Introduction

Reproductive function represents a rather unique endocrine system since its “goal” is the perpetuation of the species, not the individual. Hence its regulation in humans, which integrates both exogenous and endogenous factors, and represents a complex system with large within- and between-women variability in both the menstrual cycle length and the ovulatory cycle.

In human clinical practice various tools are routinely used to assess female reproductive status including keeping a menstrual diary, charting body temperature, or follicular ultrasound monitoring (Ecochard et al., 2001), but these methods are not appropriate for field situations, especially with less literate or illiterate subjects. Although measuring progesterone and oestradiol levels in serum is the gold standard, obtaining blood samples can be difficult in studies involving serial sampling, especially in developing countries, owing to ethical, logistical and cultural constraints. So the focus here is on the use of changes in hormonal profiles, collected by non-invasive methods, to examine the likelihood of ovulation occurring. In field situations this methodology can be applied to both literate and illiterate women.

The World Health Organisation (WHO) Collaborating Centre developed non-invasive immunoassay methods for measuring human steroid hormones from saliva and urine in the 1980s (Sufi and Donaldson, 1988; Sufi et al., 1985). This technique uses enzyme immunoassay to detect urinary pregnanediol-3-alpha-glucuronide (PdG) and oestrone-3-glucuronide (E1G), the urinary metabolites of progesterone and estradiol, respectively, throughout the menstrual cycle.

The Second Parkes Foundation Workshop was held in Cambridge in September 2007. The Foundation is a charity formed to promote the study of the biosocial sciences, i.e. the interdisciplinary area between biology and sociology, the publication of the results of such studies and to support related charitable activities of an educational or scientific nature. Each year it supports six to eight Masters and PhD students in their field studies.

The theme of the First Parkes Foundation Workshop was Human Adaptability. Here in the second volume the focus is on Reproduction and Adaptation in which four themes are explored: reproduction and the environment, energetics and adaptation, sexual maturation and ovulation assessment, and demography, health and fertility.

The editors would like to thank The Parkes Foundation for their generosity in funding the two-day meeting, and the speakers and authors of the chapters for their interest and commitment. We would like to express special thanks to Dr Rie Goto, who was responsible for most of the conference logistics, and also helped in preparing the manuscript for publication.

Introduction

The main seasonal factors that can influence fertility in a number of mammalian species are photoperiod, temperature and humidity. Although traces of the same kind of regulation are likely to be found in most species, human beings differ from most other species in that they are socialized and are non-seasonal breeders, and it is worth asking whether these seasonal factors influence fertility in humans.

This chapter will review recent observations about circannual variability of a number of measures of human fertility, including sperm quality, menstrual cycle cyclicity and hormonal rhythmicity, in addition to factors influencing birth seasonality. The ways in which climatic seasonality, as mediated through variations in photoperiod, temperature or humidity, can interfere with the physiological processes associated with fertility are also examined, as is the importance of interactions between environmental factors and social life in determining human fertility.

Seasonal variability in fertility parameters

Sperm quality

The last decade has seen an increase in medically-assisted conception in developed countries, and the number of sperm banks has risen as a consequence. Studies of sperm quality, carried out across the calendar year, show seasonal fluctuations in semen quality for both fertile and infertile males.

Analyses of semen from fertile volunteer donors in Lille (France) have shown seasonal variability in sperm count, with the highest values recorded in late winter and early spring and the lowest values recorded in late summer (Saint Pol et al., 1989). No circannual pattern was demonstrated either for sperm volume or for percentage of motile spermatozoa. A similar finding has been reported in the United States.

Introduction

The absence of mechanization and transport facilities in the Third World requires subsistence farmers, horticulturalists and hunter-gatherers to sustain moderate to high energy expenditure (Lawrence et al, 1985; Bleiberg et al, 1980; Bentley, 1985). Their diet is mainly derived from noncultivated or home-grown food. Marked seasonal food shortage and chronic malnutrition are common occurrences (Bahuchet, 1988; Pagezy, 1984; Swinton, 1988; Bailey & Peacock, 1988; Hill et aL, 1984), and a varying form of adaptation might be needed to achieve a ‘long-term energy equilibrium’ (Ferro-Luzzi, 1988).

This is particularly relevant to rural areas where the phenomena concerning fertility are more acute than those observed in urban areas in the same countries. Recent immigrants to large urban areas have generally changed their diet to a more diversified one. At the same time women seem to reduce their physical workload, as many of them work as maids or sell home-baked food in the market, which is a quite different pattern of energy expenditure from that of the daily activity of women living in the bush. Another difference between urban and rural areas is the more widespread use of modern contraceptives in towns, including sterilization for men, or the use of long-duration contraceptives for women, such as six monthly injections of Depo-Provera.

In Third World populations, breastfeeding traditionally continues for long periods of time, and this is accompanied by a long duration of postpartum amenorrhoea with long birth intervals. Health care facilities are limited and the child mortality rate up to the age of five years is generally high.