Effects of season of birth on reproduction in contemporary humans: Brief communication
S. Huber1,2,7,
M. Fieder3,4,
B. Wallner4,5,
K. Iber3 and
G. Moser3,6
1 Department of Zoology, University of Vienna, Vienna,
2 Research Institute of Wildlife Ecology, University of Veterinary Medicine, Vienna,
3 Rectors Office, University of Vienna, Vienna,
4 Department of Anthropology, University of Vienna, Vienna,
5 Office of Evaluation and Controlling, University of Vienna, Vienna and
6 Department of Internal Medicine IV, University of Vienna, AKH, Vienna, Austria
7 To whom correspondence should be addressed. e-mail: susanne.huber{at}vu-wien.ac.at
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Abstract
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BACKGROUND: At high latitudes the external environment varies with season, and therefore the season of birth may contribute to the developmental processes during the perinatal period. METHODS: We investigated the association between birth season and measures of reproductive performance (offspring count, percentage childless individuals) in a contemporary sample of women and men. RESULTS: In the male sample (n = 2342), men born in autumn had fewer offspring (mean 1.4 versus 1.62; P < 0.01) and a higher probability of remaining childless (32.6% versus 25.6%; P = 0.01) than men born in spring. The photoperiod at a males birth was significantly positively correlated with his subsequent offspring count (P = 0.023). In the female sample, an association between birth season and reproduction was not found. CONCLUSIONS: We assume that in men, among other seasonal factors, pre- or perinatal photoperiod might be involved in the underlying physiological mechanism.
Key words:
fertility rate/offspring count/photoperiod/reproduction/season
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Introduction
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Environmental factors that are present early in life, either pre- or postnatally, may have an effect on the still developing organism. At high latitudes the external environment varies with the season, and therefore season of birth may contribute to the developmental processes during the pre- or perinatal period. Accordingly, the seasonal course of factors such as photoperiod, infections, or the quality and quantity of nutrition have been assumed to affect early development of individuals with consequences for growth (Weber et al., 1998
), longevity (Doblhammer and Vaupel, 2001
) and reproductive success (Lummaa, 2003
). In addition, evidence from historical data indicates that in women, at least prior to contraception, differences in fecundity are associated with the month of birth (Smits et al., 1997
). Although semen quality has been reported to vary with season (Rojansky et al., 1992
) and birth year (Irvine et al., 1996
) in men, the relationship between season of birth and reproduction has not been investigated previously. We therefore examined birth season and its relation to offspring count in a contemporary sample of women and men.
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Materials and methods
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We used the anonymous employee database of the University of Vienna from 1981 to 2001 (6260 women and 6780 men) to analyse differences in reproductive output respective to season of birth. The empirical data are punctiliously maintained as the bureau is required to keep track of offspring count: each offspring born prior to or during the employment at the University of Vienna is registered, irrespective of marital status. Thus, our data on reproductive output are very accurate. In male employees, average offspring count (mean 1.5) tallies with the average total fertility rate (TFR) in Vienna from 1962 to 1992 (90% of the offspring in our study were born between 1962 and 1992): the average TFR in Vienna from 1962 to 1992 is 1.503 (Statistik Austria, 1990
; 2000
), whereas our female sample is weighted towards non-reproductive women (mean of 1.0 offspring/female employee;
40% childless women). The main reasons for this bias of the female dataset appear to be that due to the well known difficulty of balancing motherhood and career: (i) after childbirth, >50% of the women (particularly among scientists) leave their university positions and are thereby not kept further in the records of the employee database; and (ii) childlessness is particularly high among high educated professional women (Watkins et al., 1998
).
The seasonal variation of the external environment at birth is highly comparable within our database, as >95% of the university employees were born in Austria. We used the photoperiod in Vienna (48° 12' N, 16° 22' E; photoperiod: min = 501 min, max = 965 min) to approximate the photoperiodic conditions present during the perinatal period.
To analyse the effects of season of birth on measures of reproductive output (offspring count, percentage of childless individuals), we used the sample of employees >45 years of age (1649 women and 2342 men), as >95% of men and >99% of women have finished reproduction by the age of 45 years. We had controls for differences related to season of birth in marital status, educational level, mean age when children were born and mean age of mate; all remained non-significant (KruskalWallis H test: P > 0.2 each).
We conducted statistical analyses using the statistical package SPSS 10.0 for Windows. On basis of birthdays, we divided our samples into four groups each: men and women, respectively, born in winter (21 December to 20 March), spring (21 March to 20 June), summer (21 June to 22 September) and autumn (23 September to 20 December). We performed comparisons among these groups using KruskalWallis H test and post hoc MannWhitney U-test, as well as the
2-test. The relationship between photoperiod at a persons birth date and subsequent offspring count was tested with Spearmans rank correlation.
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Results
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We found that in the male sample >45 years of age, season of birth is associated with offspring count. On average, spring-born men have significantly more offspring than autumn-born men, and the average offspring count of men born in summer and winter is intermediate (Table IA). In addition, the percentage of childless individuals is significantly higher in autumn-born than in spring-born men, and again is intermediate in summer-born and winter-born men (Table IIA). Photoperiod at a males birth is significantly positively correlated with his subsequent offspring count (Spearman: R = 0.0468, P = 0.023, n = 2342).
In the female sample >45 years of age, an association between season of birth and offspring count was not found (Table IB). Similarly, associations between season of birth and the percentage of childless women (Table IIB), and between the photoperiod at a womans birth and her subsequent offspring count (Spearman: R = 0.31, P = 0.211, n = 1649), were not found either. Compared with the male sample, the average offspring count is significantly lower (compare Table IA with B) (MannWhitney U-test: P < 0.001), whereas the percentage of childless individuals is significantly higher in the female sample (compare Table IIA with B) (
2-test: P < 0.001).
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Discussion
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We show for the first time that in men, season of birth is related to reproductive performance. The average offspring count is higher and the percentage of childless individuals is lower in men born during spring than in men born during autumn. Moreover, the photoperiod at a males birth is positively correlated with his subsequent offspring count. In the female sample, on the other hand, an association between season of birth and offspring count was not found. This is at least partly because of the weakness of the female dataset, which is weighted towards non-reproductive women (see Materials and methods).
Many environmental factors vary with the season (e.g. photoperiod, infections, nutrition) and might therefore be involved in the underlying physiological mechanisms of the effect of season of birth on reproductive performance in men found here. The pattern showing highest average offspring count among men born in spring (long and increasing photoperiod) and lowest among men born in autumn (short and decreasing photoperiod), and the positive correlation between photoperiod around birth and subsequent offspring count, indicate that photoperiod might be among those factors that affect reproductive development during the perinatal period or during fetal life, with consequences on reproductive function far beyond infancy.
If our findings are to be confirmed, the season of birth may have an impact on the reproductive characteristics of men, including clinical implications for the treatment of subfertility. Further work is needed to elucidate the underlying physiological mechanisms.
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Acknowledgements
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We would like to thank Rector G.Winckler for making the study possible and providing access to the data.
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References
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Submitted on June 24, 2003;
resubmitted on September 11, 2003;
accepted on October 6, 2003.