Habitat characteristics and life history explain reproductive seasonality in lagomorphs

Lagomorphs show extensive seasonal variation in their reproduction. However, the factors causing this large variation have so far mostly been investigated intraspecifically and therefore provide only some exemplary comparisons of lagomorph reproductive seasonality. The present study applies both a categorical description (birth season categories 1–5) and a quantitative measure (birth season length in months) to summarize the degree of birth seasonality in the wild of 69 lagomorph species. Using a comparative approach, I tested the influence of 13 factors, comprising six habitat, five life history and two allometric variables on birth season length in lagomorphs. Leporids mainly show non-seasonal birthing patterns with high intraspecific variation. Their opportunistic breeding strategy with high reproductive output and their large distribution areas across wide latitude and elevation ranges might be the reasons for this finding. Ochotonids reproduce strictly seasonally, likely because they live at northern latitudes, are high-altitude specialists, and occur in limited distribution areas. The most important factors associated with variation in lagomorph birth seasonality are mid-latitude, mean annual temperature and precipitation of a species’ geographical range and life history adaptations including fewer but larger litters in seasonal habitats. Birth seasons become shorter with increasing latitude, colder temperatures, and less precipitation, corresponding to the decreasing length of optimal environmental conditions. Leporid species with shorter breeding seasons force maternal resources into few large litters to maximise reproductive output while circumstances are favourable. Since allometric variables were only weakly associated with reproductive seasonality, life history adaptations and habitat characteristics determine birth seasonality in Lagomorpha.

Unmasking Seasonal Cycles in Human Fertility: How holiday sex and fertility cycles shape birth seasonality

The mechanisms of human birth seasonality have been debated for over 150 years. In particular, the question of whether sexual activity or fertility variations drive birth seasonality has remained open and difficult to test without large-scale data on sexual activity. Analyzing data from half-a-million users worldwide collected from the female health tracking app Clue in combination with birth records, we inferred that birth seasonality is primarily driven by seasonal fertility, yet increased sexual activity around holidays explains minor peaks in the birth curve. Our data came from locations in both the Northern Hemisphere (UK, US, and France) and the Southern Hemisphere (Brazil). We found that fertility peaks between the autumn equinox and winter solstice in the Northern Hemisphere locations and shortly following the winter solstice in the Southern Hemisphere locations.

Early life and Adulthood Depression: Birth seasonality on demographic characteristcs of depressive symptoms

Background Environmental and biological factors in early-life in-utero can have critical health implications in adulthood. However, despite the extensive literature on the association between depressive symptoms and female gender, higher age, lower education, and lower socio-economic status, no studies have thus far investigated these depression-related demographic factors in connection with early-life environment. Here, the present study aimed to explore the effects of birth seasonality on demographic characteristics of depressive symptoms in adults. Methods We employed data from the project of Chinese Labour-forces Dynamic Survey (CLDS) 2016, containing the epidemiological data of depressive symptoms with a probability proportional to size cluster and random cluster sampling method in 29 provinces of China with final sample of 16,181 participants was analysed. Logistic regression analyses were performed to test the relations between having depressive symptoms and various demographic characteristics in the overall population and different layers driven by the season of birth (Spring: March, April, and May; Summer: June, July, and August; Autumn: September, October, and November; Winter: December, January, and February). Results In line with previous studies, female gender, higher age, lower education, lower satisfaction of family income, and northern geographical region were the depressive symptoms-related factors. Season of birth did not significantly contribute to having depressive symptoms. Gender and satisfaction of family income were significant for the linear trend in people born in spring, summer, autumn, and winter. Age was significant for linear trend in people born in spring and winter. Education was only significant in spring. The categorical variable of geographical region had different associations with depressive symptoms across seasons of birth. Conclusions Our findings indicate that although season of birth is not significantly associated with depressive symptoms, it influences the effects of the demographic factors on depressive symptoms (particularly in age). The present study sheds lights on the discussion of environmental and biological effects related to season of birth on adult mental health.

The seasonality of 65 million births in Brazil – 1997-2018

The seasonality of births has been a universal phenomenon worthy of attention since the XVIII century. Exhibiting a variety of forms, many times showing similar patterns in dissimilar societies, and dissimilar patterns in societies alike, there has never been a plausible unifying explanation for their behavior. Finding reasonable causes to understand birth seasonality worldwide is still a challenge. This paper intends to contribute to the discussion on birth seasonality in light of the Brazilian reality, analyzing data about 65,764,313 births from 1997 to 2018 in Brazil. This study investigates monthly birth seasonality, according to social-demographic characteristics of mothers (region, age, race/color, schooling, fertility, and marital status), adjusted to consider the same length for each month of the year. Brazil has two clear periods of birth peaks: the main one is observed in March, April, and May, due to winter conception. A second important peak is observed in September, related to conceptions in December. A clear valley is observed in November and December, due to summertime conceptions. In spite of the continental dimensions of Brazil, its climatic diversity and broad social differences, the Brazilian population presents the same temporal birth distribution, according to geographic spaces and population segments, due mainly to conceptions happening in the Brazilian winter season.