Gender difference in benefits of twinning in pre‐industrial humans: boys did not pay

Abstract

Summary 1.We studied how differences in the cost of producing male and female offspring in humans affected the productivity of twin vs. singleton deliveries in two ecologically different areas of pre-industrial (1752-1850) Finland. Given the higher energy requirements of male infants, we predicted sons to suffer more from increased litter size and food scarcity than daughters. 2. We found that the number of offspring surviving to adulthood from a twin delivery differed between the archipelago and mainland areas of rural Finland. On the mainland areas, where crop failures and subsequent famines were common throughout the centuries, twin deliveries were much less successful than in the south-west archipelago, where food conditions were traditionally more stable and survival was ensured by fishing. 3. Productivity of twin deliveries was modified by the gender composition of the twins; female-female twin births were generally most successful and male-male births least successful. 4. On the mainland, giving birth to twins of any gender composition never increased the mothers' reproductive success beyond giving birth to either a male or female singleton, whereas in the archipelago mothers could increase their reproductive output by producing twins. This was because in the archipelago female-female twin deliveries contributed on average more than one adult offspring to the breeding population, whereas a singleton delivery of either gender produced only about 0·7 adults. 5. Our results show that increases in litter size and variability of food conditions increased male mortality. High female twin survival in the archipelago led to higher fitness of twinning mothers in relation to mothers with only singletons. Twinning has traditionally been significantly more common in the archipelago as compared to the mainland, but contrary to the prediction, there were no apparent differences in the frequency of male-male and female-female twins being born.

Selection for Increased Brood Size in Historical Human Populations

“…These results indicate that the tendency for twinning in these historical human populations may not have been merely a maladaptive by-product of selection for other maternal reproductive traits, but likely favored by natural selection, although the strength of selection was likely to have been context dependent. A fitness advantage of delivering twins over singletons has been suggested in recent studies by Gabler and Voland (1994), Lummaa et al (1998Lummaa et al ( , 2001, and Sear et al (2001), although the way in which delivering twins conveys a fitness advantage is not known. When we replaced twin deliveries with the expected productivity of a singleton delivery, we found that twin mothers would have had higher fitness than singleton mothers independently of twinning.…”

“…The key for testing this hypothesis is to recognize the environmental and demographic factors that favor, or select against, twinning. For example, in 19th century southwestern Finland, resource stability in the archipelago areas favored twinning (greater lifetime reproductive success of mothers delivering twins), whereas in poorer nearby mainland areas it was more profitable to produce only singletons (Lummaa et al 1998(Lummaa et al , 2001. Twinning frequencies were also higher in the archipelago, supporting the idea that twinning may have been favored by natural selection in this area, and selected against in mainland areas (Lummaa et al 1998).…”

Early-life disease exposure and associations with adult survival, cause of death, and reproductive success in preindustrial humans

“…This was done to ensure that we captured longer-term effects of disease exposure and to control for variation in survival and reproductive success between families and social classes (32). Finally, because birth order (34), twinning status (33), and birth season (35) are all linked to fitness in this population, we restricted our analyses to individuals for whom this information was recorded, leaving a final sample size of 7,283 individuals, born 1751-1850. None of our study individuals moved between parishes between birth and adulthood.…”

Influence of early-life nutrition on mortality and reproductive success during a subsequent famine in a preindustrial population

“…The preindustrial period in Finland ended around the 1870s; before this, the population was characterized by high birth and death rates, poor transportation, primitive agricultural technology (24), and unreliable healthcare and contraception (44). The chief causes of mortality were diseases such as smallpox, typhus, typhoid, and whooping cough (45), epidemics of which were largely independent of food availability, only coinciding occasionally (46).…”

“…We added fixed effects potentially associated with survival, including study parish and father's social class (rich, middle, and poor), as fixed factors, measured at the same time as crop yield around birth. We also included birth status (singleton or twin) as a fixed factor, because twins show lower survival rates (44), and birth order, maternal age (49), and individual age at the end of 1866 as linear and quadratic covariates. We tested the significance of these variables by dropping them sequentially from GLMMs of survival and comparing them using Akaike's Information Criterion (AIC) values, retaining only those terms that increased model AIC by >2 when dropped.…”