1. We studied how differences in the cost of producing male and female offs
pring 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 Finl
and. 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 traditional
ly more stable and survival was ensured by fishing.
3. Productivity of twin deliveries was modified by the gender composition o
f the twins; female-female twin births were generally most successful and m
ale-male births least successful.
4. On the mainland, giving birth to twins of any gender composition never i
ncreased 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 arch
ipelago 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 c
onditions increased male mortality. High female twin survival in the archip
elago 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 predic
tion, there were no apparent differences in the frequency of male-male and
female-female twins being born.