Annual Variation in Ecology and Reproduction of Wild Simakobu (Simias concolor)

Seasonal breeding in primates is related to the degree of environmental seasonality, particularly the availability and predictability of food. Southeast Asian species in general show moderate birth seasonality due to either low environmental seasonality or unpredictable fluctuations of mast-fruiting food resources. One Southeast Asian primate, the simakobu (Simias concolor), however, has been reported to be a strict seasonal breeder with births occurring in June and July only. It is unclear whether these observations are characteristic of the species or result from a sampling bias. To address this question, we documented the annual distribution of 11 births in eight groups of simakobu over two consecutive years at Pungut, an undisturbed site on Siberut Island, Indonesia. We assessed annual variation in ecology and reproduction via rainfall, temperature, food availability, feeding time, physical condition, conceptions, and births. Mean monthly temperature was nearly constant (26.3–27.1?°C), and monthly precipitation always high (219–432?mm). Although simakobu foods were abundant year-round, there were two fruit-feeding peaks in June and September. In contrast to previous reports, we documented births in 7?mo. Most births occurred in October (45?%), the wettest month of the year, and most conceptions in March and April, following a peak in unripe fruit availability. Although sample sizes are very small, females seemed to conceive when their physical condition was best, suggesting that simakobu time conceptions flexibly to the recovery of energy reserves. Across study sites, births occurred in 10 calendar months, indicating that simakobu reproduction is not strictly seasonal.     

Seasonality of births in South-Eastern Nigeria.

A study of seasonal pattern of births in Nigeria was conducted using data on live births for the period 1971-76 from 4 climatically similar southeastern states (Anambra, Imo, Cross River and Rivers State). After pooling the data, the monthly distribution for different years was calculated. The resulting time series was graphically compared with temperature levels during time of conception, or nine months back. Data analysis revealed a seasonal pattern in the monthly distribution of births, with the peak period observed during April-June, and a trough during November-January. Counting 9 months back, the conceptions were calculated as occuring during July, August, or September of the previous year. In terms of the annual agricultural cycle of the southeastern states, July-September corresponded with the period of light farm work or relative idleness for men (main activity during this period is weeding and tending the crops, tasks primarily done by women and children); this is also a period of abundance of fruits, vegetables and palm wine, and rains after dusk, conditions conducive to mating.     

Seasonal variation of livebirths, stillbirths, extramarital births and twin maternities in Switzerland.

A study was made of the seasonal variation in all births, and births according to marital status, multiplicity and birth status (live and still) in Switzerland recorded between 1876 and 1990. To obtain seasonal variation in as pure as possible form, our analyses are based on rates. When comparing the seasonality in data sets showing markedly different levels, standardised indices were used. Assuming the length of pregnancies with twins to be about one month shorter than for pregnancies with singletons, lagged twinning rates were calculated but, in comparison with actual twinning rates, the general seasonal variation remained. Therefore, this study was based on actual twinning rates. A monotonic increase in the amplitude of the seasonal variation in general births was noted for the period 1876-1930, with strong seasonal variation holding for 1921-1980. After that, a marked decline in the amplitude can be observed. Seasonality of both all births and twin maternities showed very similar pattern for the periods 1876-1930 and 1969-1990, with maxima in the spring (March-May) and troughs in late autumn (October-December). Twin maternities showed a strong seasonality for the period 1876-1930, being about 20% higher in March than in October. The twinning rate in the period 1876-1930 was about 2.6 per thousand units higher than in the period 1969-90. For twin maternities there was also a stronger seasonal variation during the earlier period than during the later one. The pattern of the seasonal variation for extramarital births, showing a maximum in February (conceptions in May-June) and a minimum in August (conceptions in November-December) with a difference of no less than 24% was more marked than for the marital births. It seems likely that this seasonality of extra-marital maternities was due mainly to seasonal variation of coital rates and multiple ovulation in the early summer months coinciding with optima of light, temperature and food supply. A strong reduction in the rate of stillbirths (gestational age more than 29 weeks) was observed during the twentieth century. The stillbirth rate declined from about 40 per 1000 in the 1870s to fewer than 5 per 1000 in the 1980s. Irrespective of this strong decline in the stillbirth rate, the same seasonal rhythm was noticed throughout the period with high stillbirth rates among births around March and low rates during the summer and autumn.     

Seasonality of births and conceptions in a pastoral community of the province of l'Aquila (Abruzzo,Italy), 1802-1965

Natality rates and seasonality of births and conceptions were analyzed from 6,116 birth records in the pastoral community of Roio (Abruzzo, Italy) from 1802 to 1965. Gross natality rates averaged 25.5 x 1000 in the past, lower than those reported for agricultural groups. Seasonality of births showed a marked pattern: 807-67% of births occurred in the first six months of the year. The monthly distribution of conceptions was compared to that of marriages. The results show a high correlation in the 19th century and a lower one in the 20th century. These findings suggest that pastoralism acted as a primary regulator of reproduction in this community.     

Birth seasonally among peasant cultivators: The interrelationship of workload,diet, and fertility

The monthly distribution of live births was analyzed over a 51-year period, 1926–1976, for a rural Taiwan fishing community. Unlike previous studies of birth seasonality, monthly distributions of births did not deviate from what would be expected by chance. This new case is shown to be consistent with the suggestion, developed by Pasternak during a study of birth seasonality in two Taiwan farming communities, that for peasant cultivators the annual cycle of production exerts a more decisive influence on birth seasonality than time of marriage or attributes of temperature, rainfall, or workload. An hypothesis that links the productive cycle to conceptions through the intervening variable of diet is presented and successfully tested using several sets of data on monthly births. A direct effect of nutrition on human fertility, suggested by recent studies of reproductive performance under conditions of nutritional stress, may largely explain seasonality of conceptions and births in populations that experience significant seasonal variation in diet.The Cross Harbor data presented in this paper were collected as part of an ongoing investigation of the comparative demography and social structure of fishing, farming, and market town communities located within a particular Chinese regional system. The support of the National Science Foundation during the period of fieldwork is gratefully acknowledged. I wish to thank G. William Skinner, William H. Durham, Greg Acciaioli, Steven Sangren, Chuang Miao-huei, Harumi Befu, and Philip L. Ritter for their comments on earlier drafts of the present article. I owe a special debt of gratitude to Burton Pasternak (City University of New York), who intellectually inspired and personally encouraged the writing of this paper.     

Effects of maternal birth season on birth seasonality in the Canadian population during the seventeenth and eighteenth centuries

Birth records of the French-Canadian population for the period 1621-1765 were analyzed retrospectively to examine the effect of maternal birth season on the seasonal distribution of births. Preliminary examination indicated that there was a bimodal pattern in birth seasonality: a major peak in early spring, a trough in early summer, a minor peak in autumn, and a trough around December. Because this seasonality was strongly biased at the level of the first birth by the month of marriage, which was concentrated in November, the seasonality of nonfirst births (n = 32,926) was examined in relation to the four seasons of maternal birth. Mothers born in May-July showed a flatter monthly distribution of nonfirst births at a maternal age of 28 years or more. Analysis of marriage-first birth intervals indicated that mothers who married in August-October showed a lower percentage of immediate conception (intervals of 8-10 months), whereas those mothers born in May-July had a higher percentage of immediate conception. This difference in birth seasonality shown by mothers born in May-July is similar to results from early twentieth-century Japan. Some seasonal infertility factors could have affected the embryos at the earliest stage of pregnancy, modifying a part of the seasonal variation in birth rate.     

Seasonality of births is associated with seasonality of marriages in Malta

This study was carried out to quantify secular trends in seasonal variation in births in Malta, a small Mediterranean country where the vast proportion of births occur in wedlock due to a predominantly Roman Catholic population. It also related such variations to seasonal variation in marriages. Annual seasonal peaks of marriages and births were analysed over the period 1950-1996 by X11 ARIMA. A significant peak in marriages (n = 111,932) in the third quarter of the year was found for almost the entire period under study. This was paralleled by a peak in births (n = 299,558) for the period 1970-1996, which lagged after the peak in marriages by 13-14 months. For the period 1994-1996, when monthly data for monthly pregnancies were available by pregnancy order, the peak in births was caused by first pregnancies only. Seasonal patterns in births occur almost universally due to cultural and/or biometeorological factors. The best known patterns include those of the southern United States, where births decline in April and May, and in northern Europe, where births peak in March and April. In Malta, the late summer peak in births appears to be due to a practical and planned approach by Maltese couples to contraceptive planning, probably influenced by the Roman Catholic ethos and social pressures, with unprotected intercourse occurring only after marriage. In Malta, birth control, albeit by so-called natural methods, was introduced in the 1960s. Prior to this period, births peaked towards the beginning/end of the year, and this may be the more natural seasonality of births in Malta.     

Birth seasonality in the early Spanish-Mexican colonists of California (1769-1898)

The pattern of birth seasonality in California's early Spanish-Mexican colonists between 1769 and 1898 was reconstructed using genealogical data for progeny of 657 marriages. The monthly distribution of the 3,824 births in this sample shows a strong seasonal pattern, with spring and fall peaks (corresponding to peaks in conceptions during July and February) and a low point in October. This seasonal reproductive pattern is the result of a complicated set of interactions among environmental, physiological, and cultural variables. California's strongly developed winter rainfall pattern and the 19th-century agricultural cycle clearly influenced the seasonal pattern of marriages and births in this agrarian society. Several historical processes interacted with these environmental and economic factors to transform the seasonal birth pattern of the early colonists. Through time the birth pattern becomes less variable and the birth maximum shifts from spring to early winter. This appears to be, at least in part, a result of changes in labor patterns and an increase in average parity. These data suggest a multifactorial explanation for birth seasonality, in which the timing of conceptions and births is influenced by both environmental and socioeconomic factors.     

Seasonal variation in conception and various pregnancy outcomes

Abstract

U.S. vital registration data on live births and data on abortions and ectopic pregnancies from a national hospital discharge survey were used to examine the seasonality of conceptions and the influence the conception pattern has on the monthly incidence of abortions and ectopic pregnancies. We found that in the United States conceptions follow a consistent seasonal pattern with the peak in November and December. However, when the pattern for conceptions is controlled, the monthly variation for abortions and ectopic pregnancies is not significant. Therefore, we find no monthly excess for any of these outcomes of pregnancy over that expected as a consequence of the seasonality of conception. We suggest the monthly variation for the number of each of these pregnancy outcomes will best be explained when the seasonal variation in conceptions is understood.     

The Season-of-Birth Paradox

There is a seasonal pattern to births in the US: they peak in August-September, peak less strongly in December-February, and show a deep valley in April. What makes this pattern paradoxical is that, as was shown by a survey of 235 undergraduates, the preferred birth months are April-May, and the least desired months are July-August and December-January. 3 hypotheses have been put forward to explain this paradox. The Bad Data Hypothesis holds that college students are not a proper model for the decision making patterns of actual married couples. The Biological Domination Hypothesis holds that birth patterns are not under volitional control, but are determined by hormones, the pineal gland, weather patterns and fetal mortality patterns. The Misinformed Reproducer Hypothesis holds that couples underestimate the lag time between discontinuing contraception and becoming pregnant. To test this hypothesis, married women, aged 15-44, whose contraceptive and pregnancy histories were known, were selected from the National Survey of Family Growth Cycles for births in 1973-75 and 1979-81. Of these, 1271 women stopped contracepting in order to get pregnant and remembered when they had stopped contracepting. The months of stopping contraception were statistically averaged over the 6 years, and in each year there was a valley in February, March or April; and in 5 of the 6 years there was a peak in June. An 11-month dummy variable regression model was used to test the reliability of these patterns for statistical significance. The analysis showed that couples stopped using contraceptives on the assumption that pregnancy would ensue almost immediately. 10 months from February-April are December-February, and 10 months from June is April. If a 5-month pregnancy lag were added to the 9-month gestation, then births would peak in May-August and the valley would be in January-March. This pattern is still 2 months off from the actual birth distribution; however, the retrospective data probably underestimate the real pregnancy lag.     

Seasonality of births for West Malaysia's two main racial groups.

Patterns of Malay and Chinese births occurring from January 1964 through December 1969 as registered by Malaysia's Department of Statistics were analyzed by comparing them with charts of major religious and secular holidays and with the marriage distributions (by month) of the 2 ethnic groups. For Malays there was a reduction in conc eptions associated with the Moslem month of Ramadan, a period of fasting. For the Chinese, number of conceptions peaked around the Chinese New Year, the main Chinese festival. For both groups the months of high marriage rates corresponded to months of high conception rates. Seasonal fluctuations in birthrates were higher for Malays than for the Chinese, which appeared to be due to the more pervasive effect of Islamic beliefs and practices on the Malay way of life. As the Malays constitute a more rural population, climate-related factors were considered as a possible explanation of their more extreme seasonal fluctuations, but the influence of climate could not be demonstrated for either the Malays or the Chinese.     

Additional confirmation for the effect of environmental light intensity on the seasonality of human conceptions

Causality for the seasonality of human births, which affects populations wordwide, has been a profound mystery for nearly two centuries. Most explanations for seasonality fail because of inconsistent global application. In two previous studies, Cummings (2002, 2003) hypothesized that human reproduction has been responsive to changes in both seasonal environmental light intensity (surface luminosity) and photoperiod. Except at higher latitudes, photoperiod is of secondary importance to that of environmental light intensity. Because of a lack of data, the presence or lack of cloud cover is used as a general proxy for environmental light intensity. These studies show a positive correlation between conceptual seasonality and cloud cover on a worldwide basis, and propose that there is a delay between exposure to increased light and the onset of conceptions. This delay is three months at higher latitudes and one to two months for lower latitudes. Both studies suggest that an excellent means of hypothesis confirmation would be to provide one or more examples of how a seasonal change in cloud cover might alter the number of conceptions in subsequent months. The present study tests this hypothesis. The percentage of possible sunshine and averaged sunshine hours are used to investigate their influence on seasonality of births in Germany and the Netherlands. The evidence shows that a seasonal change in environmental light intensity preceded a change in the peak months for conceptions in Germany and the Netherlands. Although secondary influences are possible, the primary reason for this transition in peak conceptual months seems to be related to the seasonal changes in environmental light intensity for both countries. Although this transitional relationship was predicted in Cummings (2002, 2003), further research is required, especially with regard to more precise measurements of environmental light intensity and its physiological effect on the human endocrine system.     

Reproductive parameters of wild Trachypithecus leucocephalus: seasonality,infant mortality and interbirth interval

Understanding the reproductive parameters of endangered primate species is vital for evaluating the status of populations and developing adequate conservation measures. This study provides the first detailed analysis of the reproductive parameters of wild white‐headed langurs (Trachypithecus leucocephalus), based on demographic data collected over an 8‐year period in the Nongguan Karst Hills in Chongzuo County, Guangxi, China. From 1998 to 2002, a total of 133 live births were recorded in the population based on systematic censuses. Births occurred throughout the year, but the temporal pattern was highly correlated with seasonal variation in temperature and rainfall, with the birth peak coinciding with the dry and cold months of November–March. The average birthrate was 0.47±0.13 births per female per year and mortality for infants younger than 20 months was 15.8%. From 1998 to 2006, 14 females gave birth to 41 infants in four focal groups. The average age at first birth for female langurs was 5–6 years (n=5) and the interbirth interval (IBI) was 23.2±5.2 months (median=24.5 months, n=27). Infants are weaned at 19–21 months of age. The IBI for females with infant loss before weaning was significantly shorter than those for females whose infants survived. It appears that birth seasonality in the white‐headed langurs is influenced by seasonal changes in food availability. The timing of conceptions was found to coincide with peak food availability. The reproductive parameters for white‐headed langurs reported here are quite similar to those reported for other colobine species. One major difference is our observation of lower infant mortality in Trachypithecus. Am. J. Primatol. 71:558–566, 2009. © 2009 Wiley‐Liss, Inc.     

Marriage season, promptness of successful pregnancy and first-born sex ratio in a historical natural fertility population – evidence for sex-dependent early pregnancy loss?

 We investigated population-based vital records of the seventeenth and eighteenth century French Canadian population to assess the effects of marriage season on the outcome of the first births under natural fertility conditions (n=21,698 marriages). Promptness of the first successful conception after marriage differed according to marriage season; the proportion of marriages with a marriage-first birth interval of 8.0–10.0 months was lowest (34%) for marriages in August–October (P=0.001). Although the male/female sex ratio of the babies born with an interval of 8.0–10.0 months was generally higher (1.10) than those with an interval of 10.0–24.0 months (1.05), the marriages in August–October resulted in a significantly reduced sex ratio (0.96) among only the prompt conceptions (P=0.026). We discuss whether this seasonal reduction of the sex ratio could be partly explained by a clustered pregnancy loss of male zygotes in early pregnancy. Received: 5 January 1998 / Accepted: 9 September 1998     

Seasonality of births: stability and change in a developing country

A marked seasonality of births for the two main ethnic groups of peninsular Malaysia, far exceeding the cyclic fluctuations in births in the United States and Canada, was reported for the 1960s. A 36% excess of births over the average monthly number was observed among Malays each January. Among the ethnic Chinese in Malaysia a regular periodicity in the numbers of births was also found, but it was far less marked and the peak occurred in October or November. The peaks in both groups were due in large measure to conceptions that correlate with religious observances or holidays. Here I report on cyclic birth patterns in peninsular Malaysia for the period 1970-1985. Rapid economic development has occurred during this time and has brought with it demographic changes, such as a massive rise in contraceptive use and a decline in birth rates. These demographic changes have been accompanied by the loss of the pronounced seasonal pattern of births among the Malays. The seasonality of Malay births is now of roughly the same magnitude as the seasonality in the United States and Canada, whereas seasonality of births among the Chinese in Malaysia remains essentially unchanged.     

Reproductive life of some Gujar women of Punjab

Data was collected on current age, age at menarche, marriage age, maternal age at 1st birth, age at the birth of last child, age at menopause, total number of conceptions, live births, stillbirths, abortions, dead children and living children for a sample of 150 Gunjar women of Punjab, India, during September and October 1977 to study their reproductive life. The women ranged in age from 45-55 years. The mean age at menarche was 14.90 years for the sample. The median age at menopause was 46.20 years. The mean age at marriage of the present sample was 12.56+-2.50 years; the mean age of the mother at the birth of her 1st child was 16.85 years; and the mean age at the birth of the last child was 38.68 years. The average number of conceptions was 7.2; the average number of live births of these 150 women was 6.90. The fertility of this population was natural as they were not using any family planning method.     

Seasonality of births in nineteenth-century urban Tasmania.

The monthly distribution of births for Hobart Town, Van Diemen's Land, during the period 1839-1859 is examined. Prior research on two rural registration districts in nineteenth-century Tasmania revealed patterns of birth seasonality. The pattern was responsive to both the distinctive seasonal rhythm of regional economic activities and the birth interval and differed from the pattern for all of Tasmania in the twentieth century. Here, I argue that the aggregate monthly pattern of births in a mid-nineteenth-century urban registration district was, by contrast, not seasonal. Some seasonality was found among farmers, seamen, and dealers in foodstuffs but not in other industry groups. Class differences were not apparent. The research establishes that the seasonal distribution of mid-nineteenth-century urban births corresponds neither to nineteenth-century rural patterns nor to the patterns evident in the twentieth century.     

Statistical analysis of the seasonal variation in demographic data

There has been little agreement as to whether reproduction or similar demographic events occur seasonally and, especially, whether there is any universal seasonal pattern. One reason is that the seasonal pattern may vary in different populations and at different times. Another reason is that different statistical methods have been used. Every statistical model is based on certain assumed conditions and hence is designed to identify specific components of the seasonal pattern. Therefore, the statistical method applied should be chosen with due consideration. In this study we present, develop, and compare different statistical methods for the study of seasonal variation. Furthermore, we stress that the methods are applicable for the analysis of many kinds of demographic data. The first approaches in the literature were based on monthly frequencies, on the simple sine curve, and on the approximation that the months are of equal length. Later, "the population at risk" and the fact that the months have different lengths were considered. Under these later assumptions the targets of the statistical analyses are the rates. In this study we present and generalize the earlier models. Furthermore, we use trigonometric regression methods. The trigonometric regression model in its simplest form corresponds to the sine curve. We compare the regression methods with the earlier models and reanalyze some data. Our results show that models for rates eliminate the disturbing effects of the varying length of the months, including the effect of leap years, and of the seasonal pattern of the population at risk. Therefore, they give the purest analysis of the seasonal pattern of the demographic data in question, e.g., rates of general births, twin maternities, neural tube defects, and mortality. Our main finding is that the trigonometric regression methods are more flexible and easier to handle than the earlier methods, particularly when the data differ from the simple sine curve.     

Contents Volume 34 (2003)

Abstract

Our previous studies indicated that temperature and photoperiod were not involved as temporal variables in effecting the annual changes in human conceptions in the USA. However, temperature still receives attention in the current literature as the important variable in determining the temporal pattern of conceptions. Therefore, we present here a replication of our previous study, involving two additional 11‐year cycles of birth data in the USA, from the interval 1945 thru 1966. The data are transformed to remove linear trends, to remove over‐all mean differences, and to remove differences in amplitude. The data are then clustered into the regional groups determined by the previous study. The replication was obtained when these groups exhibited similar patterns within a 11‐year period and among 11‐year periods. The data from four 11‐year intervals are presented, representing more than 100 million Caucasian births in the USA from 1945 thru 1988. The subtle changes with time are described, and a comparison with a southern hemisphere pattern indicates, along with the other data, that temperature is irrelevant in effecting the annual pattern of changes in conceptions.     

Monthly variation in conceptions leading to induced abortion

Abstract

Abortions performed in upper New York State and in New York City over a six‐year period were examined for monthly variation in conception. Significant differences in the month of conception were observed which correlated strongly with monthly conception indices from previous studies of conception leading to illegitimate and legitimate births. Since, for the most part, aborted pregnancies were unwanted, the current findings support the notion that cyclical changes in fecundity may be responsible for seasonality of conception rather than seasonal preferences for marriage followed by an increased risk of conception in the early months after marriage.