voluntary alcohol (ethanol) consumption can be affected by lighting conditions in rodents, suggesting that photoperiodic variation may account for seasonal and geographic patterns of human alcohol consumption. Nevertheless, the existing animal data are somewhat inconsistent, and little work in this area has been performed in mice. In the present study, we monitored circadian activity rhythms and voluntary ethanol consumption under standard 12:12 light-dark (LD) cycles, and in constant light (LL) and constant darkness (DD). Experiment 1 employed male C3H/He inbred mice, while Experiment 2 employed males and females from a genetically heterogeneous line (WSC). Relative to LD conditions, ethanol intake and ethanol preference were reduced under both LL and DD in both experiments. Because similar effects were seen in both LL and DD, neither circadian disruption nor a classical photoperiodic mechanism are likely to account fully for these findings. Instead, we suggest that the absence of circadian entrainment may function as a mild stressor, resulting in reduced ethanol consumption.

Photoperiodic regulation of ovarian steroidogenesis in a tropical rodent, Funambulus pennanti: role of melatonin and MT1

ABSTRACTTo elucidate the role of photoperiodically modulated endogenous melatonin and its receptor MT1 in the ovary of a diurnal rodent, adult Indian palm squirrel Funambulus pennanti were exposed to Short Day (SD; 16L:8D) and Long Day (LD; 16L:8D) and compared with natural day controls (NDL; 12L:12D). Plasma 17-β estradiol level was significantly low in SD group while expression of StAR, P450SCC, 17-β HSD, and aromatase showed significant down-regulation in the ovary of squirrels maintained under SD when compared to LD and NDL groups suggesting an overall decrease in steroidogenesis. Expression of ERα in the ovary was significantly lower in SD than both LD and NDL groups, having a positive correlation with plasma estradiol level. Circulatory melatonin level was significantly high in SD-exposed squirrels, while expression of MT1R decreased in the ovary when compared to LD and NDL groups, showing a negative correlation with plasma melatonin levels. Expression of cell proliferation marker Proliferating Cell Nuclear Antigen (PCNA) was positively correlated to MT1R expression in the ovary being significantly low in SD-exposed squirrels. Our results suggest that even in tropical rodents seasonal variation in photoperiod is responsible for initiating female reproductive activities where circulatory melatonin via MT1R plays an important role.

Encoding seasonal information in a two-oscillator model of the multi-oscillator circadian clock.

The suprachiasmatic nucleus (SCN) is a collection of about 10000 neurons, each of which functions as a circadian clock with slightly different periods and phases, that work in concert with form and maintain the master circadian clock for the organism. The diversity among neurons confers on the SCN the ability to robustly encode both the 24-h light pattern as well as the seasonal time. Cluster synchronization brings the different neurons into line and reduces the large population to essentially two oscillators, coordinated by a macroscopic network motif of asymmetric repulsive-attractive coupling. We recount the steps leading to this simplification and rigorously examine the two-oscillator case by seeking an analytical solution. Through these steps, we identify physiologically relevant parameters that shape the behaviour of the SCN network and delineate its ability to store past details of seasonal variation in photoperiod.

The Potential of Circadian Realignment in Rheumatoid Arthritis.

In this short review, we discuss evidence supporting the modulation of peripheral circadian systems as a therapeutic strategy for rheumatoid arthritis (RA). We first review the role of proinflammatory cytokines and oxidative stress, two of the primary mediators of chronic inflammation in RA, and their regulation by circadian clock machinery. We further highlight the role of environmental and metabolic signals in regulating the central and peripheral circadian clocks, with an emphasis on seasonal variations in photoperiod and rhythmic metabolic input, respectively. Finally, we hypothesize that the entrainment and realignment of peripheral clock rhythms have the ability to modulate these mediators, improving clinical outcomes in RA patients. Our discussion emphasizes the use of light therapy and time-restricted feeding for entraining peripheral clocks either via the entrainment of the central circadian clock in suprachiasmatic nuclei (SCN) or directly by uncoupling the peripheral circadian clocks from SCN. In doing so, we highlight the use of nonpharmacologic interventions as a potential strategy for improving clinical outcomes in chronic inflammatory conditions such as RA.

Insight into a Physiological Role for the EC Night-Time Repressor in the Arabidopsis Circadian Clock.

Life cycle adaptation to seasonal variation in photoperiod and temperature is a major determinant of ecological success of widespread domestication of Arabidopsis thaliana. The circadian clock plays a role in the underlying mechanism for adaptation. Nevertheless, the mechanism by which the circadian clock tracks seasonal changes in photoperiod and temperature is a longstanding subject of research in the field. We previously showed that a set of the target genes (i.e. GI, LNK1. PRR9 and PRR7) of the Evening Complex (EC) consisting of LUX-ELF3-ELF4 is synergistically induced in response to both warm-night and night-light signals. Here, we further show that the responses occur within a wide range of growth-compatible temperatures (16-28°C) in response to a small change in temperature (Δ4°C). A dim light pulse (<1 µmol m(-2) s(-1)) causes the enhanced effect on the transcription of EC targets. The night-light pulse antagonizes against a positive effect of the cool-night signal on the EC activity. The mechanism of double-checking external temperature and light signals through the EC nighttime repressor might enable plants to ignore (or tolerate) daily fluctuation of ambient temperature within a short time interval in their natural habitats. Taken together, the EC night-time repressor might play a physiological role in tracking seasonal variation in photoperiod and temperature by conservatively double-checking both the light and temperature conditions. Another EC target output gene PIF4 regulating plant morphologies is also regulated by both the temperature and light stimuli during the night. Hence, the EC night-time repressor is also implicated in a physiological output of the PIF4-mediated regulation of morphologies in response to seasonal variation in photoperiod and ambient temperature.

The Environmental Regulation of Reproduction in Cat Fish (Clarias Batrachus) With Special Reference to the Role of Photoperiod

Development, growth and reproduction are influenced by daily and seasonal variations of photoperiod in teleost fish. Experiments were carried out to investigate the environmental regulation of reproduction in cat fish (Clarias batrachus) with special reference to the role of photoperiod. The effect of the photoperiod and temperature regimes on reproductive function was assessed by GSI and histological techniques. The Photo regimes taken up for the study were; 5hr L / 19 hr D at 22 0 C ,9 hr L / 15 hr D at 22 0 C and 18hr L / 6 hr D at 22 0 C. The results reveal the advancement in reproduction of fishes by changing

The Role of a Pseudo-Response Regulator Gene in Life Cycle Adaptation and Domestication of Beet

Life cycle adaptation to latitudinal and seasonal variation in photoperiod and temperature is a major determinant of evolutionary success in flowering plants. Whereas the life cycle of the dicotyledonous model species Arabidopsis thaliana is controlled by two epistatic genes, FLOWERING LOCUS C and FRIGIDA, three unrelated loci (VERNALIZATION) determine the spring and winter habits of monocotyledonous plants such as temperate cereals. In the core eudicot species Beta vulgaris, whose lineage diverged from that leading to Arabidopsis shortly after the monocot-dicot split 140 million years ago, the bolting locus B is a master switch distinguishing annuals from biennials. Here, we isolated B and show that the pseudo-response regulator gene BOLTING TIME CONTROL 1 (BvBTC1), through regulation of the FLOWERING LOCUS T genes, is absolutely necessary for flowering and mediates the response to both long days and vernalization. Our results suggest that domestication of beets involved the selection of a rare partial loss-of-function BvBTC1 allele that imparts reduced sensitivity to photoperiod that is restored by vernalization, thus conferring bienniality, and illustrate how evolutionary plasticity at a key regulatory point can enable new life cycle strategies.

Sunshine, Sadness and Seasonality: 25-Hydroxyvitamin D, and Depressive Symptoms in the United Arab Emirates (UAE)

Several studies have described the prevalence of vitamin D deficiency in Gulf Arab populations and focused on the physical health consequences of VTD deficiency. No work to date on this population has explored the psychological consequences of VTD deficiency or the issue of seasonal variations in deficiency and symptom severity. The aim of this study was to assess seasonal variation in depressive symptoms and 25-Hydroxyvitamin D levels, exploring the relationship between vitamin D deficiency and mood in female university students in the United Arab Emirates. A cross-sectional study design was used to assess seasonal variation in mood, and a correlational analysis to assess the relationship between vitamin D levels and depressive symptoms. Depressive symptoms were positively correlated with 25 (OH)D levels. Seasonal variation was observed for both depressive symptoms and vitamin D levels, the summer cohort having the highest levels of vitamin D deficiency and reporting greater levels of depressive symptomatology. The results support previous studies that have suggested an association between 25 (OH)D levels and depressive symptoms The observed seasonal variation supports the idea of a summer time depressive spike in the UAE, probably due to increased heat/sun avoidance rather than seasonal variations in photoperiod. These findings have implications for mental health promotion and prevention of depressive illness.

Evidence for developmental plasticity in response to demographic variation in nature

Adaptive developmental plasticity has been demonstrated across a number of taxa in response to variables such as photoperiod, resource abundance, and predator presence. Demographics also vary temporally and spatially within populations, but few studies have examined the possibility that developmental plasticity in response to changes in these variables can alter phenotypic distributions. Plastic responses to variable population density and sex ratio may play an important role in explaining phenotypic variation in nature. In this study, we examine two species of spiders (Nephila plumipes and Argiope keyserlingi) to examine whether there is evidence that males alter their development in response to demographics in natural populations. We studied spiders in which developing males can use pheromones as a cue of the density of conspecific males and females. We used published information about the mating systems and life history of each species to make predictions about expected patterns of plasticity in development time and correlated changes in adult body size in response to demographic variation. Within each species, male size and mass were positively correlated with the density of males but negatively correlated with the density of females, and as predicted, this was true only when calculated at spatial scales relevant to selection in each species. In contrast, seasonal variation in photoperiod could not explain measured variance in male size. Our results support the idea that developmental plasticity in response to demographics has a significant effect on phenotypic distributions in natural populations. Our results suggest that a proportion of variation in male phenotypes could be explained as a plasticity-mediated evolutionary response to variation in population demographics rather than as a physiological response to resource abundance and/or photoperiod.

Seasonal variation in serum concentrations of reproductive hormones and urinary excretion of 6‐sulfatoxymelatonin in men living north and south of the Arctic Circle: a longitudinal study

Seasonal variation in photoperiod or temperature may influence human reproductive biology. The present study evaluated whether seasonal changes occurred in the levels of reproductive hormones and the major melatonin metabolite, 6-sulfatoxymelatonin (aMT6s), in populations exposed to extreme variation in photoperiod and temperature. Two separate cohorts of Norwegian men were recruited from the general population in either of two locations: Tromsø (69.5 degrees N, n = 92) or Oslo (60 degrees N, n = 112), located north and south of the Arctic Circle (66.5 degrees N), respectively. Four blood and 12-h overnight urine samples were obtained on separate occasions over a 12-month period, including during the photoperiod maximum and minimum. Serum concentrations of FSH, LH, testosterone (T), oestradiol (E(2)), SHBG and the urinary excretion of aMT6s were assessed. Statistical analysis using generalized estimating equations indicated that LH levels were lowest during early winter in both locations (both P = 0.01). In Tromsø, free T and E(2) concentrations peaked during early winter (P = 0.02 and 0.003, respectively). In Oslo, free T levels were lowest during early winter (P = 0.06) whereas E(2) levels were lowest during late summer (P < 0.001). Urinary aMT6s concentrations were lowest during early summer in Tromsø and Oslo. Concentrations peaked during early winter in Tromsø (P < 0.001) and during late winter in Oslo (P < 0.001). LH levels exhibited similar changes in both locations, whereas the patterns of changes of the sex steroid concentrations differed, possibly indicating different underlying mechanisms. Excretion of aMT6s was lowest during early summer in both locations, indicating that the long natural photoperiod was sufficient to cause suppression of melatonin secretion. Whether these changes have any biological significance remains uncertain.

Endogenous control of migration and calendar effects in an intratropical migrant, the yellow-green vireo

Endogenous circannual rhythms control the time course for moult, migratory fattening and autumn migration in juveniles of several bird species that breed in the temperate zone. Exogenous factors, such as daylength, can also exert a measure of control: photoperiodic cues detected by birds that hatch late in the season induce accelerated juvenile development, assuring that late-hatched young migrate at the same time as early-hatched young. Whether these same mechanisms of control also apply to birds that migrate between breeding and nonbreeding areas entirely within the tropics at latitudes characterized by little seasonal variation in photoperiod is unknown. We conducted a common-garden experiment in Panama (9°N) in which we hand-reared wild-caught nestling yellow-green vireos, Vireo flavoviridis, under a constant photoperiod and monitored them for the expression of juvenile moult, migratory fattening and migratory activity. Even in the absence of a seasonal photoperiodic cue, juveniles moulted, accumulated fat reserves and initiated migratory activity, suggesting endogenous control of these processes. Age at the onset of moult, migratory fattening and migratory activity were each significantly negatively correlated with hatch date, however, so that the expression of these three processes was synchronized among juveniles with respect to the time of the season. We suggest that the slight differences in daylength perceived either by the nestlings themselves in the short time before they were collected (at 6–8 days of age) or by the adult females during egg production influenced rate of nestling development, thus allowing later-hatched young to moult, accumulate fat and migrate at a younger age than early-hatched young. Remarkably, photoperiod differed between the earliest and latest hatch dates in this study by only 33min.

Melatonin modulates secretion of growth hormone and prolactin by trout pituitary glands and cells in culture.

In Teleost fish, development, growth, and reproduction are influenced by the daily and seasonal variations of photoperiod and temperature. Early in vivo studies indicated the pineal gland mediates the effects of these external factors, most probably through the rhythmic production of melatonin. The present investigation was aimed at determining whether melatonin acts directly on the pituitary to control GH and prolactin (PRL) secretion in rainbow trout. We show that 2-[125I]-iodomelatonin, a melatonin analog, binds selectively to membrane preparations and tissue sections from trout pituitaries. The affinity was within the range of that found for the binding to brain microsomal preparations, but the number of binding sites was 20-fold less than in the brain. In culture, melatonin inhibited pituitary cAMP accumulation induced by forskolin, the adenyl cyclase stimulator. Forskolin also induced an increase in GH release, which was reduced in the presence of picomolar concentrations of melatonin. At higher concentrations, the effects of melatonin became stimulatory. In the absence of forskolin, melatonin induced a dose-dependent increase in GH release, and a dose-dependent decrease in PRL release. Melatonin effects were abolished upon addition of luzindole, a melatonin antagonist. Our results provide the first evidence that melatonin modulates GH and PRL secretion in Teleost fish pituitary. Melatonin effects on GH have never been reported in any vertebrate before. The effects result from a direct action of melatonin on pituitary cells. The complexity of the observed responses suggests several types of melatonin receptors might be involved.

The winter immunoenhancement hypothesis: associations among immunity, density, and survival in prairie vole (Microtus ochrogaster) populations

Seasonal variations in photoperiod, temperature, and population density have been shown to modulate immune responsiveness of animals in laboratory studies. To examine these associations under natural conditions, we monitored 3 populations of prairie voles (Microtus ochrogaster) for temporal variations in selected immunological parameters, population density, and survival rate from winter 1996 to spring 1997. Spontaneous and cytokine-stimulated T-cell proliferative responsiveness of prairie voles peaked in winter and declined in spring. Relative organ mass, hemolytic-complement activity, and in vivo hypersensitivity responses varied temporally but showed no clear seasonal trend. The population density and survival rate of all 3 prairie vole populations varied temporally and correlated with measures of immunity. Multiple regression analysis indicated that the model containing relative spleen mass, cytokine-stimulated T-cell proliferation, and in vivo hypersensitivity explained a significant amount of variability in population density, while cytokine-stimulated T-cell proliferation and relative thymus mass explained a significant amount of variability in survival rate. The results suggest that seasonal environmental changes can enhance immune responsiveness of a host and may counteract the immunoenhancing effects of photoperiod in wild populations of prairie voles. Our results also suggest that there is an association between immune function and demography in wild populations.

Reproductive patterns and birth seasonality in a South-American breeding colony of common marmosets,Callithrix jacchus

We analyzed data on captive-born and wild-caught females housed under natural conditions in a colony located in northeastern Brazil. No differences in reproductive performance were found between captive-born and wild-caught females. Twins were the most frequent litter size, followed by triplets and singletons. No parity effect was observed, with similar infant survival for nulliparous and multiparous females. No significant departures in sex ratio were detected for births and mortality of the male and female infants. The age of the females at the time of pairing showed a negative correlation with pairing-parturition length, but did not affect infant survival. The prolongation in pairing-parturition interval (PPI) and interbirth interval (IBI) was related to birth seasonality. The births were clustered in the second half of the dry season and the beginning of the wet season (November–March), and the time of pairing and the time of infant birth influenced the PPI and IBI, respectively. The use of outdoor cages, which allowed the animals to be aware of the seasonal variations in photo-period and rainfall seems to be sufficient to time the reproductive activity, even when the animals are maintained on a constant food supply.